NCERT Textbook: Cell- Structure & Functions | General Science(Prelims) by IRS Divey Sethi - UPSC PDF Download

 Page 1


CELL — STRUCTURE AND FUNCTIONS
CELL — STRUCTURE AND FUNCTIONS
Y
ou have already learnt that things
around us are either living or
non-living. Further, you may
recall that all living  organisms carry out
certain basic functions. Can you list
these functions?
Different sets of organs perform the
various functions you have listed. In this
chapter, you shall learn about the basic
structural unit of an organ, which is the
cell. Cells may be compared to bricks.
Bricks are assembled to make a building.
Similarly, cells are assembled to make
the body of every organism.
8.1 Discovery of the Cell
Robert Hooke in 1665 observed slices of
cork under a simple magnifying device.
Cork is a part of the bark of a
tree.  He took thin slices of cork and
observed them under a microscope. He
noticed partitioned boxes or
compartments in the cork slice (Fig. 8.1).
These boxes appeared like a honey-
comb. He also noticed that one box was
separated from the other by a wall or
partition. Hooke coined the term ‘cell’
for each box. What Hooke observed as
boxes or cells in the cork were actually
dead cells.
Cells of living organisms could be
observed only after the discovery of
improved microscopes. Very little was
known about the cell for the next 150
years after Robert Hooke’s observations.
Today, we know a lot about cell
structure and its functions because of
improved microscopes having high
magnification.
8.2 The Cell
Both, bricks in a building and cells in
the living organisms, are basic
structural units [Fig. 8.2(a), (b)]. The
buildings, though built of similar bricks,
have different designs, shapes and sizes.
Similarly, in the living world, organisms
differ from one another but all are made
up of cells. Cells in the living organisms
are complex living structures unlike
non-living bricks.
Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Cork cells as observed by
Robert Hooke
A hen’s egg can be seen
easily. Is it a cell or a
group of cells?
2022-23
Page 2


CELL — STRUCTURE AND FUNCTIONS
CELL — STRUCTURE AND FUNCTIONS
Y
ou have already learnt that things
around us are either living or
non-living. Further, you may
recall that all living  organisms carry out
certain basic functions. Can you list
these functions?
Different sets of organs perform the
various functions you have listed. In this
chapter, you shall learn about the basic
structural unit of an organ, which is the
cell. Cells may be compared to bricks.
Bricks are assembled to make a building.
Similarly, cells are assembled to make
the body of every organism.
8.1 Discovery of the Cell
Robert Hooke in 1665 observed slices of
cork under a simple magnifying device.
Cork is a part of the bark of a
tree.  He took thin slices of cork and
observed them under a microscope. He
noticed partitioned boxes or
compartments in the cork slice (Fig. 8.1).
These boxes appeared like a honey-
comb. He also noticed that one box was
separated from the other by a wall or
partition. Hooke coined the term ‘cell’
for each box. What Hooke observed as
boxes or cells in the cork were actually
dead cells.
Cells of living organisms could be
observed only after the discovery of
improved microscopes. Very little was
known about the cell for the next 150
years after Robert Hooke’s observations.
Today, we know a lot about cell
structure and its functions because of
improved microscopes having high
magnification.
8.2 The Cell
Both, bricks in a building and cells in
the living organisms, are basic
structural units [Fig. 8.2(a), (b)]. The
buildings, though built of similar bricks,
have different designs, shapes and sizes.
Similarly, in the living world, organisms
differ from one another but all are made
up of cells. Cells in the living organisms
are complex living structures unlike
non-living bricks.
Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Cork cells as observed by
Robert Hooke
A hen’s egg can be seen
easily. Is it a cell or a
group of cells?
2022-23
The egg of a hen represents a single
cell and is big enough to be seen by the
unaided eye.
8.3 Organisms show Variety
in Cell Number, Shape and
Size
How do scientists observe and study the
living cells? They use microscopes which
magnify objects. Stains (dyes) are used
to colour parts of the cell to study the
detailed structure.
