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Chapter 2
STRUCTURAL ORGANIZATION OF LIFE
The cell is the basic unit of life. It is the smallest entity in which the life can
exist. All the things that living organism can do are done by its cells. In fact
some living things are made up of only one cell. Each cell gets food for energy,
obtains oxygen, produces energy, gets rid of wastes, maintains homeostasis and
produces new cells. How are all these life activities carried out? The answer can
be found by examining the composition and working of its parts.
Learning objectives:
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Cell as a basic unit of living organism. 
Discovery of cell and cell theory. 
Concepts of light microscopy and electron microscopy. 
Microscopic and Ultramicroscopic structure of plant and animal cells. 
Structure and functions of different cell structures. 
Concept of Prokaryotic and Eukaryotic cells and their differences. 
Reproduction of cell, different methods. 
Mitosis and Meiosis and their significances. 
Three level of organization in living organism i.e. tissues, organs and system. 
Types of plant tissues, simple and compound tissues, their further
classification and function in different parts of plant body. 
Types of animal tissues, epithelial, connective, muscle and nervous tissues,
structure of these in relation to their function. 
Unicellular organization, Amoeba a unicellular organism. 
Multicellular organization. 
Brassica as multicellular organization, with root, stem, leaf, flower, fruit and
seed as their parts. 
Frog as multicellular organization with digestive, respiratory, circulatory,
excretory, nervous and reproductive organs and systems. 
Cell is as fundamental to biology as an atom is to chemistry. All organisms are
made of cells, which behave as basic unit of their structure and function. The
contraction of muscle cells moves your eyes as you read this book; when you
decide to turn this page, nerve cells will transmit that decision from your brain to
the muscle cells of your hand so every thing performed by organism is
fundamentally occurring at the cellular level.
2.1
DISCOVERY OF CELL AND CELL THEORY
In early classes we have studied that all living organisms are composed of cells.
The question arises here how did biologist come to know that, obviously through
observations. These observations started with the discovery of magnifying
glasses and later on with the development of microscope. (Latin word micro =
small; skopion = to see). In 1610 Galileo, an Italian astronomer and physicist
developed microscope to observe small organisms. In 1665, Robert Hook made
an improved microscope by combining lenses, called compound microscope and
examined a slice of cork under it. He found small honey comb like chambers,
which reminded him small rooms of monastery and are said cellula in Italian, so
he also named these structures as cellulae or cell (small rooms). The cork was
made from bark of oak, so he actually saw the cell-wall only.
in 1842, Dutrochet, boiled plant material in nitric acid and then examined under
microscope. It was found to consists of cells. In 1831, Robert Brown discovered a
spherical body, the nucleus in the cells of orchids. Schleiden (1838) a German
botanist, proposed that all plants are made up of cells. Next year another German
Zoologist, Theoder Schwann stated that all animals are made up of cells. He
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observed nuclei in all types of animal cells but failed to observe cell- wall in them.
From here the difference between plant and animal cell started to establish. In 1858,
Rudolf Virchow stated that new cells come only from other cells i.e animals cells
come from animal cell and plant cells from plant cell. The combined efforts of
Schleiden, Schwann and R.Virchow finally gave rise to cell theory.
The salient features of the cell theory are as under:
i)
ii)
iii)
All living organisms are composed of one or more cells.
The cell is the smallest, basic structural and functional unit of all
organisms.
New cells are formed by the division of pre-existing cells.
2.2
LIGHT MICROSCOPY AND ELECTRON MICROSCOPY
The evolution of biology as well as science often parallels the invention of
instruments that extend human senses to new limits. The discovery and early
study of cells progressed with the invention and improvement of visual
instrument, like microscope. Microscopes of various types are still important
tools for the study of cells.
Resolution
Resolution is
the capacity to
separate adjacent objects.
Resolution is maintained
upto
certain magnification.
Resolution improves
wave length
of
become shorter.
as
the
illumination
Magnification
Magnification is
A means
of
increasing size of the object.
By
increasing
magnification
resolution is disturbed.
Improve
Magnification s
with
the
focal length of lens.
The microscopes first used by scientist, as well as the microscope you use in the
biology laboratory are light microscopes. These microscope use visible light as
the source of illumination and glass lenses for magnification. These lenses reflect
the light in a way that the image of the specimen is magnified as it is projected
into the human eye. The light microscope can magnify the object upto 1000
times but its resolving power is very limited, i.e just 0.2µm (Resolving power is a
measure of the clarity of the image).
In 1935, a new type of power full microscope called Electron microscope was
invented by scientist to improve the resolving power of microscope. It uses a
beam of electron as a source of illumination. The electron beam increases its
resolving power. Modern electron microscope can achieve a resolution of about
0.2 nm, a thousand times improvement over light microscope. The electron
microscope uses electromagnet as lenses instead of glass lenses. This image
cannot focus in human eye, therefore screen or photographic plates are used to
review and focus these images.
Units of measurement
1 centimeter (cm) = 10-2 meter.
1 millimeter (mm) - 10-3 meter.
1 micrometer (µm) =10-6 meter.
1 nanometer (nm) = 10-9 meter.
Electron microscopes reveals many organelles that are impossible to be seen
with the light microscope. But the light microscope has many advantages
especially for the study of live cells. In electron microscopy, chemicals and
physical methods are used to prepare sample which kills cells.
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2.3
BASIC STRUCTURE OF CELL
Cells are of different shapes and size according to their functions. inspite of
variation found in their shape, all cells basically share many structures in
common like cell membrane, cytoplasm, nucleus, etc. In plant cell, cellmembrane is surrounded by a cell-wall.
2.3.1 Cell - Structural and Functional unit:
Microscopic studies reveal that all living organisms are composed of cells.
Therefore, cell is a unit of structure of living organisms. Cells are of different
shapes and sizes, as they have to perform different functions. All basic functional
activities, characteristic of living things, occur in the cell. Therefore, cell is also a
unit of function of all living organisms.
1. Cell-wall:
Cell-wall is the non living, outermost boundary of plant cells, bacterial cells and
fungal cells. It is not found in animal cell. It is secreted by the protoplasm of the
plant cell. In plant cell it is mainly composed of cellulose and pectin. Ultra
microscopic structure of cell-wall shows that cellulose make the fibers which are
arranged in criss cross manner. These fibers are kept in their position by a
cementing material called calcium pectate (Pectin).
Bacterial cell-wall is made up of protein and carbohydrate while fungal cell wall is
made up of fungal cellulose and chitin.
Thickness of cell-wall varies in different cells of plant. It is composed of three main
layers: middle lamella, primary wall, secondary wall and some times tertiary wall.
Middle lamella is formed between the primary walls of neighbouring cells. Primary
wall, the first wall of plant cell is chemically composed of cellulose and pectin, some
limes, lignin. Cell-wall provides protection and support to the cell. It gives a definite
shape to the cell. It also performs the function of transport of material from outside
to inside or vice versa, therefore, it is permeable in nature.
