NCERT Textbook: Cell- Fundamental Unit of Life | General Science(Prelims) by IRS Divey Sethi - UPSC PDF Download

 Page 1


While examining a thin slice of cork, Robert
Hooke saw that the cork resembled the
structure of a honeycomb consisting of many
little compartments. Cork is  a substance
which comes from the bark of a tree. This
was in the year 1665 when Hooke made this
chance observation through a self-designed
microscope. Robert Hooke called these boxes
cells. Cell is a Latin word for ‘a little room’.
This may seem to be a very small and
insignificant incident but it is very important
in the history of science. This was the very
first time that someone had observed that
living things appear to consist of separate
units. The use of the word ‘cell’ to describe
these units is used till this day in biology.
Let us find out about cells.
5.1 What are Living Organisms
Made Up of?
Activity ______________5.1
? Let us take a small piece from an onion
bulb. With the help of a pair of forceps,
we can peel off the skin (called
epidermis) from the concave side (inner
layer) of the onion. This layer can be
put immediately in a watch-glass
containing water. This will prevent the
peel from getting folded or getting dry.
What do we do with this peel?
? Let us take a glass slide, put a drop of
water on it and transfer a small piece
of the peel from the watch glass to the
slide. Make sure that the peel is
perfectly flat on the slide. A thin camel
hair paintbrush might be necessary to
help transfer the peel. Now we put a
drop of iodine solution on this piece
followed by a cover slip. Take care to
avoid air bubbles while putting the
cover slip with the help of a mounting
needle. Ask your teacher for help. We
have prepared a temporary mount of
onion peel. We can observe this slide
under low power followed by high
powers of a compound microscope.
Fig. 5.1: Compound microscope
What do we observe as we look through
the lens? Can we draw the structures that
we are able to see through the microscope,
on an observation sheet? Does it look like
Fig. 5.2?
Eyepiece
Coarse adjustment
Fine adjustment
Arm
Objective lens
Stage
Swivel
Mirror
Base
Body tube
Clip
 Microscope slide
Condenser
Fig. 5.2: Cells of an onion peel
5 5
5 5 5
T T T T THE HE HE HE HE F F F F FUNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL U U U U UNIT NIT NIT NIT NIT     OF OF OF OF OF L L L L LIFE IFE IFE IFE IFE
Chapter
Page 2


While examining a thin slice of cork, Robert
Hooke saw that the cork resembled the
structure of a honeycomb consisting of many
little compartments. Cork is  a substance
which comes from the bark of a tree. This
was in the year 1665 when Hooke made this
chance observation through a self-designed
microscope. Robert Hooke called these boxes
cells. Cell is a Latin word for ‘a little room’.
This may seem to be a very small and
insignificant incident but it is very important
in the history of science. This was the very
first time that someone had observed that
living things appear to consist of separate
units. The use of the word ‘cell’ to describe
these units is used till this day in biology.
Let us find out about cells.
5.1 What are Living Organisms
Made Up of?
Activity ______________5.1
? Let us take a small piece from an onion
bulb. With the help of a pair of forceps,
we can peel off the skin (called
epidermis) from the concave side (inner
layer) of the onion. This layer can be
put immediately in a watch-glass
containing water. This will prevent the
peel from getting folded or getting dry.
What do we do with this peel?
? Let us take a glass slide, put a drop of
water on it and transfer a small piece
of the peel from the watch glass to the
slide. Make sure that the peel is
perfectly flat on the slide. A thin camel
hair paintbrush might be necessary to
help transfer the peel. Now we put a
drop of iodine solution on this piece
followed by a cover slip. Take care to
avoid air bubbles while putting the
cover slip with the help of a mounting
needle. Ask your teacher for help. We
have prepared a temporary mount of
onion peel. We can observe this slide
under low power followed by high
powers of a compound microscope.
Fig. 5.1: Compound microscope
What do we observe as we look through
the lens? Can we draw the structures that
we are able to see through the microscope,
on an observation sheet? Does it look like
Fig. 5.2?
Eyepiece
Coarse adjustment
Fine adjustment
Arm
Objective lens
Stage
Swivel
Mirror
Base
Body tube
Clip
 Microscope slide
Condenser
Fig. 5.2: Cells of an onion peel
5 5
5 5 5
T T T T THE HE HE HE HE F F F F FUNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL U U U U UNIT NIT NIT NIT NIT     OF OF OF OF OF L L L L LIFE IFE IFE IFE IFE
Chapter
SCIENCE 58
Chlamydomonas, Paramoecium and bacteria.
These organisms are called unicellular
organisms (uni = single). On the other hand,
many cells group together in a single body
and assume different functions in it to form
various body parts in multicellular organisms
(multi = many) such as some fungi, plants
and animals. Can we find out names of some
more unicellular organisms?
Every multi-cellular organism has come
from a single cell. How? Cells divide to
produce cells of their own kind. All cells thus
come from pre-existing cells.