There are millions of living
organisms. They are of different shapes
and sizes. Their organs also vary in
shape, size and number of cells. Let us
study about some of them.
Number of Cells
Can you guess the number of cells in a
tall tree or in a huge animal like the
elephant? The number runs into
billions and trillions. Human body has
trillions of cells which vary in shapes
and sizes. Different groups of cells
perform a variety of functions.
Organisms made of more than one
cell are called multicellular (multi :
many; cellular : cell) organisms. The
number of cells being less in smaller
organisms does not, in any way, affect
the functioning of the organisms. You
will be surprised to know that an
organism with billions of cells begins
life as a single cell which is the fertilised
egg. The fertilised egg cell multiplies and
the number of cells increase as
development proceeds.
Look at Fig 8.3 (a) and (b). Both
organisms are made up of a single cell.
The single-celled organisms are called
unicellular (uni : one; cellular : cell)
Fig. 8.2 : Brick wall and onion peel
(a) Brick wall (b) Onion peel
A billion is a thousand million. A
trillion is a thousand billion.
Fig. 8.3 : (a) Amoeba (b) Paramecium
(a)
(b)
CELL — STRUCTURE AND FUNCTIONS 91
2022-23
Page 3


CELL — STRUCTURE AND FUNCTIONS
CELL — STRUCTURE AND FUNCTIONS
Y
ou have already learnt that things
around us are either living or
non-living. Further, you may
recall that all living  organisms carry out
certain basic functions. Can you list
these functions?
Different sets of organs perform the
various functions you have listed. In this
chapter, you shall learn about the basic
structural unit of an organ, which is the
cell. Cells may be compared to bricks.
Bricks are assembled to make a building.
Similarly, cells are assembled to make
the body of every organism.
8.1 Discovery of the Cell
Robert Hooke in 1665 observed slices of
cork under a simple magnifying device.
Cork is a part of the bark of a
tree.  He took thin slices of cork and
observed them under a microscope. He
noticed partitioned boxes or
compartments in the cork slice (Fig. 8.1).
These boxes appeared like a honey-
comb. He also noticed that one box was
separated from the other by a wall or
partition. Hooke coined the term ‘cell’
for each box. What Hooke observed as
boxes or cells in the cork were actually
dead cells.
Cells of living organisms could be
observed only after the discovery of
improved microscopes. Very little was
known about the cell for the next 150
years after Robert Hooke’s observations.
Today, we know a lot about cell
structure and its functions because of
improved microscopes having high
magnification.
8.2 The Cell
Both, bricks in a building and cells in
the living organisms, are basic
structural units [Fig. 8.2(a), (b)]. The
buildings, though built of similar bricks,
have different designs, shapes and sizes.
Similarly, in the living world, organisms
differ from one another but all are made
up of cells. Cells in the living organisms
are complex living structures unlike
non-living bricks.
Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Cork cells as observed by
Robert Hooke
A hen’s egg can be seen
easily. Is it a cell or a
group of cells?
2022-23
The egg of a hen represents a single
cell and is big enough to be seen by the
unaided eye.
8.3 Organisms show Variety
in Cell Number, Shape and
Size
How do scientists observe and study the
living cells? They use microscopes which
magnify objects. Stains (dyes) are used
to colour parts of the cell to study the
detailed structure.
There are millions of living
organisms. They are of different shapes
and sizes. Their organs also vary in
shape, size and number of cells. Let us
study about some of them.
Number of Cells
Can you guess the number of cells in a
tall tree or in a huge animal like the
elephant? The number runs into
billions and trillions. Human body has
trillions of cells which vary in shapes
and sizes. Different groups of cells
perform a variety of functions.
Organisms made of more than one
cell are called multicellular (multi :
many; cellular : cell) organisms. The
number of cells being less in smaller
organisms does not, in any way, affect
the functioning of the organisms. You
will be surprised to know that an
organism with billions of cells begins
life as a single cell which is the fertilised
egg. The fertilised egg cell multiplies and
the number of cells increase as
development proceeds.
