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LECTURE NOTES
Course : PST 102; PLANT FORMS AND FUNCTIONS (2 Credits /Compulsory)
Course Duration : 15hrs Teaching and 45hrs Practical
Lecturer: BELLO, Omolaran Bashir
Ph.D, M.Sc. (Ilorin), B.Sc. (Ibadan), OND (Computer Studies)
Course: PST 204 Plant Morphology (2 Credits /Compulsory)
Course Duration: 15hrs Teaching and 45hrs Practical
E-mail : [email protected],
obbello [email protected]
[email protected]
Office Location: Department of Biological Sciences
Consultation Hours; 2.30-4.00 pm Monday-Thursdays.
1. FUNCTION OF CELLS AND CELLULAR ORGANELLES
The Cell
The cell is the unit of biological function or structures or structures and of reproduction. I A cell
is also the basic and fundamental unit of life. All living things are composed of cells. Some
microscopic organisms, such as bacteria and protozoa, are unicellular, meaning consist of a
single cell. Plants, animals, and fungi are multicellular and composed of many cells working in
concert. Cells carry out thousands of biochemical reactions each minute and reproduce new cells
that perpetuate life.
Reasons for Cell Study
 The chromosomes carrying the gene resides in the cell
 Mitosis & Meiosis which are primarily responsible for continuity occur within the cell
 It is the first level or stage in the organization of life.
Plant Cell
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Cells fall into one of two categories: Prokaryotic or Eukaryotic cells. Prokaryotic cell is found
only in bacteria and archaebacteria. Eukaryotic cells contain numerous compartments, or
organelles, within each cell are found in plants, animals, fungi, and all other life forms. Plants are
multicellular eukaryote with the ability to photosynthesize.
Structure of a plant leaf
Plant Cell Organelles
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A plant Cell
(i) Chloroplasts
An examination of leaves, stems, and other types of plant tissue reveals the presence of tiny
green, spherical structures called chloroplasts, visible here in the cells of an onion root.
Chloroplasts are essential to the process of photosynthesis, in which captured sunlight is
combined with water and carbon dioxide in the presence of the chlorophyll molecule to produce
oxygen and sugars that can be used by animals. Without the process of photosynthesis, the
atmosphere would not contain enough oxygen to support animal life.
(ii) Cell wall
Cell wall is the most important feature distinguishing the cells of plants from those of animals. In
plants this wall protects the cellular contents and limits cell size. It also has important structural
and physiological roles in the life of the plant, being involved in transport, absorption, and
secretion. A plant's cell wall is composed of several chemicals, of which cellulose (made up of
molecules of the sugar glucose) is the most important. Cellulose molecules are united into fibrils,
which form the structural framework of the wall. Other important constituents of many cell walls
are lignins, which add rigidity, and waxes, such as cutin and suberin, which reduce water loss
from cells. Many plant cells produce both a primary cell wall, while the cell is growing, and a
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secondary cell wall, laid down inside the primary wall after growth has ceased. Plasmodesmata
penetrate primary and secondary cell walls, providing pathways for transporting substances.
(iii) Protoplast
Within the cell wall are the living contents of the cell, called the protoplast. These contents are
bounded by a cell membrane composed of a phospholipid bi-layer. The protoplast contains the
cytoplasm, which in turn contains various membrane-bound organelles and vacuoles and the
nucleus, which is the hereditary unit of the cell.
(iv) Vacoules
Vacuoles are membrane-bound cavities filled with cell sap, which is made up mostly of water
containing various dissolved sugars, salts, and other chemicals.
(iv) Plastids
Plastids are types of organelles, structures that carry out specialized functions in the cell. Three
kinds of plastids are important here. Chloroplasts contain chlorophylls and carotenoid pigments;
they are the site of photosynthesis, the process in which light energy from the sun is fixed as
chemical energy in the bonds of various carbon compounds. Leucoplasts, which contain no
pigments, are involved in the synthesis of starch, oils, and proteins. Chromoplasts manufacture
carotenoids.
(vi) Mitochondria
Whereas plastids are involved in various ways in storing energy, another class of organelles, the
mitochondria, are the sites of cellular respiration. This process involves the transfer of chemical
energy from carbon-containing compounds to adenosine triphosphate, or ATP, the chief energy
source for cells. The transfer takes place in three stages: glycolysis (in which acids are produced
from carbohydrates); the Krebs cycle, also called the citric acid cycle; and electron transfer. Like
plastids, mitochondria are bounded by two membranes, of which the inner one is extensively
folded; the folds serve as the surfaces on which the respiratory reactions take place.
(vii) Ribosomes
Two other important cellular contents are the ribosomes, the sites at which amino acids are
linked together to form proteins,
(viii) Golgi Apparatus
The Golgi apparatus, which plays a role in the secretion of materials from cells.
(ix) Endoplasmic Reticulum
Endoplasmic reticulum is a complex membrane system that runs through much of the cytoplasm
and appears to function as a communication system; various kinds of cellular substances are
channeled through it from place to place. Ribosomes are often connected to the endoplasmic
reticulum, which is continuous with the double membrane surrounding the nucleus of the cell.
(x) Nucleus
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The nucleus controls the ongoing functions of the cell by specifying which proteins are
produced. It also stores and passes on genetic information to future generations of cells during
cell division.
Nucleus and its components
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A typical Animal cell
Differences between plant and animal cell
S/N
Features
Plant cells
1.
Size
Plant cells are relatively large in
size
Animal cells
Animal cells are smaller in
size compared to plant cells
2.
Cell Wall
Present in Plant cell
Absent in animal cell
3.
Chloroplast
Present in Plant cell
Absent in animal cell
4.
Lysosomes
Reduced number of lysosomes
in plant cells
Large number of lysosomes
in animal cell
5.
Centrosome
Centrosome is absent in plant
cells
Centrosome is present in
animal cells
6.
Plastids
Present in plant cells
Absent in animal cells
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7.
Vacuoles
Vacuoles are more conspicuous
Vacuoles are less
conspicuous
8.
Phagocytosis
Plant cells cannot be phagocytic
Animal cells can be
Phagocytic
9.
Centrioles
Cells of higher plants lack
centrioles
Centrioles are present
10.
Plasmodesmata
Plasmodesmata is present
Plasmodesmata is absent
2. CELL DIVISION
Cell division is the process by which a parent cell divides into two or more daughter cells. In
Eukaryotes, there are two distinct type of cell division: a vegetative division, whereby each
daughter cell is genetically identical to the parent cell (mitosis), and a reductive cell division,
whereby the number of chromosomes in the daughter cells is reduced by half, to produce haploid
gametes (meiosis). Most cells divide at some time during their lives. Organisms rely on cell
division for reproduction, growth, and repair and replacement of damaged or worn-out cells.
There are hormones in an organism’s body that sends signals to the cells to prepare for division
when it is needed.
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Cell Division
Cells are classified into two categories: simple, non-nucleated prokaryotic cells, and complex,
nucleated eukaryotic cells. By dint of their structural differences, eukaryotic and prokaryotic
cells do not divide in the same way. Also, the pattern of cell division that transforms eukaryotic
stem cells into gametes (sperm cells in males or ova – egg cells – in females) is different from
that of the somatic cell division in the cells of the body.
The primary concern of cell division is the maintenance of the original cell's genome. Before
division can occur, the genomic information that is stored in chromosomes must be replicated,
and the duplicated genome must be separated cleanly between cells. A great deal of cellular
infrastructure is involved in keeping genomic information consistent between "generations".
1. MITOSIS
Mitosis is the portion of the cell cycle when the cells nucleus is replicated and divided into two
identical nuclei containing genetically identical material. Mitosis results in two cells that are
genetically identical, a necessary condition for the normal functioning of virtually all cells.
Mitosis is vital for growth; for repair and replacement of damaged or worn out cells; and for
asexual reproduction, or reproduction without eggs and sperm. Mitosis forms somatic cells,
which are also referred to as the cells of the body (having a 2 n or diploid number of
chromosomes). This is the type of cell division all of the cells in the body do except for those
responsible for “sex cell” production. Mitosis maintains the cell's original ploidy level (for
example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two
haploid n cells; etc.).
There are four (4) main stages in mitosis:
i.
Prophase
This is the first stage of mitosis. In this stage the sister chromatids also condense to a visible
form. The DNA is arranged and condensed into chromosomes. The nuclear envelope and break
up, exposing the chromosomes. The spindle fibers begin to form the centrioles. You can see how
the DNA is arranged and condensed into chromosomes in the picture.
Prophase stage
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The nuclear envelope also breaks up, exposing the chromosomes. The spindle fibers begin to
form, extending from the centrioles. These are made up of microtubules and attach to the
centromere of the sister chromatids. The centrioles slowly migrate to opposite sides of the cell.
ii.
Metaphase
This is when the chromosomes are lined up along the metaphase or equatorial plate, an
imaginary line in the center of the cell. The chromosomes are moved here with the help of the
spindle fibers and the centrioles.
iii.
Anaphase
The centromere of each chromosome are pulled apart by the spindle fibers, causing the sister
chromatids to separate, creating two daughter chromosomes. One of the daughter chromosomes
is pulled to one side of the cell, while the other is pulled to the opposite pole. This process is
critical, because it ensures that the soon to be daughter cells will each have full, identical sets of
chromosomes, also being identical to the parent cell.
iv.
Telophase
The new nuclei begin to form around the new sets of chromosomes, at each end of the cell. The
chromosomes also begin to unravel, back into their loose form. By the end of this phase, the
spindle fibers are also disassembled. At the same time, cytokinesis begins, and the cell is
pinched” into two new cells. As mitosis comes to an end, the two new nuclei must end up in two
new cells; this is where cytokinesis comes in.
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Cytokinesis is when the cell’s cytoplasm divides into two, making two new cells called daughter
cells. Each of these new cells receives one of the new nuclei, making the daughter cells
genetically identical to the parent cell. Animal cells and plant cells complete mitosis and
cytokinesis differently.
Significance of Mitosis
(i) Genetic stability: Mitosis produces two nuclei which have the same number of chromosomes
as the parent cell. Daughter cells are genetically identical to the parent cell and no variation
in genetic information can therefore be introduced during mitosis.
(ii) Growth: The number of cells within an organism increases by mitosis and this is the basis of
growth in multicellular organisms.
(iii) Cell replacement: Replacement of cells and tissues also involve mitosis. Cells are constantly
dying and being replaced, an obvious example being in the skin.
(iv) Regeneration: Some animals are able to regenerate whole parts of the body such as legs in
crustacean and arm in starfish. Production of new cells involves mitosis.
(v) Asexual reproduction: Mitosis is the basis of asexual reproduction, the production of new
individuals of a species by one parent organism. Many organisms undergo asexual
reproduction.
Differences between mitosis in animal and plant cells
Plant Cell
Animal Cell
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No centriole present.
Centrioles present.
No asters form.
Aster form.
Cell division involves formation of cell plate.
Cell division involves furrowing and cleavages
of cytoplasm.
Occurs mainly at meristems.
Occurs in tissues throughout the body.
MEIOSIS
Meiosis is the formation of gametes, performed by reproductive cells only. This will result in a
reduction of the chromosome number, forming haploid cells (n). So, unlike mitosis, which
produces two daughter cells with identical chromosomes, meiosis produces 4 daughter cells,
each with half the number of chromosomes that are not identical to each other. In a process
called meiosis, the homologous pairs of chromosomes are separated and end up in 4 daughter
cells. This happens in two main states, Meiosis I and Meiosis II. The pairs separate during
meiosis I, and meiosis II is when the sister chromatids separate, much like in mitosis.
Meiosis I
Meiosis I is a 4 step process. The start of this is much like mitosis, the cells have gone through
the G1, S and G2 phases, and thus the DNA has been replicated and is in the form of sister
chromatids, connected at the centromere. The main difference here is that the homologous pairs
of chromosomes pair up and then proceed as follows:
Prophase I
It is the longest phase of meiosis. During prophase I, DNA is exchanged between homologous
chromosomes in a process called homologous recombination. This often results in chromosomal
crossover. The new combinations of DNA created during crossover are a significant source of
genetic variation, and may result in beneficial new combinations of alleles. The paired and
replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four
chromatids, with one chromosome coming from each parent. The process of pairing the
homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross-over
at points called chiasmata (plural; singular chiasma).The prophase I has six (6) sub-stages:
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(a).. Leptotene:
The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words
meaning "thin threads". In this stage of prophase I, individual chromosomes—each consisting of
two sister chromatids—change from the diffuse state they exist in during the cell's period of
growth and gene expression, and condense into visible strands within the nucleus. However the
two sister chromatids are still so tightly bound that they are indistinguishable from one another.
During leptotene, lateral elements of the synaptonemal complex assemble. Leptotene is of very
short duration and progressive condensation and coiling of chromosome fibers takes place.
Leptotene
(b) Zygotene:
The Zygotene stage, also known as zygonema, from Greek words meaning "paired threads"
occurs as the chromosomes approximately line up with each other into homologous chromosome
pairs. This is called the bouquet stage because of the way the telomeres cluster at one end of the
nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes
takes place, facilitated by assembly of central element of the synaptonemal complex. Pairing is
brought about in a zipper-like fashion and may start at the centromere (procentric), at the
chromosome ends (proterminal), or at any other portion (intermediate). Individuals of a pair are
equal in length and in position of the centromere. Thus pairing is highly specific and exact. The
paired chromosomes are called bivalent or tetrad chromosomes.