Activity ______________5.2
? We can try preparing temporary
mounts of leaf peels, tip of roots of
onion or even peels of onions of different
sizes.
? After performing the above activity, let
us see what the answers to the following
questions would be:
(a) Do all cells look alike in terms of
shape and size?
(b) Do all cells look alike in structure?
(c) Could we find differences among
cells from different parts of a plant
body?
(d) What similarities could we find?
Some organisms can also have cells of
different kinds. Look at the following picture.
It depicts some cells from the human body.
Nerve Cell
Fat cell
Sperm
Bone
cell
Smooth
muscle
cell
Blood
cells
Ovum
Fig. 5.3: Various cells from the human body
More to  know
We can try preparing temporary mounts
of peels of onions of different sizes. What do
we observe? Do we see similar structures or
different structures?
What are these structures?
These structures look similar to each other.
Together they form a big structure like an
onion bulb! We find from this activity that
onion bulbs of different sizes have similar
small structures visible under a microscope.
The cells of the onion peel will all look the
same, regardless of the size of the onion they
came from.
These small structures that we see are
the basic building units of the onion bulb.
These structures are called cells. Not only
onions, but all organisms that we observe
around are made up of cells. However, there
are also single cells that live on their own.
Cells were first discovered by
Robert Hooke  in 1665. He observed
the cells in a cork slice with the help
of a primitive microscope.
Leeuwenhoek (1674), with the
improved microscope, discovered the
free living cells in pond water for the
first time.  It was Robert Brown in
1831 who discovered the nucleus in
the cell.  Purkinje in 1839 coined the
term ‘protoplasm’ for the fluid
substance of the cell. The cell theory,
that all the plants and animals are
composed of cells and that the cell is
the basic unit of life, was presented
by two biologists, Schleiden (1838)
and  Schwann (1839). The cell theory
was further expanded by Virchow
(1855) by suggesting that all cells
arise  from pre-existing cells.  With the
discovery of the electron microscope
in 1940, it was possible to observe and
understand the complex structure of
the cell and its various organelles.
The invention of magnifying lenses led to
the discovery of the microscopic world. It is
now known that a single cell may constitute
a whole organism as in Amoeba,
Page 3


While examining a thin slice of cork, Robert
Hooke saw that the cork resembled the
structure of a honeycomb consisting of many
little compartments. Cork is  a substance
which comes from the bark of a tree. This
was in the year 1665 when Hooke made this
chance observation through a self-designed
microscope. Robert Hooke called these boxes
cells. Cell is a Latin word for ‘a little room’.
This may seem to be a very small and
insignificant incident but it is very important
in the history of science. This was the very
first time that someone had observed that
living things appear to consist of separate
units. The use of the word ‘cell’ to describe
these units is used till this day in biology.
Let us find out about cells.
5.1 What are Living Organisms
Made Up of?
Activity ______________5.1
? Let us take a small piece from an onion
bulb. With the help of a pair of forceps,
we can peel off the skin (called
epidermis) from the concave side (inner
layer) of the onion. This layer can be
put immediately in a watch-glass
containing water. This will prevent the
peel from getting folded or getting dry.
What do we do with this peel?
? Let us take a glass slide, put a drop of
water on it and transfer a small piece
of the peel from the watch glass to the
slide. Make sure that the peel is
perfectly flat on the slide. A thin camel
hair paintbrush might be necessary to
help transfer the peel. Now we put a
drop of iodine solution on this piece
followed by a cover slip. Take care to
avoid air bubbles while putting the
cover slip with the help of a mounting
needle. Ask your teacher for help. We
have prepared a temporary mount of
onion peel. We can observe this slide
under low power followed by high
powers of a compound microscope.
Fig. 5.1: Compound microscope
What do we observe as we look through
the lens? Can we draw the structures that
we are able to see through the microscope,
on an observation sheet? Does it look like
Fig. 5.2?
Eyepiece
Coarse adjustment
Fine adjustment
Arm
Objective lens
Stage
Swivel
Mirror
Base
Body tube
Clip
 Microscope slide
Condenser
Fig. 5.2: Cells of an onion peel
5 5
5 5 5
T T T T THE HE HE HE HE F F F F FUNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL U U U U UNIT NIT NIT NIT NIT     OF OF OF OF OF L L L L LIFE IFE IFE IFE IFE
Chapter
SCIENCE 58
Chlamydomonas, Paramoecium and bacteria.
These organisms are called unicellular
organisms (uni = single). On the other hand,
many cells group together in a single body
and assume different functions in it to form
various body parts in multicellular organisms
(multi = many) such as some fungi, plants
and animals. Can we find out names of some
more unicellular organisms?
Every multi-cellular organism has come
from a single cell. How? Cells divide to
produce cells of their own kind. All cells thus
come from pre-existing cells.
Activity ______________5.2
? We can try preparing temporary
mounts of leaf peels, tip of roots of
onion or even peels of onions of different
sizes.