Look at Fig 8.3 (a) and (b). Both
organisms are made up of a single cell.
The single-celled organisms are called
unicellular (uni : one; cellular : cell)
Fig. 8.2 : Brick wall and onion peel
(a) Brick wall (b) Onion peel
A billion is a thousand million. A
trillion is a thousand billion.
Fig. 8.3 : (a) Amoeba (b) Paramecium
(a)
(b)
CELL — STRUCTURE AND FUNCTIONS 91
2022-23
SCIENCE 92
organisms. A single-celled organism
performs all the necessary functions
that multicellular organisms perform.
A single-celled organism, like
amoeba, captures and digests food,
respires, excretes, grows and
reproduces. Similar functions in
multicellular organisms are carried out
by groups of specialised cells forming
different tissues. Tissues, in turn, form
organs.
Activity 8.1
The teacher may show a permanent
slide of amoeba and paramecium
under a microscope. Alternatively,
the teacher can collect pond water
and show these organisms by
preparing the slides.
Shape of Cells
Refer to Fig. 8.3 (a). How do you define
the shape of amoeba in the figure? You
may say that the shape appears
irregular. Infact, amoeba has no
definite shape, unlike other organisms.
It keeps on changing its shape.
Observe the projections of varying
lengths protruding out of its
body. These are called pseudopodia
(pseudo : false; podia : feet), as you
learnt in Class VII. These projections
appear and disappear as amoeba
moves or feeds.
A white blood cell (WBC) in human
blood is another example of a single cell
which can change its shape. But while
WBC is a cell, amoeba is a full fledged
organism capable of independent
existence.
What shape would you expect in
organisms with millions of cells? Fig.
8.4 (a, b, c) shows different cells such
as blood, muscle and nerve of human
beings. The different shapes are related
to their specific functions.
Generally, cells are round, spherical
or elongated [Fig. 8.4(a)]. Some cells are
long and pointed at both ends. They
exhibit a spindle shape [Fig. 8.4(b)].
Cells sometimes are quite long. Some
are branched like the nerve cell or a
neuron [Fig. 8.4(c)]. The nerve cell
receives and transfers messages, thereby
The change in shape is due to
formation of pseudopodia which
facilitates movement and help
in capturing food.
Fig. 8.4 : (a) Spherical red blood cells
of humans, (b) Spindle
shaped muscle cells,
(c) Long branched nerve cell
(a)
(b)
(c)
What advantage does
amoeba derive by
changing shape?
2022-23
Page 4


CELL — STRUCTURE AND FUNCTIONS
CELL — STRUCTURE AND FUNCTIONS
Y
ou have already learnt that things
around us are either living or
non-living. Further, you may
recall that all living  organisms carry out
certain basic functions. Can you list
these functions?
Different sets of organs perform the
various functions you have listed. In this
chapter, you shall learn about the basic
structural unit of an organ, which is the
cell. Cells may be compared to bricks.
Bricks are assembled to make a building.
Similarly, cells are assembled to make
the body of every organism.
8.1 Discovery of the Cell
Robert Hooke in 1665 observed slices of
cork under a simple magnifying device.
Cork is a part of the bark of a
tree.  He took thin slices of cork and
observed them under a microscope. He
noticed partitioned boxes or
compartments in the cork slice (Fig. 8.1).
These boxes appeared like a honey-
comb. He also noticed that one box was
separated from the other by a wall or
partition. Hooke coined the term ‘cell’
for each box. What Hooke observed as
boxes or cells in the cork were actually
dead cells.
Cells of living organisms could be
observed only after the discovery of
improved microscopes. Very little was
known about the cell for the next 150
years after Robert Hooke’s observations.
Today, we know a lot about cell
structure and its functions because of
improved microscopes having high
magnification.
8.2 The Cell
Both, bricks in a building and cells in
the living organisms, are basic
structural units [Fig. 8.2(a), (b)]. The
buildings, though built of similar bricks,
have different designs, shapes and sizes.
Similarly, in the living world, organisms
differ from one another but all are made
up of cells. Cells in the living organisms
are complex living structures unlike
non-living bricks.
Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Cork cells as observed by
Robert Hooke
A hen’s egg can be seen
easily. Is it a cell or a
group of cells?
2022-23
The egg of a hen represents a single
cell and is big enough to be seen by the
unaided eye.
8.3 Organisms show Variety
in Cell Number, Shape and
Size
How do scientists observe and study the
living cells? They use microscopes which
magnify objects. Stains (dyes) are used
to colour parts of the cell to study the
detailed structure.
There are millions of living
organisms. They are of different shapes
and sizes. Their organs also vary in
shape, size and number of cells. Let us
study about some of them.
Number of Cells
Can you guess the number of cells in a
tall tree or in a huge animal like the
elephant? The number runs into
billions and trillions. Human body has
trillions of cells which vary in shapes
and sizes. Different groups of cells
perform a variety of functions.
Organisms made of more than one
cell are called multicellular (multi :
many; cellular : cell) organisms. The
number of cells being less in smaller
organisms does not, in any way, affect
the functioning of the organisms. You
will be surprised to know that an
organism with billions of cells begins
life as a single cell which is the fertilised
egg. The fertilised egg cell multiplies and
the number of cells increase as
development proceeds.
Look at Fig 8.3 (a) and (b). Both
organisms are made up of a single cell.
The single-celled organisms are called
unicellular (uni : one; cellular : cell)
Fig. 8.2 : Brick wall and onion peel
(a) Brick wall (b) Onion peel
A billion is a thousand million. A
trillion is a thousand billion.
Fig. 8.3 : (a) Amoeba (b) Paramecium
(a)
(b)
CELL — STRUCTURE AND FUNCTIONS 91
2022-23
SCIENCE 92
organisms. A single-celled organism
performs all the necessary functions
that multicellular organisms perform.
A single-celled organism, like
amoeba, captures and digests food,
respires, excretes, grows and
reproduces. Similar functions in
multicellular organisms are carried out
by groups of specialised cells forming
different tissues. Tissues, in turn, form
organs.
Activity 8.1
The teacher may show a permanent
slide of amoeba and paramecium
under a microscope. Alternatively,
the teacher can collect pond water
and show these organisms by
preparing the slides.
Shape of Cells
Refer to Fig. 8.3 (a). How do you define
the shape of amoeba in the figure? You
may say that the shape appears
irregular. Infact, amoeba has no
definite shape, unlike other organisms.
It keeps on changing its shape.
Observe the projections of varying
lengths protruding out of its
body. These are called pseudopodia
(pseudo : false; podia : feet), as you
learnt in Class VII. These projections
appear and disappear as amoeba
moves or feeds.
A white blood cell (WBC) in human
blood is another example of a single cell
which can change its shape. But while
WBC is a cell, amoeba is a full fledged
organism capable of independent
existence.
What shape would you expect in
organisms with millions of cells? Fig.
8.4 (a, b, c) shows different cells such
as blood, muscle and nerve of human
beings. The different shapes are related
to their specific functions.
Generally, cells are round, spherical
or elongated [Fig. 8.4(a)]. Some cells are
long and pointed at both ends. They
exhibit a spindle shape [Fig. 8.4(b)].
Cells sometimes are quite long. Some
are branched like the nerve cell or a
neuron [Fig. 8.4(c)]. The nerve cell
receives and transfers messages, thereby
The change in shape is due to
formation of pseudopodia which
facilitates movement and help
in capturing food.
Fig. 8.4 : (a) Spherical red blood cells
of humans, (b) Spindle
shaped muscle cells,
(c) Long branched nerve cell
(a)
(b)
(c)
What advantage does
amoeba derive by
changing shape?
2022-23
CELL — STRUCTURE AND FUNCTIONS 93
the elephant be much bigger than those
in a rat. The size of the cell is related to
its function. For example, nerve cells,
both in the elephant and rat, are long
and branched. They perform the same
function, that of transferring messages.