2. Cell- membrane:
The
cell-membrane
or plasma
membrane
surrounds nucleus and
cytoplasm in all types of cells. However in bacteria and plants, plasma
membrane itself is surrounded by a cell-wall. It can repair itself to some extent.
Different models have been presented to understand the structure of cell
membrane. The most acceptable model among them is
Fluid mosaic model
presented by Singer and Nicholson
(1972). According to it, cell membrane
consists of lipid (Phospho-lipid) bilayer, in which protein molecules like float
iceberg in the sea. This basic structure is found in all the membranes of
mitochondria, chloroplast etc. Therefore, it is also called unit membrane.
Cell membrane is a selectively permeable membrane because it regulates selective
movement of molecules. In many animal cells the cell membrane infolds, taking in
materials in the form of vacuoles. This process is called endocytosis.
3. Nucleus or Karyon:
Nucleus (discovered by Robert Brown in 1831) is an important arid prominent
structure present inside the cell. It controls all the activities of cell. It may be
spherical or irregular in shape. In animal cell it is usually present in the center
but in plant cell, due to presence of large vacuole it is pushed towards cellmembrane. Nucleus is enveloped by a double membrane called nuclearmembrane. This membrane possesses large number of nuclear pores. Nucleus is
filled with a gel like substance called nucleoplasm. The nucleoplasm contains
nucleoli and a network of thread like structures called chromatin network. The
threads of chromatin become prominent during cell-division. Each thread is
called chromosome. These structures of major importance. They are composed
of Deoxyribo nucleic acid (DNA) and protein. DNA plays significant role in the
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inheritance of characters as well as in controlling or regulating the cell activities.
The number of chromosomes in the cells of all individual of the same species
always remains constant.
Cells of organism
Man
Frog
Chimpanzee
Drosophila (fruit fly)
Onion
Potato
Garden pea
No. of Chromosomes
46
26
48
08
16
48
14
4.
Cytoplasm:
It is the translucent fluid portion of the cell lying in between plasma membrane
and nucleus. It consists of an aqueous ground substance called cytosol and
granular portion called cytoplasmic organelles.
Chemically cytoplasm is about 90% water and forms a solution and serves as
store house of vital chemicals. It is a site of metabolic reactions like protein
synthesis, glycolysis etc. Many reactions can occur at the same time in different
regions of the cytoplasm.
Some important cytoplasmic organelles found in eukaryotic cells.
1.
3.
5.
7.
Endoplasmic reticulum
Mitochondria
Centrioles
Vacuoles
2.
4.
6.
Golgi complex
Plastids
Ribosomes
1. Endoplasmic reticulum:
(Endo= inside, plasma = protoplasm, reticulum=net work).
It is a network of membranous channels or tubules extending throughout the
cytoplasm. The channels seem to be in contact with plasma membrane as well
as nuclear membrane. There are two types of endoplasmic reticulum.
i)
Rough endoplasmic reticulum having ribosomes at its outer surface which
are involved in protein synthesis.
ii)
Smooth endoplasmic reticulum without ribosome.
Endoplasmic reticulum plays important role in the synthesis and transport of
material within the cell. It also provides mechanical support to the cell so that its
shape is maintained. It detoxifies the harmful effects of drugs.
2. Golgi complex:
They were discovered
or apparatus". They
flattened, fluid filled
enzymes. Golgi bodies
by Camillo Golgi and thus called Golgi complex or bodies
are set of smooth membranes that are stacked into
sacs or vesicles have carbohydrate, glycoproteins and
are mainly concerned with the cell secretions.
3. Mitochondria (Sing; mitochondrion):
They are generally rod-like or bean shaped organelles consisting of double
membrane. The inner membrane is folded. These infoldings are called cristae
while the fluid present inside is called matrix. Mitochondria contain enzymes
which break the food for the production of energy. As producers of energy they
are called Power house of the cell. The number of mitochondria in cell relates
to its activities.
4. Plastids:
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Plastids are found in the cells of all the higher plants. These are the organelles
which contain different types of pigments. Plastids are of three types on the
basis of their pigment or colour (Fig: 2.11)
i)
Chloroplasts have green pigment i.e. chlorophyll found in leaves and
other green parts of a plant. They manufacture carbohydrates by the
process of photosynthesis.
ii)
Chromoplast have coloured pigments other than green found in fruit,
flower, petals and other coloured parts of plants .
iii)
Leucoplast (leucos = white or colourless) are colourless, found in the cells of
underground parts of plants. They store food in the form of starch.
5. Centrosome and Centrioles:
A rounded structure, the centrosome is present near the nucleus in animal cells.
A centrosome contains two centrioles (Fig: 2.12). Each centriole consists of a
cylindrical array of 9 rows of microtubules. They form fibrous protein spindle
which help in movement of chromosomes towards poles during animal cell
division.
6. Ribosome:
They are granules, rich in ribonucleic acid (RNA). They serve as sites where proteins
are synthesized hence called protein factories of cell. They are found free in
cytoplasm as well as attached on the surface of rough endoplasmic reticulum.
7. Vacuole:
They are the fluid (other than cytoplasm) filled sacs surrounded by a membrane
called tonoplast. In animal cell they are numerous, small but temporary
structures while in plant cell they are permanent and very large in size, one or a
few in number. They are concerned with storage of cell sap.
2.4
PROKARYOTIC AND EUKARYOTIC CELL
There are two types of cells, Prokaryotic and eukaryotic cells. Prokaryotes have
prokaryotic cell while eukaryotes have eukaryotic cells.
Prokaryotic (pro: before; karyon: nucleus) cell does not possess true nucleus. It
means its nuclear material is not enclosed in a proper nuclear membrane. These
types of cells are found in bacteria and cyanobacteria (blue green algae). Such
organisms are called prokaryotic organisms.
Eukaryotic (eu: true, karyon: nucleus) cell possesses proper nucleus where
nuclear material is enclosed in a proper nuclear membrane. Plants and animals
are composed of this type of cells and are called eukaryotic organisms.
Followings are the differences found between them.
Prokaryotic cell
1. Nuclear membrane is absent
therefore prokaryotic cells do not
possess distinct nucleus.
2. They do not have many of the
membrane bound structures e.g.
mitochondria E.R, Golgi
apparatus etc.
3. Ribosomes are of small size
and freely
scattered in
cytoplasm.
4. Nucleoplasm is absent.
5. Single chromosome is found.
6. Respiratory enzymes are located on
the inner surface of the cell
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Eukaryotic cell
1. A double nuclear membrane is
present. They have well defined
nucleus.
2. They have membrane bounded
structures (organelles).
3. Ribosomes are of large size and
present either on endoplasmic
reticulum or free in cytoplasm.