? After performing the above activity, let
us see what the answers to the following
questions would be:
(a) Do all cells look alike in terms of
shape and size?
(b) Do all cells look alike in structure?
(c) Could we find differences among
cells from different parts of a plant
body?
(d) What similarities could we find?
Some organisms can also have cells of
different kinds. Look at the following picture.
It depicts some cells from the human body.
Nerve Cell
Fat cell
Sperm
Bone
cell
Smooth
muscle
cell
Blood
cells
Ovum
Fig. 5.3: Various cells from the human body
More to  know
We can try preparing temporary mounts
of peels of onions of different sizes. What do
we observe? Do we see similar structures or
different structures?
What are these structures?
These structures look similar to each other.
Together they form a big structure like an
onion bulb! We find from this activity that
onion bulbs of different sizes have similar
small structures visible under a microscope.
The cells of the onion peel will all look the
same, regardless of the size of the onion they
came from.
These small structures that we see are
the basic building units of the onion bulb.
These structures are called cells. Not only
onions, but all organisms that we observe
around are made up of cells. However, there
are also single cells that live on their own.
Cells were first discovered by
Robert Hooke  in 1665. He observed
the cells in a cork slice with the help
of a primitive microscope.
Leeuwenhoek (1674), with the
improved microscope, discovered the
free living cells in pond water for the
first time.  It was Robert Brown in
1831 who discovered the nucleus in
the cell.  Purkinje in 1839 coined the
term ‘protoplasm’ for the fluid
substance of the cell. The cell theory,
that all the plants and animals are
composed of cells and that the cell is
the basic unit of life, was presented
by two biologists, Schleiden (1838)
and  Schwann (1839). The cell theory
was further expanded by Virchow
(1855) by suggesting that all cells
arise  from pre-existing cells.  With the
discovery of the electron microscope
in 1940, it was possible to observe and
understand the complex structure of
the cell and its various organelles.
The invention of magnifying lenses led to
the discovery of the microscopic world. It is
now known that a single cell may constitute
a whole organism as in Amoeba,
THE FUNDAMENTAL UNIT OF LIFE 59
The shape and size of cells are related to
the specific function they perform. Some cells
like Amoeba have changing shapes. In some
cases the cell shape could be more or less
fixed and peculiar for a particular type of cell;
for example, nerve cells have a typical shape.
Each living cell has the capacity to
perform certain basic functions that are
characteristic of all living forms. How does a
living cell perform these basic functions? We
know that there is a division of labour in
multicellular organisms such as human
beings. This means that different parts of the
human body perform different functions. The
human body has a heart to pump blood, a
stomach to digest food and so on. Similarly,
division of labour is also seen within a single
cell in many cases. In fact, each such cell
has got certain specific components within it
known as cell organelles. Each kind of cell
organelle performs a special function, such
as making new material in the cell, clearing
up the waste material from the cell and so
on. A cell is able to live and perform all its
functions because of these organelles. These
organelles together constitute the basic unit
called the cell. It is interesting that all cells
are found to have the same organelles, no
matter what their function is or what
organism they are found in.
uestions
1. Who discovered cells, and how?
2. Why is the cell called the
structural and functional unit of
life?
5.2 What is a Cell Made Up of?
What is the Structural
Organisation of a Cell?
We saw above that the cell has special
components called organelles. How is a cell
organised?
If we study a cell under a microscope, we
would come across three features in almost
every cell; plasma membrane, nucleus and
cytoplasm. All activities inside the cell and
interactions of the cell with its environment
are possible due to these features. Let us see
how.
5.2.1 PLASMA MEMBRANE OR CELL
MEMBRANE
This is the outermost covering of the cell that
separates the contents of the cell from its
external environment. The plasma membrane
allows or permits the entry and exit of some
materials in and out of the cell. It also
prevents movement of some other materials.
The cell membrane, therefore, is called a
selectively permeable membrane.
How does the movement of substances
take place into the cell? How do substances
move out of the cell?
Some substances like carbon dioxide or
oxygen can move across the cell membrane
by a process called diffusion. We have studied
the process of diffusion in earlier chapters.
We saw that there is spontaneous movement
of a substance from a region of high
concentration to a region where its
concentration is low.
Something similar to this happens in cells
when, for example, some substance like CO
2
(which is cellular waste and requires to be
excreted out by the cell) accumulates in high
concentrations inside the cell. In the cell’s
external environment, the concentration of
CO
2
 is low as compared to that inside the
cell. As soon as there is a difference of
concentration of CO
2
 inside and outside a cell,
CO
2
 moves out of the cell, from a region of
high concentration, to a region of low
concentration outside the cell by the process
of diffusion. Similarly, O
2 
enters the cell by
the process of diffusion when the level or
concentration of O
2
 inside the cell decreases.
Thus, diffusion plays an important role in
gaseous exchange between the cells as well
as the cell and its external environment.