8.4 Cell Structure and
Function
You have learnt that each living
organism has many organs. You have
studied in Class VII about the digestive
organs which together constitute the
digestive system. Each organ in the
system performs different functions
such as  digestion, assimilation and
absorption. Similarly, different organs
of a plant perform specific/specialised
functions. For example, roots help in the
absorption of water and minerals.
Leaves, as you have learnt in Class VII,
are responsible for synthesis of food.
Each organ is further made up of
smaller parts called tissues. A tissue is
a group of similar cells performing  a
specific function.
Paheli realised that an organ is made
up of tissues which in turn, are made
up of cells. The cell in a living organism
is the basic structural unit.
8.5 Parts of the Cell
Cell Membrane
The basic components of a cell are cell
membrane, cytoplasm and nucleus
(Fig. 8.7). The cytoplasm and nucleus
are enclosed within the cell membrane,
also called the plasma membrane. The
membrane separates cells from one
another and also the cell from the
surrounding medium. The plasma
membrane is porous and allows the
helping to control and coordinate the
working of different parts of the body.
Can you guess, which part of the cell
gives it shape? Components of the cell
are enclosed in a membrane. This
membrane provides shape to the cells
of plants and animals. Cell wall is an
additional covering over the cell
membrane in plant cells.  It gives shape
and rigidity to these cells (Fig. 8.7).
Bacterial cell also has a cell wall.
Size of Cells
The size of cells in living organisms may
be as small as a millionth of a metre
(micrometre or micron) or may be as large
as a few centimetres. However, most of the
cells are microscopic in size and are not
visible to the unaided eye. They need to
be enlarged or magnified by a microscope.
The smallest cell is 0.1 to 0.5 micrometre
in bacteria.  The largest cell measuring
170 mm ×130 mm, is the egg of an ostrich.
Activity 8.2
Boil a hen’s egg. Remove the shell.
What do you observe? A white
material surrounds the yellow part.
White material is albumin which
solidifies on boiling. The yellow part
is yolk. It is part of the  single cell.
You can observe this single cell
without any magnifying device.
Are the cells in an
elephant larger than the
cells in a rat?
The size of the cells has no relation
with the size of the body of the animal or
plant. It is not necessary that the cells in
2022-23
Page 5


CELL — STRUCTURE AND FUNCTIONS
CELL — STRUCTURE AND FUNCTIONS
Y
ou have already learnt that things
around us are either living or
non-living. Further, you may
recall that all living  organisms carry out
certain basic functions. Can you list
these functions?
Different sets of organs perform the
various functions you have listed. In this
chapter, you shall learn about the basic
structural unit of an organ, which is the
cell. Cells may be compared to bricks.
Bricks are assembled to make a building.
Similarly, cells are assembled to make
the body of every organism.
8.1 Discovery of the Cell
Robert Hooke in 1665 observed slices of
cork under a simple magnifying device.
Cork is a part of the bark of a
tree.  He took thin slices of cork and
observed them under a microscope. He
noticed partitioned boxes or
compartments in the cork slice (Fig. 8.1).
These boxes appeared like a honey-
comb. He also noticed that one box was
separated from the other by a wall or
partition. Hooke coined the term ‘cell’
for each box. What Hooke observed as
boxes or cells in the cork were actually
dead cells.
Cells of living organisms could be
observed only after the discovery of
improved microscopes. Very little was
known about the cell for the next 150
years after Robert Hooke’s observations.
Today, we know a lot about cell
structure and its functions because of
improved microscopes having high
magnification.
8.2 The Cell
Both, bricks in a building and cells in
the living organisms, are basic
structural units [Fig. 8.2(a), (b)]. The
buildings, though built of similar bricks,
have different designs, shapes and sizes.
Similarly, in the living world, organisms
differ from one another but all are made
up of cells. Cells in the living organisms
are complex living structures unlike
non-living bricks.
Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Fig. 8.1: Cork cells as observed by
Robert Hooke
A hen’s egg can be seen
easily. Is it a cell or a
group of cells?
2022-23
The egg of a hen represents a single
cell and is big enough to be seen by the
unaided eye.