4. Nucleoplasm is present
5. Proper chromosomes in diploid
numbers are present.
6. Respiratory enzymes are
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membrane.
7. These cells are simple and
comparatively smaller in size
i.e. average 0.5 -l0nm in
diameter.
8. Bacteria and cyanobacteria are
examples of prokaryotes.
present in mitochondria.
7. These cells are complex and
comparatively larger in size i.e. 10l00nm in diameter average.
8. Fungi, algae, animal and plants are
examples of eukaryotes.
2.5
CELL DIVISION
Cells reproduce and increase in number by division. After growing to a certain
maximum size, a cell may undergo the process of cell division. During this
process the nucleus divides first. This is followed by division of the cytoplasm.
This nuclear division is called Karyokinesis (karyon=nucleus; kinesis = division)
while the cytoplasmic division is called Cytokinesis. Thus two daughter cells arise
from a single division of a cell. There are two main types of cell division found in
living organisms.
(1) Mitosis
(2) Meiosis
1. Mitosis:
In this type of cell division a parent cell divides into two daughter cells in a way
that the number of chromosomes in the daughter cells remains the same as in
the parent cell.
Although mitosis is a continuous process, its karyokinesis can be divided for
convenience into four phases which are Prophase, Metaphase, Anaphase and
Telophase. Let us now study mitosis is an animal cell.
i)
ii)
iii)
iv)
Prophase:
During early prophase chromatin material condenses and becomes
visible as thick coiled, thread like structures called chromosomes.
Each chromosome at this stage is already double, i.e. consists of two
chromatids. The chromatids are attached to each other at
centromere. The nuclear membrane gradually disappears and at the
same time centrosome divides to form two centrioles, each moves
towards the opposite pole of the cell and forms the spindle fibres. The
centrioles are absent in plant cells.
Metaphase:
During this phase each chromosome arranges itself on the equator of
the spindle. Each chromosome is attached to separate spindle fibre by
its centromere.
Anaphase:
In this phase the centromere of a chromosome divides and the
chromatids of each chromosome separates from each other and begin
to move towards opposite poles. In this way one set of the
chromatids (each chromatid is now an independent chromosome)
move towards one pole while the other set towards the other pole.
Telophase:
This is a stage when the chromatids (now called chromosomes) reach
the poles and their movement ceases. Each pole receives the same
number of chromosomes as were present in the parent cell. The
nuclear membrane is reformed around each set of chromosomes. In
this way two daughter nuclei are formed in each cell. Soon the
cytoplasm of the cell also divides and two daughter cells arise. The
nucleus of each daughter cell contains the same
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chromosome number as in their parent cell. In this way the daughter
cells are exact copies of their parent cell.
Significance of mitosis:
Mitosis plays an important role in the life of an organism. It is responsible for
development and growth of organisms by increasing exact copies of cells. With
few exceptions all kinds of asexual reproduction and vegetative propagation take
place by mitosis. The production of new somatic cells, such as blood cells
depends on mitosis. The healing of wounds, repair of wear and tear within
organism is also dependent upon the mitotic division.
2. Meiosis:
Meiosis or reduction cell-division is a special type of cell-division S| which a
parent cell finally divides into four daughter cells in a way that the number of
chromosome in each daughter cell reduce to half of their parent cell. Thus it is
the reduction of the diploid (2n) number of chromosomes to the haploid (n)
number. In animals meiosis produces gametes (sperms and eggs) while in plants
it gives rise to spores.
The process of meiosis involves two consecutive divisions.
(a) Meiosis I - First meiotic division or reduction phase
(b) Meiosis II - Second meiotic division or meiotic mitotic phase
(a) Meiosis I - First meiotic division or Reduction Phase:
This division consists of the following phases.
i)
ii)
iii)
iv)
Prophase I: Those chromosomes in the cell which 'are similar to each
other in shape and size are called homologous chromosomes.
Homologous chromosomes occur in pairs. The difference between
mitosis and meiosis starts at this point. In mitosis individual
chromosomes remain separate from each other while in meiosis the
homologous chromosomes come together and form pairs. In each
homologous pair, there are four chromatids, since each member
(chromosome) of the pair has already doubled itself. Homologous
chromosomes join to exchange their parts at certain places. This
exchange is called crossing over. During crossing over exchange of
genetic material takes place and new combination of genes result. The
nuclear membrane disappears and at the same time spindle fibres are
formed.
Metaphase I: During this phase pairs of homologous chromosomes
arrange themselves on the equator of the spindle. Unlike mitosis, it is
the homologous pair and not the individual chromosomes which attach
at separate fibre of the spindle.
Anaphase I: The members of the homologous pairs now begin to
separate and move towards the opposite poles.
Telophase I: In this phase the chromosomes come to rest at the
poles. The nuclear membranes are reformed around each set of
chromosomes resulting in formation of two daughter nuclei. On
completion of nuclear division, the cytoplasm also divides and two
daughter cells are formed. Each daughter cell has half (haploid) the
number of chromosomes present in the parent cell (compared with the
cell in prophase) .Thus, the first meiotic division reduces the 2n
(diploid-2 sets) chromosomes to n (haploid-half or one set).
(b) Meiosis II - Second meiotic division or Equational Division:
During second meiotic division the details are almost similar to those seen in
mitosis. During prophase, spindles are formed and the nuclear membrane
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disappears. In metaphase, the chromosomes (each consisting of two chromatids)
arrange themselves on the equator. Their chromatids separate from each other
in anaphase and migrate to the opposite poles. In telophase, the nuclear
membrane reappears around each set of chromatids (now called chromosomes)
and the cytoplasm divides forming two daughter cells. So at the end of meiosis
four daughter cells are produced in total, each possessing a haploid nucleus.
Thus meiosis produces cells (gametes or spores) with a haploid number of
chromosomes.
Significance of meiosis:
Meiosis plays very important role in keeping chromosome number constant in a
species from generation to generation. When the haploid male gamete (sperm)
fertilizes i.e. fuses with the haploid female gamete (ovum) to form a zygote, the
diploid number of chromosomes is restored (n + n = 2n).
Meiosis is responsible for genetic variability i.e. the individuals of a given species
differ from one another. It is due to crossing over which takes place during
prophase I. This genetic variability provides the basis of evolution by providing
raw material for it.
2.6 ORGANIZATION OP CELLS TO FORM TISSUES, ORGANS AND ORGAN
SYSTEM
So far you have learnt about the cell as the basic structural and functional unit of
life. The question now is how can a cell express itself as an independent living
thing?
You know that some small organisms (Amoeba) are made of only one cell These
organisms are called unicellular organisms. They represent single cells capable of
independent existence by making use of their organelles. Once capable of
independent existence, the cell has become an organism. Such an organism
represents the unicellular level of organization of life. In some cases, cells have
come together to form loose assemblies and live together as a colony. In others,
cell with similar structure and function have formed groups. Both have laid down
the foundation of multicellular level of organization of life.
