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UNE
ESCO-NIGE
ERIA TECH
HNICAL &
V
VOCATION
NAL EDUCA
ATION
REVIT
TALISATIO
ON PROJEC
CT-PHASE II
NATIIONAL
L DIPLO
OMA IIN
SCIENC
S
CE LAB
BORATORY TECH
HNOLO
OGY
CEL
LL BIOL
B
OGY
Y
COU
URSE CODE: STB
S
121
YE
EAR I- S
SE MEST
TER I
THE
EORY
Version
V
1: December
D
r 2008
List of Contents
WEEK 1……………………………………………………………………………..3 WEEK 2…………………………………………………………………………….. 11 WEEK 3……………………………………………………………………………..16 WEEK 4……………………………………………………………………………..25 WEEK 5……………………………………………………………………………..29 WEEK 6……………………………………………………………………………..35 WEEK 7……………………………………………………………………………..43 WEEK 8……………………………………………………………………………..48 WEEK 9……………………………………………………………………………..54 WEEK 10……………………………………………………………………………..60 WEEK 11……………………………………………………………………………..67 WEEK 12……………………………………………………………………………..73 WEEK 13……………………………………………………………………………..77 WEEK 14……………………………………………………………………………..81 WEEK 15……………………………………………………………………………..90 2
WEEK 1:
1.1
The Living Cell
The cell is the basic structural and functional unit of an organism. It is therefore, the
simplest, the smallest and basic unit of life. All living things are made up of cells.
Fundamentally, the cell is regarded as the basic unit of all living things because it
under take all life activities such as reproduction, excretion, growth, adaptation,
respiration and definite life span e.t.c. All these activities possessed by a cell formed
the characteristics of living organisms. Living organisms are classified into two major
groups based on the number of cells. Unicellular (acellular) organisms consist of only
one cell (e.g. Amoeba, Chlamydomonas, Euglena, Paramecium. On the other hand
multicellular organisms consist of two or more cells (e.g. volvox, hydra, spirogyra,
flowering plants, fish, bird and man).
1.2
The Cell Theory
The idea that all living things are composed of cells developed over many years and is
strongly linked to the invention and refinement of the microscope. Early microscopes
in the 1600’s (such as Leeuwenhoek’s) opened up a whole new field of biology – the
study of cell biology and microorganism. The cell theory is a fundamental idea of
biology.
1.3
Milestones in Cell Biology
Many scientists contributed to the history of the cell. The following are the
milestones in cell biology;
I.
1500s – convex lenses with a magnification greater than x5 became available
II.
Early 1600s – first compound microscopes used in Europe (used 2 convex lenses
to make objects look larger). Suffererred badly from color distortion; an effect
called spherical aberration.
III.
1632 – 1723 – Antoni Van Lecuwenhoel of Leyden, Holland, produced over 500
single lens microscope. Discovered bacteria, human blood cells, spermatozoa,
and protozoa, friend of Robert Hooke of England.
IV.
1661 – Marcello malpighi used lenses to study insects. Discovered capillaries and
may have described cells in writing of ‘globules’ and ‘saccules’
3
V.
1662 – Robert Hooke introduced the term cell in describing the microscopic
structure of corse. He believed that the cells walls were the important part of
otherwise empty structures. Published micrographia in 1665.
VI.
1672 – Nehemlah Grew wrote the first of two well illustrated books on the
microscopic anatomy of plants
VII. 1838 – 1839 – Botanist Mathias Schleiden and zoologist Theodor Schwann
proposed the cell theory for plants and animals: Plants and animals are composed
of groups of cells and that the cell is the basic unit of living organisms.
VIII. Rudelph Virchow – extended the cell theory by stating that: new cells are formed
only by the division of previously existing cells.
IX.
August Weismann added to Virchow’s idea by pointing out that: all the cells
living today can trace their ancestry back to ancient times (the link between cell
theory and evolution).
Fundamentally, the milestones in cell biology composed ideas which were formulated
by the early biologists to form the cell their: the cell theory states that:
1.
All living things are composed of cells and cell products
2.
New cells are formed only by the division of pre-existing cells
3.
The cell contains inherited information (genes) that are used as instruction for
growth, functioning, and development.
4.
The cell is the functioning unit of life; the chemical reactions of life take place
within cells.
5.
All living things are either single cells (unicellular) or group of cells
(multicellular)
1.4
Types Of Cells
Living things (organisms) are made up of one or more cells (i.e. unicellular and
multicellular respectively). Under the five kingdom system, cells can be divided into
two basic kinds: the prokaryotes, which are single cells without a distinct, membrane
– bound nucleus (e.g. bacteria) and the more complex eukaryotes with clearly
discernible nucleus bounded by nuclear membrane as found in majority of organisms,
including all animals and plants.
4
1.5
S/No
Comparison of Prokaryotic and Eukaryotic Cells
Characteristics
Prokaryotic cell
Eukaryotic cell
1.
Cell size
Mostly small (1 – 10μm)
Mostly large (10 - 100μm)
2.
Genetic system
DNA not associated with proteins; DNA
no chromosomes
3.
Cell division
Direct
by
Sexual system
binary
fission
Nutrition
or Some form of mitosis; centrioles,
partners; gametes that fuse
Absorption
by
most, Absorption,
absent;
and
Energy
Mitochondria
metabolism
enzymes bound to cell membrane, enzymes packaged therein, unified
Intracellular
oxidative Mitochondria
Organelles
on
present;
oxidative
oxidative
metabolism
variation in metabolic pattern
throughout
None
Cytoplasmic streaming, phagocytosis,
movement
8.
ingestion
photosynthesis
not packaged separately, great pattern
7.
in
Absent in most, highly modified if Present in most male and female
photosynthesis by some
6.
proteins
miotic spindle present
present
5.
with
chromosomes
budding; no mitosis
4.
complex
pinocytosis
Lack organelles – cell fused with Full complement of membrane bound
cytoplasm
but
no
membrane organelles in the cytoplasm. It has
bound nucleus. It has plasma plasma membrane, cytoplasm and
membrane, cell wall, nucleoid, nucleus.
plasmids, ribosomes, mesosomes, nucleolus
chromatophores,
flagella
1.6
capsule
and Cytoplasm
The
nucleus
and
contains
chromosomes.
contains
cytosol,
membrane bound organelles
Similarities
At the chemical level the prokaryotic and eukaryotic cells are fundamentally similar.
Therefore, they both have DNA, ATP and much the same range of enzymes and
coenzymes.
1.7
Plant and Animal Cells
Eukaryotic cells are typical of most organisms and are characterized by possession of
a nucleus and membrane bound organelles. Typical examples of eukaryotic cells are
the plant and animal cells.
5
Plant Cells: The plant cells are enclosed in a cellulose cell wall. The cell wall
protects the cell, maintains its shape, and prevents excessive water up take. It does not
interfere with the passage of materials into and out of the cell. Plant cells may be
specialized to perform particular functions e.g. thickened cell wall and plasma
membrane may form sclerenehyma cell (Sclereid). Other specialized plant cells
include: Root hair cells, guard cells, phloem cells e.t.c.
6
Fig. 1.1 A Typical Plant cell
Animal Cells: Animal cells, unlike plant cells, do not have a regular shape. Infact,
some animal cells (such as phagocytes) are able to alter their shape for various
purposes (e.g. engulfment of foreign materials). Animal cells differ form plant cells in
other ways too: they lack a cell wall, and some of their structures and organelles are
7
different. Animal cells are often not generalized, but they may become specialized to
perform particular functions. Typically, many animal cells have special features that
allow them to perform their role in the animal body to which they are a part of e.g.
leukocyte, red blood cell, muscle cell, sperm cell, fat (adipose cell), neurone, retina
(rod) cell, and endocrine gland cell e.t.c
8
Fig. A Typical Animal Cell
9
10
WEEK 2:
2.1
Description of Cell Inclusions and Organelles With Their Functions
1.
Cell wall: It does not act as a physiological boundary. Its main function is
mechanical – supporting the cell and multicellular structures and preventing the
outer membrane from busting as a result of the hydrostatic pressures that develop
inside the cell. Equally helps plant resist infection due to its impenetrability to
invaders.
The primary wall is cheaply of cellulose. The nuddle lamella contains pectin and
calcium pectate. It helps connects twp adjacent cells. The plasmodesmata enables
the movement of materials form one cell to another.
The cell wall allows water, gases to pass except when certain substances such as
ligmn (in xylem) or suberin (in cork) are deposited on it. It equally give shape and
firmness to the cell.
2.
Membranes: All the properties of living cells depends to some extent upon the
properties of their membranes.
A cell is surrounded by a membrane that separate it from its environment and
enables it to control the entry and exit of substances selectively. Moreover,
virtually all of the sub cellular organelles are made of, or surrounded by
membranes, and much of the cellular enzyme machinery is mounted on or
associated with membranes. (composing globules of phospholipids coated with
protein).
Most sub cellular structures such as nucleus, mitochondria or chloroplast are
surrounded by double membranes; however, the outermost living layer of the
cytoplasm, the cell membrane or plasmalemma, as well as the inner membrane
living the vacuole, the tonoplast, consist of single unit membranes.
11
3.
Nucleus: The largest and most prominent organized inclusion in most cells. It
contains a large part of the cell’s genetic material, the Deoxyribonucleic Acid
(DNA) strands that are present in protein complexes forming the nucleoprotein
(present as strands of chromatin, but during cell division they form into distinct
chromosomes.
The nucleus usually contains 1 – 4 nucleoli, densely staining spherical bodies that
appear to be Ribonucleic Acid (RNA) reserves, presumably used during the
decoding of the DNA message of the chromatin.
The nucleus is surrounded by a double membrane that has many small pores, thus
permitting the exit of informational material from the nucleus to the cytoplasm.
Nucleotide = Nitrogenous base + D Ribose
e.g Adenine + D-Ribose = Adenosine
nucleotide = Nucleoside + esterified phosphoric acid
e.g. Adenosine + phosphoric acid – AMP
nucleic acids = polynucleotides of high
4.
Endoplasmic Recticulum: Is a network of membranes that ranified throughout
the cytoplasm of most metabolically active cells. The rough endoplasmic
recticulum has large numbers of ribosomes, whereas no ribosomes attached to
smooth endoplasmic reciculum.
The ribosomes are the sites of protein synthesis. The endoplasmic recticulum is
directly concern with the synthesis and play a part in assembling the sub-units for
the protein synthesis and distributing the products.
Larger organelles such as chloroplast mitochondria and nucleus may have
ribosomes associated with their internal membranes.
5.
Golgi Apparatus and Dictyosomes: Dictyosomes are saucer – shaped bodies
made up of several layers of flat vesicles (cristernae, singular = cisterna) with unit
12
membranes. One or many distyosomes constitute the golgi apparatus. It is
primarily important as a major transport system of materials to the outside of the
cell.
6.
Mitochondira: They are larger and oval in structures. They contain much of the
cells metabolic machinery. Present in large members is numbers in metabolically
active cells but not abundant in resting (senescent) cells.
The mitochrodria have small knob like structure called F1 – ATPpase attached to
their inner surface by stalk (FO – particles). These structures are concerned with
the synthesis of adenosine triphosphate (ATP) – the cell’s energy mobilization
compound. Mitochondria provide the energy through the controlled break down
of respiratory substrates, for the synthesis of a large part of the cell’s ATP, which
is in turn uses to drive energy – requiring syntheses and reactions.
The mitochondria contain DNA which may be concerned with synthesis of some
structural proteins but not enzymatic proteins (which are normally programmed
by nuclear DNA).
7.
Plastids: Present in many plant cells. Most familiar are chloroplasts, which
contain the photosynthetic pigments, mainly chrolophylls, and carry on
photosynthesis. Leucoplasts are coloouless, often the site of starch granule
development, have called amyloplasts. The exact nature of the plastid may
depend on the presence or observe of light. Chromoplasts are specialized plastids
that contain pigments other than chlorophyll and are not involved on
photosynthesis. For instance, the red colour of berries, tomatoes and water melon
is due to chromoplasts that contain carotene.