Water also obeys the law of diffusion. The
movement of water molecules through such
a selectively permeable membrane is called
Q
Page 4


While examining a thin slice of cork, Robert
Hooke saw that the cork resembled the
structure of a honeycomb consisting of many
little compartments. Cork is  a substance
which comes from the bark of a tree. This
was in the year 1665 when Hooke made this
chance observation through a self-designed
microscope. Robert Hooke called these boxes
cells. Cell is a Latin word for ‘a little room’.
This may seem to be a very small and
insignificant incident but it is very important
in the history of science. This was the very
first time that someone had observed that
living things appear to consist of separate
units. The use of the word ‘cell’ to describe
these units is used till this day in biology.
Let us find out about cells.
5.1 What are Living Organisms
Made Up of?
Activity ______________5.1
? Let us take a small piece from an onion
bulb. With the help of a pair of forceps,
we can peel off the skin (called
epidermis) from the concave side (inner
layer) of the onion. This layer can be
put immediately in a watch-glass
containing water. This will prevent the
peel from getting folded or getting dry.
What do we do with this peel?
? Let us take a glass slide, put a drop of
water on it and transfer a small piece
of the peel from the watch glass to the
slide. Make sure that the peel is
perfectly flat on the slide. A thin camel
hair paintbrush might be necessary to
help transfer the peel. Now we put a
drop of iodine solution on this piece
followed by a cover slip. Take care to
avoid air bubbles while putting the
cover slip with the help of a mounting
needle. Ask your teacher for help. We
have prepared a temporary mount of
onion peel. We can observe this slide
under low power followed by high
powers of a compound microscope.
Fig. 5.1: Compound microscope
What do we observe as we look through
the lens? Can we draw the structures that
we are able to see through the microscope,
on an observation sheet? Does it look like
Fig. 5.2?
Eyepiece
Coarse adjustment
Fine adjustment
Arm
Objective lens
Stage
Swivel
Mirror
Base
Body tube
Clip
 Microscope slide
Condenser
Fig. 5.2: Cells of an onion peel
5 5
5 5 5
T T T T THE HE HE HE HE F F F F FUNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL U U U U UNIT NIT NIT NIT NIT     OF OF OF OF OF L L L L LIFE IFE IFE IFE IFE
Chapter
SCIENCE 58
Chlamydomonas, Paramoecium and bacteria.
These organisms are called unicellular
organisms (uni = single). On the other hand,
many cells group together in a single body
and assume different functions in it to form
various body parts in multicellular organisms
(multi = many) such as some fungi, plants
and animals. Can we find out names of some
more unicellular organisms?
Every multi-cellular organism has come
from a single cell. How? Cells divide to
produce cells of their own kind. All cells thus
come from pre-existing cells.
Activity ______________5.2
? We can try preparing temporary
mounts of leaf peels, tip of roots of
onion or even peels of onions of different
sizes.
? After performing the above activity, let
us see what the answers to the following
questions would be:
(a) Do all cells look alike in terms of
shape and size?
(b) Do all cells look alike in structure?
(c) Could we find differences among
cells from different parts of a plant
body?
(d) What similarities could we find?
Some organisms can also have cells of
different kinds. Look at the following picture.
It depicts some cells from the human body.
Nerve Cell
Fat cell
Sperm
Bone
cell
Smooth
muscle
cell
Blood
cells
Ovum
Fig. 5.3: Various cells from the human body
More to  know
We can try preparing temporary mounts
of peels of onions of different sizes. What do
we observe? Do we see similar structures or
different structures?
What are these structures?
These structures look similar to each other.
Together they form a big structure like an
onion bulb! We find from this activity that
onion bulbs of different sizes have similar
small structures visible under a microscope.
The cells of the onion peel will all look the
same, regardless of the size of the onion they
came from.
These small structures that we see are
the basic building units of the onion bulb.
These structures are called cells. Not only
onions, but all organisms that we observe
around are made up of cells. However, there
are also single cells that live on their own.
Cells were first discovered by
Robert Hooke  in 1665. He observed
the cells in a cork slice with the help
of a primitive microscope.
Leeuwenhoek (1674), with the
improved microscope, discovered the
free living cells in pond water for the
first time.  It was Robert Brown in
1831 who discovered the nucleus in
the cell.  Purkinje in 1839 coined the
term ‘protoplasm’ for the fluid
substance of the cell. The cell theory,
that all the plants and animals are
composed of cells and that the cell is
the basic unit of life, was presented
by two biologists, Schleiden (1838)
and  Schwann (1839). The cell theory
was further expanded by Virchow
(1855) by suggesting that all cells
arise  from pre-existing cells.  With the
discovery of the electron microscope
in 1940, it was possible to observe and
understand the complex structure of
the cell and its various organelles.