8.3 Organisms show Variety
in Cell Number, Shape and
Size
How do scientists observe and study the
living cells? They use microscopes which
magnify objects. Stains (dyes) are used
to colour parts of the cell to study the
detailed structure.
There are millions of living
organisms. They are of different shapes
and sizes. Their organs also vary in
shape, size and number of cells. Let us
study about some of them.
Number of Cells
Can you guess the number of cells in a
tall tree or in a huge animal like the
elephant? The number runs into
billions and trillions. Human body has
trillions of cells which vary in shapes
and sizes. Different groups of cells
perform a variety of functions.
Organisms made of more than one
cell are called multicellular (multi :
many; cellular : cell) organisms. The
number of cells being less in smaller
organisms does not, in any way, affect
the functioning of the organisms. You
will be surprised to know that an
organism with billions of cells begins
life as a single cell which is the fertilised
egg. The fertilised egg cell multiplies and
the number of cells increase as
development proceeds.
Look at Fig 8.3 (a) and (b). Both
organisms are made up of a single cell.
The single-celled organisms are called
unicellular (uni : one; cellular : cell)
Fig. 8.2 : Brick wall and onion peel
(a) Brick wall (b) Onion peel
A billion is a thousand million. A
trillion is a thousand billion.
Fig. 8.3 : (a) Amoeba (b) Paramecium
(a)
(b)
CELL — STRUCTURE AND FUNCTIONS 91
2022-23
SCIENCE 92
organisms. A single-celled organism
performs all the necessary functions
that multicellular organisms perform.
A single-celled organism, like
amoeba, captures and digests food,
respires, excretes, grows and
reproduces. Similar functions in
multicellular organisms are carried out
by groups of specialised cells forming
different tissues. Tissues, in turn, form
organs.
Activity 8.1
The teacher may show a permanent
slide of amoeba and paramecium
under a microscope. Alternatively,
the teacher can collect pond water
and show these organisms by
preparing the slides.
Shape of Cells
Refer to Fig. 8.3 (a). How do you define
the shape of amoeba in the figure? You
may say that the shape appears
irregular. Infact, amoeba has no
definite shape, unlike other organisms.
It keeps on changing its shape.
Observe the projections of varying
lengths protruding out of its
body. These are called pseudopodia
(pseudo : false; podia : feet), as you
learnt in Class VII. These projections
appear and disappear as amoeba
moves or feeds.
A white blood cell (WBC) in human
blood is another example of a single cell
which can change its shape. But while
WBC is a cell, amoeba is a full fledged
organism capable of independent
existence.
What shape would you expect in
organisms with millions of cells? Fig.
8.4 (a, b, c) shows different cells such
as blood, muscle and nerve of human
beings. The different shapes are related
to their specific functions.
Generally, cells are round, spherical
or elongated [Fig. 8.4(a)]. Some cells are
long and pointed at both ends. They
exhibit a spindle shape [Fig. 8.4(b)].
Cells sometimes are quite long. Some
are branched like the nerve cell or a
neuron [Fig. 8.4(c)]. The nerve cell
receives and transfers messages, thereby
The change in shape is due to
formation of pseudopodia which
facilitates movement and help
in capturing food.
Fig. 8.4 : (a) Spherical red blood cells
of humans, (b) Spindle
shaped muscle cells,
(c) Long branched nerve cell
(a)
(b)
(c)
What advantage does
amoeba derive by
changing shape?
2022-23
CELL — STRUCTURE AND FUNCTIONS 93
the elephant be much bigger than those
in a rat. The size of the cell is related to
its function. For example, nerve cells,
both in the elephant and rat, are long
and branched. They perform the same
function, that of transferring messages.
8.4 Cell Structure and
Function
You have learnt that each living
organism has many organs. You have
studied in Class VII about the digestive
organs which together constitute the
digestive system. Each organ in the
system performs different functions
such as  digestion, assimilation and
absorption. Similarly, different organs
of a plant perform specific/specialised
functions. For example, roots help in the
absorption of water and minerals.