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Zygotene stage
(a) Pachytene:
The pachytene stage, also known as pachynema, from Greek words meaning "thick threads”
is the stage when chromosomal crossover (crossing over) occurs. Non-sister chromatids of
homologous chromosomes may exchange segments over regions of homology. Sex
chromosomes, however, are not wholly identical, and only exchange information over a
small region of homology. At the sites where exchange happens, chiasmata form. The
exchange of information between the non-sister chromatids results in a recombination of
information; each chromosome has the complete set of information it had before, and there
are no gaps formed as a result of the process. Because the chromosomes cannot be
distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable
through the microscope, and chiasmata are not visible until the next stage.
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Pachytene stage
Diplotene:
During the Diplotene stage, also known as diplonema, from Greek words meaning "two threads"
the synaptonemal complex degrades and homologous chromosomes separate from one another a
little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However,
the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions
where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed
in anaphase I. In human fetal oogenesis all developing oocytes develop to this stage and stop
before birth. This suspended state is referred to as the dictyotene stage and remains so until
puberty.
Diplotene Stage
Diakinesis:
Chromosomes condense further during the Diakinesis stage, from Greek words meaning
"moving through". This is the first point in meiosis where the four parts of the tetrads are
actually visible. Sites of crossing over entangle together, effectively overlapping, making
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chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles
prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into
vesicles, and the meiotic spindle begins to form
.
Diakinesis
Metaphase I
This is where the homologous pairs are lined up next to each other, along the equatorial plate.
Homologous pairs move together along the metaphase plate: As kinetochore microtubules from
both centrioles attach to their respective kinetochore, the homologous chromosomes align along
an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on
the bivalents by the microtubules emanating from the two kinetochore of homologous
chromosomes. The physical basis of the independent assortment of chromosomes is the random
orientation of each bivalent along the metaphase plate, with respect to the orientation of the other
bivalents along the same equatorial line.
Anaphase I
The homologous pairs are now separated, due to the spindle fibers pulling them apart, from the
centromere. Each chromosome still has two sister chromatids. Kinetochore (bipolar spindles)
microtubules shorten, severing the recombination nodules and pulling homologous chromosomes
apart. Since each chromosome has only one functional unit of a pair of kinetochore, whole
chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome
still contains a pair of sister chromatids. During this time disjunction occurs, which is one of the
processes leading to genetic diversity as each chromosome can end up in either of the daughter
cells. Non kinetochore microtubules lengthen, pushing the centrioles farther apart. The cell
elongates in preparation for division down the center.
Telophase I
The nuclear membrane may or may not reform, depending on the species, but in any case,
cytokinesis does occur, resulting in two new cells, each with the haploid number of
chromosomes, which are still in the form of sister chromatids. The first meiotic division
effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the
number of chromosomes but each chromosome consists of a pair of chromatids. The
microtubules that make up the spindle network disappear, and a new nuclear membrane
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surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the
pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells,
occurs, completing the creation of two daughter cells. Sister chromatids remain attached during
telophase I.
TELOPHASE I
The Stages of Meiosis I
Meiosis II
In the second meiotic division the cell moves directly into prophase II, skipping the interphase
replication of DNA. This is what follows after telophase I and cytokinesis. The daughter cells
from meiosis I are what go into this phase. They divide again, but this time occurring much the
same as mitosis. The only difference is that there are n number of chromosomes rather than 2n,
and we end up with a total of4 daughter cells rather than2.Again, since there were two daughter
cells produced in meiosis I, and each of them divide again, the result of meiosis is 4 daughter
cells, each with n number of chromosomes, which are not identical to the original parent cell or
each other. This is done through 4 more steps: prophase II, metaphase II, anaphase II and
telophase II, followed by the final cytokinesis.
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Prophase II
During Prophase II, nuclear envelopes (if they formed during Telophase I) dissolve, and spindle
fibers reform. All else is as in Prophase of mitosis. In prophase II we see the disappearance of the
nucleoli and the nuclear envelope again as well as the shortening and thickening of the
chromatids. Centrioles move to the Polar Regions and arrange spindle fibers for the second
meiotic division. Indeed Meiosis II is very similar to mitosis.
The Prophase II Stage
Metaphase II
In metaphase II, the centromeres contain two kinetochore that attach to spindle fibers from the
centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90
degrees when compared to meiosis I, perpendicular to the previous plate.
Anaphase II
During Anaphase II, the centromeres split and the former chromatids (now chromosomes) are
segregated into opposite sides of the cell. The centromeres are cleaved, allowing microtubules
attached to the kinetochore to pull the sister chromatids apart. The sister chromatids by
convention are now called sister chromosomes as they move toward opposing poles.
The Metaphase II and Anaphase II stage
Telophase II
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Telophase II is identical to telophase I. Cytokinesis separates the cells. Telophase II completes
meiosis. The spindle fibers disappear and a new nuclear membrane forms around each new
group of chromosomes to form four haploid cells.
The Telophase II stage
DIAGRAMMATICAL SUMMARY OF THE STAGES IN MEIOSIS II
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Significance of Meiosis
(i).Sexual Reproduction: Meiosis occurs in all organisms carrying out sexual reproduction.
During fertilization the nuclei of the gamete cells fuse. Each gamete has one set of chromosomes
(is haploid n). The product of fusion is a zygote which has two sets of chromosomes 9the haploid
condition 2n).If meiosis does not occur fusion in gametes would result in the doubling of the
chromosomes foe each successive sexually reproduced generation.
(ii) Genetic Variation: meiosis also produces opportunities foe new combinations of genes to
occur in the gametes. This leads to genetic variation in the offspring produced by fusion of the
gametes.
Comparison of mitosis and meiosis
Mitosis
1. It occurs in the vegetative or somatic cells.
2. After this division two daughter cells are produced.
3. Daughter cells process the same chromosome number as that of parent cell.
4. It is of short duration.
5. Daughter cells are similar to parent cell genetically.
6. There is no crossing over in mitosis.
7. With the splitting of centromere, chromatids are pulled apart towards the respective poles.
Each chromatid behaves as an independent chromosome.
8. This is an equational division.
9. Homologous chromosomes are not arranged in pairs in the equatorial plate.
10. Genetic variation does not occur between daughter cells.
11. DNA synthesis is completed in the interphase.
12. In metaphase, the centromeres are lined up on the equatorial plane and the arms extend into
the cytoplasm.
13. Mitosis maintains ploidy level.
Meiosis
1. It occurs in the reproductive cells. The testes in males and the ovaries in females, in males,
each of the meiotic divisions result in four equally sized haploid cells that mature into functional
sperm cells. In females, the meiotic divisions are uneven, resulting in three tiny cells called polar
bodies and one large egg that can be fertilized.
2. Four daughter cells are produced after meiotic division.
3. Chromosome number of daughter cells is reduced to half.
4. It is of long duration. The complete process is divided in to two divisions.
5. Daughter cells have genetic differences from the parent cell.
6. Crossing over takes place between the non-sister chromatids.
7. Whole chromosome moves apart in anaphase I of meiosis because there is no splitting of
centromere.
8. In meiosis, the first division is reductional and the second one is equational.
9. Homologous chromosomes are arranged in pairs in the equatorial plate.
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10. Exchange between maternal and paternal chromosomal segments does not render the
daughter cell identical.
11. DNA synthesis is not completed in the interphase.
12. In metaphase I, the centromeres of the homologous chromosomes lie towards the two
opposite poles and their arms extend towards the equator.
13. Meiosis reduce ploidy level
Comparison of Mitosis and Meiosis Stages
Stages
Mitosis
Prophase
Homologous chromosomes
remain separate.
No formation of chiasmata.
No crossing over.
Meiosis
Homologous chromosomes
pair up.
Chiasmata form.
Crossing over may occur.
Metaphase
Pairs of chromatids line up on
the equator of the spindle.
Pairs of chromosomes line up
on the equator.
Anaphase
Centromeres divide.
Chromatids separate.
Centromeres do not divide.
Whole chromosomes
separate.
Separating chromosomes and
their chromatids may not be
identical due to crossing over.
Separating chromatids are
identical.
Telophase
Same numbers of
chromosomes present in
daughter cells as parents cells.
Both homologous
chromosomes present in
daughter cells if diploid.
Half the number of
chromosomes present in
daughter cells.
Only one of each pair of
homologous chromosomes
present in daughter cell.
3. HEREDITY
Heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by
which an offspring cell or organism acquires or becomes predisposed to the characteristics of its
parent cell or organism. Through heredity, variations exhibited by individual can accumulate and
cause some species to evolve. The study of heredity in biology is called genetics.
Types of Allele
There are mainly two types of allele namely;
i. Dominant
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This is the relationship between alleles of a gene, in which the phenotypic effect of one allele
masks the phenotypic effect another allele at the same locus
Intermediate (also called "codominant"). Codominant – a phenomenon in which a single gene
has more than one dominant allele.
ii. Recessive
A recessive gene is an allele that causes a phenotype(visible or detectable characteristics) that is
only seen in a homozygous genotype(an organism that has two copies of the same allele) and
never in a heterozygous genotype.
4. NUTRITION IN PLANTS
Nutrition can be defined as the process by which an organism obtains food which is used to
provide energy and materials for its life sustaining activities. Thus the term nutrition includes the
means by which an organism obtains its food and also the process by which the nutrients in the
food are broken down to simpler molecules for utilization by the body.
Modes of nutrition
The chemical substances that provide nourishment to living organisms are called nutrients.
Depending on the mode of nutrition the organism are classified as autotrophs and heterotrophs
Autotrophs
Organisms which utilize carbon dioxide as their sole source of carbon for the formation of
organic food by the process of photosynthesis are called autotrophs. (Self nourishing). In
addition to carbon dioxide, autotrophs require water and several in organic ions. If the autotrophs
prepare their own food by utilizing chemical energy they are called Chemoautotrophs.
Photosynthesis
A green plant differs from other living things in their mode of nutrition, because of their ability
to utilize light energy in order to manufacture their food. This is accomplished through a process
called photosynthesis. This ability to manufacture food is important, not only to plant but to
animals which depends directly or indirectly on plants for food. Photosynthesis therefore is the
process whereby green plants manufacture feeding materials from carbon dioxide and water
using light energy and chlorophyll , this can represented by the following equation.
Sunlight
6Co2 + 6H20 →
C6H12O6 + 6O2
Chlorophyll
(Glucose) (Oxygen)
From the chemical equation above water combines with carbon dioxide in the presence of
sunlight within the chlorophyll of leaves to manufacture food (Glucose) and oxygen is librated as
by product. Photosynthesis takes place in the chlorophyll alls of leaves and stem and other green
part of plants. The cells have chloroplast containing chlorophyll chloroplast are present in guard
cells of stomata, green stems, green floral plants (e.g sepals, petals, stigma) and developing
fruits. The main photosynthetic organs are the leaves and the main photosynthesing material
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required for photosynthesis are water from the soil and carbon dioxide from the atmosphere.
These are conveyed to the chlorophyllous cells where high energy containing compounds like
carbonhydrate and oxygen. The carbohydrate is distributed to all part of the plant while the
oxygen I given off through the stomata into the atmosphere.
Chemistry of Photosynthesis
Photosynthesis occurs in two stages namely
•
Light reaction
2. Dark reaction.
•
Light Reaction
This occurs during the day or in the presence of sunlight. The light energy is captured by the
chlorophyll and electrons are emitted (released). The energy so trap is used to split water
molecules into hydrogen ion (H+) and hydroxyl ion (OH), this splitting of water into hydrogen
ion (H+) and hydroxyion (OH-) is called photolysis of water.
Sunlight
Chlorophyll
2H20 ---------------→
2OH + 2H+
The importance of this stage is to transfer the light energy to chemical energy of ALP
(Adenosine triphosphate) and make reduce NADP (Nicotinamide Adenosine de nucleon tide
phosphate.
In the dark, this energy will be converted to chemical energy in organic compounds. Photolysis
of water can also be represented as.
4H2O
Water
light →
Chlorophyll
4H+ +
hydrogen
ion
4OH
hydroxyl
ion
The hydrogen ion is converted to water 4OH – 2H2O + O2 during this process oxygen is given
out as by produce. A compounds co. enzymes or NADP is reduced by hydrogen ion to NADP
and ATP is formed.
Dark Reaction:
This occurs at night or in the absence of light together with the energy provided by the ATP, the
reduced compound NADP leads to the breakdown of carbon dioxide. This is controlled by
specific enzymes whereby 3 carbon compound (CH2O) or sugar is formed.
4H+ + Co2 Enzyme → CH2 O + H20.
CH2o is the carbon structure from which simple sugar fat & oil, protein etc are formed during the
dark reaction.
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Materials and Conditions Necessary for Photosynthesis
(i)
Carbon dioxide: Carbon dioxide from the atmosphere diffuses into intercellular spaces
through the stomata of leaves then into the Mesophyii all containing chloroplasts.
(ii)
Water and Mineral Salt: these are derived from the soil, they pass into the root of plant
through the root hairs by a process called OSMOSIS, water and mineral salts are
conducted by Xylem through the stem and finally to the Mesophyii cells containing
chloroplants.