2.6.1 Tissues:
A major step in the direction of multicellular organization of life has been the
formation of tissues. A tissue consists of a group of cells which are similar in
structure and function. Both plants and animals tissues have achieved increasing
complexity by formation of organs and organ systems.
1.
Plant tissues:
In plants there are two basic types of tissues which are as follows.
i)
Meristematic tissue: This tissue contains cells which have ability
to divide, so that the number of cells increases and the organism can
grow . Meristematic cells are smaller in size with comparatively thin walls
and a nucleus in the center. This tissue is commonly present in root tips
and shoot apex and helps to increase the length of the root and the shoot
by adding primary tissue.
ii)
Permanent tissue: Permanent tissue is formed from meristematic
cells. This tissue is different from meristematic tissue because its cells do
not divide. The walls of these cells are thick enabling them to maintain
their shape. Permanent tissue may be classified into two groups i.e.
simple tissue and complex tissue. Simple tissue is made up of one type
of cells forming a homogeneous or uniform mass and a complex tissue
is made up of more than one type of cells working together as a unit.
a) Simple tissue: Simple tissues may further be divided into following type
on the basis of their structure, i.e. Parenchyma, Collenchyma and
Sclerenchyma.
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i.
Parenchyma: It consists of living cells which are more or less equally
expanded on all sides. These cells have intercellular spaces. They are
present in all the soft parts of plant. It is food storing tissue.
ii.
Collenchyma: It consists of some what elongated cells with the
corners filled with cellulose and pectin. Collenchyma occurs in a few
layers under the epidermis of herbaceous dicotyledons.
iii.
Sclerenchyma: Sclerenchyma (scleros =hard) consist of very long,
narrow thick walled and lignified cells. They are dead cells. They
become hard by deposition of chemical like lignin and thus provide
support to the plants. They are found in xylem and hard fruit coats
etc.
b) Complex or Compound tissues: Compound tissues are mainly of two
types: (a) Xylem
(b) Phloem. These will be discussed later under
conducting tissues.
Types of permanent tissues on the basis of function:
i) Epidermal tissues: The cells of these tissues are rectangular in shape.
These tissues form the outer layer of root, stem and leaf .The cells in it
are very compactly arranged so that there is no space between them.
However, in the stem and leaves, pores called stomata are present
through which gases are exchanged. These tissues protect the inner parts
of plant.
ii) Ground tissues: Ground tissues are composed of thin walled
parenchymatous cells, which are formed from meristematic tissue. These
cells are basically meant for storing food. These tissues are present in all
parts of the plant except the epidermal and the vascular tissues.
iii) Supporting tissues: When cells reach a maximum size their cell wails
become thick due to deposition of special material and become dead.
Such cells make up supporting tissue. This tissue is of various shapes and
provides rigidity and support to the plant. Sclerenchyma (thick walled,
lignified and elongated) and collenchyma (living cells with thick cellular
walls with few small intercellular spaces) are examples of the supporting
tissues.
iv) Conducting or Vascular tissues: These tissues consist of elongated
cells with thick or thin walls. Xylem and Phloem are examples of this
tissue. The xylem consists of sclerenchyma vessels and fibers, which
conducts water and salts from the soil to the leaves and also provides
support. The phloem is made up of living cells like sieve tubes, which
conducts food from leaves to various parts of the plants. Xylem and
phloem together form vascular bundle in the stem while they remain
separate from each other in the roots.
2.
Animal tissues:
Like plants, animals have tissues which form organs and organ system. Some
important types of animal tissues are:
i)
ii)
Epithelial tissue: The cells of this tissue occur in a single layer and are
closely packed together. This tissue forms surface layer under lines of the
tubular organs of the body. Epithelial tissue occurs in glands where it is
variously folded.
Connective tissues: These tissues provide support to other tissues and
organs and bind them together. They consist of a ground substance, cells
and fibres. They range from soft to very hard tissues. Fatty tissues are
examples of the soft type. Cartilage and bone are special types of these
tissues and are hard. Blood is also a special connective tissue with cells
suspended in a fluid medium. It transports materials in the body.
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iii)
iv)
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Muscular tissues: This tissue is formed of muscle fibres. Each muscle
fibre is an elongated cell, which has the ability to contract and relax.
These tissues are responsible for movement of the body and body parts.
Nervous tissues: These tissues are formed of cells called neurons or
nerve cells. Nerve cells are specialized to conduct messages in the form of
electrical currents. The nervous system (brain, spinal cord, nerves) is
made up of this tissue. '
2.6.2 Organs:
Your arm is an organ because it consists of various kinds of tissues such as
epithelial tissue, muscular tissue, connective tissue and nervous tissue. All of
these tissues have come together in the arm to make it an organ. Your heart,
kidney, liver and many others structures are organs made in the same way.
Similarly, in a plant the root, the stem and the leaves arc organs. The stem, for
example, consists of several tissues such as epidermal tissue, ground tissue and
conducting tissue.
2.6.3 Organ systems:
Organs work together as a unit to perform a particular function to make an
organ system. For example, the digestive system is made of organs such as
mouth, gut, liver and pancreas are all working together to digest food. There are
other systems in the animal body such as transport, respiratory, excretory,
muscle, skeletal, nervous and reproductive systems. In plants also, the tissues
and organs (root, stem, and leaves) are organized to form systems. However,
the systems, here are not so clearly organized as in the animals. It is usual to
study these in plants, as conduction, storage, supporting systems, or root and
shoot systems.
In this chapter you are studying life at various levels of organization from the
simplest to the most complex. A simple diagram of this organization is given
below:
Cells Tissues Organs  Systems  Organism
2.7
UNICELLULAR ORGANISMS
Those animals and plants, which are single-celled, are called unicellular
organisms. Amoeba is one of the example.
Amoeba:
It is a unicellular aquatic organism found in stagnant water pools and ponds. It is
microscopic in size measuring about 0.25 millimeter. It does not possess a
permanent form and' keeps on changing its shape.
The structure of Amoeba is very simple. The nucleus and cytoplasm are
surrounded by a protective cell membrane.
Cytoplasm is differentiated into two parts. Its outer portion, which is clear and
transparent is called ectoplasm. The inner viscous, translucent and granular part
is called endoplasm. The endoplasm contains many food vacuoles of different
size, a contractile vacuole and other cells organelles.
Nucleus is usually present in the centre but as the Amoeba moves, the nucleus
changes its position. The contractile vacuole functions to remove excess water
from the body. The food vacuoles contain food particles. The animal moves by
producing temporary finger-like projections called pseudopodia (Pseudo =
false, podia a feet). The pseudopodia are also used to capture food particles,
which enter the body as food vacuoles. Amoeba respires by exchanging gases
with the surrounding water through its surface.