Chloroplasts contain a substantial amount of DNA and are evidently capable of
programming the synthesis of some of their own structural components.
13
8.
Glyoxysomes and Peroxisomes: Are microscopic bodies found in many plant
cells. They appear to be essentially “packaged units” of enzymes concerned with
a specific sequence of reactions, much as mitochondria are concerned with Kreb’s
cycle oxidation and ATP synthesis and chloroplast with photosynthesis.
Glyosomes contain the enzymatic machinery of the glyozylate path way of fat
metabolism, which is important in the conversion of fats to sugars (e.g. during
germination of fat storing seeds such as casto beans).
Peroxisomes contain the enzymatic machinery for the oxidation of glycolate
produced in photosynthesis. They also contain the enzyme, catalase, which breaks
down the poisonous substance, hydrogen peroxide, formed during the oxidation
of glycolate.
2.2
Other Sub Cellular Structures
*
Centrosome: Present in the cells of primitive plants, it’s associated with the
mechanism of cell division. It’s now prominent in animal cells and became
extinct in plan cells.
*
The Vacuole: Is physiologically important to the cell because it affords a
storage place for materials not immediately required, and it provides a
dumping ground for cellular wastes that plants. Lacking an excretory system,
must store internally. Hydrolytic or destructive enzymes are secreted into the
vacuole; these enzymes degrade waste material into simple substances that
may be reabsorbed by the cytoplasm. The vacuole also functions as water
reserve in the cell, it maintains the cell’s structure and rigidity by exerting
pressure on the cell will, prevents it from distorting or collapsing.
The vacuole membrane, the tonoplast may be involved in the secretion of substances
into the vacuole.
14
The vacuole may contain a range of dissolved substances: sugars, salts, acids,
nitrogenous compounds, such complex compounds as alkaloids, glycosides, small
droplets or emulsions of fats, oils, protein may be found.
15
WEEK 3:
3.1
Description of Cell Inclusions and Organelles With Their Functions
2.
Cell wall: It does not act as a physiological boundary. Its main function is
mechanical – supporting the cell and multicellular structures and preventing the
outer membrane from busting as a result of the hydrostatic pressures that develop
inside the cell. Equally helps plant resist infection due to its impenetrability to
invaders.
The primary wall is cheaply of cellulose. The nuddle lamella contains pectin and
calcium pectate. It helps connects twp adjacent cells. The plasmodesmata enables
the movement of materials form one cell to another.
The cell wall allows water, gases to pass except when certain substances such as
ligmn (in xylem) or suberin (in cork) are deposited on it. It equally give shape and
firmness to the cell.
2.
Membranes: All the properties of living cells depends to some extent upon the
properties of their membranes.
A cell is surrounded by a membrane that separate it from its environment and
enables it to control the entry and exit of substances selectively. Moreover,
virtually all of the sub cellular organelles are made of, or surrounded by
membranes, and much of the cellular enzyme machinery is mounted on or
associated with membranes. (composing globules of phospholipids coated with
protein).
Most sub cellular structures such as nucleus, mitochondria or chloroplast are
surrounded by double membranes; however, the outermost living layer of the
cytoplasm, the cell membrane or plasmalemma, as well as the inner membrane
living the vacuole, the tonoplast, consist of single unit membranes.
16
3.
Nucleus: The largest and most prominent organized inclusion in most cells. It
contains a large part of the cell’s genetic material, the Deoxyribonucleic Acid
(DNA) strands that are present in protein complexes forming the nucleoprotein
(present as strands of chromatin, but during cell division they form into distinct
chromosomes.
The nucleus usually contains 1 – 4 nucleoli, densely staining spherical bodies that
appear to be Ribonucleic Acid (RNA) reserves, presumably used during the
decoding of the DNA message of the chromatin.
The nucleus is surrounded by a double membrane that has many small pores, thus
permitting the exit of informational material from the nucleus to the cytoplasm.
Nucleotide = Nitrogenous base + D Ribose
e.g Adenine + D-Ribose = Adenosine
nucleotide = Nucleoside + esterified phosphoric acid
e.g. Adenosine + phosphoric acid – AMP
nucleic acids = polynucleotides of high
4.
Endoplasmic Recticulum: Is a network of membranes that ranified throughout
the cytoplasm of most metabolically active cells. The rough endoplasmic
recticulum has large numbers of ribosomes, whereas no ribosomes attached to
smooth endoplasmic reciculum.
The ribosomes are the sites of protein synthesis. The endoplasmic recticulum is
directly concern with the synthesis and play a part in assembling the sub-units for
the protein synthesis and distributing the products.
Larger organelles such as chloroplast mitochondria and nucleus may have
ribosomes associated with their internal membranes.
5.
Golgi Apparatus and Dictyosomes: Dictyosomes are saucer – shaped bodies
made up of several layers of flat vesicles (cristernae, singular = cisterna) with unit
17
membranes. One or many distyosomes constitute the golgi apparatus. It is
primarily important as a major transport system of materials to the outside of the
cell.
6.
Mitochondira: They are larger and oval in structures. They contain much of the
cells metabolic machinery. Present in large members is numbers in metabolically
active cells but not abundant in resting (senescent) cells.
The mitochrodria have small knob like structure called F1 – ATPpase attached to
their inner surface by stalk (FO – particles). These structures are concerned with
the synthesis of adenosine triphosphate (ATP) – the cell’s energy mobilization
compound. Mitochondria provide the energy through the controlled break down
of respiratory substrates, for the synthesis of a large part of the cell’s ATP, which
is in turn uses to drive energy – requiring syntheses and reactions.
The mitochondria contain DNA which may be concerned with synthesis of some
structural proteins but not enzymatic proteins (which are normally programmed
by nuclear DNA).
7.
Plastids: Present in many plant cells. Most familiar are chloroplasts, which
contain the photosynthetic pigments, mainly chrolophylls, and carry on
photosynthesis. Leucoplasts are coloouless, often the site of starch granule
development, have called amyloplasts. The exact nature of the plastid may
depend on the presence or observe of light. Chromoplasts are specialized plastids
that contain pigments other than chlorophyll and are not involved on
photosynthesis. For instance, the red colour of berries, tomatoes and water melon
is due to chromoplasts that contain carotene.
Chloroplasts contain a substantial
amount of DNA and are evidently capable of programming the synthesis of some
of their own structural components.
18
8.
Glyoxysomes and Peroxisomes: Are microscopic bodies found in many plant
cells. They appear to be essentially “packaged units” of enzymes concerned with
a specific sequence of reactions, much as mitochondria are concerned with Kreb’s
cycle oxidation and ATP synthesis and chloroplast with photosynthesis.
Glyosomes contain the enzymatic machinery of the glyozylate path way of fat
metabolism, which is important in the conversion of fats to sugars (e.g. during
germination of fat storing seeds such as casto beans).
Peroxisomes contain the enzymatic machinery for the oxidation of glycolate
produced in photosynthesis. They also contain the enzyme, catalase, which breaks
down the poisonous substance, hydrogen peroxide, formed during the oxidation
of glycolate.
3.2
Other sub cellular structures
*
Centrosome: Present in the cells of primitive plants, it’s associated with the
mechanism of cell division. It’s now prominent in animal cells and became
extinct in plan cells.
*
The Vacuole: Is physiologically important to the cell because it affords a
storage place for materials not immediately required, and it provides a
dumping ground for cellular wastes that plants. Lacking an excretory system,
must store internally. Hydrolytic or destructive enzymes are secreted into the
vacuole; these enzymes degrade waste material into simple substances that
may be reabsorbed by the cytoplasm. The vacuole also functions as water
reserve in the cell, it maintains the cell’s structure and rigidity by exerting
pressure on the cell will, prevents it from distorting or collapsing.
The vacuole membrane, the tonoplast may be involved in the secretion of substances
into the vacuole.The vacuole may contain a range of dissolved substances: sugars,
19
salts, acids, nitrogenous compounds, such complex cmpounds as alkaloids,
glycosides, small droplets or emulsions of fats, oils, protein may be found.
3.3
Structure and Functions of DNA And RNA
Nucleic acids are a special group of chemical in cells concerned with he transmission
of inherits information. They have the capacity to store the information that controls
cellular activity. The central nucleic acid is deoxyribonucleic acid (DNA). DNA is a
major component of chromosomes found primarily in the nucleus. Small amount is
found in mitochondria and chloroplasts. Other ribonucleic acids (RNA) are involved
in the reading of the DNA information. All nucleic acids are made up of simple
repeating units known as nucleotides, linked together to form chains or strands, often
of great length such as DNA molecule. The strands vary in the sequence of the bases
found on each nucleotide. It is this sequence which provides the genetic code for the
cell.
20
Fig. 3.1 Structures of RNA and DNA Molecules
21
3.4
The Building Block of Nucleic Acids
The fact that nucleotides are the building block of nucleic acids signifies that DNA
and RNA are polynucleotide, DNA being a particularly stable polynucleotide. A
nucleotide consists of three molecules linked together a pentose sugar, phosphoric
acid and an organic base.
A penetos sugar has basically the same structure as a hexose sugar such as glucose
except that there is one less carbon atom in the ring. Thus the molecule is constructed
as follows:
This particular sugar is ribose and it is found in ribonucleic acid.
Deoxyribonucleic acid has a different sugar, deoxyribose, which differs from ribose
only in that the hydroxyl group at position 2 is replaced by a hydrogen atom. Thus,
deoxyribose has one less oxygen atom than ribose hance the name deoxyribose.
The second construction of a nucleotide is phosphoric acid (H3PO4) with structural
formula thus:
The third component is the organic base. DNA contains four different organic bases
adenine, guanine, cytosine, and thymine, abbreviated respectively to A, G, C, and T.
RNA too contains adenine, cytosine and guanine, but has uracil (abbreviated to U)
rather than thymine. All these five bases are ring compounds composed of carbon and
nitrogen atoms simply represented as follows:
Ribonucleic acid (RNA): comprises a single strand of nucleotides linked together
DNA replication and its significance.
The replication of DNA is a necessary preliminary step for cell division (both mitosis
and meiosis). This process creates the two chromatids that are found in chromosomes
that are preparing to divide. By this process, the whole chromosome is essentially
duplicated, but is still held together by a common Centromere. Enzyme are
responsible for all the key events.
22
The main stypes involve in DNA replication are: unwinding the DNA molecule,
making bew DNA strands and rewinding the DNA molecule.
A normal chromosome consists of single NDA molecule parked into a single
chromatid.
The long molecule of double stranded DNA must be untwisted at high speed at its
replication fork by tow enzymes helicase unwinds the parental strands, DNA gyrase
then relives the strain that this generates by cutting, windings and rejoining the DNA
strands.
Sept 2: The formation of new DNA is carried out mostly by an enzyme complex
called DNA polymerase and series of proteins that cause the two strands to break a
parts.
•
On one side (the leading strand), nucleotides are assembled in a continuous
fashion.
•
On the other side (the lagaing strand) fragments of single stranded DNA between
1000-2000 nucleotides long are created.
•
These will be later joined together to form one continuous length.
Step 3: each of the two new double-helix DNA molecule has one strand of the original
DNA and one strand that is newly synthesized.
•
The two DNA molecules rewind into their corkscrew double helix is then coiled
around histone proteins and further wrapped up to form separate chromatids skill
joined a common cemtromere)
•
The two chromatids will become separated in the cell deivision process to form
two separate chromosomes.
23
3.5
Role of Rns in Protein Sunthesis
DNA in the nucleus acts as the basis for template for the production of another sort of
nucleic acid called messenger RNA. Messeger RNA has the ability to convey the
instruction needed for protein synthesis from the nucleus to the cytoplasm. The idea
that DNA makes protein via an intermediated, RNA is known as the central dogma of
molecular genetics. When messenger RNA gets out into the cytoplasm it attaches it
self to a ribosome where it causes amino acids to assemble in the right order. This it
acid know as transfer RNA. The transfer RNA molecules transfer (carry) amino acids
to ribosomes transfer RNA main property in their ability to bind to amino acids at one
end and to messenger RNA at the other
24
WEEK 4:
4.1
Introduction to cell division
Growth and sexual reproduction depend on the division of cells. The division of cells
occurs in two ways: mitosis and meiosis.