(iii) Sunlight: is obtained from solar energy. This is trapped by Chlorophyii of the leaves to
split water molecules into hydrogen ions and hydro xylem ion (photolysis).
(iv)
Optinum temperature: temperature is derived from solar energy and from chemical
reaction within the leaves during which heat is generalized temperature ratume is
essential for the proper functioning of enzymes during photosynthesis.
(v)
Chlorophyll: this is the green pigment found in the spongy Mesophyii of leaves. This is
where food is help to trappe solar energy converting it to chemical energy.
(vi)
Enzymes: this is also an internal condition needed for photosynthesis. It acts as a catalyst
that ensure that reaction is completed.
Uses of Photosynthesis
The main produce formed during photosynthesis is simple sugar notably Glucose. The simple
sugar formed is partly used by the plant and excess of heat. It is converted to starch immediately
for storage the starch is then transported to other part of the plant through the phloem vessels.
This process is known as Translocation.
Importance of Photosynthesis
•
Production of food
•
Purification of the atmosphere
•
Release of oxygen into the environments oxygen needed for respiration is release in the
environment during photosynthesis.
•
Building block for other substances such as protein, fat and oil.
Nutrient Elements
•
The primary macronutrients: nitrogen (N), phosphorus (P), potassium (K)
•
The three secondary macronutrients: calcium (Ca), sulphur (S), magnesium (Mg)
•
The micronutrients/trace minerals: Silicon (Si), boron (B), chlorine (Cl), manganese
(Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), selenium (Se),
and sodium (Na)
Three fundamental ways plants uptake nutrients through the root:
(1) Simple Diffusion, occurs when a nonpolar molecule, such as O2, CO2, and NH3 that follow a
concentration gradient, can passively move through the lipid bilayer membrane without the use
of transport proteins.
(2) Facilitated Diffusion, is the rapid movement of solutes or ions following a concentration
gradient, facilitated by transport proteins.
23
(3) Active transport, is the active transport of ions or molecules against a concentration gradient
that requires an energy source, usually ATP, to pump the ions or molecules through the
membrane.
Functions of Different Nutrients
a. Macronutrients
Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and enzymes
essential for plant life. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative
growth). Too much can delay flowering and fruiting. Deficiencies can reduce yields, cause
yellowing of the leaves and stunt growth.
Phosphorus is necessary for seed germination, photosynthesis, protein formation and almost all
aspects of growth and metabolism in plants. It is essential for flower and fruit formation. Low pH
(<4) results in phosphate being chemically locked up in organic soils. Deficiency symptoms are
purple stems and leaves; maturity and growth are retarded. Yields of fruit and flowers are poor.
Premature drop of fruits and flowers may often occur. Phosphorus must be applied close to the
plant's roots in order for the plant to utilize it. Large applications of phosphorus without adequate
levels of zinc can cause a zinc deficiency.
Potassium is necessary for formation of sugars, starches, carbohydrates, protein synthesis and
cell division in roots and other parts of the plant. It helps to adjust water balance, improves stem
rigidity and cold hardiness, enhances flavor and color on fruit and vegetable crops, increases the
oil content of fruits and is important for leafy crops. Deficiencies result in low yields, mottled,
spotted or curled leaves, scorched or burned look to leaves.
Sulfur is a structural component of amino acids, proteins, vitamins and enzymes and is essential
to produce chlorophyll. It imparts flavor to many vegetables. Deficiencies show as light green
leaves. Sulfur is readily lost by leaching from soils and should be applied with a nutrient
formula. Some water supplies may contain Sulfur.
Magnesium is a critical structural component of the chlorophyll molecule and is necessary for
functioning of plant enzymes to produce carbohydrates, sugars and fats. It is used for fruit and
nut formation and essential for germination of seeds. Deficient plants appear chlorotic, show
yellowing between veins of older leaves; leaves may droop. Magnesium is leached by watering
and must be supplied when feeding. It can be applied as a foliar spray to correct deficiencies.
Calcium activates enzymes, is a structural component of cell walls, influences water movement
in cells and is necessary for cell growth and division. Some plants must have calcium to take up
nitrogen and other minerals. Calcium is easily leached. Calcium, once deposited in plant tissue,
is immobile (non-translocatable) so there must be a constant supply for growth. Deficiency
causes stunting of new growth in stems, flowers and roots. Symptoms range from distorted new
growth to black spots on leaves and fruit. Yellow leaf margins may also appear.
24
b. Micronutrients
Iron is necessary for many enzyme functions and as a catalyst for the synthesis of chlorophyll. It
is essential for the young growing parts of plants. Deficiencies are pale leaf color of young
leaves followed by yellowing of leaves and large veins. Iron is lost by leaching and is held in the
lower portions of the soil structure. Under conditions of high pH (alkaline) iron is rendered
unavailable to plants. When soils are alkaline, iron may be abundant but unavailable.
Applications of an acid nutrient formula containing iron chelates, held in soluble form, should
correct the problem.
Manganese is involved in enzyme activity for photosynthesis, respiration, and nitrogen
metabolism. Deficiency in young leaves may show a network of green veins on a light green
background similar to an iron deficiency. In the advanced stages the light green parts become
white, and leaves are shed. Brownish, black, or grayish spots may appear next to the veins. In
neutral or alkaline soils plants often show deficiency symptoms. In highly acid soils, manganese
may be available to the extent that it results in toxicity.
Boron is necessary for cell wall formation, membrane integrity, calcium uptake and may aid in
the translocation of sugars. Boron affects at least 16 functions in plants. These functions include
flowering, pollen germination, fruiting, cell division, water relationships and the movement of
hormones. Boron must be available throughout the life of the plant. It is not translocated and is
easily leached from soils. Deficiencies kill terminal buds leaving a rosette effect on the plant.
Leaves are thick, curled and brittle. Fruits, tubers and roots are discolored, cracked and flecked
with brown spots.
Zinc is a component of enzymes or a functional cofactor of a large number of enzymes including
auxins (plant growth hormones). It is essential to carbohydrate metabolism, protein synthesis and
internodal elongation (stem growth). Deficient plants have mottled leaves with irregular chlorotic
areas. Zinc deficiency leads to iron deficiency causing similar symptoms. Deficiency occurs on
eroded soils and is least available at a pH range of 5.5 - 7.0. Lowering the pH can render zinc
more available to the point of toxicity.
Copper is concentrated in roots of plants and plays a part in nitrogen metabolism. It is a
component of several enzymes and may be part of the enzyme systems that use carbohydrates
and proteins. Deficiencies cause die back of the shoot tips, and terminal leaves develop brown
spots. Copper is bound tightly in organic matter and may be deficient in highly organic soils. It is
not readily lost from soil but may often be unavailable. Too much copper can cause toxicity.
Molybdenum is a structural component of the enzyme that reduces nitrates to ammonia. Without
it, the synthesis of proteins is blocked and plant growth ceases. Root nodule (nitrogen fixing)
bacteria also require it. Seeds may not form completely, and nitrogen deficiency may occur if
plants are lacking molybdenum. Deficiency signs are pale green leaves with rolled or cupped
margins.
Chlorine is involved in osmosis (movement of water or solutes in cells), the ionic balance
necessary for plants to take up mineral elements and in photosynthesis. Deficiency symptoms
25
include wilting, stubby roots, chlorosis (yellowing) and bronzing. Odors in some plants may be
decreased. Chloride, the ionic form of chlorine used by plants, is usually found in soluble forms
and is lost by leaching. Some plants may show signs of toxicity if levels are too high.
Nickel has just recently won the status as an essential trace element for plants according to the
Agricultural Research Service Plant, Soil and Nutrition Laboratory in Ithaca, NY. It is required
for the enzyme urease to break down urea to liberate the nitrogen into a usable form for plants.
Nickel is required for iron absorption. Seeds need nickel in order to germinate. Plants grown
without additional nickel will gradually reach a deficient level at about the time they mature and
begin reproductive growth. If nickel is deficient plants may fail to produce viable seeds.
Sodium is involved in osmotic (water movement) and ionic balance in plants.
Cobalt is required for nitrogen fixation in legumes and in root nodules of non-legumes. The
demand for cobalt is much higher for nitrogen fixation than for ammonium nutrition. Deficient
levels could result in nitrogen deficiency symptoms.
Silicon is found as a component of cell walls. Plants with supplies of soluble silicon produce
stronger, tougher cell walls making them a mechanical barrier to piercing and sucking insects.
This significantly enhances plant heat and drought tolerance. Foliar sprays of silicon have also
shown benefits reducing populations of aphids on field crops. Tests have also found that silicon
can be deposited by the plants at the site of infection by fungus to combat the penetration of the
cell walls by the attacking fungus. Improved leaf erectness, stem strength and prevention or
depression of iron and manganese toxicity, have all been noted as effects from silicon.
Essential Elements in Plants
Element
Form available
Macronutrients
Carbon
CO2
Major functions
Major components of plant, organic
components
Oxygen
O2
Major components of plants organic
compound
Hydrogen
H2O
Major components of plants organic
compound.
Nitrogen
NO-3 or NH+4
Component of nucleic acids, proteins,
hormone chlorophyll, co enzymes.
Potassium
K+
Cofactors that function in protein
photosynthesis. major solute functioning in
water balance operation of stomata.
Calcium
Ca2+
Important in formation of stability of the cell
walls and in the maintenance of a membrane
26
structure and permeability activates some
enzymes regulate many response of cell to
stimuli.
Magnesium
Mg2+
Phosphorus
H2PO-4,HPO2-4 Components of nucleic acid . Phosphorus
ATP, Several coenzymes.
Sulphur
SO42-
Components of protein and co enzymes.
Micronutrients
Chlorine
Cl-
Require for water splitting step of
photosynthesis functions in water balances.
Iron
Fe2+, Fe2+
Components of cytochromes, activates some
enzymes.
Manganese
Mn2+
Active in formation of amino acids. Activate
some enzymes : required for water splitting
step of photosynthesis.
Boron
H2BO3
Cofactor in chlorophyll synthesis, may be
involved in carbohydrate transport and
nucleic acid synthesis; role in cell wall
formation
Zinc
Zn2+
Active in formation of chlorophyll , activate
some enzymes
Copper
Cu+ or Cu2+
Component of any redox and lignin
biosynthetic enzymes
Nickel
Ni2+
Cofactor for an enzyme functioning in
nitrogen metabolism
Molybdenum
MO2-
Essential for symbiotic relationship with
nitrogen fixing bacteria ; cofactor in nitrate
reduction
Components of chlorophyll: activate many
enzymes.
27
5. RESPIRATION IN PLANTS
Respiration is the exchange of oxygen and carbon dioxide between the atmosphere and the body
cells. It can also be defined as the metabolic processes by which living cells breakdown
carbohydrates, amino acids, and fats to produce energy in the form of Adenosine Triphosphate
(ATP).
Cellular Respiration
Cellular Respiration, process in which cells produce the energy they need to survive. In cellular
respiration, cells use oxygen to break down the sugar glucose and store its energy in molecules
of adenosine triphosphate (ATP). Cellular respiration is critical for the survival of most
organisms because the energy in glucose cannot be used by cells until it is stored in ATP. Cells
use ATP to power virtually all of their activities—to grow, divide, replace worn out cell parts,
and execute many other tasks. Cellular respiration occurs within a cell constantly, day and night,
and if it ceases, the cell—and ultimately the organism—dies.
Cellular Respiration
Two critical ingredients required for cellular respiration are glucose and oxygen. The glucose
used in cellular respiration enters cells in a variety of ways. Plants, algae, and certain bacteria
make their own glucose through photosynthesis, the process by which plants use light to convert
carbon dioxide and water into sugar. Regardless of how they obtain it, cells must have a steady
supply of glucose so that ATP production is continuous. Oxygen is present in the air, and also is
found dissolved in water and diffuses into cells plants.
28
Chemical Reactions and Metabolic Pathways
To understand cellular respiration, it is necessary to understand the nature of chemical reactions.
Chemical reactions can occur outside of living organisms—the rusting of a car, for example, is a
chemical reaction—or they can occur within organisms, where they are termed biochemical
reactions. In a chemical or biochemical reaction, the bonds between atoms that hold molecules
together break apart, and the atoms rearrange to form new molecules. Water molecules, for
example, are composed of hydrogen and oxygen atoms, and under certain conditions, the bonds
between these atoms can break and reform to yield separate molecules of hydrogen and oxygen
gas. In living organisms, most biochemical reactions occur with the help of enzymes, specialized
proteins designed to carry out specific reactions. All biochemical reactions release energy in the
form of heat as they occur.
Cells carry out biochemical reactions to create needed molecules—such as proteins or starch—or
to destroy these molecules once they are no longer needed. If certain molecules are built or
destroyed in a single biochemical reaction, the reaction may release too much heat, which could
incinerate the cell. To control the release of heat, cells build up and break down most molecules
in a linked series of small reactions that release only a little bit of heat at a time. The series of
linked biochemical reactions is called a metabolic pathway.
Cellular respiration is one of the most important metabolic pathways found in cells. This
enzyme-assisted, step-by-step process not only protects the cell from lethal temperature increases
but also provides the cell with a mechanism of transferring the energy of glucose to ATP in a
controlled manner.