2.8
MULTICELLULARORGANISMS
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The majority of living organisms consist of many cells and are called
multicellular organisms. Brassica and frog have been selected here as
representative examples of multicellular plants and animals, respectively.
2.8.1 Brassica:
Brassica campestris is the botanical name of mustard (sarsoun). You are very
familiar with this plant since its oil (mustard oil) is used for cooking and its
leaves are used as vegetable (saag).
Structure of Brassica:
This plant consists of roots, stem, leaves, flowers, fruit and seeds. These parts
can be divided into two categories on functional basis i.e. vegetative parts and
reproductive parts.
The vegetative parts are those which do not directly take part in sexual
reproduction. These parts are root, stem, branches and leaves.
The reproductive parts consist of sex organs which are directly related to
sexual reproduction. These are flowers.
1.
Vegetative parts:
i)
Root: The root is that part, which grows under the soil and
develops from the radicle of the seed. The first part of the root to arise
from the radicle is known as the primary root. During its growth it gives
off secondary and tertiary roots. The primary roots are thicker than the
secondary and tertiary roots. The tips of all the roots bear a cap, the root
cap. The root bears fine, thin root hairs. The plant absorbs water and
minerals from the soil through the root hairs only, the rest of the root fix
the plant to the soil.
Internal structure:
The outer part of a root is the epidermis (epi=above; derma=skin), which
protects the root. Root hairs are outgrowths of epidermal cells. Next to epidermis
is the cortex. Cortex is composed of parenchyma cells. Parenchyma cells store
food material. Within the cortex is a central cylinder region called the stele. The
stele of the root is surrounded on the outside by a layer of cells called
endodermis. Next to the endodermis is a layer of cells called pericycle. Branch
of the root originate from the pericycle. The central part of the stele is occupied
by a star shaped xylem. In between the arms of the xylem is phloem. Rest of
the stele is made of parenchyma cells.
ii) Stem: This part of plant develops from the plumule of the seed and
grows away from the soil. It bears branches and flowers. The point, on
the stem or on a branch, which gives rise to leaf, is known as the node.
The part between two adjacent nodes is called the internode. The stem
and the branches transport water and salts from the root to the leaves. It
also transports prepared food from the leaves to all parts of the plant. In
addition, the stem supports the leaves and the branches in the air, thus
enabling the leaves to receive maximum amount of sun light for
photosynthesis. The stem and its branches also bear flowers, which are
the reproductive organs.
Internal structure:
A cross section of Brassica stem shows that it is surrounded on the outside by a
single layered epidermis. Next to the epidermis is cortex. The cortex is made up of
parenchyma and collenchyma cells. Food material is stored in the cortex. Next to the
cortex is a ring of vascular bundles . Each bundle consists of xylem and phloem.
Xylem is located towards the inside and phloem towards the outside. In between
xylem and phloem, there is a region consisting of meristematic cells
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called cambium. The centre of the stem is occupied by pith. It is made up of
parenchyma cells and stores food material.
iii) Leaf: Leaves grow out on the stem and its branches from the nodes.
Generally, the leaf of Brassica consists of two parts. The lower stalk like
part is the petiole and upper green expanded portion is the lamina.
Young leaves are without petioles and their margins are entire or smooth
but in mature leaves the margin is wavy. There is a swollen vein in the
middle of the leaf which is known as midrib. The branch veins emerge
and spread in the leaf like a net. These veins are actually vascular
bundles consisting of xylem and phloem. This network of veins supports
the leaf and keeps its lamina in an expanded position. New branches of
the plant arise from buds present in the axil of the leaf. The function of
the leaf is to prepare food. Therefore, all of its tissues are arranged in
such a way that photosynthesis can take place easily.
The leaves are arranged on the stem and branches in such a way that
their upper surfaces remain directly exposed to sunlight while the lower
surface does not get the same amount of light. Due to this difference the
upper and lower surfaces are slightly different from each other. Leaves
having different upper and lower surfaces are called bifacial leaves.
Internal structure:
A leaf is composed of several distinct cell layers. The upper layer of a leaf is
called the upper epidermis. The lower layer of the leaf is called the lower
epidermis, which contains stomata (Sing: Stoma). Each stoma has a pore and
two guard cells. The tissue between upper and lower epidermis is called the
mesophyll. The mesophyll cells below the upper epidermis are longer than
broad and are closely packed. It is called the palisade layer. The cells next to
the palisade layer are irregular in shape and loosely arranged having spaces like
sponge and is called the spongy layer.
Photosynthesis takes place in palisade and spongy mesophylls. Running through
the leaf are many vascular bundles or veins. The veins are composed of xylem
and phloem. Xylem is located towards the upper side and the phloem towards
the lower epidermis.
2. Reproductive parts
Flower:
With growing age, Brassica plant bears small, yellowish flowers. Flowers are the
most beautiful and important parts of the plant. They are arranged on young
branches in a special way. This special arrangement of the flowers on the stem is
called inflorescence.
Parts of the flower:
The flower in Brassica is situated on a stalk known as pedicel. The tip of the
pedicel bears thalamus. The floral leaves are arranged in four whorls on the
thalamus. These whorls, starting from the outermost to the central one, are in
the following order.
i)
ii)
Calyx: This is the outermost whorl and consists of four free sepals. The
sepals are light greenish in young flowers but as the flower matures,
their colour also becomes yellowish like that of the petals. The most
important function of the calyx is to cover the inner parts of the flower
and to protect them from sunlight and rain.
Corolla: This is the second whorl and is composed of four free yellow
petals. Because of the petals, the flower becomes very conspicuous that
honey bees, butterflies and other insects are easily attracted and thus
help in pollination.
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iii)
Androecium: The androecium lies inside the petals. It makes the third
whorl of the floral leaves. Its parts are not leaf-like. The androecium
consists of six free stamens which are the male reproductive organs of
the flower. In Brassica flower, the stamens are arranged in two circles.
The outer circle has two small stamens. The inner circle has four long
stamens. Each stamen has two well defined parts, a lower delicate stalk
called the filament and an upper swollen part called the anther. Each
anther contains numerous pollen grains. When the anther matures a
longitudinal slit in its wall enables the pollen grains to escape. There are
dark green nectaries of small size at the base of the androecium. These
nectaries contain nectar (a honey-like substance). This nectar is the food
of insects. When the insects are attracted towards the flowers to collect
this nectar pollen grains get attached to their bodies and are transferred
from one flower to another. This results in the pollination of flowers.
iv)
Gynoecium: This is fourth whorl occupying the central position in the flower.
The parts of the gynoecium are called carpels, who are the female
reproductive organs of the plant. In Brassica, gynoecium is formed by the
union of two carpels. Each carpel is divisible into three main parts. The
lower swollen part is the ovary. Above the ovary carpel extends into a
thin stalk, the style. The style has swollen tip, which is called stigma. In
the ovary many ovules are present, which ripen into seeds. The ovary
ripens and is converted into fruit. The fruit of Brassica is a long dry
capsule with many seeds. The seeds are very small and light. They can be
easily dispersed by air currents. When these seeds fall on a suitable place
they germinate and produce new Brassica plants.