1.
MITOSIS (KARYOKINESIS)
Mitosis refers to the division of a body cell (somatic cell), and consists of the
division of the nucleus into two, followed by a division of the cytoplasm into two
to form two daughter cell.
The biological importance mitosis lies in the fact that it leads to the formation of
two daughter nucleus having the same nucleus, and carrying the same genetic
information. Mitosis takes place in all growth (somatic) cells, but not in those
concerned with garmete formation. However, the primitive stages of gameto
genesis is an exception. Mitosis is most noticeable in the actively growing parts a
plants such as the apical meristems (shoot a pex), root tips (e.g onions and lilies)
and the cambium (eg casscualr cambium found in the stem and root which
divides to produce secondary xylem and secondary phloem). Mature tissues of
plants such as xylem, ploem and the wood fibres do not divide. In animals,
mitosis is active in the epidermal cells (the outermost layer of cells of the body of
an animal) and the bone marrow cells (bone marrow = soft tissue contained
within the central cavity and internal spaces of a bone.
The division of the nucleus in mitosis occurs in series of five stages, phases
namely: prophase, promataphase, metaphase, anaphase, and telophase. The period
between two phases of mitosis is known as interpahse. At the beginning of
interphase, chromosomes are not visible. Division of the cells begins with the
spiral action of the chromosomes. At this time the cell contains one or two
25
nucleoli within the nuclear evelope. Interphase ends when the chromosomes
becomes visible as double threads.
i.
PROPHASE:
This is the first phase of the division of the nucleus. It strabds when the
chromosomes become visible as a result of the sprialized and nature of the
chromosome threads. The nuclear envelope and nucleoli remain intact as the
chromosomes become shorter and thicker. Prophase ends when the nuclear
envelope and nucleoli disintegrate.
ii.
PROMETAPHASE
Prometaphase begins with the disappearance of the nuclear envelop. As the
nuclear envelope disappears, protein fibres (spindle fibre) appear in the
cytoplasm. These spindle ficbres connect each centromare to each of the two
points in the cytoplasm (poles). There are also spindle fibres which run from one
pole to the other without being connected to chromosomes. At this time the
chromosomes are scattered near the centre of the cell. The spindle fibres then start
to contract and chromosomes are pulled by their centromeres towards the equator
of the cytoplasm promataphase ends when the chromosomes reach the equator of
the cell.
iii.
METAPHASE
At this phase the chromosomes lie at the equator or centre of the cells but the
arms of the chromosomes may be lie in any position. Metaphase ends when all
the chromosomes divide longitudinally.
iv.
ANAPHASE
This phase starts when the chromosomes have divided.
Each of the former chromotids now has its own centromere and has become, by
definition, a chromosome. Each of these chromosomes is a single thread. Spindle
26
fibres continue to contract and the centromeres, followed by the chromosome
arms are drawn to the poles of the cells. Anaphase ends when the chromosomes
are gathered at the poles of the cells and a nuclear envelope starts to form around
each group of chromosomes.
v.
TELOPHASE
During this phase, the chromosomes despiralize (i.e stop their spiral action).
Nuclear envelope are formed around each group of chromosomes, nucleoli are
formed within each group. At this stage the division of the cytoplasm ends and a
new cell is formed.
The cell plate begins to form during this period. It gradually widens until a very
clear division between the two daughter nuclei cells is achieved. This is the
process known as cytokinesis.
THE MAIN STAGES OF MITOSIS ARE ILLUSTRATED BELOW:
27
4.2
Significance of Mitosis
1.
Basis for the growth of all higher plants and animas typically the growth of the
human foetus begins by the mitotic divisions of the zygote.
2.
All the cells that arise by mitosis are genetically alike.
28
WEEK 5
5.1
Meiosis
Meiosis is distinctly different from mitosis. It is a special type of calle division
associated with diploid (2nO organisms which occurs during gameted (sex cells)
formation. The main characteristics is the reduction of the chromosomes number from
double (2n) to a single (n) set in a gamete. This means that instead of being dipaloid, a
mature gamete from either plants or animals carries half (haploid –n) the chromosome
umber of the parent. It is for this reason that meiosis is also calles reduction division.
Meisosis comprises two nuclear divisions followed by two cell divisions. It always
result in the formation of four nucleoli. The first deivsion of meiosis is more
complicated than the second. Each of these divisions is made of five phases:
prophase, prometaphase, metaphase anaphase and telophase.
5.2
First Division of Meiosis
i.
PROPHASE
The complication associated with the first division of meiosis is as a result of
the different stages of the prophase. They are
a.
Leptotene
b.
Zygotene,
c.
Pachytene
d.
Diplotene
e.
Diakinesis
a.
LEPTOTENE
At this stage the chromosome thread appears to be single, and not
divided into chromatids
29
b.
ZYGOTENE
At this stage homologous chromosomes (chromosomes that are the
same) of the diploid complement pair with each other pairing starts at
certain points along the lengths of homologous chromosomes and
continues until they are closely paired along their whole lengths. Each
pair of homologous chromosome is now BIVALENT (A bialent is
made of a paternal and maternal chromosome).
c.
PACHYTENE
At this stage, homologous chromosomes are closely paired and each
chromosomes can now be seen to be composed of two chromotids.
d.
DIPLOTENE
This stage begins when the chromosomes of each bivanlent start to
repel each other.
They however, do not separate because chromatides of homologous
chromosomes have exchanged parts at certain point along their length.
Hence they are prevented from separating completely.
e.
DIAKINESIS
At this stage the shortening of chromosomes and repulsion between
homologous chromosomes of each bivalent reaches its maximum. This
is the last stage of prophase of the first division of meiosis
f.
PROMETAPHASE
At this stage the nuclear envelope and nucleoli disintegrate. Spindle
fibres. Form and each centromere receives a single spindle fibre. Of
the chromosomes of each bivalent, one becomes connected to one pole
30
of the spindle and the other is connected to the other pole of the
spindle.
g.
METAPHASE
The bivalents at this stage are at the equator of the spindle centromeres
do not divide.
h.
ANAPHASE
This phase begins when the chromosomes of each bivalent are pulled
apart by the spindle fibres. Each chromosome still as two chromatids,
connected by a single centromere, as it is drawn to a pole of the
spindle.
i.
TELOPHASSE
This phase begins as nuclear envelopes are formed. Lytokinesis may or
may not take place at this time, as the nucleus enter interphase.
5.3
Second Division of Meiosis
PROPHASE II
Chromosomes appear in both of the nuclei formed in the first division of meiosis.
There is no pairing of chromosomes and chiasmata do not develop as in prophase.
PROMETAPHASE II
The nuclear membranes disappear and the chromosomes become attached by their
centromeres to the spindle fibres at the equator. The to chromatides of each
chromosome are now easily seen.
METAPHASE II
The chromosomes are drawn to the equator of the cytoplasm as a result of the
contraction of the spindle fibres. Towards the end of metaphase II, the chromosomes
divide into two.
31
ANAPHASE II
The centromeres of the chromosomes break in two and the chromatids are pulled
towards opposite poles of the cell.
TELEPHASE
The chromatids come together at opposite poles of the cells. Here become surrounded
by nuclear membranes, uncoil and the nucleoli appear. The spindle fibres disappear
and cleavage of the cytoplasm follows. Altogether four cells, each with half the
number of chromosomes, are produces from each cell which divides by meiosis.
Meiosis includes two cell divisions. In this
figure, the original cell is 2n-4. After two meiotic divisions each resulting cell
1n-2
32
5.4
Significance of Meiosis
1.
The haploids (n) gamete from each sex (male and female) produced by
meiotix division fuse at gertisation to make a complete (diploid -2n)
chromosome set for the zygote formed. By this way, the diploid state is
restored and maintained in all sexually reproducing organisms.
2.
Another important feature of meiosis is the inter changes of genetic materials
between two parental chromosomes through a process called crossing over,
this is the phenomenon that creates variation in individuals.
5.5
Differences between Mitosis And Meiosis
MITOSIS
MEIOSIS
1.
Chromosome number constant
Chromosome number haploid (n)
2.
Equational division
Reduction division
3.
Occure in somatic (meristematic ot body Occurs in reproductive mother cells (cells
cells)
4.
5.
that produce the gametes)
In prophase, chromosomes with double Indentical
chromosomes
threads
threads in its prophase
No pairing
Pairing
of
identical
with
single
(homologous)
chromosome occure.
6.
The prophase is short
Prophase prolonged and divided into
substages
7.
In the metaphase, the centomere are In the metaphase, the centromeres of the
lined up in the equatorial plane and the homologous chraomosomes lie toward the
arms extended into the cytoplasm
to opposite poles of the spindle near the
equator aonther arms extended toward the
equator
33
8.
No fusion of diploid cells (nucleic taking The haploid (n) gametes formed fuse by
place)
9.
fertilization to form the zygote (2n)
The centromere divides in metaphase Chiasmata formation occur
and the sister chromatids move to the
opposite poles chiamata not formed
10.
Two daughter cells produces
Four new types of cells prodiced
THE MAIN STAGES OF MEIOSIS ARE ILLUSTRATED BELOW
34
WEEK 6
6.1
Hydrogen Ion Concentration (pH):
pH indicates the acidity or alkalinity of a substance and is a measurement of hydrogen
or hdyroxyl ion activity. Acidic solutions with a high hydrogen ion concentration
have a low pH, and alkaline solutions with a low hydrogen ion concentration have a
high pH. The acidity of a solution is expressed as its pH. This is the negative
logarithm to the base 10 of the hydrogen ion concentration in moles per dm3 of
solution (measure of hydrogen ion concentration of the solution). A pH of 7.0
represents neutrality. A solution with a pH of less than 7.0 is acidic, and the lower the
figure the higher the acidity (i.e the greater the hydrogen ion concentration). A
solution with pH greater than 7.0 is basic or alkaline and the higher the figure the
more basis is the solution.
Buffers: Are compounds which behave in such a way as to resist changes in pH on
dilution or addition of moderate amounts of acid or alkali. Typically sodium
bicarbonate can resist either a decrease or an increase in pH by mopping up e
hydrogen or hydroxyl ins as appropriate. Much more important are phosphate both of
which play an important part in suppressing the hydrogen ion concentration in the
blood. The phosphate combines with free hydrogen ions to form the dihydrogen
phosphate.
35
6.2
Biological Molecules
The molecules that make up the bodies of living things can be grouped into five
classes: water, carbohydrate, lipids, proteins and nucleic acid.
Fig 6.1 Biological Molecules
36
6.2.1
Isomers and formation of simple carbohydrate
37
6.2.2
Amino Acid
38
6.2.3
Isomers of Amino Acid
39
6.3
Structures of Protein
The main protein structures are: Primary, secondary, tertiary and quaternary structures
Protein may be denaturated by various agents (Fig. 6.2).
Fig. 6.2 Main Structure Protein and Protein Denaturation
40
6.4
Fats and Oil
Fig. 6.3 Fats and oil
41
42
WEEK 7
7.1
Nucleic Acids
Nucleic acids are wonderful discoveries of modern times. They are universally
present in the nucleus and the cytoplasm of all living cells, and are now definitely
known to form the chemical basis of life. They are very complex organic compounds
made of phosphate, pentose sugar (ribose, as in RNA or deoxyribose as in DNA) and
nitrogen bases (purine and pyrimindine; see p. 124). Nucleic acid molecules are very
large, even larger than protein molecules, and consist of infinite numbers of repeating
nucleotide units linked in any sequence into a long chain. They are, thus, high
polymers of nucleotides and have very high molecular weights. Depending on the
sequence of nucleotide units in the chain, the nucleic acids may be of an infinite
variety of structures. A nucleotide is a molecular unit (monomer) of a nucleic acid
molecule (macro-), and consists of three sub-units: a phosphate, a pentose sugar
(ribose or deoxyribose) and a nitrogen base (purine or pyrimidine). Phosphate and
sugar link. There are a few types of nucleotides, each with a specific nitrogen base.