How Cellular Respiration Works
The process of cellular respiration occurs in four stages: glycolysis; the transition stage; the
Krebs cycle, also known as the citric acid cycle; and the electron transport chain. Each stage
accomplishes different tasks. Glucose is the primary fuel used in glycolysis, the first stage of
cellular respiration. This all-important molecule is found in the cell’s cytoplasm, the gel-like
substance that fills the cell. Glucose consists of 6 carbon, 12 hydrogen, and 6 oxygen atoms
bonded together, along with electrons (negatively charged atomic particles) associated with each
atom. Of these components, only the hydrogen atoms and certain electrons participate directly in
glycolysis.
29
30
Glycolytic pathways
In glycolysis, glucose is broken down with the help of enzymes and other molecules found in the
cytoplasm. Enzymes first attach two phosphate groups to glucose to make it more reactive. A
phosphate group is a cluster of one phosphorus and four oxygen atoms. The addition of the two
phosphate groups prepares glucose for the action of another enzyme. This enzyme splits glucose
in half to produce two three-carbon molecules, each with one phosphate group attached.
In the next step, an enzyme removes one hydrogen atom and two electrons from each threecarbon molecule. Both hydrogen atoms are modified to hydrogen ions, positively charged
particles. A hydrogen ion and two electrons from each three-carbon molecule are transferred as a
unit to a large molecule called nicotine amide adenine dinucleotide (NAD+) to form two
molecules of NADH. The hydrogen ions and electrons stored in each molecule of NADH are
used to make ATP in later stages of cellular respiration.
In the final steps of glycolysis, two hydrogen atoms are removed from each three-carbon
compound. These hydrogen atoms bond to free-floating oxygen atoms in the cytoplasm to form
water. This step prepares the two three-carbon compounds for action by the next enzyme in the
pathway. This enzyme removes the phosphate group from each three-carbon compound. Each
phosphate group then bonds to a single molecule of adenosine triphosphate (ADP). ADP is
composed of three carbon-based rings and a tail of two phosphate groups. The addition of the
third phosphate group to the tail forms ATP. In this step, two new ATP molecules are produced.
When cells require energy, another enzyme breaks off the third phosphate group, releasing
energy that powers the cell. The removal of the third phosphate from ATP converts ATP back to
ADP, which is used again in cellular respiration to make more ATP. When the two three-carbon
compounds are separated from the phosphate groups, the three-carbon compounds are converted
to two molecules of pyruvate, each composed of three carbon, three oxygen, and three hydrogen
atoms. When glycolysis is complete, important products used in the next steps of cellular
respiration have been produced: two molecules of NADH and two molecules of pyruvate. The
two ATP molecules gained in glycolysis are used for reactions in the cell that require energy.
The pyruvate molecules move from the cytoplasm to special structures in the cell called
mitochondria, where the remaining steps of cellular respiration are carried out. Each
mitochondrion contains a membrane that is folded back and forth many times. This extensive
membrane is studded with hundreds of thousands of enzymes that direct cellular respiration. The
numerous enzymes enable great quantities of ATP to be produced simultaneously in one
mitochondrion. Without mitochondria or a similar structure, most cells could not generate
enough ATP to survive.
The transition stage is a short biochemical pathway that links glycolysis with the Krebs cycle.
In this brief stage, enzymes transfer hydrogen’s and electrons from the two pyruvate molecules
to two molecules of NAD+ to form two more molecules of NADH. Another enzyme breaks off
one carbon and two oxygen atoms from each pyruvate molecule. These atoms combine to form
carbon dioxide, the primary waste product of cellular respiration, which diffuses out of the cell.
As a result of these reactions, each pyruvate molecule is transformed into a two-carbon
compound called an acetyl group. The two acetyl groups unite with two molecules of coenzyme
31
A to form two acetyl coenzyme A molecules. The acetyl coenzyme A molecules are the
molecules that enter the Krebs cycle.
Krebs cycle: The oxidation of pyretic acid into co2 and water is called Krebs Cycle. This circle
is also citric acid cycle because the cycle begins with the formation citric acid. The citric acid is
a carboxylic acid containing 3cooh groups. Hence this cycle is also called as tricarboxylic acid
cycle or TCA cycle. The cycle occurs only in the presence of oxygen. Hence it is an aerobic
process. It takes place in the mitochondria.
32
During the Krebs cycle, the acetyl coenzyme A molecules are processed. As this complex
pathway progresses, six molecules of NADH are formed. Additional carbon dioxide is created,
and this process releases energy that is used to build two molecules of ATP from a pool of ADP
and phosphate groups in the mitochondria. Hydrogen’s and electrons then are transferred to a
33
molecule of flaking adenine dinucleotide (FAD++)to form FADH2, a molecule like NADH that
temporarily stores hydrogen and electrons for later use. By the end of the Krebs cycle, most of
the usable energy from the original glucose molecule has been transferred to ten molecules of
NADH (two from glycolysis, two from the transition stage, and six from the Krebs cycle); two
molecules of FADH2; and four molecules of ATP, two of which were formed in glycolysis.
Electron Transport Chain
.
The reactions of the electron transport chain occur in several closely spaced molecules embedded
in the mitochondrial membrane. Acting like specialized delivery trucks, the NADH and
FADH2molecules dump off their load of electrons and hydrogen ions near these electron
transport chain molecules. The first molecule in the chain has an attraction for electrons and
grabs them, but the molecule next to it in the chain has an even stronger attraction and grabs the
electrons away from the first molecule. The electrons are passed down the chain in this manner,
until they reach oxygen, the final molecule in the chain. Oxygen has a stronger appetite for
electrons than any molecule in the chain, and the electrons therefore are held by oxygen. They
are joined by the hydrogen ions that were dropped off by NADH and FADH2 at the beginning of
the electron transport chain. The combination of the electrons, hydrogen ions, and oxygen forms
water, used by the cell in other biochemical reactions. As NADH and FADH2 release hydrogen
and electrons in the electron transport chain, they are converted back to NAD+ and FAD++,
respectively, providing the cell with a steady supply of these molecules so that cellular
respiration can be carried out over and over again.
As the electrons flow down the electron transport chain, they release a veritable windfall of
energy that is used by an enzyme to make more ATP, again from the pool of ADP and phosphate
groups in the mitochondria. In most cells, the electron transport chain produces 32 molecules of
ATP. Together with the two ATP molecules gained in glycolysis and the four generated in the
Krebs cycle, cellular respiration produces a grand total of 38 molecules of ATP for every
molecule of glucose processed. Glucose molecules enter the cell by the hundreds of thousands.
They are processed simultaneously to generate millions of ATP molecules every second. Some
of the ATP molecules remain in the mitochondria to supply it with needed energy, but the
majority stream from the mitochondria to the cytoplasm, where they fuel the cell’s activities. It is
estimated that a single human brain cell uses a staggering 10 million ATP molecules per second
to carry out its tasks.
Although glucose is the primary fuel for cellular respiration, cells can rely on other molecules to
produce ATP. The cellular respiration pathway is connected to other metabolic pathways that can
donate molecules to cellular respiration at different steps along the way. For example, glycerol, a
breakdown product of fat, can enter the cellular respiration pathway in the middle of glycolysis.
Another product of fat digestion, fatty acids, can enter at the transition stage. Glycerol is
modified in glycolysis to pyruvate, and fatty acids are modified to acetyl coenzyme A in the
transition stage. The pyruvate and acetyl coenzyme A are processed through the remaining steps
of cellular respiration to yield ATP. The ability to use alternate molecules enables cells to keep
ATP production going even if they run out of glucose. Marathon runners, for example, first use
up their glucose reserves to make ATP, and then draw on their fat reserves to generate the ATP
needed to get to the finish line.
34
Anaerobic Pathways to ATP
Although most organisms on Earth carry out cellular respiration to generate ATP, a few
rely on alternative pathways to make this vital molecule. These pathways are anaerobic—that is,
they do not require oxygen. Fermentation is a type of anaerobic pathway used by certain species
of bacteria that live in anaerobic environments, such as stagnant ponds or decaying vegetation.
Some cells produce ATP using both anaerobic and aerobic pathways. For example, muscle cells
typically carry out cellular respiration, but if they do not receive enough oxygen, as can occur
during strenuous exercise, muscles switch to fermentation. Yeast cells also carry out both
pathways, depending on whether they are in an aerobic environment, such as soil, or an
anaerobic one, such as inside a wet lump of dough. Fermentation is much less efficient than
cellular respiration, however, producing only two molecules of ATP for every glucose molecule
processed.
Fermentation occurs in two stages: glycolysis and the recycling stage. The glycolysis pathway in
fermentation is virtually the same as the glycolysis pathway of cellular respiration, relying on
enzymes, NAD+, and other molecules to transfer the energy of glucose to two molecules each of
pyruvate, NADH, and ATP. This is the only stage of fermentation that yields ATP, and while it
produces a relatively small amount, it is ample for simple cells. Molecules of NAD+, like other
molecules required in glycolysis, must be in constant supply for glycolysis to continue without
interruption. In the recycling stage of fermentation, the NADH produced in glycolysis is recycled
back into NAD+ to be used again in glycolysis.
While all cells that carry out fermentation use the recycling stage to make more NAD+, slight
variations exist, depending on the organism involved. Many variations produce a number of
interesting by products that are waste as far as the cell is concerned but are useful to humans. In
yeast, for example, the recycling stage produces the carbon dioxide gas that makes dough rise in
bread making and the alcohol that makes beer and wine intoxicating. In certain kinds of bacteria,
the recycling stage produces the lactic acid that turns milk into yogurt or cheese.
Other types of anaerobic pathways are carried out by certain species of archaebacteria that live in
extreme environments, such as hydrothermal vents, springs of hot water in the deep ocean. These
anaerobic pathways begin with glycolysis and are followed by two subsequent stages that
resemble the Krebs cycle and electron transport chain of cellular respiration. However, these
anaerobic pathways yield about half the amount of ATP that cellular respiration yields.
6. POLLINATION
Pollination is defined as the transfer of mature pollen grains from the anther of one flower to the
mature stigma of the same flower or another flower of the same plant closely related species.
Pollination is the first step which leads to the eventual coming together of male and female
gametes for the sake of fertilization. In spite of a common perception that pollen grains are
gamete, like the sperm cells of animals, this is incorrect; pollination is a phase in the alternation
of generation: achieve each pollen grain is a male haploid plant, a gametophyte, adapted to being
transported to the female gametophyte, where it can fertilization by producing the male gametes
(or gametes in the process of double fertilization). Sperm gets transported to the stigma, where
its two gametes travel down the tube to where the gametophytes(s) containing the female
35
gametes are held within the carpel. One nucleus fuses with the polar bodies to produce the
endosperm tissues, and the other with the ovum to produce the embryo. Hence the term” double
fertilization”. In gymnosperms the ovule is not contained in a carpel, but exposed on the surface
of dedicated support organs such as the scale of a cone, so that the penetration of carpel tissues is
unnecessary. Details of the process vary according to the division of gymnosperms in question.
The receptive part of the carpel is called a stigma in the flowers of angiosperms. The receptive
part of gymnosperm ovule is called the micropyle. Pollination is the necessary step in the
reproduction of flowering plants, resulting in the production of offspring that are genetically
diverse.






The diagrams below shows the flowering parts of pants that deals with pollination. The diagrams
clearly show:
The male part of flower comprising the anther and filament (together called the stamen) and
The female part of the flower: the stigma and style with the ovary containing the ovule at the
base of the flower (the carpel).
Pollen grains lands and sticky stigma.
A pollen tube grows down the style, followed by the male sperm nuclei.
The sperm nuclei fuse with the female ovules.
The ovules develop into seed and the ovary develops into fruits
A complete flower
36
A Diagrammatic Complete Flower
The Diagrammatic Representation of Plant Pollination
37







Importance of Pollination
Pollens are essentially plant sperm, so pollination is essential to continue the lifecycle of
flowering plants.
Flowering plant are the basis of the animal, if plant are not pollinated then there will be no
progeny and all animal that depend on them would starve.
It helps wide flowers reproduce and produce enough seed for dispersal and propagation.
Maintain genetic diversity within a population.
Develop adequate fruit to entice seed dispersal.
It helps in cleaning air, carbon cycling and sequestration.
It helps to purify water and prevent erosion.
Classes of Pollination
i. Wind pollination
ii. Insect pollination
iii. Water pollination
iv. And others.
v.
Wind Pollination
Process of Wind Pollination.
The male flowers ripen before the female flowers on the same plant. This favors cross
pollination. The wind carries pollen grains of one maize plant to another plant with ripe female
flowers. If the pollen falls on ripe stigma, cross pollination takes place. Examples of wind
pollinated flowers are Maize, guinea, rice, millet and wheat.








Characteristics of wind pollinated flowers. (Anemophilous flowers)
They have small, inconspicuous petals/sepals.
Flowers are usually dull coloured.
There is absence of scent.
There is absent of nectars.
Large quantity of pollen grains is produced.
Pollen grains are small, smooth, light and not sticky.
Stigma is elongated and sticky with large surface area.
Anthers are attached to the flowers in such a way that they readily swing in the air and release
the pollen grains.
Insect Pollination (Entomophilies)
This often occurs on plants that have developed colored petals and a strong scent to attract
insects such as, bees, wasps and occasionally ants (Hymenoptera), beetles (Coleoptera), moths
and butterflies (Lepidoptera), and flies (Diptera). The existence of insect pollination dates back
to the dinosaur era.