The invention of magnifying lenses led to
the discovery of the microscopic world. It is
now known that a single cell may constitute
a whole organism as in Amoeba,
THE FUNDAMENTAL UNIT OF LIFE 59
The shape and size of cells are related to
the specific function they perform. Some cells
like Amoeba have changing shapes. In some
cases the cell shape could be more or less
fixed and peculiar for a particular type of cell;
for example, nerve cells have a typical shape.
Each living cell has the capacity to
perform certain basic functions that are
characteristic of all living forms. How does a
living cell perform these basic functions? We
know that there is a division of labour in
multicellular organisms such as human
beings. This means that different parts of the
human body perform different functions. The
human body has a heart to pump blood, a
stomach to digest food and so on. Similarly,
division of labour is also seen within a single
cell in many cases. In fact, each such cell
has got certain specific components within it
known as cell organelles. Each kind of cell
organelle performs a special function, such
as making new material in the cell, clearing
up the waste material from the cell and so
on. A cell is able to live and perform all its
functions because of these organelles. These
organelles together constitute the basic unit
called the cell. It is interesting that all cells
are found to have the same organelles, no
matter what their function is or what
organism they are found in.
uestions
1. Who discovered cells, and how?
2. Why is the cell called the
structural and functional unit of
life?
5.2 What is a Cell Made Up of?
What is the Structural
Organisation of a Cell?
We saw above that the cell has special
components called organelles. How is a cell
organised?
If we study a cell under a microscope, we
would come across three features in almost
every cell; plasma membrane, nucleus and
cytoplasm. All activities inside the cell and
interactions of the cell with its environment
are possible due to these features. Let us see
how.
5.2.1 PLASMA MEMBRANE OR CELL
MEMBRANE
This is the outermost covering of the cell that
separates the contents of the cell from its
external environment. The plasma membrane
allows or permits the entry and exit of some
materials in and out of the cell. It also
prevents movement of some other materials.
The cell membrane, therefore, is called a
selectively permeable membrane.
How does the movement of substances
take place into the cell? How do substances
move out of the cell?
Some substances like carbon dioxide or
oxygen can move across the cell membrane
by a process called diffusion. We have studied
the process of diffusion in earlier chapters.
We saw that there is spontaneous movement
of a substance from a region of high
concentration to a region where its
concentration is low.
Something similar to this happens in cells
when, for example, some substance like CO
2
(which is cellular waste and requires to be
excreted out by the cell) accumulates in high
concentrations inside the cell. In the cell’s
external environment, the concentration of
CO
2
 is low as compared to that inside the
cell. As soon as there is a difference of
concentration of CO
2
 inside and outside a cell,
CO
2
 moves out of the cell, from a region of
high concentration, to a region of low
concentration outside the cell by the process
of diffusion. Similarly, O
2 
enters the cell by
the process of diffusion when the level or
concentration of O
2
 inside the cell decreases.
Thus, diffusion plays an important role in
gaseous exchange between the cells as well
as the cell and its external environment.
Water also obeys the law of diffusion. The
movement of water molecules through such
a selectively permeable membrane is called
Q
SCIENCE 60
osmosis. The movement of water across the
plasma membrane is also affected by the
amount of substance dissolved in water.
Thus, osmosis is the passage of water from a
region of high water concentration through a
semi-permeable membrane to a region of low
water concentration.
What will happen if we put an animal cell
or a plant cell into a solution of sugar or salt
in water?
One of the following three things could
happen:
1. If the medium surrounding the cell has
a higher water concentration than the
cell, meaning that the outside solution
is very dilute, the cell will gain water
by osmosis. Such a solution is known
as a hypotonic solution.
Water molecules are free to pass
across the cell membrane in both
directions, but more water will come
into the cell than will leave. The net
(overall) result is that water enters the
cell. The cell is likely to swell up.
2. If the medium has exactly the same
water concentration as the cell, there
will be no net movement of water
across the cell membrane. Such a
solution is known as an isotonic
solution.
Water crosses the cell membrane
in both directions, but the amount
going in is the same as the amount
going out, so there is no overall
movement of water. The cell will stay
the same size.
3. If the medium has a lower
concentration of water than the cell,
meaning that it is a very concentrated
solution, the cell will lose water by
osmosis. Such a solution is known as
a hypertonic solution.
Again, water crosses the cell
membrane in both directions, but this
time more water leaves the cell than
enters it. Therefore the cell will shrink.
Thus, osmosis is a special case of diffusion
through a selectively permeable membrane.
Now let us try out the following activity:
Activity ______________5.3
Osmosis with an egg
(a) Remove the shell of an egg by dissolving
it in dilute hydrochloric acid. The shell
is mostly calcium carbonate. A thin
outer skin now encloses the egg. Put
the egg in pure water and observe after
5 minutes. What do we observe?
The egg swells because water passes
into it by osmosis.
(b) Place a similar de-shelled egg in a
concentrated salt solution and observe
for 5 minutes. The egg shrinks. Why?
Water passes out of the egg solution
into the salt solution because the salt
solution is more concentrated.
We can also try a similar activity with dried
raisins or apricots.