They may also occur free in the cytoplasm as ATP, DPN, TPN, COA (coenzyme A),
etc. A nucleotide is formed when a phosphate group is added to a nucleoside. A
nucleoside is a compound consisting of two sub-units: a pentose sugar and a nitrogen
base. It is the precursor of a nucleotide. There are two kinds of nucleic acids, viz.
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). The latter occurs in three
forms: messenger RNA (mRNA), transfer RNA or soluble RNA (tRNA or sRNA) and
ribosomal RNA (rRNA), as detailed on p. 265. a summary of nucleic acid formation
may be given thus: pentose sugar + nitrogen base --- nucleoside; nucleoside +
phosphate group – nucleotide; nucleotide + nucleotide + --- nucleic acid.
DNA and RNA. Occurance. DNA occurs almost exclusively in the chromosome, and
to a small extent only, as is now known, in chloroplasts and mitochondria. RNA
43
occurs mostly in the cytoplasm (about 90% of a cells RNA occurs here), nucleolus
and ribosomes, and to some extent in the chromosomes, of course, in three different
forms, as already mentioned. A big portion of the RNA formed in the nucleolus,
possibly under the control of DNA moves to the surrounding cytoplasm.
DNA or RNA with a certain protein in each case, is the predominant constituent of
most virus particles (see fig V/64). This is also true of many bacterial cells.
Chemistry. DNA and RNA are close chemical relatives. The principal different
between the two lies in the kind of pentose sugar present in their molecules. RNA
contains a 5-carbon atom (pentose) sugar, ribose whereas NDA contains ‘deoxrybose’
also a 5 – carbon (pentose) sugar, but it has one in RNA structure both occur as
macromolucules but RNA molecules are single stranded, while DNA molecules are
double stranded (with but few exceptions in each case). The DNA and RNA bases are
the same except that RNA has urcil, while DNA has thymine. Functions DNA is the
sole genetic material (analogous to genes) migrating intact from generation to
generation through the reproductive units or gametes, and is responsible for the
development of specific characteristics in successive generations. DNA is the
controlling center of all the vital activities of a living cell and is responsible for all
biosynthetic processes, including protein synthesis. Biologists now believe that all
secrets of life are confined to and controlled by the DNA of a living cell. RNA, under
the instructions of DNA, is directly connected with the synthesis, of proteins.
44
Fig. 7.1 DNA replication
DNA Molecule. The DNA molecule of a single chromosome is very long and
complex (macromolecule) forming the backbone of each chromosome. While
investigation nucleic acids in 1953, Watson and Crick (see footnote on p. 123)
proposed a double helix model of the DNA molecule (Watson - Crick model),
universally accepted since then. According to them, DNA occurs as double-stranded
molecule, with the two strands profusely coiled and entwined about each other
throughout their whole length. The structure is like a ladder twisted in a helical
fashion. Each spiral strand is made of groups (micromolecules) of deoxyribose sugar
(a 5-carbon or pentose sugar), alternating with groups of phosphate, and an infinite
45
number of cross-links is made of two distinct types of nitrogenous bases-purines and
pyrimidines each attached to a sugar. Each pair of bases is loosely linked by hydrogen
bonds. Altogether, there are two purines (adenine and guanine) and two pyrimidines
(thymine and cytosine). It is the rue that a specific purine always pairs with a specific
pyrimidine as alleles, (i.e. complementary pairs), e.g. adenine with thymine (A-T) and
guanine with cytosine (G-C). It may be noted that each base is a part of a nucleoside
(see p. 123). It important to note that the pairs of nitrogenous bases occur in infinite
sequences in a DNA molecule, enabling the latter to coin an infinite number of
chemical codes (messages or information) and transmitting the appropriate codes
through its working partner, RNA, to the surrounding cytoplasm for its manifold
activities. In summary, a DNA strand is made of four types of nucleotides PDT, PDA,
PDG and PDC, evidently including four types of nucleosides DT, DA, DG and DC,
and also four kinds of nitrogenous bases T, A, G and C. although such bases combine
in only four specific pairs T-A, A-T, G-C and C-G, they may occur in infinite
sequences in a DNA molecule.
46
Fig. 7.1 Base Pairing Rule of Nucleotides
47
WEEK 8
8.1
Various types of cells/tissues
Atoms are organized into complex molecules such as proteins. These form the
components of cells, which are the function and structural units of all living
organisms. Some organisms consist of single sells, but others are collections of many
cells, organized into tissues and organs. Thus the multicellular organisms, consisting
numerous cells of one or more types are generally grouped together to form tissues.
The function of a tissue depends on what kind of cell it is composed of. Moreso, in
more complex organisms different tissues are combined to form organs. The study of
tissues and the way they are arranged in organs is known as histology.
Tissues can conveniently be classified based on the function they perform in the body.
On this basis tissues are classified as:
1.
Animal Tissues: Which may be divided into epithelial tissue (epithelium), connective
tissue, skeletal tissue, blood tissue, nerve tissue, muscle tissue and reproductive
tissue?
2.
Plant Tissue: Which may be divided into meristematic tissue, epidermal tissue
(epidermis), parenchyma, collenchyma, silerenchyma, vascular tissue and cork?
1.
Animal Tissues
Epithelium: Refers to living tissue. It covers the surface of the animal and the organs,
cavities and tubes within it. It simply consists of a sheet of cells closely fit together;
resting on a basement membrane with a free surface on the other side.
The basement membrane which is produced by the epithelial cells consists of collagen
(mesh work of fine protein fibre) embedded in a jelly like matrix). It supports the
epithelium and serves some control over what passes through it.
The epithelium on the outer surface of an animal is known as the epidermis. In
arthropods, the epidermis secretes a protective cuticle. The epithelium which forms
the inner living of cavities and tubes inside the body such as the heart, blood vessels
and lymph vessels is called endothelium.
8.2
Types of Epithelia
Epithelia are of two kinds namely: Epithelia consisting one layer of cells and epithelia
consisting many layers of cells.
48
1.
Epithelia consisting of one layer of cells
This form of epithelium consists of cells
This form of epithelium consists of
(a) Squamous epithelium
(d) Ciliated epithelium
(b) cuboid epithelium
(e) Glandular epithelium
(c) columnar epithelium
2.
Epithelia consisting of many layers of cells (stratified epithelium). It is thicker
than the single layer epithelia and more effective as a protective covering. It
comprises the epidermis of the skin and the lining of certain cavities and tubes
inside the body, such as the vagina and oesophagus.
8.3
Connective Tissue
The connective tissue binds organs and tissues together and fills the spaces between
them. It consists of a jelly like matrix (ground substance) in which several types of
cells and protein fibres are embedded. The main kinds of connective tissue are:
Areolar tissue, collagen tissue, elastic tissue and adipose tissue.
8.4
Skeletal Tissue
The skeletal tissue supports the body and provides a strong frame work whose rigid
components can move against each other and smoothly articulating joints. It also
consists of cells embedded in a matrix (in this case the matrix is hard).
The main kinds of skeletal tissue occur in vertebrates: cartilage and bone.
Cartilage: The cartilage is softer than bone. It is useful as cushioning material. The
matrix of cartilage, chondrin consists of mucopoly saccharide in which are embedded
spherical cells known as chondroblasts.
The main types of cartilages are: Hyaline cartilage, fibrocartilage, and elastic
cartilage.
Hyaline Cartilage: Is found in the wall of the trachea where its function is to prevent
the wall caving in. it is also found in the ends (epiphyses) of limb bones where it is
associated with the formation of bone tissue (ossification), and at the joints where it
performs a cushioning function and provides a smooth articulating surface.
49
Fibrocartilage (white fibrous cartilage): It is like hyaline cartilage but it contains
collagen fibres. It is found in the invertebral discs of the vertebral column where it
performs a cushioning function.
Elastic cartilage (yellow elastic cartilage): It contains elastic fibres. It is tough but
bendable. It is found in the pinna of the ear.
8.5
Bone Tissue
This consist of an organic matrix impregnated with mineral salts containing calcium
and phosphate. Cells called osteoblasts produces both the organic matrix and mineral
salts.
The two types of bone tissue are compact bone and spongy bone.
Compact Bone: This has haversian canals with the surrounding lamellae packed
tightly together to give a very dense material.
Spongy Bone: This is of looser construction and forms a three – dimensional network
of interconnected strands with spaces in between.
8.6
Plant Tissues
Plants have unique tissues which are related to their way of life. Other tissues found in
plants apart from conducting tissues are: meristematic, epidermal, cork, parenchyma,
collenchyma and selerenchyma tissues.
8.6
Vascular Bundles (Tissues) in Plants
It is concerned with transport. It is functionally equivalent to the circulatory system of
animals. The two main forms of vascular tissue are xylem and phloem.
8.7
Xylem tissue
Consists of elongated, lignified tubes which are either vessels or tracheids. They are
like sclerenchyma fibres as they begin as living cells but looses their cell contents and
die with the lignification of their walls. Their main function is to transport water and
mineral salts from the roots to the leaves.
8.8
Phloem Tissue:
50
Consists mainly of unlignified living cells called sieve tubes. The adjacent cells are
called companion cells. Both of these transport soluble food substances from one part
of the plant to another.
8.9
Meristematic Tissue:
This gives rise to all other plant tissues. The cells are small, immature and with thin
walls. The cell lack chloroplasts and large vacuoles characteristic of mature plant
cells. They are found at the growth regions of plants such as the tip of the stem and
root. The cells have ability to divided by mitosis and subsequently differentiate into
other cell types.
8.1.0
Epiderminal Tissue:
This is equivalent to animal epithelium. It is located at the surface of stems and
leaves. The cells are flattened and irregular in shape. They form a protective covering
for delicate tissues beneath. Their outer walls are tick and covered with a waxy cuticle
that is impermeable to water and also prevents excessive evaporation in dry
conditions. Plant epidermal cells lack chloroplasts with the exception of stomata
guard cells.
8.1.1
Cork
This is a multilayered under the epidermis of the stems and branches of shrubs and
trees, where it forms the hard part of the bark. The cells are small and more or less
spherical and, as they develop, their walls become impregnated with a fatty substance
called suberin which renders them impervious lose their contents. Therefore cork is a
dead tissue. However. Tissue underneath it are alive. Cork function to protect these
living tissues from physical attack, insects and cold cork is useful in making bottles
using the bark.
8.1.2
Parenchyma
Is a packing tissue. Its main function is to fill the spaces between other tissues. The
cells are roughly spherical in shape, with flattened faces where they press against each
other. If the cells are fully turgid and tightly packed, they help maintain the plant
shape and firmness.
Parenchyma may perform
some other functions in different parts of the plant.
Typically in the roots, the cells contain starch granules and thus performing storage
51
function. In the leaves they are found to contain chloroplasts and thus useful in
photosynthesis, and hence named as chlorenchyma. In aquatic plants, aerenchyma
(with air filled spaces between the cells) which are parenchyma enable the plants to be
buoyant and allow store of air for respiration.
8.1.3
Collechyma
This tissue is composed of living cells with thickened cellulose walls at the corners.
Collenchyma is found in the outer part of stems and in the mid rid of leaves.
Functionally, it provides strength with flexibility.
8.1.4
Sclerenchyma
This is much stronger than collechyma and plays a major role in support. It is found in
stems and in the midribs of leaves where is takes the form of elongated sclerenchyma
fibres. The cells start as living before became impregnated with lignin
(complexatomatic compound) which make them strong and impervious to water,
gases and solute. In the absence of oxygen and nutrients, the cell contents die and
degenerate, leaving a hollow fibre of lignin with tapering ends.
Strength and flexibility of sclerenchyma tissue varies from one type of plant to
another depending on the length of the fibres, thickness of their walls and their
arrangement. It provides raw materials for the textile industry and useful in making
paper.
8.1.5
Vascular Tissue
Is concerned with transport and is functionally equated to the circulatory system of
animals. Basically there are two types of vascular tissue, viz: xylem and phloem.
-
Xylem Tissue: It consists of elongated, lignified tubes with either vessels or
tracheids. Like sclerenchyma fibres, they start as living cells but with the
lignification of their walls lose their cell contents and die. Therefore, they finish
up as hollow tubes whose function is to transport water and mineral salts from the
roots to the leaves.