Process of pollination by insects (e.g. Pride of Barbados).
The insect that normally pollinate pride of Barbados are swallowing tail Butterfly and bees.
When the insect lands on the standard petals it uncoils its proboscis and inserts it through the
38
furrow in the standard petals that leads to the nectar. During this process, the hairy body or wings
of the butterfly or bees touches the pollen grains and these stick to the body or wings of the
insect. When the insect visit another pride of Barbados flowers, the pollens grains on its body or
wings may touch the stigma of this flower thereby bringing about cross pollination.
39
A European honey bee collects nectar, while pollen collects on its body.







Characteristics of insect pollinated flowers. (Entomophilous flower).
They have large conspicuous petals/sepals.
Flowers are usually brightly colored.
They posses scent.
Nectar is also present.
Pollen grains are rough, sticky and relatively few.
The stigma is flat with sticky surface to enable it receive pollen grains.
Petal are shaped and arranged to enable visiting insect becomes dusted with pollen grains.
Examples of insect pollinated flowers include: Hibiscus, Delonix, Cowpea, Crotalaria, Pride of
Barbados, e.t.c.
Water Pollination
Water pollinated plants are aquatic and pollen is released into the water. Water currents
therefore, act as a pollen vector in a similar way to wind currents. Their flowers tend to be small
and inconspicuous with lots of pollen grains and large feathery stigmas to catch the pollen.
However, this is relatively uncommon (only 2%of pollination is hydrophilic). And most aquatic
plants are insect- pollinated, with flowers that emerge into the air.
zoophily:
pollination
is
performed
by
vertebrates.(
such
as birds and bats,
particularly, hummingbirds, sunbirds, spider hunters ,honeyeaters, and fruit bats.): Plants adapted
to using bats or moths as pollinators typically have white petals and a strong scent, while plants
that use birds as pollinators tend to develop red petals and rarely develop a scent (few birds rely
on a sense of smell to find plant-based food).
Anthropophily, pollination by humans: often artificial pollination used in hybridization
techniques.
1
Differences between Insect Pollinated Flower and Wind Pollinated Flowers.
Insect pollinated flowers
Wind pollinated flowers
Flowers are usually large and conspicuous.
Flowers are usually small and
inconspicuous.
2
Flowers are usually brightly coloured.
Dull coloured.
3
There is present of scent.
There is absent of scent.
4
Pollen grains are rough, sticky and relatively
few.
Pollen grains are light, smooth very
numerous.
5
Anthers may or may not be enclosed by the
petals.
Filaments are long so that anthers hang
outside the flowers.
6
Flowers may be held above the leaves
Flowers are carried above the leaves
where they are exposed to the wind.
40
7
Stigma is flat or lobed with sticky surfaces for
easy adherences of pollen grains.
Stigma is a large and feathery hanging
outside the flowers providing large
surface area for easy trapping of pollen
grains.
8
The shapes and flora pairs are such that they
enable insect get dusted with the pollen grains
during visiting.
There is particularly adaptive shape as
flowers are small and exposed.
9
Nectars are present.
Nectars are absent.
The mechanics involve in pollination.
Pollination can be accomplished by cross-pollination or by self-pollination:
Cross-pollination: This is also called allogamy, occurs when pollen is delivered to a flower from
a different plant. Plants adapted to outcross or cross-pollinate often have taller stamens than
carpel or use other mechanisms to better ensure the spread of pollen to other plants' flowers.
Conditions or device which aid cross pollination.
Some plants may have conditions or device which may aid cross pollination to take place. These
are dichogamy, Unisexuality and self-sterility.
Dichogamy: Dichogamy refers to the ripening of the anthers and stigmas of a bisexual flower at
different times. Dichogamy occurs in two ways. These are Protandry and protogyny.
(a) Protandry: This refers to the condition in which the anthers of a flower mature earlier than the
stigmas that flowers or other flowers of the same plant so that the mature pollen grains are only
useful to flowers of other plant which have mature stigma to receive them.
(b) Protogyny: Protogyny refers to the condition in which the stigma of flowers matures earlier than
its own pollen grains or those of others flowers of the same plant so that it can only receive
pollen grains from flowers of other plant.
Examples include blue bells and white dead nettle.
Unisexuality: Unisexuality is a situation in which some plant bears only male or female flowers
and not both on the same plant, e.g. pawpaw. Such plant are said to be dioecious plant. Cross
pollination may occur under this condition.
On the other hand, in a monoecious plant the male and female flowers are borne by the same
plant, the female flowers are usually situated higher than the male flowers so that pollen grains
may not reach the stigma of the female flowers. Hence, they will be received only by stigmas of
female flowers of other plants.
Self-sterility: This is refers to situation in which some plants make themselves sterile. The
presence of pollen on their stigmas is injurious to further development of the plant. For
41
examples, they may wither and die. However, when pollen grains come from other plants,
fertilization can take place in such plant. Examples are found in passion flowers and tea.
Self incompatibility: This mechanism ensures that the pollen tube does not develop unless it
has a different genetic composition from that of the stigma. This prevents self pollination.




Advantages of cross pollination.
Cross pollination leads to the production of healthier offspring than self pollination.
It also produces viable seeds.
Offspring or individuals produced are more adapted to the environment conditions.
It also leads to the formation of new varieties with good characteristic.
Disadvantages of cross pollination.
 It relies on external agent such as wind and insect whose presence at the right time cannot be
guaranteed.
 It may leads to wastage of pollen grains especially pollination by wind.

Self-pollination: This occurs when pollen from one flower pollinates the same flower or other
flowers of the same individual. It is thought to have evolved under conditions when pollinators
were not reliable vectors for pollen transport, and is most often seen in short-lived annual species
and plants that colonize new locations. Self-pollination may include autogamy, where pollen
moves to the female part of the same flower; orgeitonogamy, when pollen is transferred to
another flower on the same plant. Plants adapted to self-fertilize often have similar stamen and
carpel lengths. Plants that can pollinate themselves and produce viable offspring are called selffertile. Plants that cannot fertilize themselves are called self-sterile, a condition which mandates
cross pollination for the production of offspring.
Conditions or device which aid self pollination.
Some plants have conditions or device which aid self pollination to take place. These conditions
are cleistogamy and homogamy.
Cleistogamy: is self-pollination that occurs before the flower opens. The pollen is released from
the anther within the flower or the pollen on the anther grows a tube down the style to the ovules.
It is a type of sexual breeding, in contrast to asexual systems such as apomixis.
Some cleistogamous flowers never open, in contrast to chasmogamous flowers that open and are
then pollinated. Cleistogamous flowers by necessity are self-compatible or self-fertile plants.
Many plants are self-incompatible, and these two conditions are end points on a continuum.
Homogamy: Homogamy refers to the ripening of the anthers and stigma of a bisexual flower at
the same time. Under this condition, self pollination may occur in the following ways:
1. A gentle breeze may blow the mature pollen grains which may be shed on mature stigma on
mature stigma that are situated below.
42
2. A visiting insect may transfer the mature pollen grains to the stigma of the same flower.
3.Self pollination may also occur when mature stigma push their way out of the corolla tube
during which they are brushed against the anthers and in the process pollen grains are collected.
4. In a situation where the filaments are longer than the stigma, the filament may recoil to touch
the mature stigmas.
5. In like manner, self pollination may occur where the style are longer than the filament; the
style may also bend or recoil to make the stigmas touch the anthers.
Advantages of self pollination.
1. It is a sure way of ensuring pollination especially in bisexual flowers.
2. It may not waste pollen grains.
Disadvantages of self pollination.
1. It leads to the production of weak offspring as a result of continuous or repeated self pollination.
2. The offspring or individuals produced are less adapted to the environment.
Geranium incanum, like most geraniums and pelargoniums, sheds its anthers, sometimes its
stamens as well, as a barrier to self-pollination. This young flower is about to open its anthers,
but has not yet fully developed its pistil.
43
These Geranium incanum flowers have opened their anthers, but not yet their stigmas. Note the
change of colour that signals to pollinators that it is ready for visits.
This Geranium incanum flower has shed its stamens, and deployed the tips of its pistil without
accepting pollen from its own anthers. (It might of course still receive pollen from younger
flowers on the same plant.)
Differences between self pollination and cross pollination.
Self pollination
Cross pollination
1
Self pollination takes place only in bisexual
Cross pollination takes place in both
plants.
unisexual and bisexual plant.
2
Only one parent is involved.
Two parents are involved.
3
Pollination may occur without an external
agent.
This requires external agent, e.g. insect
and wind.
4
It does not ensure new variation.
It results in the formation of new
varieties.
5. Pollen grains are effectively utilized.
Much of the pollen grains are wasted.
Pollinator and pollenizer
Pollination also requires consideration of pollenizer and pollinator.
44


A pollinator is the agent that pollinates plant i.e. involves in the transfer of pollen grains from
anthers to stigma, mostly insects like: flies bats moths or birds.
A pollenizer is the plant that serves as the pollen source for the other plants; some plants can
pollinate themselves i.e. they act as their own pollenizer.
A good example is a plant that provides compatible viable and plentiful pollen and blooms at the
same time as the plant that is to be pollinated.
The wasp Mischocyttarus rotundicollis transporting pollen grains of Schinus terebinthifolius
Pollen vectors
They are usually insects but also reptiles birds mammals, and others that transport pollen grains
and play a vital role in pollination. Any kind of animal that often visits or encounters flower is
likely to be a pollen vector to some extent.
Plants that rely on pollen vectors tend to be adapted to their particular type of vector. For
example day pollinated species tends to be brightly coloured, but if they are pollinated largely by
birds or specialist mammals, they tend to be larger nectar and have larger nectar reward than
species that are strictly insect pollinated. As for types of pollinator, reptiles pollinators are known
but they form minority in most ecological situations. Most species of lizards in the families that
seem to be significant in pollination seem to carry out pollen only accidentally especially the
larger area species such as varanidae and Iguanidae, but especially several species of the
Gekkonidae are active pollutant. Mammals are not generally thought of as pollinators, but some
rodents, bats and marsupials are significant and some even specialize in the activities. Examples
of pollen vectors include many species of wasps that transport pollen of many plant species,
being potential or even efficient pollinators. After of pollination in plants there occurs another
important process called fertilization.
Fertilisation
Fertilization is the union of the male and female gametes to form a zygote. Since the male and
female gametes are haploid (n) when the two unite the zygote is diploid (2n). Fertilization starts
when a pollen grain lands on the stigma. The pollen grain then grain germinates forming a pollen
tube. The tube nucleus controls the growth of the pollen tube. The pollen tube is an example of
chemotropism since it is growing toward chemicals produced from the ovule. The generative
nucleus travels down the pollen tube. It undergoes mitosis forming two haploid male gamete
45
nuclei. The pollen tube enters the ovule by way of the micropyle. The two male gamete nuclei
are released into the embryo sac. The tube nucleus disintegrates.
Double Fertilization
Since there are 2 sperm nuclei that have reached the embryo sac both nuclei will fuse with
female gametes. One sperm nuclei will fuse with the egg cell to form the zygote (2n) while the
other sperm nucleus fuses with the 2 polar nuclei in the embryo sac to form an endosperm
nucleus (3n).
Problems in Fertilization
From the time that the pollen comes in contact with the stigma, there are a number of problems
that can arise. If the ambient temperatures are too high the pollen can become denatured and if
it’s too low then the pollen will just sit until the temperature rises, although the flower’s
receptivity is short lived. If the relative humidity is too low at this stage then the stigma or the
46
pollen can desiccate, preventing the germination of the pollen. Low relative humidity has been
found to be the culprit in a poor seed set for these reasons in a number of instances in vegetable
seed production in the arid western states.
The next step in the process of fertilization, the pollen tube growing down through the style can
also be derailed by unfavorable weather conditions. The pollen tube is essentially a free living
miniature plant (the gametophyte generation) and requires temperatures similar to the mother
plant to grow vigorously. The pollen tube’s life cycle is usually 24 hours or less and it must
make the trip from stigma to ovule in this period or not be successful in fertilizing the ovule. If
the ambient temperature during this short period of time is colder or hotter than temperatures
favorable to normal growth of the species than the pollen tube will stop growing and fail restart
when the temperature comes back into a favorable range for growth? This will result in no
fertilization for that particular pollen tube. In a cool loving crop like spinach that produces
luxuriant growth at 58 – 65F (15 – 18C) and virtually stops growth at 78F (25.5C), this means
that when it gets hotter than 78F (25.5C) as spinach seed crops are flowering there can be serious
damage done to the yield due to poor fertilization of the ovules.
Alternately, a heat loving crop like tomatoes can suffer blossom drop producing fewer fruit with
low seed yields when tomatoes are exposed to cold night time temperatures during flow.
 Any Modern Biology or Advance Biology text book.
7. FERTILIZATION AND SEED FORMATION
Fertilization
Fertilization is a biological term that describes the union of male and female gametes to produce
an offspring. In agriculture, it refers to the process of enriching the soil with extra nutrients
needed for healthy plant growth.
Fertilization is the union of the male and female gametes to form a zygote. Since the male and
female gametes are haploid (n) when the two unite the zygote is diploid (2n). Fertilization starts
when a pollen grain lands on the stigma. The pollen grain then grain germinates forming a pollen
tube. The tube nucleus controls the growth of the pollen tube. The pollen tube is an example of
chemotropism since it is growing toward chemicals produced from the ovule. The generative
nucleus travels down the pollen tube. It undergoes mitosis forming two haploid male gamete
nuclei. The pollen tube enters the ovule by way of the micropyle. The two male gamete nuclei
are released into the embryo sac.