2.8.2 Frog:
The frog lives both in water as well as on land. It swims in water and moves by
jumping when on land. There is a membranous skin between its toes which helps
in swimming. There are five toes in each foot but the hand has only four fingers
because the thumb is rudimentary. In male frog the first finger is thicker than
the others.
Frog has neither a neck nor a tail. As the head is directly attached to the trunk
frog cannot move it as we can. The conical head has two large bulging eyes.
Behind each eye is a circular area called tympanic membrane. These membranes
help in hearing. At the tip of the snout it has two openings called external
nostrils by which frog breathes. The skin of the frog is loose and slippery. It is
slippery due to secretions produced by glands present in it.
Frogs are found in abundance in the rainy season during which they lay eggs.
They hibernate during the winter season by burying themselves in the mud and
stay there throughout the winter. This phenomenon is called hibernation or
winter sleep.
Internal organs:
The internal organs are located in the body cavity, which is also called coelom.
These organs make up various systems, which perform specific functions. These
are as follows:
1. Digestive system: The organs involved in the breakdown of complex food
into simpler form (digestion) constitute the digestive system. This system is
composed of a tube, the alimentary canal and special glands associated with
it. The alimentary canal consists of buccal cavity, pharynx, oesophagus,
stomach and intestine.
i)
Buccal cavity: Food enters into the buccal cavity through mouth. The
upper jaw has a row of weak but pointed teeth. They are not meant for
chewing food but prevent it from slipping out of the mouth. The tongue
of frog is unique in being attached in front to the floor of
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ii)
iii)
iv)
v)
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the buccal cavity and being free behind. This allows the animal to
throw it outward.
Frog feeds mainly on insects. On seeing the prey, it suddenly throws
out its tongue. The prey sticks to its sticky tongue. The tongue is
then immediately withdrawn and the prey is swallowed.
Pharynx: The buccal cavity opens into a short but narrow pharynx,
which leads into a wide tube,, the oesophagus. Immediately behind
the tongue on the floor of the pharynx is a slit like opening, the
glottis, which opens into the lungs. When the food passes into the
oesophagus, the glottis is closed and during respiration it is opened.
Oesophagus and stomach: Pharynx opens into a wide tube called
oesophagus or gullet; It transports food into the stomach. Stomach
is a thick walled, muscular and glandular sac. Anterior end of
stomach is called cardiac end while posterior end is called pyloric
end. Food is grounded in stomach and mixed with enzyme pepsin,
which partially digest proteins. Food in stomach changes into a paste
like substance called chyme.
Intestine: The intestine is a long narrow coiled tube. It is divisible
into small and a large intestine. The partially digested food from the
stomach enters the small intestine through pyloric end, where its
digestion is completed. The digested food is absorbed into blood.
The undigestible parts of the food enter the large intestine, also
called rectum. The short terminal part of rectum is called cloaca.
From cloaca undigested food is expelled out as faeces through its
opening called cloacal aperture. This is a common aperture for urine,
reproductive and undigested food discharge.
Liver and pancreas: The liver is a large reddish-brown gland
located adjacent to the stomach. Its secretion is known as bile.
Between the lobes of the liver is a rounded pouch called gall
bladder, which stores bile. A bile duct arises from it. On its way,
this duct passes through pancreas and joins the pancreatic duct. The
pancreas lies between stomach and duodenum, the first part of
small intestine. Its secretion, pancreatic juice, is carried by the
pancreatic duct. The pancreatic duct and the bile duct join to form a
common hepato-pancreatic duct, which then opens into
duodenum. The bile and the pancreatic juice help in the complete
digestion of the food in the small intestine.
Digestion is a process by which the complex insoluble food
substances are converted into soluble form by the action of
enzymes. The digested food is then absorbed into the blood through
the intestinal walls.
2.
Respiratory system:
Energy is required by every organism to carry on all the life activities. It is
produced by the oxidation of food specially glucose. This process takes place in
the cells. For oxidation the cells require oxygen and as a result of oxidation of
food they produce CO2 as waste product. This entire process called respiration,
divided into two phases.
a)
b)
Gaseous exchange or Extra-cellular respiration
Cellular respiration.
We will restrict our discussion upto gaseous exchange as respiration. Frog has
three types of respiration on the basis of organs involved in the gaseous
exchange. These are:
i)
Pulmonary respiration
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ii)
ii)
Cutaneous respiration
Buccal respiration
i)
Pulmonary respiration:
The gaseous exchange, which takes place in lungs is called pulmonary
respiration.
The frog has two lungs, which are balloon like structures. Their outer
surface is smooth but their inner surface has numerous folds which
increase the area for gaseous exchange. The lungs are richly supplied
with blood vessels. Each lung has a bronchus at its upper end. The two
bronchi open into a larynx. The glottis opens into the larynx.
During respiration air is taken in by the external nostrils. It passes into the
buccal cavity through the internal nostrils. From here it enters the glottis,
passes through the larynx and bronchi finally reach the lungs. In the
lungs, exchange of gases between air and blood takes place i.e. oxygen is
taken up by the blood and CO2 is given out, which leaves the body
through same route.
Aresting human breathes out about 500 litres of CO2
ii)
iii)
every 24 hours.
Cutaneous respiration:
Gaseous exchange carried out by skin, is called cutaneous respiration.
Frog uses skin as a respiratory organ during swimming and hibernation.
Oxygen diffuse into blood through skin while CO2 diffuses out from the
network blood capillaries in skin.
Buccal respiration:
The lining of buccal cavity is thin, moist and richly supplied with blood
capillaries. Here also exchange of gases takes place between the air and
blood. This type of respiration is called buccal respiration.
3.
Circulatory system:
Every cell requires a supply of oxygen and nutrient molecules and must get rid of
waste products. For this purpose a transport system usually called circulatory
system is required. Blood transports these materials during its circulation
through out the body.
Frog has a closed type circulatory system in which blood circulates in the closed
circuit of blood vessels being pumped by an organ called heart, This system is
also called cardiovascular system. It consists of:
i)
ii)
Heart- strong muscular pumping organ.
Three kinds of blood vessels:
(a)
Arteries － Which carry blood away from heart.
(b)
Veins － Which return blood to the heart.
(c)
Capillaries － Exchange material between tissues and blood.
Heart:
Heart is a conical, muscular pumping organ, located in the anterior region of
body cavity. It is enclosed in a membrane called pericardium. It contracts and
expands continuously through out the life. This contraction and expansion of
heart is called heart beat, due to which blood circulates continuously in the body.
Frog heart consists of three chambers.
(i)
(ii)
(iii)
Right auricle or Atrium.
Left auricle or Atrium.