-
Phloem Tissue: Have unlignified living cells known as sieve tubes. With the aid
of adjacent companion cells, the sieve tubes transport soluble food substances
from one part of the plant to another.
52
Fig. 8.1 The Main plants tissues
53
WEEK 9
9.1
Process of Photosynthesis
Photosynthesis is the single most important physic-biochemical process on
which the existence of life depends on.
It is ability of green plants to utilize energy of light to produce carbon
containing organic material from stable inorganic material.
It is from carbohydrate produced that all of the countless number of organic
compounds which compose of living are derived.
The oxidation of organic compounds release stored energy to be utilize by
organisms to derive essential processes.
Photosynthesis can be defined as the formation of carbon containing
compounds from carbon dioxide and water, Illuminating green cells , water
and oxygen being the bi-products.
Above: showing growing green plants absorbing light energy from the
sun
54
Light energy
Simplest equation ; 6CO2+6H20
9.2
chlorophyll
C6H12O6 + 6O2
The Chloroplasts
The bulk of the photosynthesis of higher plants takes places in the green
leaves. Chloroplasts are plastids and concern with photosynthesis.
Chloroplasts are green in colour, the corresponding structures of red brown
algae are called chromoplasts,
While the blue-green are called chromatophores.
Chloroplasts are small, green, discoid in shape.
There may be patches of irregular shaped bodies composed of numerous
starch platelets called pyrenoid.
Chloroplasts in higher plants are found distributed in the palisade.
Approximate number of 20 -40 chloroplasts have been found in the leaf of a
higher plant.
Chemically, chloroplasts may compose in them these chemical constituent:
protein, lipid, carbohydrates,
Chlorophylls a and b carotenoid e.g. xanthophyll and carotene,nucleic acid
e.g DNA and RNA, vitamins K and E as well as metallic atoms such as Fe,
Cu, Mn, Mg. and Zn.
9.3
Importance of Stoma and Grana in The Chloroplasts
Chloroplasts have a heterogeneous structure made up of small granules
called grans.
9.4
Embedded Within The Stroma /Matrix.
Chloroplast is bounded by a double membrane, within which numerous
55
sheet-like lamellae, Running from one end to the other, each lamella consist
of a pair of unit membrane, Lamella is distinguish in to two regions; granal
and intergranal regions, the granal region constitutes a granum [pl. grana],
the function of lamella is to hold the chlorophyll molecules in a position
suitable for trapping the maximum amount light.
The stroma contains amongst other thing, the enzymes responsible for the
reduction of carbon dioxide and numerous starch granules.
In a nutshell, the absorption of light and splitting of water molecules into
oxygen takes place in the lamellae.
The subsequent building up of carbohydrates takes place in the stroma.
9.5
Light Stage / Phase
The light stage involves the photochemical splitting of water, the light stage
has the ability of trapping
the radiant energy
by chlorophyll, later
converted into chemical energy, the energy is used for splitting water
molecules into hydrogen and oxygen, oxygen is released as a bi-product
and the hydrogen ions; H+ from the water molecules, later enter the dark
phase in which carbon dioxide is reduced to form carbohydrate.
Light energy
C02 + 2H20
CH20 + H20 + O2
Chlorophyll
9.6
Dark stage
It involves the reduction of carbon dioxide to form carbohydrate, its an
endergonic process requiring energy, the energy is supplied by the splitting
of ATP formed in the light stage, the hydrogen for the carbon dioxide is
provided by the reduced NADP H2, also formed in the light phase, the
56
reduction of carbon dioxide and subsequent synthesis of carbohydrates
takes place in a series of small steps, each of which is controlled by a
specific enzyme, in the first step, the carbonadoed combines with a 5
carbon organic compound / ribulose diphosphate, Diphosphate is
responsible for fixing the carbon dioxide into the photosynthetic machinery
of the plant. The combination of carbon dioxide with the ribulose 1, 5 diphosphate gives An unstable 6 - carbon compound which split into two
molecules of a 3 - carbon compound phosphoglyceric acid next step; PGA
reduced to phosphoglyceraldehyde; a 3 carbon sugar/triose phosphaste, by
the help of NADP H2 , supported by the ATP and an enzyme,
Phosphoglyceraldehyde molecule is converted into dihydroxyacetone in
the
presence
of
an
enzyme.
phosphoglyceraldehyde and
Then
,
the
unification
of
one
dihyroxyacetone to form fructose 1, 6
diphosphate, With addition of energy, Next step involves, the synthesis of
a
series
of
organic
compounds,
to
form
fructose
1,6
diphosphate/glucose/starch the compounds are glucose or starch via,
Fructose 6, phosphate for storage.
Not all of the 3 -carbon sugar is converted into 6 -carbon sugar, Some of it
enters a series of reactions which later results in the regeneration of ribulose
1,5 diphosphate,
The regeneration of ribulose 1,5 diphosphate help in
ensuring the constant supply of the compound which is required in CO2.
Starch is not the only end product of photosynthesis. Other products include
lipids and proteins, these are the products of metabolic pathways leading
from PGA.
PGA is regarded as a kind of crossroads in photosynthesis, from it
carbohydrates, fats and proteins can ultimately be formed, these are end
57
products of dark reactions and occur in the stroma of the chlorophyll.
Factors affecting photosynthesis.These factors can be divided into two broad categories; the internal and
external. Internal factors-these include;
Chlorophyll: The amount of chlorophyll present has a direct relationship
with the rate of photosynthesis because it is a pigment which is
photoreceptive and is directly involve in trapping the light energy.
Photosynthetic Enzyme System- The amount and nature of enzymes play a
direct role on the rate of photosynthesis. Greater enzyme activity at higher
light intensity increases the capacity of a leaf to absorb more light needed
for photosynthesis.
Demand for photosynthetic: Rapid growing plants show increased rate of
photosynthesis in comparison to mature plants.
Role of Hormones- It has been observed that photosynthesis may be
regulated by plants hormone system. It was found that fibercillic acid and
cytokinins increase the photosynthetic and activity in plants.
Leaf Age: Newly expanding leaves generally show a maximum
photosynthetic activity while fairly old leaves show less photosynthetic
activity.
External factors these are as follows:Carbon dioxide: It is one of the raw materials for photosynthesis and its
concentration affects the rate of photosynthesis greatly. Due to its low
concentration in the atmosphere it acts as a limiting factor in
photosynthesis. At optimum temperature and light intensity, the rate of
photosynthesis increased with an increased in supply of carbon dioxide, but
rapidly decrease if the carbon dioxide concentration increases beyond the
58
maximum level.
Light: It affects the rate of photosynthesis because the energy stored by
green plants in carbohydrate molecules during photosynthesis can be supply
by light. Light affects the rate of photosynthesis in many ways, reflected or
absorbed or transmitted light, the intensity of light the quality and the
duration of light available are all semi-factor which falls under light factor.
Temperature- The effect of temperature on rate of photosynthesis is little
than other processes. Very high and very low temperatures affect the
photosynthetic rate adversely. The rate of photosynthesis increases with rise
in temperature from 6o - 37oc beyond which there is a rapid fall.
Photosynthesis increases with temperature but declines with It is then called
time factor.
Water: Water has an indirect effect on the rate of photosynthesis although
it may be considered as one of the raw materials for the process because it
affect the water relation of plants thus affecting photosynthesis Cells of the
plants become flaccid when there is scarcity of water. Depending on the
degree of water intensity in plants, the rate of photosynthesis may be
decreased from 10 -90%.
Oxygen: Oxygen is a bi-product of photosynthesis. Oxidation may be
essential for photosynthesis but it has been discovered that oxygen
accumulation in cells of plants may retard the rate of photosynthesis.
Mineral Nutrient Elements: Some minerals elements such as copper may
form a component of the photosynthetic enzymes or magnesium which is a
component of chlorophylls affect the rate of photosynthesis indirectly by
affecting the synthesis of photosynthetic enzymes and chlorophyll
respectively.
59
WEEK 10:
10.1
Respiration (Tissue Respiration)
All living cells require energy to carry out a variety of life processes;
1.
To do chemical work such as synthesis of organic compounds used for growth
and reproduction
2.
Movement
3.
Pumping of ions and solutes against electrical and concentration gradients
4.
production of heat to maintain body temperature
5.
Generation of electricity – in electric tissues and cells
6.
Emission of light as in fireflies, some bacteria and some fish ATP.
These activities and numerous others are energized by the hydrolysis of the terminal
phosphate group of ATP. The hydrolysis of the high energy bond yields 34 kilo joules
energy per molecule.
ATPase
ATP
+ H2O
-
ADP + H3PO4 + Energy (34kj)
The essential purpose of respiration is the breakdown of organic compounds
usually carbohydrates (hexose sugars) or fats, or occasionally proteins.
-
Thro and orderly sequence of enzyme – catalysed reactions, the energy is
liberated
The four stages in the biological oxidations of respiration are:
1.
Glycolysis – conversion of sugar to pyruvic acid
2.
Oxidative decarboxylation of pyruvate
3.
The citric acid (Kreb) cycle
4.
Terminal oxidation of hydrogen (oxidative phosphorylation).
60
Glycolysis
Glucose
Pyruvate
Acetyl Co A
Citric Acid Cycle
Oxidative
phosphorylation
61
Glycogen – animals
Starch – plants
1.
Glycolysis – sugar is broken down step by step by step to pyruvic acid
Glucose
6C
ATP
ADP
Glucose phosphorylated
Hexokinase
Glucose - 6 – phosphate 6C
Phosphoisomerase
Fructose – 6 – phosphate 6C
ATP
Phosphohexokinase
ADP
Fructose 1, 6 diphosphate 6C
Aldolase
Split into two 3C
Phospho -
2.
2NAD
Dihydroxy
Glyceraldehydes (3C) acetone phosphate (3C)
2H+ P
2NADH
2 Diphosphoglyceric acid (3C)
2ADP
2ATP
(2) phosphoglyceric acid (3C)
2ADP
(a)
2ATP
Glycolysis begins with phosphorylation of sugar. Energy required is derived form
2 pyruvic acid (3C)
splitting of ATP which supplies the terminal phosphate group attachment for the
sugar molecule.
62
(b)
In the next stage the phosphorylated sugar is split into two 3 – carbon (triose)
sugar.
These are in equilibrium. Under the conditions prevailing in the cytoplasm, each is
converted to pyruvic acid.
(c)
In the first step towards the conversion of the 3 – carbon sugar to pyruvic acid,
two hydrogen atoms are removed from the triose sugar.
As this happens in the cytoplasm outside the mitochondria where none of the other
respiratory carriers are present, no energy can be derived from it.
(d)
Subsequent steps however yields some energy
Net energy yield (net ATP yield) during glycolysis from one molecule of glucose is:
Utilization – 2 molecules of ATP
Production – 4 ATP
Net gain
- 2 moles/mole of glucose
Also 2 moles of NADH2 is produced during glycolysis.
Glycolysis proceeds whether oxygen is present or not.
If O2 is present
-
Pyruvic acid enters the mitochondrion where it converted into a two – carbon
derivative of acetic acid called ACETYL COENZYME A. (Acetyl CoA)
-
CO2 is given off in this reaction
-
Pyruvic loses two hydrogen atoms 2H+ which are passed through the carrier
system with the formation of 3 molecules of ATP.
-
Acetyl CoA links glycolysis with the next series of reactions
63
-
Since Acetyl CoA is formed in the breakdown of fats and proteins, it is a very
important point in oxidative metabolism.
1.
Acetyl CoA (2C) reacts with a 4C compound oxaloacetic acid present in the
mitochondria to form citric acid
2.
A series of reaction in which citric acid is gradually converted back to oxaloacetic
acid follows.
3.
Two of the steps involve the loss of CO2 (decarboxylation)
4.
Four of the steps involve the removal of hydrogen atoms which are passed
through carrier systems with the formation of ATP.
Three of the carrier systems are NAD and yield 3ATP for every pair of hydrogen
atom transferred.
5.