Activity ______________5.4
? Put dried raisins or apricots in plain
water and leave them for some time.
Then place them into a concentrated
solution of sugar or salt. You will
observe the following:
(a) Each gains water and swells when
placed in pure water.
(b) However, when placed in the
concentrated solution it loses water,
and consequently shrinks.
Unicellular freshwater organisms and
most plant cells tend to gain water through
osmosis. Absorption of water by plant roots
is also an example of osmosis.
Thus, diffusion is important in exhange
of gases and water in the life of a cell. In
additions to this, the cell also obtains
nutrition from its environment. Different
molecules move in and out of the cell through
a type of transport requiring use of energy in
the form of ATP.
The plasma membrane is flexible and is
made up of organic molecules called lipids
and proteins. However, we can observe the
structure of the plasma membrane only
through an electron microscope.
The flexibility of the cell membrane also
enables the cell to engulf in food and other
material from its external environment. Such
processes are known as endocytosis. Amoeba
acquires its food through such processes.
Page 5


While examining a thin slice of cork, Robert
Hooke saw that the cork resembled the
structure of a honeycomb consisting of many
little compartments. Cork is  a substance
which comes from the bark of a tree. This
was in the year 1665 when Hooke made this
chance observation through a self-designed
microscope. Robert Hooke called these boxes
cells. Cell is a Latin word for ‘a little room’.
This may seem to be a very small and
insignificant incident but it is very important
in the history of science. This was the very
first time that someone had observed that
living things appear to consist of separate
units. The use of the word ‘cell’ to describe
these units is used till this day in biology.
Let us find out about cells.
5.1 What are Living Organisms
Made Up of?
Activity ______________5.1
? Let us take a small piece from an onion
bulb. With the help of a pair of forceps,
we can peel off the skin (called
epidermis) from the concave side (inner
layer) of the onion. This layer can be
put immediately in a watch-glass
containing water. This will prevent the
peel from getting folded or getting dry.
What do we do with this peel?
? Let us take a glass slide, put a drop of
water on it and transfer a small piece
of the peel from the watch glass to the
slide. Make sure that the peel is
perfectly flat on the slide. A thin camel
hair paintbrush might be necessary to
help transfer the peel. Now we put a
drop of iodine solution on this piece
followed by a cover slip. Take care to
avoid air bubbles while putting the
cover slip with the help of a mounting
needle. Ask your teacher for help. We
have prepared a temporary mount of
onion peel. We can observe this slide
under low power followed by high
powers of a compound microscope.
Fig. 5.1: Compound microscope
What do we observe as we look through
the lens? Can we draw the structures that
we are able to see through the microscope,
on an observation sheet? Does it look like
Fig. 5.2?
Eyepiece
Coarse adjustment
Fine adjustment
Arm
Objective lens
Stage
Swivel
Mirror
Base
Body tube
Clip
 Microscope slide
Condenser
Fig. 5.2: Cells of an onion peel
5 5
5 5 5
T T T T THE HE HE HE HE F F F F FUNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL UNDAMENTAL U U U U UNIT NIT NIT NIT NIT     OF OF OF OF OF L L L L LIFE IFE IFE IFE IFE
Chapter
SCIENCE 58
Chlamydomonas, Paramoecium and bacteria.
These organisms are called unicellular
organisms (uni = single). On the other hand,
many cells group together in a single body
and assume different functions in it to form
various body parts in multicellular organisms
(multi = many) such as some fungi, plants
and animals. Can we find out names of some
more unicellular organisms?
Every multi-cellular organism has come
from a single cell. How? Cells divide to
produce cells of their own kind. All cells thus
come from pre-existing cells.
Activity ______________5.2
? We can try preparing temporary
mounts of leaf peels, tip of roots of
onion or even peels of onions of different
sizes.
? After performing the above activity, let
us see what the answers to the following
questions would be:
(a) Do all cells look alike in terms of
shape and size?
(b) Do all cells look alike in structure?
(c) Could we find differences among
cells from different parts of a plant
body?
(d) What similarities could we find?
Some organisms can also have cells of
different kinds. Look at the following picture.
It depicts some cells from the human body.
Nerve Cell
Fat cell
Sperm
Bone
cell
Smooth
muscle
cell
Blood
cells
Ovum
Fig. 5.3: Various cells from the human body
More to  know
We can try preparing temporary mounts
of peels of onions of different sizes. What do
we observe? Do we see similar structures or
different structures?
What are these structures?
These structures look similar to each other.
Together they form a big structure like an
onion bulb! We find from this activity that
onion bulbs of different sizes have similar
small structures visible under a microscope.
The cells of the onion peel will all look the
same, regardless of the size of the onion they
came from.
These small structures that we see are
the basic building units of the onion bulb.
These structures are called cells. Not only
onions, but all organisms that we observe
around are made up of cells. However, there
are also single cells that live on their own.