47
Double Fertilization
Since there are 2 sperm nuclei that have reached the embryo sac both nuclei will fuse with
female gametes. One sperm nuclei will fuse with the egg cell to form the zygote (2n) while the
other sperm nucleus fuses with the 2 polar nuclei in the embryo sac to form an endosperm
nucleus (3n).
Steps involved in fertilization
The act of fusion of the sperm and the method of approach, entry and eventual fusion of the male
and female nuclei constitutes the mechanism of fertilization. This is believed to take place in the
following stages.
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1. Movement of the sperm towards the egg:
This is the first step which brings the sperm in physical contact with the ovum. As the male and
female gametes are produced in different individuals, there are various mechanisms to bring
them nearer. The initial attraction of the sperms towards the egg is supposed to be chmotactic.
The sperms swim towards the egg collides with it. Usually several sperms attach themselves to
the egg. In individuals with external fertilization large number of eggs and sperms are released
outside so that they have a favourable chance of meeting with each other. In individuals with
internal fertilization the sperms are discharged into the genital tract of the females by the male
individual. From here the sperms move upward and reach the egg present in the uterus.
2. Capacitation and contact:
When a large number of sperms approach the egg for contact, the fertilizin and antifertilizin
reaction ensures that only a few spermatozoa are allowed to reach the ovum. The initial
attachment of the sperm to the egg is believed to be due to the chemical bonding of fertilizin and
antifertilizin.
3. Penetration of sperm into ovum:
The acrosome portions of the sperm produces some lytic enzymes called sperm lycines which
have the ability to dissolved the egg membrane allowing the entry of the sperm into the egg
cytoplasm. Usually only the head and the middle piece of the sperm enter the egg while the tial is
left outside.
4. Cortical reaction:
The entry of the sperm head into the egg brings about several changes in the egg surface. These
are the cortical changes and the development of fertilization membrane. In some echinoderm
eggs some fine granules are visible in the ooplasm after the entry of the sperm head.
The vitelline membrane starts lifting itself up from the point of sperm entry and a perivitelline
space is formed between it and the egg surface. The cortical granules from the egg cortex release
their contents into the privitelline space. These contents attach themselves to the inner surface of
the vitelline membrane forming what is known as a fertilization membrane. The development of
the fertilization membrane prevents the entry of other sperms into the egg.
5. Activation of the ovum:
The mature egg will be generally in a hibernating condition with very low rates of metabolism
and inactive nucleus. The penetration of the sperm triggers the egg into activity. The metabolic
rates increase allowing for entry of water and other particles. Metabolically the rate of protein
synthesis goes up as these are needed for further cell divisions. At this stage the nucleus
(pronucleus) of the egg which has remained in the metaphse II stage (of the meiotic II division)
becomes active completing its second division and releases the second polar body.
6. Fusion of male and female pronuclei (amphimixis):
In this process there is fusion of the male and female nuclei. Initially the two nuclei remain close
together and at the point of contact the nuclear membranes disappear and the chromosomes come
to lie on the equator. Finally the nuclear fusion is completed and it becomes a zygote nucleus.
49
The egg is said to have been fertilized and it becomes a zygote. It is now ready to undergo
cleavage to develop into the embryo.
Germination
The embryo will germinate from the seed if the proper environmental conditions are present.
When this occurs the embryo resumes its growth. In order for germination to occur the following
conditions must be present:
i. Water must be present. This allows the seed to swell and enzymes to function.
ii. Oxygen must be present in the soil.
iii. The temperature must be suitable for the species of plant. Suitable temperatures
are
usually between 5-30 degrees Celsius depending on the species.
iv. The dormancy period must be complete.
v. Some seeds need light and others need darkness.
·
·
·
·
·
·
·
·
·
·
Events of germination
When germination begins the first thing that happens is water is absorbed by the seed through the
micropyle and through the testa. Enzymes in the soil now digest the foods stored in the seeds:
Oils become fatty acids and glycerol
Starch becomes glucose
Protein becomes amino acids
These foods now are absorbed by the embryo.
The glucose and amino acids make new structures such as cell walls and enzymes.
The fats and glucose are used in cellular respiration to produce energy.
The stored food of the seed is being used up as the embryo grows larger.
The radicle grows larger and breaks through the testa. It becomes the roots of the new plant.
The plumule grows larger and emerges above the ground.
Leaves form.
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Germination occurs differently in different plants. In some plants the cotyledon remains
underground while in other plants the cotyledon emerges above ground. The diagrams below
show these 2 methods of germination.
51
Problems in Fertilization
From the time that the pollen comes in contact with the stigma, there are a number of problems
that can arise. If the ambient temperatures are too high the pollen can become denatured and if
it’s too low then the pollen will just sit until the temperature rises, although the flower’s
receptivity is short lived. If the relative humidity is too low at this stage then the stigma or the
pollen can desiccate, preventing the germination of the pollen. Low relative humidity has been
found to be the culprit in a poor seed set for these reasons in a number of instances in vegetable
seed production in the arid western states. The next step in the process of fertilization, the pollen
tube growing down through the style can also be derailed by unfavorable weather conditions.
The pollen tube is essentially a free living miniature plant (the gametophyte generation) and
requires temperatures similar to the mother plant to grow vigorously. The pollen tube’s life cycle
is usually 24 hours or less and it must make the trip from stigma to ovule in this period or not be
successful in fertilizing the ovule. If the ambient temperature during this short period of time is
colder or hotter than temperatures favorable to normal growth of the species than the pollen tube
will stop growing and fail restart when the temperature comes back into a favorable range for
growth. This will result in no fertilization for that particular pollen tube. In a cool loving crop
like spinach that produces luxuriant growth at 58 – 65F (15 – 18C) and virtually stops growth at
78F (25.5C), this means that when it gets hotter than 78F (25.5C) as spinach seed crops are
flowering there can be serious damage done to the yield due to poor fertilization of the ovules.
Alternately, a heat loving crop like tomatoes can suffer blossom drop producing fewer fruit with
low seed yields when tomatoes are exposed to cold night time temperatures during flowering.
Seed Formation
Seed formation can be described as a process whereby a pollen grain sends grain to the stigma
and a flower or seed is formed.
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The fertilized becomes the seed. The integuments become the wall of the seed called the testa.
The micropyle closes. The endosperm nucleus leads to the formation of triploid endosperm, a
food tissue. The diploid zygote, by mitosis, develops into a plant embryo. The developing
embryo draws nourishment from the endosperm. The embryo ceases development and goes
dormant. The ovule becomes a seed, which contains a dormant plant embryo, food reserve, and
the protective coat called the testa.
The Embryo
The embryo is made up of the radicle or future root and the plumule or future shoot. The
endosperm cells divide many times and absorb the nucleus. This is the nutrition (mainly fats, oils
and starch) for the embryo.
There are 2 types of seeds. Some are endospermic while others are non-endospermic. In
endospermic seeds the food reserve is the endosperm, which is outside the plant embryo.
Examples of this type of seed are maize and wheat. Non-endospermic seeds have food reserve
within the cotyledon(s) of the plant embryo. This occurs in broad beans.
53
Monocots and Dicots
Monocots have one cotyledon in the seed while dicots have two cotyledons. The cotyledons are
food reserves for the young plant after it germinates from the soil. It uses these food reserves
until it is capable of making its own food. In monocots the food is absorbed from the endosperm
while in dicots the food is stored in the cotyledons.
Monocot
Dicot
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Fruit Development
The ovary becomes a fruit. The wall of the ovary becomes the wall of the fruit called the
pericarp. The fruit protects the developing seeds and plays an important role in seed dispersal.
This process is controlled by auxins produced by the seeds. Once the fruit forms the rest of the
flower parts die and fall away.
Fruit and Seed Dispersal
Seed dispersal is the scattering of offspring away from each other and from the parent plant. As a
result of dispersal there is an improved chance of success by reducing competition and
overcrowding. Dispersal also enables colonization of new suitable habitats and thus, there is an
increased chance of species survival.
55
Methods of Seed Dispersal
Wind: The seeds of wind-dispersed plants are lightweight seeds. They have a high air resistance
so they can be carried far away from the mother plant.
Water Dispersal
Fruits which float such as those of the water lily and the coconut palm are carried by water.
Coconuts can travel for thousands of kilometres across seas and oceans. The original coconut
palms on South Sea Islands grew from fruits, which were carried there from the mainland by
ocean currents.
Mangroves in the swamp regions of countries such as Thailand are another example.
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Animal Dispersal
Some plants have juicy fruit that animals like to eat.
The animal eats the fruit but only the juicy part is digested.
The stones and pips pass through the animal's digestive system and are excreted to form new
plants. This can be far away from the parent plant.
Blackberry, cherry and apple seeds are dispersed in this way.
Birds also like to eat fruit and they help to disperse seeds to other areas through their droppings.
57
Mistletoe has sticky fruits that are attractive to birds. The sticky seeds stick to the bird's beak.
They then rub their beaks clean on the bark of trees. The sticky seeds are left on the bark to grow
into new mistletoe plants - mistletoe is a parasitic plant.
Squirrels collect nuts like acorns and bury them for winter food, but they often forget where they
have buried them and these grow into new trees.
Some fruits like that of the burdock plant have seeds with hooks.
These catch on the fur of animals and are carried away.
Self-Dispersal: Some plants have pods that explode when ripe and shoot out the seeds. Lupins,
gorse and broom scatter their seeds in this way. Pea and bean plants also keep their seeds in a
pod. When the seeds are ripe and the pod has dried, the pod bursts open and the peas and beans
are scattered.
Dormancy
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Dormancy is a period of inactivity. There is very little cellular activity and no growth. One or
many of the following reasons bring about dormancy:
Auxins that inhibit growth- Growth Inhibitors
Process of seed formation
 Pollination is the first step in reproduction. Pollen grains are shed from the anthers and
fall onto the feathery stigmas.

Fertilization is the second step in seed formation. The pollen that reaches the stigma
germinates and forms a pollen tube that carries the male nuclei inside the ovary for fusion
with the egg nuclei.
 The complete process from pollination to fertilization takes from 18 to 24 hours.
9. ECOLOGICAL ADAPTATION OF PLANT ON AQUATIC HABITAT
Ecology
The word ecology can be defined as the study of plant and animal in relation to their
environment. Ecology is derived from a Greek word Oikos which means or dwelling place. In
other word ecology can be define as a field of study which deals with the relationship of living
organism with one another and with the environment in which they live. Ecology is often
described as environmental biology.
Ecology is divided into two main branches:
(a). Autecology: Autecology is concerned with the study of an individual organism or a single
species of organism and its environment. For example the study of a single rat and its
environment.
(b). Synecology: synecology is concerned with the study of inter-relationships between groups of
organisms or species living together in an area. For example the study of different organism in a
river in relation to their aquatic environment.
Ecological Concept
There are some important concepts commonly used in the study of ecology which enable one to
understand the subject matter. Some of these ecological concepts include:
Environment: The environment includes all the factors external and internal, living and nonliving which affect an organism.
Biosphere or Ecosphere: The biosphere or ecosphere is the zone of earth occupied by living
organisms.it is a layer of life which exists on the earth surface. The biosphere is a narrow zone
where complex biological and chemical activities occurred.it can be found on land, soil, water
and air.it provide habitat for plant animal and micro-organisms.
Lithosphere: The lithosphere is the solid portion of the earth.it is the outermost layer of the earth
crust.it is made up of rock and mineral materials, and it also represent 30% 0f the earth surface.
Lithosphere forms the basis of all human settlement.
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Hydrosphere: hydrosphere is the liquid or aquatic part of the earth or living world. It covers
about 70% of the earth’s crust. It holds water in various forms- solid (ice), liquid (water) and as
gases (water vapour).Examples of hydrosphere are lake, pools spring, ocean or sea, ponds, oasis,
rivers and stream.
Atmosphere: The atmosphere is the gaseous portion of the earth. It is a layer of gases
surrounding the earth. Over 99% of the atmosphere lie within 30km of the earth surface.it
contains 78%nitrogen, 21% oxygen, 0.03% carbon dioxide and 0.97 rare or inert gases.
Habitat: Habitat is defined as an area occupied by a biotic community. In other words, habitat is
any environment in which an organisms live naturaly.it is the natural home of organisms. For
example, the habitat of the fish is water.
The various types of habitat are aquatic habitat (i.e. live in water) such as rivers, lakes, ponds,
stream, lagoons, sea, ocean and terrestrial habitats (live on land) e.g. savannah, forest, desert etc.
Biotic Community or Biome: A biotic community is any natural occurring group of different
organisms living together and interacting in the same environment. A biome is the largest
community of organisms, e.g. rain forest, guinea savannah etc.
Ecological Niche: Ecological niche refers to the specific portion of a habitat which is occupied
by a particular species or organisms. For example, a caterpillar and an Aphid which live on the
same plant occupy different position or ecological niches on the plant. The caterpillar lives
mainly on the leaves and feed on them while the Aphid lives on the young shoot and sucks sap
from it. Although both organisms live on the same habitat, each has it living space and source of
food.