The Final Oxidation Steps
-
At various stages in the respiratory process especially during the Kreb’s cycle
pairs of hydrogen atoms are removed from intermediate compounds by
hydrogen carriers or acceptors
-
The intermediate compounds are therefore oxidized while the acceptors are
reduced
-
The first carrier passes the 2 hydrogen atoms to a second carrier which is in
turn reduced while the first carrier becomes oxidized
-
At the transfer, sufficient energy is released for the synthesis of ATP
molecule.
-
The process of oxidation and reduction is repeated with further carriers, the
hydrogen atoms finally combining with oxygen to form water.
AH2
NAD
FAD
Quinone
Cysts
Cyt a3 O2
64
Or cytochrome oxidase
The first two carriers are dinucleotides. The first is nicotinamide adenine dinucleotide
(NAD).
The second carrier is FAD derived from vitamin B2. The third carrier is a cytochrome,
a protein pigment with iron prosthetic group.
The 4th is cytochrome oxidase, an enxzyme
The first three are coenzyme which are responsible for the transfer of hydrogen to the
cytochrome oxidase.
Although hydrogen atoms are removed from the substrate and finally join up with
oxygen, it is mainly electrons which are actually transferred from one carrier to the
next.
10.2
Anaerobic Respiration
-
Respiration in the absence of O2 e.g. fungi, yeast bacteria.
-
Not efficient as far as energy released is concerned since the bulk of the energy
yield in respiration comes from the transfer of hydrogen through the carrier
system.
-
For the carrier system to operate O2 must be available to accept the hydrogen
-
In anaerobic respiration, no O2 no Krebs circle
-
The only source of energy is glycolysis
-
The net yield of energy in glycolysis is 2 ATP out of 32 ATP in aerobic
respiration.
-
As if to make up for the poor yield of energy, the process occurs at an accelerated
rate in anaerobic organisms
-
The process of glycolysis is the same in both anaerobic and aerobic respiration
65
-
And the product is the same pyruvic acid. However the fate of pyruvic acid in
anaerobic respiration is different.
Glycolysis
Pyruvic acid In Plants
In Animals
Acetaldehyde
Ethanol
Lactic acid
2H
66
WEEK 11
11.1
Process of Transpiration
Module 8.1
Some vascular plants are the ferns (pteriodophyte) pinus (Gymnospermae) sunflower
and maize (angiospermae). These plants develop channels through which water flows
in their body. The materials for transportation (gases, soil water, solute, manufactured
frod, hormones and metabolic wastes) are carried in water that flows in these
channels. The strus that serve as transport channels are called vascular bundles
(tissues) comparable to the blood vessels and lymphatic vessels of animals.
The vascular bundles are the conducting elements of the plants. Each vascular bundle
consists of the phloem on the outside and the xylem inside.
The phloem cells translocate manufactured food from the leaves down to all living
cells and storage organs of the plant, while the xylem cells are responsible for the
upward conduction of soil solution containing dissolved mineral substances from the
roots to the leaves.
Transpiration: Is the loss of water vapour for different parts of the plant shoot. These
parts include the stomata of leaves, the cuticle of both leaves and young stems, and
the lenticels on tree trunks. Transpiration is described according to the venue through
which the water vapour is lost. Hence water loss through the stomata is called stomata
transpiration. Water loss through the cuticle and lenticels are called cuticular and
lenticular transpiration, respectively.
The greatest amount of transpiration occurs through the stomata. The water that is lost
through the leaves is originally absorbed by the root hairs from the soil into the root
cortext. The water is passed into the root xylem from where it is translocated to the
stem xylem, to the veins and mesophylls of the leaf and finally to the stomatal
67
surfaces from where the water evaporates into the atmosphere. This unbroken chain of
water existing between the soil and the leaves constitutes the transpiration stream
Module 8.2: List the different types of transpiration in plants
i.
Stomatal transpiration
ii.
Lenticular transpiration
iii.
Cuticular transpiration
Module 8.3: Differentiate between Transpiration and Guttation
Transpiration
1.
Guttation
Loss of water through evaporation in Excessive uptake of water by plants
plants
2.
Guard cells or other cells control the Cellulose cell walls impose a natural limit on the
process
3.
amount of water that can be taken in
It is restricted to stomatal, cuticular Pressure builds up and water exude from the leaves,
and lenticular cells
either through the stomata or from special structures
called hydathodes
4.
The rate at which water is transpired Guttation is particularly common in tropical rain
may be considerable, particularly if forest, because of the high rainfall and humid
5.
the atmosphere is warm and dry.
atmosphere.
Water loss in form of vapour/gas
Water loss in water state but not pure.
Module 8.4: Explain the mechanism of Stomatal Movement in Plants
Stomata are very minute openings formed in the epidermal layer in the green aerial of
the plant, particularly the leaves. Each stoma is surrounded by two semi-linear cells,
known as the guard cells. The guard cells are living and always contain chloroplast.
68
Their inner walls are thicker and outer walls inner. They guard the stoma or the
passage, i.e. regulate the opening and closing of the stoma like lips. Sometimes, the
guard cells are surrounded by two or more cells which are distinct from the epidermal
cells. Such cells are called accessory cells. Under normal conditions, the stomata
remain closed at night, i.e. in the absence of light. They remain open during the day
time, i.e. in the presence of light. In most plants they open fully only in bright light,
but in certain plants the stomata do so in diffuse light. Usually the open fully in the
morning and close towards the evening. They may close at day time, when very active
transpiration takes place from the surface of the leaf under certain conditions. Such as
dryness of air, blowing of dry wind and deficient supply of water in the soil. The
intensity of light markedly affects the degree of stomatal opening. The guard cells
movement regulates opening and closing of the stomata. When the guard cells
become turgid, i.e. full of water, expanding and bulging outward, the stoma open.
When the guard cells become flaccid by losing water, the stoma closes.
The turgidity or flaccidity of the guard cells is due to the presence of sugar or starch
in them. In light, the sugar manufactured by the chloroplasts of the guard cells
accumulates in them and being soluble, increases the concentration of the cell – sap.
Under this condition, the guard cells absorb water from the neighbouring cells and
become turgid, opening the stoma. In darkness, on the other hand, the sugar present in
the guard cells become converted into starch; an insoluble compound. The
concentration of the cell sap is, therefore, lower than that of the neighbouring cells.
Under this condition, the guard cells lose water and shrink the closing stoma. The
transformation of sugar into starch at night and vice versa at daytime is due to the
acidity and alkalinity of the cell – sap of the guard cells. In the absence of
photosynthesis at night, carbon dioxide accumulates in the guard cells and the cells’
contents becomes weakly acid. Under carbon dioxide is utilized in photosynthesis,
69
and thus the cell contents become slightly alkaline under this condition, starch is
converted into sugar.
In colloidal hypothesis, the cell contents become alkaline as a result of the effect of
sunlight on the guard cells and this causes the colloids present in them to swell, apart
from the fact it results in the transformation of starch into sugar. The swelling of the
colloids, according to this theory, causes the guard cells to bulge out and the stoma to
open. At night, the acidity of the guard cells increases and causes the colloids to
shrink again, thus closing the stoma, apart from the fact that the increased acidity
brings about the conversion of sugar into starch.
Module 8.5: Importance of Transpiration
1.
Cooling Effect: Transpiration involves the evaporation of water which is a
cooling process. It is believed that this cooling effect is likely to prevent the plant
from overheating on a very hot day.
2.
Translocation of Mineral Salts: Water absorption by the plant root is a passive
process which does not involve the use of metabolic energy. Salt absorption is an
active process which involves the use of energy and is independent of water
absorption. However, once the salt reaches the xylem vessels in the root, it is
translocation to the other parts of the plant under the influence of transpiration
pull.
3.
Good Growth of the Plant: Most plants do no grow well when they are
maintained under conditions of high humidity conditions which do not favour
transpiration. Often there is a great reduction in the size of the plant. In some
cases the bud fails to grow and no flower is produced. It can be argued that under
conditions of high humidity, the rates of water absorption and transpiration are
70
greatly reduced and the plant lacks the required amount of water to carry out the
vital metabolic processes. Transpiration is a process which ensures the continuous
availability of water in the plant body.
Module 8.6: List and explain the factors affecting transpiration in plants.
These factors can be divided into internal and external factors
Internal factors: The most important internal condition affecting transpiration is the
state of the stomata: their number, distribution, structural features and how open they
happen to be any factor that influences the opening an closing of the stomata will
obviously affect transpiration.
External Factors: External conditions affecting transpiration include;
-
Temperature: A high temperature provides latent heat of vapourization and
therefore encourages evaporation from the mesophyll cells.
-
Relative Humidity: The degree to which the atmosphere is saturated with water
vapour, is important because it determines the saturation deficit, i.e. the humidity
difference between the inside and outside of the leaf. Normally the relative
humidity in the sub-stomatal chambers is very high. The lower the relative
humidity of the surrounding atmosphere, the greater will be the saturation deficit
and the water potential gradient, and the faster will water vapoour escape through
the stomata.
Air movements – water vapour tends to build up close to the surface of the leaf as it
diffuses out of the stomata. Obviously the atmosphere will be most highly saturated
immediately outside each stoma and become progressively less saturated as water
vapour diffuses away. Water vapour molecules are deflected by the perimeter of a
71
stoma and the closer they are to the perimeter the greater is the deflection. The
diffusion path of the water vapour molecules therefore describe a hemisphere around
the stoma, called a diffusion shell. If the air is still, diffusion shells build up around the
stomata and the rate of evaporation from the mesophyll cells inevitably decreases. Air
movements blow away these diffusion shells, thereby increasing the rate of evaporation
from the leaf.
As a result, the path of diffusion of molecules along the water potential gradient is
curved (shown by the arrowed lines). This means that the molecules near the edge of
the pore escape more readily than those in the centre.
-
Atmospheric Pressure: The lower the atmospheric pressure the greater is the rate
of evaporation. For this reason alpine plants which live at high altitudes where the
atmospheric pressure is lower than at se level, are liable to have a high rate of
transpiration and many of them therefore have adaptation which prevent
excessive loss of water.
-
Light: If the light intensity is increased, the rate of evaporation from a plant
increases. The reason is not that light affects evaporation as such, but that it
causes the stomata to open, thereby increasing water loss from the plant
-
Water supply transpiration depends on the walls of the mesophyll cells being
thoroughly wet. For this to be so the plant must have an adequate water supply
from the soil sooner or later the stomata close, thus reducing the rate of
transpiration.
72
WEEK 12
12.1
Process of Translocation
Module 9.1
Translocation is the transport of manufactured food substances from the leaves to all
parts of the plant. The movement is usually described as downwards from the leaves
to the roots but it is also known to occur on the reverse direction especially in the nongrowing season. While the downward movement will take food and mineral salts to
growing branches, root tips and storage organs such as root and stem tuber, corms,
bulbs and rhizomes, upwards and side way movement of manufactured food
substances are necessary to serve the growing shoot tips, flowers and storage organs
such as seeds and fruits. Photosynthesis takes place in the leaves and the
manufactured food substances such as sucrose is traced to the phloem tissue in the
veins of the leaves and stem of plants.
Module 9.2
One of the evidences to support translocation through the phloem is the ringing
experiments. This is a process where all the living tissues are removed in a ring from
around the central core of vessels and tracheids in a woody stem and the plant is then
placed in a solution containing radioactive phosphate. Removal of the living cells in
no way impedes the upward movement of the radioactive phosphate, which can
subsequently be detected in the leaves by means of a Geiger – Muller tube. It so
happens that in a mature truck the phloem is confined to the inner part of the bark. If a
ring of bark is stripped off a tree trunk it can be shown that the sugar concentration
increases immediately above the ring and decreases below it, indicating that the
downward movement of sugar is blocked at that point.
73
Critically investigations have been carried out with radioactive tracers. Thus if a plant
is exposed to carbon dioxide labeled with radioactive
14
C, the
14
C becomes
incorporated into the products of photosynthesis which are subsequently detected in
the parts of the plant that are served by the intact phloem. If the phloem is removed by
ringing, the photosynthetic products cannot get through. That these substances are
confined to the phloem can be shown by cutting sections of the stem, placing the
sections in contact with photographic film and making autoradiographs. It is found
that the sites of radioactivity correspond to the positions of the phloem.