Cells were first discovered by
Robert Hooke  in 1665. He observed
the cells in a cork slice with the help
of a primitive microscope.
Leeuwenhoek (1674), with the
improved microscope, discovered the
free living cells in pond water for the
first time.  It was Robert Brown in
1831 who discovered the nucleus in
the cell.  Purkinje in 1839 coined the
term ‘protoplasm’ for the fluid
substance of the cell. The cell theory,
that all the plants and animals are
composed of cells and that the cell is
the basic unit of life, was presented
by two biologists, Schleiden (1838)
and  Schwann (1839). The cell theory
was further expanded by Virchow
(1855) by suggesting that all cells
arise  from pre-existing cells.  With the
discovery of the electron microscope
in 1940, it was possible to observe and
understand the complex structure of
the cell and its various organelles.
The invention of magnifying lenses led to
the discovery of the microscopic world. It is
now known that a single cell may constitute
a whole organism as in Amoeba,
THE FUNDAMENTAL UNIT OF LIFE 59
The shape and size of cells are related to
the specific function they perform. Some cells
like Amoeba have changing shapes. In some
cases the cell shape could be more or less
fixed and peculiar for a particular type of cell;
for example, nerve cells have a typical shape.
Each living cell has the capacity to
perform certain basic functions that are
characteristic of all living forms. How does a
living cell perform these basic functions? We
know that there is a division of labour in
multicellular organisms such as human
beings. This means that different parts of the
human body perform different functions. The
human body has a heart to pump blood, a
stomach to digest food and so on. Similarly,
division of labour is also seen within a single
cell in many cases. In fact, each such cell
has got certain specific components within it
known as cell organelles. Each kind of cell
organelle performs a special function, such
as making new material in the cell, clearing
up the waste material from the cell and so
on. A cell is able to live and perform all its
functions because of these organelles. These
organelles together constitute the basic unit
called the cell. It is interesting that all cells
are found to have the same organelles, no
matter what their function is or what
organism they are found in.
uestions
1. Who discovered cells, and how?
2. Why is the cell called the
structural and functional unit of
life?
5.2 What is a Cell Made Up of?
What is the Structural
Organisation of a Cell?
We saw above that the cell has special
components called organelles. How is a cell
organised?
If we study a cell under a microscope, we
would come across three features in almost
every cell; plasma membrane, nucleus and
cytoplasm. All activities inside the cell and
interactions of the cell with its environment
are possible due to these features. Let us see
how.
5.2.1 PLASMA MEMBRANE OR CELL
MEMBRANE
This is the outermost covering of the cell that
separates the contents of the cell from its
external environment. The plasma membrane
allows or permits the entry and exit of some
materials in and out of the cell. It also
prevents movement of some other materials.
The cell membrane, therefore, is called a
selectively permeable membrane.
How does the movement of substances
take place into the cell? How do substances
move out of the cell?
Some substances like carbon dioxide or
oxygen can move across the cell membrane
by a process called diffusion. We have studied
the process of diffusion in earlier chapters.
We saw that there is spontaneous movement
of a substance from a region of high
concentration to a region where its
concentration is low.
Something similar to this happens in cells
when, for example, some substance like CO
2
(which is cellular waste and requires to be
excreted out by the cell) accumulates in high
concentrations inside the cell. In the cell’s
external environment, the concentration of
CO
2
 is low as compared to that inside the
cell. As soon as there is a difference of
concentration of CO
2
 inside and outside a cell,
CO
2
 moves out of the cell, from a region of
high concentration, to a region of low
concentration outside the cell by the process
of diffusion. Similarly, O
2 
enters the cell by
the process of diffusion when the level or
concentration of O
2
 inside the cell decreases.
Thus, diffusion plays an important role in
gaseous exchange between the cells as well
as the cell and its external environment.
Water also obeys the law of diffusion. The
movement of water molecules through such
a selectively permeable membrane is called
Q
SCIENCE 60
osmosis. The movement of water across the
plasma membrane is also affected by the
amount of substance dissolved in water.
Thus, osmosis is the passage of water from a
region of high water concentration through a
semi-permeable membrane to a region of low
water concentration.
What will happen if we put an animal cell
or a plant cell into a solution of sugar or salt
in water?
One of the following three things could
happen:
1. If the medium surrounding the cell has
a higher water concentration than the
cell, meaning that the outside solution
is very dilute, the cell will gain water
by osmosis. Such a solution is known
as a hypotonic solution.
Water molecules are free to pass
across the cell membrane in both
directions, but more water will come
into the cell than will leave. The net
(overall) result is that water enters the
cell. The cell is likely to swell up.
2. If the medium has exactly the same
water concentration as the cell, there
will be no net movement of water
across the cell membrane. Such a
solution is known as an isotonic
solution.
Water crosses the cell membrane
in both directions, but the amount
going in is the same as the amount
going out, so there is no overall
movement of water. The cell will stay
the same size.