Population: Population is defined as the total number of organisms of the species living together
in a given area. For example, the total number of Tilapia fish in a pond constitutes the population
of Tilapia fish in that habitat.
Ecosysyem: An ecosystem refers to a community of plants and animals functioning together
with their non-living environment. In other words, ecosystem consist of the living factors (plant
and animals) interacting with the non-living factors in an environment.
Ecological Factors
Ecological factors are those factors in the environment which can influence living organisms or
cause changes in any habitat, be it aquatic or terrestrial habitat. Ecological factors are grouped
into two categories-Biotic factors and Abiotic factors.
Ecological factors common to all habitats
Factors affecting or common to all habitats include: Temperature, Rainfall, Light, Wind,
Pressure and Hydrogen ion concentration (pH). Of these factors, temperature and rainfall
determine the major biomes of the world.
Temperature
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Temperature determines the vegetation of an area.
It is necessary for the germination of seeds
It regulates the activities of majority of the living things
High temperature affects evapotranspiration and reduces the performance of animals.
It affects the wilting of field crops, ripening and maturity of crops.
It leads to loss of soil nutrients through volatilization.
vii) Unfavourable temperature may result in seed dormancy.
Rainfall
i) Rainfall determines seasons in some places, e.g Nigeria where we have rainy and dry season.
ii) It determines the type of vegetation in an area.
iii) It determines the distribution of plants and animals.
iv) Rainfall is necessary for seed germination.
V) Rainfall provides a dwelling place or habitat for some organisms, e.g. fish, crab, shrimps, sea
weeds etc
vi) it helps to dissolve nutrients in the soil thereby making them available to plants.
Plants use water for photosynthesis.
viii) It is the main source of water in rivers, ponds, lakes, oceans etc.
Wind
Wind determines seasons in Nigeria, for instance, the south-West wind is responsible for rainy
season while the North-East wind brings harmattan or dry season.
It helps in the distribution of rainfall.
It can aid the spread of diseases.
It aids the pollination of flowers.
It also aid the dispersal of seeds and fruits.
High velocity wind may cause wind erosion.
Wind is responsible for water currents and waves.
Light
Sunlight is necessary for photosynthesisto take place in green plants.
It affects evapo-transpiration.
It affects the productivity of crops due to length of day,i.e.,photoperiodism.
Light affect flowering and fruiting in plants.
Light is the ultimate source of energy for all organisms.
It affects the activities of animals,e.g.some animals are active during the day while others are
active at night.
Pressure
Atmospheric pressure decreases as one goes up from the sea level.
In aquatic environment, pressure increase as one moves down the water.
Plants and animals have special adaptions to a particular level of pressure to enable them survive
Too high or too low pressure will affect the lives and activities of plants and animals.
Pressure is responsible for the movement of winds
61
Hydrogen ion concentration (pH)
PH values range from 1 to 14, with pH 1 being very acidic, PH
7 neutral and PH 14 very
alkaline.
Living organisms are highly sensitive to pH changes.
Too high or too low pH will affect the lives and activities of plant and animals.
Plants and animals are adapted to special pH values; e.g pH of fresh water is low while marine
PH is high.
Most plants thrive well in neutral or slightly alkaline soil while acidic soils support very little
vegetation.
Aquatic Habitat
Aquatic habitat is a body of water in which certain organisms live naturally. In other words,
acquatic habitats are habitats or places that relates to lives in water. Organisms that live in water
are called acquatic organisms. Examples of aquatic organisms are fish, crabs, toads, plants etc.
Other Ecological Factors Common to Aquatic Habitat
Salinity: Salinity is defined as the degree of saltiness or concentration of salt solution in water.
Salinity is low in fresh water, high in sea water and moderate in brackish water. Aquatic
organisms need to maintain the osmotic balance between their body fluids and the aquatic
surroundings to survive. For example, organisms living in fresh water will require some adaptive
features to enable them get rid of excess water that enters their bodies while those in sea water
equally have adaptations to enable them cope with excess water in their bodies.
Turbidity/Transparency: Turbidity is caused as a result of suspended materials in water. Clear
water has low turbidity. Turbidity is also influenced by season. It is higher during the raining
season than in dry season. Turbidity reduces light penetration into the water, resulting in the in
ability of aquatic green plant to carry out photosynthesis, and it causes pollination.
Dissolved Gases: Dissolved gases in the case refer to oxygen and carbon dioxide. The oxygen
concentration of water decreases with depth. Oxygen is required by most aquatic organisms for
respiration. It is also required for the decaying of organic substances. Carbon dioxide is required
as raw material for photosynthesis.
Density: Density of water varies with the type of aquatic habitats. While the density of pier fresh
water is 1.00, that the sea water is 1.028 at atmospheric pressure and 0c. Organisms like fish
have streamlined bodies which enable them to move them easily through water while other
organisms which float on the water surface are sensitive to changes in density.
Currents: Water currents increase aeration and the turbidity of the water. It also affects the
distribution of aquatic organisms. The type of organisms found in an aquatic habitat is affected
by the speed of water current. For example, animal living in fast moving water usually have
structures of attaching themselves in rock surfaces so that they cannot be swept away.
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Tidal Movement And Waves: Tidal movements and waves affect the organisms in aquatic
environment. Most organisms in certain level of the water attach themselves to substances or
may even live in burrows. Some may possess hard body cover to prevent evaporation of water
their bodies. In the open sea, waves cause the aeration of the surface waters, enabling aquatic
organisms to have sufficient supply of dissolved gases for their respiration.
Types of Aquatic Habitats
There are three types of aquatic habitats. These are marine or salt water habitats, estuarine or
brackish water habitats and fresh water habitats.
Marine Habitats
Marine habitats refers to aquatic habitats which contain salt water. Marine habitats include the
ocean, lakes, shore and the open seas.
Characteristics of Marine Habitat
The marine or salt water habitat has the following characteristics:
Salinity: Salinity is defined as the degree of saltiness or concentration of salt solution in oceans.
The marine habitats have a high salinity and its average salinity is put at 35.2 per 1000. In other
words, the average salinity of the ocean is 35.2 parts of salts by weight per 1000 parts of water.
Density: The destiny of marine water is high, hence many organisms can float in it. While the
destiny of ocean water is about 1.028, that of fresh water is higher than that of fresh water.
Pressure: Water pressure increases in depth at the rate of one atmosphere for every ten meters.
In other words, pressure varies from one atmosphere at the surface level to about 1000
atmosphere at the greatest depth. This is why animals in marine habitats have features which
enables them to adapt especially at the deep level of the sea.
Size: Marine habitats represent the largest of all the habitats. The ocean alone occupies over 70%
or 360 million square kilometers of the earth’s total area of 510 million square kilometeres.
Examples of oceans are atlantic ocean, Indian ocean, Pacific ocean ( the largest) etc.
Currents: Currents are always produced down the ocean as a result of certain variations such as
salinity and changes in temperature.
Tides:Tides are the alternate rise and fall of the ocean approximately twice a day. This alternate
rise and fall in water level is due to the gravitational effects of the moon and sun.
Oxygen Concentration:The concentration of oxygen in the ocean is highest at the surface while
it decreases with depth, and in the very deep parts of the oceans there is practically no oxygen.
Hydogen Ion Concentration:Salt water is known to be alkaline in nature with pH of about 8.09.0 near the surface.
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Waves:Waves are movement of surface waters of the oceans and it can take any direction and
are caused by winds. Waves bring about the mixing of sea water especially on the surface of the
ocean.
Light Penetration:Light penetrates the ocean water only to a maximum depth of 200metres.
Therefore, plant life is limited to the upper layers of the ocean where light can penetrate.
Penetration of light depends on the water turbidity.
Major zones of the marine habitats.
The major ecological zones of the marine habitats include:
Supratidal or Splash Zone:This is the exposed zone of the marine habitat. It has occasional
moisture since it is the area where water splashes when the waves break at the shore.
Intertidal or Neritic Zone:This zone which is also called planktonic or euphotic zone is only
exposed at low tide or covered by water at high tide. It has high photosynthetic activities because
of abundant sunlight. There is also fluctuation of the water temperature.
Litoral or Subtidal Zone:This zone is about 200m deep. It is constantly under water, it has
abundant sunlight and therefore abundant nutrients.
Benthic Zone:Benthic zone is also under water and is about 500m deep. It has low light
penetration and low nutrients.
Pelagic or Abyssal Zone:This zone is about 7000m deep. It has low temperature, low light
penetration, high pressure, low photosynthetic activities and the primary production of food is
by chemosynthesis.
Hadal or Aphotic Zone:This is the deepest zone of the marine habitat. It is over 7000m deep. It
forms the floor or bed of the ocean. There is no light penetration and no photosynthetic activities.
On the basis of depth or light penetration or vertical zoning of marine habitat, there exist three
major zones. These are euphotic, disphotic and aphotic zones:
(1)Euphotic Zone:This is an area which is directly connected with sunshine. Producers,
consumers and decomposers are present here. There is enough light penetration for
photosynthesis to take place.
(2)Disphotic Zone:Disphotic zone is a region of dim light. Consumers and decomposers are
also found here. Light penetrates the water but the intensity is too low for photosynthesis to take
place.
(3)Aphotic Zone:This represents the bottom or bed of the seas and oceans. It is characterised
by cold dark water without light penetration and very few living organisms are found in this
zone.
Distribution of organisms in marine habitats and their adaptive features.
The organisms in marine habitats include plants and animals.
Plant in marine habitats.
Sea Weeds:They possess hold-fast for attachment. They have divided leaves, floating devices or
air bladder for bouyancy.
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Algae, eg. Sargassum: The Algae possess chlorophyll for photosynthetic activities, small size or
large surface area for drifting or floating.
Sesuvium:Sesuvium possesses thick leaves or reduced leaves for water conservation.
Planktons, e.g. Diatoms:They possess air spaces in their tissues, rhizoids (fake feet) for
attachment to rocks and air bladder for buoyancy.
Food chain in marine habitat.
A typical food chain in marine habitat could be up to three or four trophic levels. The
phytoplanktons, e.g. diatoms serve as the major producers which support the food chain. Some
examples of food chain include:
(1)Diatoms - Zooplanktons - Tilapia - Shark.
(2)Diatoms - Crabs - Tilapia.
Factors Affecting Marine Habitats.
Some of the major factors affecting marine habitats are temperature, sunlight, wind, density, pH
and salinity. These factors have been explained under the characteristics of marine habitat.
Estuarine Habitats.
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Estuarine habitat is a body of water formed at the coast as a result of the action of tides which
mix salt water from the sea with fresh water from the land. The mixing of salt water and fresh
water results in the formation of a brackish water. This brackish water is what is called
estuarine.
Types of estuaries.
Estuary is found in the following bodies of water:
(1)Delta: A delta is where a river divides into many channels before entry into the ocean or sea.
Brackish water or estuary (delta) is formed at the mouth of a river as it enters the sea.
(2)Lagoon: Lagoon is a body of ocean water that enters into the land through a canal and
therefore has the opportunity of mixing with fresh water from rivers and streams.
(3)Bay: Bay is a little or small portion of the sea water which enters into the land and mixes up
with fresh water from rivers and streams. It should be noted that a lagoon is bigger than a bay
and it may be long enough to join the sea at another end while bay is very small and not long
enough to rejoin the sea in another end.
Characteristics of Estuarine Habitats.
The followings are the characteristics of the estuarine habitats:
(1)Fluctuation in Salinity:Salinity fluctuates in this habitat.Salinity is lower at the mouth of a
river and gets higher towards the sea. Salinity is also affected by season. While rainy season
reduces salinity due to addition of fresh water, dry season increases it.
(2)Turbidity:Turbidity of estuarine habitat increases especially during the rainy season when
lotsof debris are brought down by rivers to the habitat. This high turbidity also reduces the rate of
photosynthesis and respiration by organisms.
(3)Shallowness of Water:Unlike the sea water which is deep, the water in estuarine habitat is
very shallow.
(4)Low Species Diversity:The estuarine habitat has low diversity of species compared to marine
habitat. Common plant species are phytoplanktons algae, marsh vegetation etc while animal
species are crabs, oysters, lobsters, fishes etc.
(5)Water is affected by tides:Sea water usually flows rapidly into estuaries at high tides and
rushes back into the ocean at low tides.
(6)High Level of Nutrients:The estuarine habitat contains abundant nutrients especially the
organic detritus which form the bulk of producers in the habitat.
(7)Low Oxygen Content:Oxygen content of estuarine habitat is generally 5very low and as a
result, much of the microbiological activities are anaerobic.
Distribution of plants in estuarine habitats.
Plant species and their adaptive features
(1) Planktons, e.g. Diatoms:They possess air spaces in their tissues, rhizoids or false feet for
attachment to rock shores and air bladder for buoyancy.
(2)Algae:Algae possess chlorophyll for photosynthetic activities and small size or large surface
area for floating.
(3)Red Mangrove Rhizophora racemosa): It has silt roots which grow down from the stem into
the soft mud and develop numerous rootlets which have air spaces for conducting air to the
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tissues of the roots. The roots also provide support and prevent plants from being washed away
by the tides. Again the seeds of red mangrove germinate while they are still on the parent plant
thereby preventing the seedlings from being carried away by water current.
Red Mangrove
(4)White Mangrove(Avicennia Nitida):It has pneumatophores or breathing roots for exchange of
gases.