Leaves, as you have learnt in Class VII,
are responsible for synthesis of food.
Each organ is further made up of
smaller parts called tissues. A tissue is
a group of similar cells performing  a
specific function.
Paheli realised that an organ is made
up of tissues which in turn, are made
up of cells. The cell in a living organism
is the basic structural unit.
8.5 Parts of the Cell
Cell Membrane
The basic components of a cell are cell
membrane, cytoplasm and nucleus
(Fig. 8.7). The cytoplasm and nucleus
are enclosed within the cell membrane,
also called the plasma membrane. The
membrane separates cells from one
another and also the cell from the
surrounding medium. The plasma
membrane is porous and allows the
helping to control and coordinate the
working of different parts of the body.
Can you guess, which part of the cell
gives it shape? Components of the cell
are enclosed in a membrane. This
membrane provides shape to the cells
of plants and animals. Cell wall is an
additional covering over the cell
membrane in plant cells.  It gives shape
and rigidity to these cells (Fig. 8.7).
Bacterial cell also has a cell wall.
Size of Cells
The size of cells in living organisms may
be as small as a millionth of a metre
(micrometre or micron) or may be as large
as a few centimetres. However, most of the
cells are microscopic in size and are not
visible to the unaided eye. They need to
be enlarged or magnified by a microscope.
The smallest cell is 0.1 to 0.5 micrometre
in bacteria.  The largest cell measuring
170 mm ×130 mm, is the egg of an ostrich.
Activity 8.2
Boil a hen’s egg. Remove the shell.
What do you observe? A white
material surrounds the yellow part.
White material is albumin which
solidifies on boiling. The yellow part
is yolk. It is part of the  single cell.
You can observe this single cell
without any magnifying device.
Are the cells in an
elephant larger than the
cells in a rat?
The size of the cells has no relation
with the size of the body of the animal or
plant. It is not necessary that the cells in
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SCIENCE 94
movement of substances or materials
both inward and outward.
Activity 8.3
In order to observe the basic
components of the cell, take an
onion bulb. Remove the dry pink
coverings (peels). You can easily
separate these from the fleshy white
layers of the bulb with the help of
forceps or even with your hand. You
can also break the onion bulb and
separate out thin  layers. Place a
small piece of the thin onion peel
in a drop of water on a glass slide.
The thin layer can be cut into
smaller pieces with the help of a
blade or forceps. Add a drop of
methylene blue solution to the layer
and place a coverslip on it. While
placing the coverslip ensure that
there are no air bubbles under the
coverslip. Observe the slide under
the microscope. Draw and label. You
may compare it with Fig. 8.5.
between the nucleus and the cell
membrane is called cytoplasm.
I want to know why plant
cells need cell walls?
You have learnt earlier that the cell
membrane gives shape to the cell. In
addition to the cell membrane, there is
an outer thick layer in cells of plants
called cell wall. This additional layer
surrounding the cell membrane is
required by plants for protection. Plant
cells need protection against variations
in temperature, high wind speed,
atmospheric moisture etc. They
are exposed to these variations
because they cannot move. Cells can
be observed in the leaf peel of
Tradescantia, Elodea or Rhoeo. You
can prepare a slide as in the case
of onion.
Paheli asks Boojho if he can also
observe animal cells.
Activity 8.4
Take a clean tooth pick, or a
matchstick with the tip broken.
Scrape inside of your cheek without
hurting it. Place it in a drop of water
on a glass slide. Add a drop of iodine
and place a coverslip over it.
Alternatively, add 1-2 drops of
methylene blue solution. Observe it
under the microscope. You may
notice several cells in the scraped
material (Fig. 8.6). You can identify
the cell membrane, the cytoplasm
and nucleus. A cell wall is absent
in animal cells.
The boundary of the onion cell is the
cell membrane covered by another thick
covering called the cell wall. The central
dense round body in the centre is called
the nucleus. The jelly-like substance
Fig. 8.5 : Cells observed in an onion peel
nucleus
cytoplasm
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