3. If the medium has a lower
concentration of water than the cell,
meaning that it is a very concentrated
solution, the cell will lose water by
osmosis. Such a solution is known as
a hypertonic solution.
Again, water crosses the cell
membrane in both directions, but this
time more water leaves the cell than
enters it. Therefore the cell will shrink.
Thus, osmosis is a special case of diffusion
through a selectively permeable membrane.
Now let us try out the following activity:
Activity ______________5.3
Osmosis with an egg
(a) Remove the shell of an egg by dissolving
it in dilute hydrochloric acid. The shell
is mostly calcium carbonate. A thin
outer skin now encloses the egg. Put
the egg in pure water and observe after
5 minutes. What do we observe?
The egg swells because water passes
into it by osmosis.
(b) Place a similar de-shelled egg in a
concentrated salt solution and observe
for 5 minutes. The egg shrinks. Why?
Water passes out of the egg solution
into the salt solution because the salt
solution is more concentrated.
We can also try a similar activity with dried
raisins or apricots.
Activity ______________5.4
? Put dried raisins or apricots in plain
water and leave them for some time.
Then place them into a concentrated
solution of sugar or salt. You will
observe the following:
(a) Each gains water and swells when
placed in pure water.
(b) However, when placed in the
concentrated solution it loses water,
and consequently shrinks.
Unicellular freshwater organisms and
most plant cells tend to gain water through
osmosis. Absorption of water by plant roots
is also an example of osmosis.
Thus, diffusion is important in exhange
of gases and water in the life of a cell. In
additions to this, the cell also obtains
nutrition from its environment. Different
molecules move in and out of the cell through
a type of transport requiring use of energy in
the form of ATP.
The plasma membrane is flexible and is
made up of organic molecules called lipids
and proteins. However, we can observe the
structure of the plasma membrane only
through an electron microscope.
The flexibility of the cell membrane also
enables the cell to engulf in food and other
material from its external environment. Such
processes are known as endocytosis. Amoeba
acquires its food through such processes.
THE FUNDAMENTAL UNIT OF LIFE 61
Activity ______________5.5
? Find out about electron microscopes
from resources in the school library or
through the internet. Discuss it with
your teacher.
uestions
1. How do substances like CO
2
 and
water move in and out of the cell?
Discuss.
2. Why is the plasma membrane
called a selectively permeable
membrane?
5.2.2 CELL WALL
Plant cells, in addition to the plasma
membrane, have another rigid outer covering
called the cell wall. The cell wall lies outside
the plasma membrane. The plant cell wall is
mainly composed of cellulose. Cellulose is a
complex substance and provides structural
strength to plants.
When a living plant cell loses water
through osmosis there is shrinkage or
contraction of the contents of the cell away
from the cell wall. This phenomenon is known
as plasmolysis. We can observe this
phenomenon by performing the following
activity:
Activity ______________5.6
? Mount the peel of a Rheo leaf in water
on a slide and examine cells under the
high power of a microscope. Note the
small green granules, called
chloroplasts. They contain a green
substance called chlorophyll. Put a
strong solution of sugar or salt on the
mounted leaf on the slide. Wait for a
minute and observe under a
microscope. What do we see?
? Now place some Rheo leaves in boiling
water for a few minutes. This kills the
cells. Then mount one leaf on a slide
and observe it under a microscope. Put
a strong solution of sugar or salt on
the mounted leaf on the slide. Wait for
a minute and observe it again. What
do we find? Did plasmolysis occur now?
What do we infer from this activity? It
appears that only living cells, and not dead
cells, are able to absorb water by osmosis.
Cell walls permit the cells of plants, fungi
and bacteria to withstand very dilute
(hypotonic) external media without bursting.
In such media the cells tend to take up water
by osmosis. The cell swells, building up
pressure against the cell wall. The wall exerts
an equal pressure against the swollen cell.
Because of their walls, such cells can
withstand much greater changes in the
surrounding medium than animal cells.
5.2.3 NUCLEUS
Remember the temporary mount of onion peel
we prepared? We had put iodine solution on
the peel. Why? What would we see if we tried
observing the peel without putting the iodine
solution? Try it and see what the difference
is. Further, when we put iodine solution on
the peel, did each cell get evenly coloured?
According to their chemical composition
different regions of cells get coloured
differentially. Some regions appear darker
than other regions. Apart from iodine solution
we could also use safranin solution or
methylene blue solution to stain the cells.
We have observed cells from an onion; let
us now observe cells from our own body.
Activity ______________5.7
? Let us take a glass slide with a drop of
water on it. Using an ice-cream spoon
gently scrape the inside surface of the
cheek. Does any material get stuck on
the spoon? With the help of a needle
we can transfer this material and
spread it evenly on the glass slide kept
ready for this. To colour the material
we can put a drop of methylene blue
solution on it. Now the material is ready
for observation under microscope. Do
not forget to put a cover-slip on it!
? What do we observe? What is the shape
of the cells we see? Draw it on the
observation sheet.
Q