Module 9.4
12.2
Mechanism of Translocation
In search for a mechanism of translocation, plant scientists attach increasing
significance to the fine protein filaments which span the sieve cells from end to end.
These filaments are continuous from one sieve cell to the next via the pores in the
sieve plates. High magnification electron micrographs suggest that in the vicinity of
the sieve plate the protein filament take the form of microtubules approximately
20mm wide, but as they traverse the sieve cell they break up into finer strand. It has
been suggested that solutes might be transported by streaming along these protein
filaments, the necessary energy coming from the sieve tubes themselves or the
companion cells. It is envisaged that some strands convey solutes downwards, while
others convey them upwards, thus accounting for the bi-directional flow of materials
that is known to occur in the sieve tubes. However, cytoplasmic streaming have been
seen in sieve tubes.
Another mechanism which have been put forward is surface spreading. The idea ere is
that solute molecules might spread over the interface between two different
cytoplasmic materials, just as oil spreads at a water – air interface. The molecular film
74
so formed could be kept moving by molecules being added at one end and removed at
the other. A major objective here is that the films would be so thin that a very large
number of them would need to be formed to account for the known rates of
translocation. However, sieve tube do contain numerous membranes and filaments
which collectively might provide the necessary surface.
Active mechanism an independent movement of different substances that strongly
suggest translocation involves some sort of active mechanism. This is supported by
other lines of evidence. For example, phloem tissue has a high rate of respiration and
there is a close correlation between the speed of translocation and the metabolic rate.
Again, lowering the temperature and treatment with metabolic poisons both reduce
the rate of translocation, suggesting that it is an active energy requiring process which
can not be explained by physical forces alone.
Mass flow theory – Among the many mechanisms proposed over the years, one which
has gained some support from experimental work is the mass flow hypothesis. The
way mass flow is believed to occur in the sieve tubes. Water enters the left had funnel
by osmosis through the partially permeable membrane. The hydrostatic pressure
developed causes the sugar solution to flow into the right hand funnel and forces
water out through the partially permeable membrane on that side. There is therefore a
flow of solution from left to right which will cease when the concentration in the two
funnels are equal.
Of course in the living plant the flow must be continuous. In order to maintain a
continuous flow, sugar would have to be loaded into the sieve tubes at one end
(source) and off – loaded at their destination (sink). The loading of sugars into the
phloem is achieved in the leaf by active transport. This creates a high sugar
concentration at the source, which draws water into the sieve tubes by osmosis.
75
At the root end of the system, sugars are removed for use in metabolic processes, so
water flows out into the intercellular spaces. The continual input of sugars and water
at the top of the system and their removal at the bottom creates a pressure gradient
which maintains the downward flow of fluid in the sieve tubes. The mass flow
hypothesis is an attractive way of explaining the movement of substances in the sieve
tubes.
Fig 12.1 The mass flow Hypothesis
76
WEEK 13
13.1
Process of Ion Absorption in Plants
The substances absorbed from the soil may be classified into two groups, namely:
water (and also sugar) which undergo no or little ionization and may enter the cells by
following the simple laws of diffusion and other physical process, while the second
group consists of mineral salts which undergo extensive ionization. The ionized
particles of such salts are taken up by the cells, where they accumulate, sometimes in
heavy concentrations. They ions may transport as such, or they combine into suitable
compounds.
Ions are atoms or groups of atoms which carry either a positive or negative charge of
electricity. When an ionizable material in water is subjected to electrolysis, its
molecules break up into two or more ions of different kinds those charged with
positive electricity are said to be electropositive ions, such as K+, Na+, Ca++, Mg++ and
also H+, and those charges with negative electricity are said to be electro-negative
ions, such as Cl-, Br- NO3-, H2PO4-, OH- ans SO4-. The process is reversible as the
following examples will show: NaCl ↔ Na+ + Cl-; HCl ↔ H+ + Cl-. The breaking
up of molecules may not always be complete.
Absorption of mineral salts (ionic theory or electrolytic dissociation theory).
It have been shown through physical process of absorption of salt form of ions,
although certain compounds may probably enter the plant cell through the plasma
membrane by the physical process of diffusion from the region of higher
concentration of the soil solution to that of lower concentration of the cell-sap.
However, it has been seen in may cases that the concentration of the cell-sap is higher
than that of the soil solution. This being so, the process of absorption cannot be
explained on the basis of simple diffusion (stiles, 1924). As a matter of fact, many
mineral salts which may undergo extensive ionization do not follow simple laws of
77
diffusion. In 1936
Hoagland definitely proved that it is the ions (and not the
undissociated molecules of salts in the soil solution) that make their entrance into the
cell, independent of the rate of absorption of water, from the region of lower
concentration (cell-sap). The physico-chemical nature of the plant cell is very
complex, changing continually in response to its environment, and at the same time,
the soil itself is a heterogenous medium. So the forces concerned must be of varied
nature. As already proved by many workers on the basis of experiments conducted by
them, avsorption of salts takes place in the form of ions (+ and - ) produces by
electrolytic dissociation (or ionization) of the molecules of different salts are taken up
individually and independently of one another. The special feature of living cells is
that they can accumulate individual ions (and not salts) to a concentration that far
exceeds that of the surrounding medium. Several workers (notably Hoagland, 1944)
have actually proved this by experimental work on Nitella and other plants.
Passive and Active Absorption: In modern research, passive absorption of salts and
active absorption are distinguished from each other according to their dependence on
non-metabolic energy and metabolic energy, respectivle. One speaks of passive or
non-metabolic absorption when the forces driving the salts through the membrane
originate in the environment of the cell, i.e. these forces are physical and nonmetabolic. One speaks of active metabolic absorption when it is dependent on
metabolic energy which originates in the ell as a result of metabolic activity
(particularly respiration) within it. Active uptake, as explained below, is known to be
the principal method of salt absorption although some salts are absorbed. Sometimes
rapidly for a time, by the passive method. The interaction between the cell and its
environment is essential to maintain a certain concentration within the cell in order to
sustain life. By passive transport, an exchange of ions takes place between the
external solution (soil colloids readily yield ions on electrolysis) and the cell. It is
78
known that the cell membrane, possibly in all cases, maintains differences in electric
potential between the inner side and the outer side, evidently acting as a driving force.
This influences passive uptake of ions through the membrane into the peripheral or
outer plasm (see below). The ions in this phase may move freely and even out of the
cell. It may be noted that ions move by diffusion through the cell wall and the
cytoplasm in their water phase, and that the plasma membrane has the ability to select
and permit the entrance of certain ions and greatly restrict others. However, by
passive uptake, soon an equilibrium is reached between the outer plasm and the
external medium. Ions may move upwards through the transpiration current along
with the mass flow of water. This being so, transpiration may help in the absorption of
ions. By active transport, which is slow but steady, ions are brought into the inner or
central plasm, i.e. from the region of lower concentration to that of higher
concentration. There is supposed to be a dividing line or membrane in the cytoplasm,
though not clearly demarcated, between the outer plams and the inner plasm. This
membrane is regarded as impermeable to the exchange of free ions between the two
sides. This leads to the conception of a ‘specific carries’ which can pick up ions from
the outer plasm through the so-called membrane. Since this ‘carrier’ moves only in
one direction (from the outer to the inner), ions once released into the inner plasm
cannot reach out of the cell and, thus, cannot be exchanges for those in the external
solution. Evidently, the ions may accumulate there received support from many
investigators. It has been suggested that lecithin, phospholipids, may act as such a
‘carrier’. Active transport is closely connected with metabolic energy in the form of
ATP (an energy rich phosphate compound) formed in the living cell. The chemical
energy required for active transport of ions is believed to be supplied by ATP. ATP,
in its turn, receives this energy from glucose as a result of oxidation of the latter in
root respiration. It is known that young roots respire vigorously. Synthesis of lecithin
79
also depends on the availability of ATP. Thus, the absence of ATP in cell interferes
with active transport. Specific enzymes may also help the passage of certain ions
through the cell membrane. The concentration of ions in the cells is not even in all
cases, the maximum accumulation being K+ ions and also some other cations (see
footnote, p. 235). Ions of both the electric charges must be taken up by the cell in
order to maintance an electric balance both on its inside and outside. For example, a
negative ion releases by the ectoplasm establishes a difference of potential between
the two media. Thus, to equalize the charge, the soil solution yields positive ion to the
ectoplasm. In fact, an interchange of ions takes place between the cell and the
surrounding solution.
Conditions: Absorptions of salts depends on a number of conditions, viz. aerobic root
respiration, amount of light, rate of transpiration, permeability of the plasma
membrane, metabolic activity of the cell, influence of temperature, hydrogen-ion
concentration, etc.
80
WEEK 14
14.1
Growth
The growth of a plant is a complex phenomenon associated with numerous
physiological processes – both constructive and destructive. The formed
(constructive) lead to the formation of various nutritive substances and the
protoplasm, and the latter (destructive) to their breakdown. The
protoplasm
assimilates the protein food and increases in the bulk while the carbohydrates are
mainly utilized in respiration and in the formation of the cell – wall substances
namely cellulose. The cells divide and numerous new cells are formed. These increase
in size and become fully turgid, and the plant grows as a whole. Growth is therefore, a
complex process brought about by the protoplasm. It can simple be defined as a
permanent and irreversible increase in size and form, attended by an increase in
weight. The plant growth can accurately be measured using auxanometer.
14.2
Conditions Necessary for Growth
The fact that growth is brought about by the protoplasm, the conditions necessary for
growth are the same as those that maintain the activity of the protoplasm. The
conditions are therefore the following:
a.
Supply of nutritive material: Growth can only take place when the protoplasm
of the growing region is supplied with nutritive materials and builds up the
body of the plant.
b.
Supply of water: Adequate supply of water is absolutely necessary to maintain
the turgidity of growing cells. Turgidity is the initial step towards growth. The
protoplasm works only when it is saturated with water. An abundant supply of
water addresses the loss caused by transpiration. However, only a small
quantity is required for actual growth.
81
c.
Supply of oxygen: Free oxygen supply is necessary for respiration of cells.
Respiration is an oxidation process by which the potential energy stored in the
food is released in the form of kinetic energy and utilized by the protoplasm
for its activities.
d.
Suitable temperature: The protoplasm requires a suitable temperature for its
activities. It ceases to perform its functions or done it slowly at a low
temperature, while a temperature of 45oC coagulates and kills it. The
protoplasm performs its activities within a certain range of temperature (due to
thermotonic effect of temperature). The optimum temperature averages from
28oC to 30oC, and the maximum lies at about 4oC.
e.
Light: it is not absolutely necessary for the initial stages of growth. In fact,
plants grow more rapidly in darkness than in light. Although light has a
retarding effect on growth, the protoplasm remains healthy and the plant
becomes sturdy, the stem and green leaves developing normally, when there is
a certain intensity of light. The stomata remain open and the chloroplasts
function maintain only in the presence of light. This is due to the phototonic or
stimulating effect of light.
f.
Force of gravity: It determines the direction of growth of particular organs of
the plant body. The root grows towards the force of gravity, and the stem away
from it.
Others are:
g.
pH
h.
Accumulation of metabolic products. The internal factors include hormones
such as auxins and gibberellins.
82
14.3
Phases of Growth
Growth does not take place throughout the whole length of the plant body, but is
localized in special regions called meristems, which may be apical, lateral or
intercalary. The growth in length is due to gradual enlargement and elongation of the
cells of the apical meristems (root apex, and stem apex). In dicotyledons and
gymnosperms, the growth in thickness is due to the activity of the lateral meristems
i.e. interfascicular cambium, fascicular cambium and cork cambium. Three phases can
be identified in the growth of any organ of a plant.
(i)
The Formative Phase: This is restricted to the apical meristem of the root and the
stem. The cells of this region are constantly dividing and multiplying. They are
characterized by abundant protoplasm, a large nucleus and a thin cellulose wall.