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White Mangrove
Food Chain in Estuarine Habitat.
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A t ypical food chain in an estuarine habitat may have up to three, four or five trophic levels. The
phytoplanktons such as diatoms and detritus form the basic producers which support the food
chain. Some examples of food chains in the estuarine habitat include:
(1)Detritus – Worms – Snails – Birds.
(2)Diatoms – Shrimps – Fishes.
Factors affecting estuarine habitat.
The factors which affect estuarine habitats are common to aquatic habitats and these include
temperature, wind, relative humidity, light, pH etc.
Fresh Water Habitats.
Fresh water habitat is a body of water formed mainly from inland waters and contain very low
level of salinity. Examples of fresh water habitats are rivers, ponds, streams, springs and lakes.
Types of Fresh Waters.
Fresh waters are classified on the basis of their mobility. Based on this, two types are identified.
These are:
(1)Lotic Fresh Waters: These include all running waters which can flow continuously in a
specific direction. In other words, these are flowing or running waters, e.g. rivers, springs and
streams.
(2)Lentic Fresh Waters: These include standing or stagnant waters. These do not flow nor move.
Example of lentic fresh waters are lakes, ponds, swamps, and dams.
Characteristics of Fresh Water Habitats.
The following characteristics are associated with fresh water habitats:
Low Salinity: Fresh water habitats normally contain very low level of salts.It has about 0.5% of
salt compared to about 3.5%for sea water.
Small in Size: Fresh water habitat is usually very small compared to the
ocean water which is about 75%of the earth surface.
Variation in Temperature: The temperation of fresh water habitat usually varies with season and
depth. Temperature at the surface of the water varies slightly with that of at the bottom of the
water.
High Concentration of Oxygen Content: Oxygen is usually available in all parts of the fresh
water especially in the surface of the water.
Shallowness of Water:Most fresh water habitats are very shallow hence sunlight can easily
penetrate through the water to the bottom.
Seasonal Variation: Some fresh water habitats like streams and rivers normally dry up during the
dry season while others have their volume reduced. The volume of water in rivers also increases
during the rainyseason. Turbidity and fast flow of rivers are also high during the rainy season
than in dry season.
Currents: Currents can affect the distribution of gases, salts and small organisms in fresh water
habitats such as rivers and streams.
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Major Ecological Zones of Fresh Water Habitats
The zones of a lentic fresh water habitat, e.g. lake are similar to those of the marine habitats but
there are no supratidal and intertidal zones.
There are two major zones in a lentic fresh water habitat. These are littoral and benthic zones.
(1)Littoral Zone: Littoral zone is the shallow part of fresh water habitat. It contains several
plants and animals. The littoral zone has rooted vegetation at its base. It has the highest level of
primary production because sunlight can easily penetrate the zone, hence photosynthetic
activities are common. Plants associated with this zone include spirogyra, water lettuce,
chlamydomonas, water fern,duckweed, diatoms and sedges. Animals associated with this zone
include water fleas, water snails, flatworms, frogs, toads, water skaters, ducks, snakes,
crocodiles, tadpoles, hippopotamus, and Hydrae.
(2)Benthic Zone: Benthic zone is the deepest parts of the lentic fresh water habitat. The benthic
zone does not have rooted vegetation like the littoral zone although flowering plants may occur
at its surface. Plants associated with the benthic zone have well developed root system in the
mud. These plants include water lily, water arum, ferns, crinum lily, commelina and grasses.
Animals associated with the benthic zone include protozoa, rotifers, Hydrae, Tilapia fish, mud
fish, cat fish, leeches, caddish fly larvae, larvae and pupae of mosquito, water snail, water spider,
crayfish, water scorpion, water boatman and water bugs.
Lotic Fresh Water Habitat
In a lotic fresh water habitat e.g. rivers, there exist two zones. These are:
(1)Pool Zone: In this zone, water is relatively slow and calm.
(2)Rapid Zone:In this zone, is fast.The loctic fresh water is habitat is not as stratified as the lentic
fresh water habitat.
Adaptive features of some organisms fresh in water habitat
(1)Water Lily (Nymphaea):The plant has air bladders,expanded shape and light weight which
keeps it afloat.It has a long petioles attached at the centre of leaf blade which prevent them from
being drawn under water by the current.
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Water Lily.
(2)Water Hycinth (Ipomea Grassipsis): They have cavities and intercellular airspace which
gives them the ability to float or to maintain buoyancy on water.
Water Hycinth
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Spirogyra
(3)Spirogyra: The plant has mucilagenous cover which protects them in water.
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(4)Water Lettuce (Pistia):Water lettuce hairs on their leaves which help them to trap air and
enable them to float.
Water Lettuce
ECOLOGICAL ADAPTATION OF PLANTS TO TERRESTRIAL HABITAT
Habitat
A habitat is a place where an organism is commonly found. It may be a physical area, some
specific part of the earth surface, air, soil or water. Habitat is a place where organism of different
species dwells and communicates.
Adaptation enables plants to survive, live and grow in different areas. Adaptation enables plants
and animals to live in a particular habitat. It may also be difficult for some plants to adapt in
places. Adaptation expanciate on why plants are found in different environment. Example the
plants that are found in mangrove swamp forest are different from the plants found in Sudan
savannah.
Terrestrial habitat is refers to the part of the earth land. It is inhabited by plants which lives and
grow on land. The vegetation is influenced by rainfall and temperature. The abundant rainfall
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and humidity throughout the year flourish the presence of forest while the cold temperature and
low rainfall causes poor vegetation on land. Terrestrial habitat is divided into four:
i. Rainforest habitat ,
ii. Savannah or grassland habitat,
iii. Marsh or swamp habitat and
iv. Arid habitat.
Rainforest Habitat
It is an extensive community of plants dominated by tall trees which are of different species and
heights. The tropical rainforest is hot and it rains a lot, about 80 to 180 inches per year. This
abundance of water can cause problems such as promoting the growth of bacteria and fungi
which could be harmful to plants. Heavy rainfall also increases the risk of flooding, soil erosion,
and rapid leaching of nutrients from the soil (leaching occurs when the minerals and organic
nutrients of the soil are "washed" out of the soil by rainfall as the water soaks into the ground).
Plants grow rapidly and quickly use up any organic material left from decomposing plants and
animals. This results is a soil that is poor. The tropical rainforest is very thick, and not much
sunlight is able to penetrate to the forest floor. However, the plants at the top of the rainforest in
the canopy, must be able to survive 12 hours of intense sunlight every day of the year. There is a
great amount of diversity in plant species in the tropical rainforest.
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Characteristics of Rainforest Habitat
Presence of tall trees
They have thin bark
They have buttress roots
They are hosts to epiphytes.
Features of plants that makes them adapt in rainforest
Plants have buttress root and tap root which enables them to support the weight of the plants.
Examples iroko tree, afara tree e.t.c.
They have broad leaves which enhances their mood of respiration and
feeding[photosynthesis].example walnut
They allow epiphytes on them. Example orchid, mistletoe
Drip tips and waxy surfaces allow water to run off, to discourage growth of bacteria and fungi
Buttresses and prop and stilt roots help hold up plants in the shallow soil
Some plants climb on others to reach the sunlight
Some plants grow on other plants to reach the sunlight
Flowers on the forest floor are designed to lure animal pollinators since there is relatively no
wind on the forest floor to aid in pollination
Smooth bark and smooth or waxy flowers speed the run off of water
Plants have shallow roots to help capture nutrients from the top level of soil.
Many bromeliads are epiphytes (plants that live on other plants); instead of collecting water with
roots they collect rainwater into a central reservoir from which they absorb the water through
hairs on their leaves
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Epiphytic orchids have aerial roots that cling to the host plant, absorb minerals, and absorb water
from the atmosphere.
Drip-tips on leaves help shed
excess water.
Prop roots help support
plants in the shallow soil.
Some
plants
rainwater into a
reservoir.
collect
central
Savannah or Grassland Habitat
This habitat lies between rainforest and arid. It is dominated by grass, the rainfall is not enough
to support the growth of the plant.
Types of Grassland
There are two types of grasslands which are:
a. The tropical grassland [savannah]
b. The temperate grassland.
Tropical Grassland or Savannah
The tropical grassland is located around the equator i.e. between 50 and 200North and South of
the equator. Areas where this grassland is found includes; Africa where it is called savannah, in
Brazil where it is called Campos, and in South America where it is called llanos. In Nigeria the
savannah has three major belts. Guinea savannah, Sudan savannah and Sahel savannah. The
luxuriant nature of the grassland decreases North wards from guinea to Sudan and finally to
Sahel savannah.
Temperate grassland or savannah.
The temperate grassland is found in the interior continent of Asia, North America, South
America and South Africa where the grasses appear to be uniform.
Adaptive features of plants in savannah habitat
1. They have underground stem which helps the plants to withstand intense heat fire and dry season
spear grass
2. They have broad and succulent trunks for storage of excess water. Example, baobab.
3. They have long root to search for underground water. Example acacia
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4. They have a heavy and thick bark which help in reduction of transpiration and also protects them
from bush fire. Example elephant grass.
Marsh or Swamp Habitat
Marshes are permanently or periodically covered with nutrient-rich water. Marshes are
characterized by emergent vegetation that is adapted to saturated soils and by submerged
vegetation that lives at deeper depths. Plants living in marshes are exposed to three
environmental stresses:
1. They are frequently covered by water so they must be able to cope with low oxygen content,
2. They are often exposed to the atmosphere so they can be exposed to factors such terrestrial
herbivores and fire, and
3. They are sometimes exposed to the effects of wave action or water movement. Thus, these
factors have selected for the herbaceous plants with well developed root systems (that provide
anchorage and storage). Salt marshes are found in estuarine areas with high (and fluctuating) salt
content. Thus, salt marsh plants must have adaptations for dealing with high salt content in the
water that surrounds them, a fourth type of stress.
These are low land habitat which is usually flooded all the time. There are two major marsh
habitats:
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a. Fresh water marshes [FWM].
b. Salt water marshes.
Fresh Water Marshes
Fresh water marshes occur mainly on land beyond limit of the salt water marsh and beyond the
areas influenced by tides. Fresh water from rivers overflows the river banks to flood the
adjoining low lands resulting in the formation of fresh water marshes.
Salt Water Marshes
Salt water marshes occur along the coastal areas and they are influenced by tides. The action of
the tides in the ocean causes the flooding of adjoining lowlands with brackish water resulting in
the formation of salt water marshes.
1.
2.
3.
4.
5.
6.
7.
Characteristics of marsh habitat.
Lowland habitat
High flooding
Nature of soil
Presence of stagnant water
Presence of organic mater
High rate of organic decomposition
High relative humidity.
Plants Found in Marshes
Examples of plants commonly found in marshes includes Algae, water lettuce [Pistia], sword
grasses, duckweed [Lemma], water lilies [Nymphaea], hornwort, sedges, white mangrove, red
mangrove and raphia palm.
Low soil oxygen content area
Wetland soils have been affected by the permanent cover of water. One problem faced by plants
living in marshes is the lack of oxygen in the soil. Oxygen is used by plants (and most other
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organisms) in the process of cellular respiration in which the energy from glucose (produced by
photosynthesis) is released so that the organisms can use the energy to do “biological work.”
When glucose is broken down in the presence of oxygen, aerobic respiration occurs and the
organisms are able to use a great deal of the stored energy in the glucose. In situations where
oxygen is lacking, glucose is broken down by the process of anaerobic respiration which does
not release as much energy from each molecule of glucose (aerobic respiration releases about 18
times more energy than anaerobic respiration). Not only is anaerobic much less energy efficient
than aerobic respiration, but by products of anaerobic respiration, are toxic.
Arid Land or Desert Habitat
This refers to the area of very low rainfall and high evaporation rate. Arid lands are the driest
habitat receiving less than 25cm of annual rainfall.
Types of Arid Land (Deserts)
There are two major types of arid lands which are hot and cold desert.
Hot Desert
Hot desert of the world are located on the western coast of the continent within latitude 15 0 to300
north and south of the equator. Example of hot desert is Sahara desert [North Africa, Great
Australia desert and Atacama Desert of South America].
Cold Desert
They are located or found in the interior of the continent around 450 to 600 North and South of
the equator. The desert is found in the interior of Eurasia, north America and in Patagonia [South
America].
1.
2.
3.
4.
5.
6.
7.
8.
1.
2.
3.
4.
Characteristics of Arid Land Habitat
Scarcity of water
Hot temperature
Presence of sandy soil
High sunshine
Poor vegetation
Predominance of strong winds
Low relative humidity
Presence of drought resistance plant.
Adaptive Features of Plants
Cactus: It is a leafless plant with prickles to reduce transpiration. It has a thick succulent stem
and side branches to store water for long drought.
Acacia: This is a drought resistance plant. It has deep root which absorb underground water deep
down in the soil.
Baobab tree: The leaves are waxy, hairy or needle shape to help reduce the rate of transpiration
Wiring grass: It has narrow and slender leaves which help to reduce the rate of transpiration in
the plant.
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5. Oleander: This plant has extremely deep root which is able to absorb underground water deep
down the soil.
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Text Books
Ecology and environment biology by Puroht Agrawal
Botany for degree student by A.C Dutta
A textbook on plant ecology by Schond, R.S Shukla, P.S Chadel
A functional approach biology [4th edition] by MBV Roberts.
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