Ventricle.
The longest heart stoppage was 4 hours. A Norwegian fell into the sea in December 1987.
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Survival was due to the low temperature of his body in the sea.
The truncus arteriosus originates from ventral side of the ventricle and divide
into two branches each of which divides into three arches (arteries). Another thin
walled triangular sac called sinus venosus formed by major veins opens into
right atrium. Both of these structures are not true chambers of heart but often
called accessory chambers.
In heart, the flow of blood is continuous and moves in two path ways. The
oxygenated blood from the lungs enters the left auricle through pulmonary veins.
The deoxygenated blood from all other parts of the body enters the sinus
venosus. From sinus venosus, it reaches the right auricle. When the two auricles
contract, the blood is pushed into the single ventricle. When ventricle contracts it
is pumped into the truncus arteriosus From here it enter (a) the pulmonary
arteries, which carry the blood to lungs for oxygenation and (b) the systemic
arteries, which supply it to all parts.pf the body and (c) the carotid arteries,
which supply it to the brain.
A human being contains about 70 ml of blood per Kilogram of body weight. For an adult,
this is about 4 or 5 liters.
The opening between various chambers of the heart are guarded by valves,
which prevent the flow of blood in reverse direction.
4.
Arterial system:
A blood vessel, which carries blood away from heart to the various body parts is
called an artery. The arterial system can be simply stated to comprise of the
following three main components.
Pulmocutaneous arteries: They supply deoxygenated blood to lungs and skin
where it gives up carbon dioxide and picks up oxygen.
Carotid arteries: These vessels arise from the truncus arteriosus, and supply
the oxygenated blood to various parts of the head region such as brain, tongue,
head muscles, eyes, ears etc.
Systemic arteries: These vessels carry oxygenated blood to all the parts of the
body except the head and lungs. They fuse together to form a major vessel of
this system called aorta, which gives off branches to various parts of the body
such as fore and hind limbs, digestive system, liver, pancreas, kidneys, genital
organs and muscles.
5.
Venous system:
It is a set of blood vessel, called veins, which bring the blood from all the parts
of the body towards heart. The venous system consists of the following major
components.
(i)
The oxygenated blood from the lungs is collected by pulmonary
veins, which bring it to the left auricle of the heart.
(ii) The deoxygenated blood from head and fore limbs is collected
through several veins, which join together to form one major
precaval vein, on each side.
(iii) Blood from all the lower parts of the body such as stomach,
intestine, liver, pancreas, genital organs, muscles, hind limbs etc, is
collected through veins, which join together and form one major
vein called post caval. Both the pre-cavals and the post-caval open
into the sinus venosus from where the blood is pumped into the
right auricle of the heart.
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Portal system: Set of veins, which collect the blood from one organ and
discharge it into another organ, is called portal veins. The set of veins draining
their blood into liver are included in hepatic portal whereas the set opening into
the kidneys are called renal portal system.
Hepatic portal system: The veins collecting the blood from the digestive
system do not carry it directly to the heart. They all join to form a hepatic portal
vein, which breaks up into capillaries in the liver to allow transfer of some of the
digested food into the liver for storage. From the liver, blood enters the postcaval, which carries it to the sinus venosus. Some of the blood from hind limbs is
collected in an abdominal vein, which joins the hepatic portal vein in the liver
and drains blood into liver from where it is collected by the post caval vein.
These veins are also included hepatic portal system.
Renal portal system: The alternate route of blood from the hind limbs is by the
way of a renal portal vein. These veins begin in capillaries in the hind limbs and
breaks up into capillaries in the kidneys. From the kidneys, this blood enters the
post caval by renal veins and ultimately returns to the heart. These veins arc
included in renal portal system.
6.
Excretory system:
It is the set of organs involved in the process of excretion i.e. the removal of
metabolic waste matters from the body. This function is performed by kidneys,
which filter out the excretory matter from the blood and pass it out in the form
of urine. There are two kidneys in frog. They are elongated reddish brown organs
attached to the dorsal wall of the body cavity. The urine is carried from the
kidneys by a pair of tubes called ureters, which open into the cloaca. From the
cloaca, it is either passed out directly through cloacal aperture or is stored for
some time in a bag, the urinary bladder.
7.
Reproductive system:
Reproduction is the process of production of new babies by their parents. Organs
involved in this process are included in reproductive system. Sexes are separate
in the frog. The reproductive organs consist of gonads and their ducts. The
gonads produce germ cells and the ducts pass them but of the body. The male
gonad is known as the testis (plural testes) and the female gonad is called the
ovary. The gonads are paired structures and are located near the kidneys. The
sex cell of male is sperm and that of female is ovum (Plural ova) or egg. The ova
are released into water through the cloaca in the mating season.
The male produces sperms during the breeding season. Sperms are also released
from the testes into water near the eggs. Each sperm fuses with a egg and a
new frog starts developing.
8.
Nervous system:
The set of organs, which control and co-ordinate all the activities of the body is
called nervous system. It is composed of two parts, i.e the central nervous
system and the peripheral nervous system. The central nervous system includes
the brain and the spinal cord whereas the peripheral nervous system comprises
of nerves, which connect the central nervous system with various parts of the
body, i.e. muscles, glands and sense organs.
The animal with the heaviest brain is the sperm whale.
Its brain weighs about 9.2 Kg.
The brain is protected in the skull and the spinal cord is enclosed in the vertebral
column. The brain consists of the following regions:
(i)
The most anterior region comprises the olfactory lobes.
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(ii)
Immediately behind the olfactory lobes, there are two large
outgrowths called the cerebral hemispheres.
(iii) Behind the hemispheres are located two prominent outgrowths
called the optic lobes.
(iv) The part between the optic lobes and hemispheres is known as the
diencephalon. On the dorsal surface of diencephalon is present a
pineal body while the pituitary gland is attached to its ventral
surface.
(v) The last part of the brain consists of the cerebellum and the
medulla oblongata.
The spinal cord starts from the posterior end of the medulla oblongata like a
thick thread. It passes from the skull through a hole and enters the canal of the
vertebral column. The vertebral column protects the spinal cord just as the skull
protects the brain. Brain and spinal cord are not solid. They have a system of
canals which is filled with a fluid.
Olfactory lobes are associated with the sense of smell and transfer of olfactory
sensation to the cerebral hemispheres, which are the seats of intelligence and
memory. The diencephalon receives a variety of messages from the internal and
external environment of the body and also controls the secretion of hormones
from the pituitary gland. The optic lobes are associated with the eyes and vision.
The cerebellum and the medulla oblongata co-ordinate body movements and
maintain balance of the body. The medulla also controls respiration, circulation
and digestion. The spinal cord controls the movements of the trunk area, and
many other functions independently.
The peripheral system connects body parts with the central nervous system. This
system consists of cranial and spinal nerves. Cranial nerves connect and
establish communication between various parts of brain and parts of the head
while the spinal nerves connect structures of the trunk area with the spinal cord.