(ii)
The phase of elongation: It lies immediately behind the formative phase. The
cells no longer divide, but increase in size. They begin to enlarge and elongate
until they reach their maximum dimension. In the root, this phase is a few
millimeters long and in stems, a few centimeters. It may be longer in some
climbers.
(iii) The phase of maturation: It lies further back. The cells have already reached their
permanent size. The thickening of the cell wall takes place in this phase.
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Fig. 14.1 Growth of Meristemetic Tissues
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14.4
Parameters Used to Assess Growth
It is sometimes very difficult to determine growth in organisms. Growth varies from
organism to another. Parameters used to measure growth in organisms includes:
(i)
Mass
(ii)
Length, height, or width
(iii)
Area
or
volume
In most growth studies, mass may be measured as wet mass and dry mass.
(i)
Wet Mass: Is the mass of the organisms under normal conditions. It is not a
reliable indication of growth.
(ii)
Any Mass: Refers to the mass of an organism after all the water in it has been
removed. Although reliable, the organisms get killed in the process. Thus not
able to measure growth in the same organism. It study growth by measuring
dry mass, a study must be done on a large number of organism. Growth can
then be estimated by removing a given number of organisms at a time and
estimating their dry weight.
(iii)
Size and length: Can be measured at successive intervals on the same
organism e.g. height of man, length of lizard e.t.c.
(iv)
Increase in number of cells: It also proof growth of a population. Typically in
yeast, by budding (asexual reproduction) or dividing into two, into four and
four into eight e.t.c. The yeast culture continues to double its number as long
as none of the cells dies or loses its power of division.
Module 11: Water Absorption
Root Absorption: Water and various dissolved substances are absorbed through the
root hairs. A root – hair is simply an outgrowth of one of the cells of pilierous layer,
having the form of a slender tube close at the free end. It has a very thin wall of
cellulose which is lined with a thin layer of cytoplasm surrounding a large central
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vacuole. The vacuole contains the cell sap which his a strong solution of a sugar,
organic acids, mineral salts, e.t.c. The root hairs come in close contact with the soil
particles, each of which is surrounded by a film of water (hygroscopic water)
containing mineral salts in dilute solution. Thus the film of watery solution adhering
to the soil particles are separated from the cell sap of the root hairs by their walls and
thin parietal layers of protoplasm. The cell sap is a stronger solution than that
surrounding the soil particles.
The soil water is absorbed by the root hairs by the process of imbibitions and osmosis.
The water enters the space between the cellular particles of the cell – wall and force
them apart. The cell wall swells up and thus becomes more porous, offering very little
resistance to the diffusion of soluble substances. The protoplasmic lining of the root
hairs also imbibes water and swells up.
By mere imbibition the absorption of water by root hairs cannot go on continuously.
Hence osmosis takes place to ensure continuous absorption of water. The cell wall is a
permeable membrane which allows water and almost all dissolved substances to pass
through it. The thin layer of protoplasm inside the cell wall, however, forms a semi
permeable membrane which allows water to pass freely through it, but prevents or
greatly restricts the outwards passage of most of the substances dissolved in the cell
sap. Since the cell sap is comparatively a strong solution, the hygroscopic soil water
with dissolved mineral salts flows into the root hairs through the cell wall. The layer
of protoplasm lining the cell – wall regulates the flow of dissolved substances into the
root hair. It has the power of selection, in as much as it allows some soluble
substances to pass through it but does not allow other to do so. At the same time, it
prevents the escape of the cell – sap from the root hairs into the soil. Thus the
absorption of solutions from the soil by root hair is a process of osmosis which is
controlled and modified by the activity of the layers of protoplasm lining their walls.
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Carbondioxide exhale by the living roots dissolved in the surrounding soil water and
renders many insoluble substances, such as calciumtrioxocarbonate IV, many
silicates, e.t.c, soluble. The absorption of soil water through the root hairs is known as
root absorption.
As a result of absorption of water by endosmosis, the root hairs become turgid and
their cell – sap becomes less concentrated than that of the adjacent cortical cells.
Under these conditions the water from the root hairs flows by osmosis into the
neighbouring cortical cells and then into the cell farther and farther away from the
root hairs. Thus the water and dissolved mineral salts which are continuously
absorbed by the root hairs from the soil gradually diffuse by osmosis from the outer to
the inner layers of the cortex till they reach the xylem. But no osmosis takes place
between the innermost cortical cells and the xylem, because the xylem vessels are
empty and dead.
Module 11.2
Several theories have been advanced to explain the ascent of sap in tall trees against
the force of gravity.
-
Force of capillarity: Water rises in the capillary glass tube above the level of
water in the outer vessel, and smaller the bore of the tube, the higher the water
rise. Thus according to this theory the capillary or extreme narrowness of the
xylem vessel is responsible for the movement of water up to the leaf.
-
Atmospheric pressure: According to this theory the atmospheric pressure is
believed to be the cause of the ascent of water. The loss of water by transpiration
from the surface of the leaves creates a partial vacuum in the xylem vessels, and
consequently water is forced up by the atmospheric pressure from below.
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-
Osmotic Pressure: It has been suggested that water diffuses upwards through the
parenchymatous tissues of the plant.
-
Root Pressure: According to this theory, the root pressure which drives water
from the cortical cells of the roots into the xylem vessels causes its movement
upwards to the leaves. The bleeding at the cut surface of the stem of vine and the
exudation of water drops at the tips of the leaves of grasses indicate the driving
force of root – pressure.
-
Imbibition Theory: According to this theory, water passes upwards by imbibition
only along the lignified walls of the xylem vessels and not through their cavities.
-
Transpiration and force of cohesion (cohesion theory) particles of water cohere
strongly together, and form continuous columns in the vessels, extending from
the roots to the leaves. Hence the pull at the upper end of the water column is
transmitted to its lower end. As water is lost by transpiration from the surface of
the leaf, the cell – sap of the mesophyll cells becomes concentrated and water is
withdrawn osmotically from the tracheids of the vein lets. Accordingly a vacuum
is created in the tracheids of the veinlets, and water columns in the xylem
elements of the leaf and stem are bodily pulled upwards.
The cohesion theory which seeks to explain the upwards movement of water in
terms of the cohesion strength of water (cell –sap) and the pulling or suction force
of transpiration is regarded as the most plausible one. It has been shown that,
owning to the cohesive strength of water columns in xylem vessels remain
unbroken and that the pulling force of transpiration is so strong that it lifts water
to the height of the tallest trees. It is also be lived that transpiration gives water
column a pull from above and root – pressure a push from below, and that the
combined action of these two forces causes the ascent of sap.
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-
Goldlewski’s Vital Theory: According to this theory, the ascent of water take
place through the vital activity of the living protoplasm of the xylem parenchyma,
the xylem vessels merely act as reservoirs in which water is temporarily stored
up.
-
Bose’s pulsation theory: According to Sir. J.C. Bose, the ascent of water in the
plants is due to active pulsation of the living cortical cells just outside the
endodermis. That is, the cells of the innermost layer of the cortex pulsate and
pump the water upwards. Bose demonstrated the pulsating movements of these
cells by means of a fine electric probe, connected at one end with the
galvanometer. When the electric probe, on being gradually thrust into the stem,
reached the internal layer of cortical cells, there was a sudden deflection in the
needle of the galvanometer, showing the pulsating activity of these cells. Bose
does not regard transpiration or root pressure as the cause of the ascent of sap, for
he believes that water passes upwards through the pulsating layer of cortical cells
even in their absence. The xylem vessels, in his view, are mere reservoirs.
Morphological and experimental evidence does not seem to support Bose’s
explanation of the ascent of sap.
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WEEK 15
MODULE 13
15.0
Movement in Plants
15.1
Irritability (Sensitivity)
This refers to the response of living organisms to the stimuli of both their internal and
external environments so that they can maintain the most suitable condition of life.
Whereas animals can show a very quick response to an external stimuli by perhaps
moving the whole body away from the direction of stimuli (taxis), the response of
plants is very slow and usually involves the growth of the simulated part towards or
away from the direction of stimulus.
The unilateral growth of plants part in response to an external stimulus is referred to
as tropsim.
15.2
Tropisms
Plants respond to the stimuli of:
7.
Light (phototropism)
8.
Gravity (geotropism)
9.
Water (hydrotropism)
These responses which are controlled by certain plant hormones called auxins are
experienced in the growth regions of the plants (regions close to the root or shoot tips).
Auxins are produced at the rips of shoots. The chemicals diffuse vertically downwards
to stimulate growth in both the shoot and the root.
Auxins like the animals hormone are produced in small quantities. The effect of auxins
is to increase the growth of plants. The growth rate of plants increase as the
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concentration of auxins becomes increased. But after a certain concentration, a further
increase will inhibit rather than promote the growth of that plant.
15.3
Phototropism
This is to response of plant parts to the stimulus of light, especially sunlight. Plant
shoots normally grow towards the direction of light. Plant shoots are therefore
positively phototrophic.
15.4
Klinostat
Is an instrument used to demonstrate or control the effect of light and gravity on
growth of plants.
15.5
Geotropism
This is the response of plant parts to the stimulus of gravity. Plant shoots grow away
from gravity of the earth and are said to be negatively geotropic. Roots grow towards
gravity and are said to be positively geotropic.
Gravity is thought to cause a redistribution of auxin in root and shoot. The effect of
gravity on the root is opposite to its effection the shoot.
15.6
Hydrotropism
This is the response of roots to the stimulus of water in the soil. Plant roots are
positively hydrotropic. They grow toward areas where there is water which is vital to
life. Shoots do not absorb water and consequently. They do not respond to the
presence of water.
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15.7
Other Responses in Plants
1.
Chemotropism: The response of plant parts to chemical e.g. pollen tubes grow
toward the ovary after pollination in response to the sugary chemical secreted
by the stigma.
2.
Thignotropism: Some plants respond to the stimulus of touch e.g. tendrils,
leaves of sensitive plant (e.g. Mimosa pudica) and the carnivorous plants (e.g.
sundew and venues flytrap).
15.8
Nastic/Sleeping Movements
The pinnate and bipinnate leaves of many leguminous plants open and close as a
result of certain changes in temperature, light intensity or humidity of the atmosphere.
The process which is however gradual is known as “sleeping movement” and occurs
is such leaves as St. Thomas plant, and Cassia. Flowers of certain plants are seen to
open only at certain time of the day e.g. “four O’clock” plant. Sun flower changes its
direction to face the sun. the flowers of “snake tomato” remain closed during hot day
and open at night or under conditions of high humidity. These various responses of
plant parts to external stimuli of the weather are known as NASTIC RESPONSES
(MOVEMENTS)
15.9
Taxis (Tactic Movement)
This is a locomotory movement of an entire organism or cell (e.g. gamete) in response
to a directional stimulus. If the locomotory movement is towards the stimulus, it is
positive taxis, if away its negative taxis. This is commonly found in animals. For
instance Euglena moves toward source of light (positive phototaxis) and many fresh
water fish (e.g. Tilapia) move against current positive rheotaxis).
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15.1.0 Factors Responsible for Irritability
Reaction of cell to the environment (irritability) is governed by the factors:
(ii)
Water
(iii) pH
(iv)
(v)
(iv) light
Gravity
Temperature and
(vi)
Humidity
The response is specific as follows:
a.
Roots: Response positively to water and gravity
b.
Humidity: Leaves and flowers respond positively
c.
Light: Shoots generally respond positively
d.
Temperature: Flowers and shoot respond positively
15.1.1 Differences Between Nastic/Sleep Movement and Tropism
Nastic/Sleeping Movement
Tropism
1.
Movements are reversible
Not easily reversible
2.
Response does not lead to growth
Response leads to growth
3.
Movements are due to changes in cell tugor
Movements are not due to changes in cell
tugor
4.
Response to stimulus is not directed towards Response to stimulus is towards the
the direction of stimulus
direction of stimulus
5.
Response is fast
Response is slow
6.
Responses are due to changes in temperature, Responses are due to water, light, gravity
humidity, light intensity, or touch
7.
and chemicals
Responses is usually associated with the time Not associated with the time of the day.
of the day
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