Some special nerves are present in the head and trunk region called the
autonomic nerves, which work automatically. Autonomic nerves control the
internal organs of the body such as heart, lungs, stomach, smooth muscles of
the intestine, vessels and glands.
2.9 SENSE ORGANS
To get the informations about the environment organisms like frog have receptor
organs. These receptors send sensations to the central nervous system via
nerves. The frog has many types of receptors. Its skin has many small
microscopic receptors for the sense of touch. For smell, there are olfactory
receptor located in the nostrils. For the sense of taste sensory cells are present in
taste buds on the tongue. Ears are used for receiving sound waves and
maintaining balancing and eyes for receiving light i.e. sight.
The animal with the most acute sense of smell is the male emperor moth. Using
its antennae, it can detect a female emperor moth 11.Km upwind
The biggest mammalian tongue that has ever been weighed belonged to a Blue
whale caught by Russian trawlers in 1947. Its tongue weighed 4.3 tonnes.
1.
Ear:
In frog, like the other vertebrates, the organ of hearing is the ear. Its outer most
part is the tympanic membrane. An external ear called pinna is absent in frog.
On the inner side of this membrane is a cavity known as tympanic cavity. The
cavity contains three small rod- like bones called ossicles Its one end is
attached to the tympanic membrane and the other with the internal ear.
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High pitched sounds are rapid vibrations of the molecules In air. Human can hear sounds
which vibrate at about 19 KHz. Some bats can hear ultrasonic sounds upto about 160 KHz.
The internal ear (Fig: 2.43) is a very delicate organ. It consists of three semicircular canals. These canals are filled with a fluid and sensory cells are located
at special places in these canals. When sound waves strike the tympanic
membrane it is set into vibration. This vibrates ossicles which in turn, vibrate the
internal ear and thus, sound waves stimulate the hearing receptors in the inner
ear. The internal ear, in addition to hearing, also keeps the balance of the body.
2.
Eye:
The frog has two eyes one on each side of the head (Fig: 2.44). If we make
vertical section of the eye, we find that the innermost layer of the ball is the
sensory retina. The retina contains photoreceptor cells. Outside the retina is the
choroid, which is richly supplied with blood capillaries supplying nutrients to the
retina. The sclerotic is the hard, outer most layer of the eye. It provides shape to
the eye ball. The anterior transparent part of the eye is called cornea. Behind the
cornea is iris. The iris has a window called the pupil. Behind the pupil is the lens
of the eye. The cornea, pupil and lens focus light on the retina. A watery fluid is
present in between the cornea and lens. Similarly a jelly like fluid is present
between the lens and retina, through which light passes before it strikes retina.
Optic nerve takes the sensory messages from the eye to the brain.
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SUMMARY
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Cell is the basic unit of living organisms. 
All the living organisms are made up of one or more cell. 
Cells are observed under light and electron microscopic. 
Resolution power of electron microscope is very much high. 
Plant cell consist of cell-wall, cell membrane, cytoplasm and nucleus, in an
animal cell all these parts are present except cell-wall. 
Prokaryotic cell is the type of cell with out proper nucleus while eukaryotic
cell contains proper membrane bound nucleus. 
Cell wall is non-living part of cell, mainly composed of cellulose and pectin. 
Cell membrane is a thin, differentially permeable membrane which limits the
cytoplasm. 
Translucent material filled in between nucleus and plasma membrane is
called cytoplasm. 
Cytoplasm contains many granular bodies called cytoplasmic organelles like,
endoplasmic reticulum, golgi bodies, lysosome, ribosome, mitochondria,
plastids etc. 
Cells divide to increase in numbers. 
Mitosis is the equational type of cell-division in which chromosome number
does not change. 
Meiosis is the reductional cell-division in which number of chromosomes
reduces to half. 
Tissues are the groups of similar cells. 
Amoeba is an example of unicellular organization, Brassica and frog of
multicellular organization. 
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1.
Sindh Text Book Board, Jamshoro.
EXERCISE
Fill in the blanks with appropriate terms:
i)
Animals which consist of only one cell are called________
ii)
Finger like projection in amoeba are called________
iii)
Botanical name of mustard plant is__________
iv)
Reproductive part of a higher plant is_______
v)
Part of stem from where leaf arises is called
2.
Write whether the following statements are true or false:
i)
Gonads are the organs responsible to produce germ cells.
ii)
Brain and spinal cord are the parts of peripheral nervous system.
iii)
Pulmonary vein contains de-oxygenated blood.
iv)
Truncus arteriosus and sinus venosus are the true chambers of the
heart of frog.
v)
During pulmonary respiration gaseous exchange takes place
through skin.
3.
Encircle the appropriate answer:
i)
ii)
iii)
iv)
v)
Nucleus of cell was discovered by
(a) Robert Hooke
(b) Schleiden
(c) Robert Brown
(d) Virchow
The type of lenses of light microscope is
(a) Biconcave
(b) Convex
(c) Concave
(d) Electromagnetic
DNA is found in
(a) Nucleus
(b) Chromosome
(c) Cell
(d) Nucleoplasm
Prokaryotic cells do not contain
(a) Nucleus
(b) Membrane bound nucleus
(c) Ribosome
(d) Cell-membrane
The number of chambers in the heart of frog is
(a) Two
(b) Three
(c) Four
(d) Five
4.
Write detailed answers of the following questions:
i)
Make a chart of different types of plant tissues:
ii)
Draw a diagram showing different parts of a typical plant cell.
iii)
Describe those organelles which are only found in plant cell.
iv)
Describe digestive System of frog with the help of diagram.
v)
What is the difference between cellular respiration and gaseous
exchange? Describe different methods of gaseous exchange in frog.
vi)
What do you mean by cell-division? Describe the method of celldivision by which gametes or spores are produced.
5.
i)
ii)
iii)
Give scientific reason of the following:
Why is cell membrane differentially permeable?
Why is cell called the basic structural and functional unit of life?.
How do the chromosome number remains the same in the members of
same species?
Why is meiosis called reductional cell-division?
Why is upper surface of dicot leaf much darker than the lower surface?
iv)
v)
6.
Define the following terms:
i) Cell
ii) Tissue
iv) Eukaryotic cell
v) Chromosomes
vii) Connective tissue viii) Meiosis
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iii) Prokaryotic cell
vi) Mitochondria
ix) Meristematic tissue
38
Biology
Sindh Text Book Board, Jamshoro.
x) Compound tissue xi) Mitosis
xiii) Neuron or Nerve cells
7.
Distinguish between:
i)
Prokaryotic and eukaryotic cell.
ii)
Cell-wall and cell-membrane.
iii)
Mitochondria and plastids.
iv)
Light and electron microscope.
v)
Arteries and veins.
xii)Hibernation