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Basic Biology
Learning Objectives
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Discovery of the cell
Cell theory
Organization of cells in living beings
Variation in cell number, shape and size
Structure of the cell
Differences between plant and animal cells
Cell division and its importance
Just as houses are made up of bricks, similarly all living organisms whether plants, animals or human
beings are made up of cells. Since, bricks constitute the basic structure of the house, so we call them
as the structural unit of the house. Similarly, the cells constitute the basic structure of a living thing.
However, any function performed by an organism is an outcome of the activity of the cells in its body.
That means the cells perform all life functions. Thus, we can say that the cell is the structural as well
as the functional unit of living organisms.
Discovery of the Cell
The discovery of the cell was possible only because of the
invention of the microscope. The first microscope was
developed by Antonie van Leeuwenhoek. It was a simple
microscope which was able to magnify things to 300 times
their original size. He used this microscope to see the
blood cells in the capillaries of the foot-web of a frog and
recognized them as living units of living beings. He also
observed minute single-celled organisms in a drop of water.
Robert Hooke, an English scientist, developed a compound
microscope with two lenses to observe different objects. He
examined a very thin slice of cork under his microscope and
found that it was made up of small box-like structures. He
called them cells. The word ‘cell’ is derived from the Latin
word cellula meaning a small room. Thus, Robert Hooke was
the first one to discover and name cell in 1665.
Fig. 1.1: Simple microscope and
compound microscope
Fig. 1.2: Cork cells
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Cork is the dead bark of a tree. Its cells were once living but now dead. So, Robert Hooke observed
only the walls of the dead cells.
Cell Theory
Later in the nineteenth century, with the advancement of technology and the improvement of
microscopes, more scientists studied about the cells.
A German botanist, Matthias Jakob Schleiden (1838), observed different parts of the plants under
the microscope and found that all of them are made up of cells.
A German zoologist, Theodor Schwann (1839), observed parts of the animal body and noticed that
they were also made up of cells.
A German doctor and biologist, Rudolph Carl Virchow (1858), studied that all cells arise from the preexisting cells. In other words, the existing cells divide to form new cells.
The research and observations of these scientists led to the formulation of the cell theory.
The main points of the cell theory are as follows:
• All living beings are made up of cells.
• Cells are the basic structural and functional unit
of all living beings.
• New cells arise by the division of pre-existing
cells.
Organization of cells in living beings
Fig. 1.3: Cell
We already know that cells in an organism are grouped together to make tissues, organs and organ
systems. An arrangement of different organ systems working together form the organism.
Variation in cell number, shape and size
There is a variety in cell number, shape and size in different organisms.
Number of Cells
Depending on the number of cells, all organisms can be divided into two groups.
1. Unicellular organisms: Organisms which are made up of a single cell are called unicellular
organisms. For example, Amoeba, Paramecium, Euglena, bacteria, etc. In these organisms the
single cell performs all the activities of life like growth, movement, nutrition and reproduction.
2.
Multicellular organisms: Most organisms including all plants and animals are made up of
many cells. They are called multicellular organisms. They are made up of millions and billions
of cells. For example, neem, peepal, rose, fish, frog, lion, etc. In these organisms, cells are
differentiated to perform different functions.
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Shape of Cells
The shape of cells is determined by the functions they perform. Therefore, cells show a lot of variation
in their shape as follows:
•Oval: This shape of the cell is found in the unicellular green alga Chlamydomonas.
•Irregular: The single-celled body of Amoeba is irregular in shape. White blood cells are also
irregular in shape which helps them to engulf and destroy the harmful material in the blood.
•Oblong: Paramecium is slipper-shaped or oblong.
•Elongated: Muscle cells are elongated and contractile
(capable of shrinking or contracting). They can become
longer or shorter and thus help in the movement of
bones.
•Thread-like: Nerve cells are long and thread-like. This
enables them to send messages over long distances
in the body.
Epithelial cells
White blood cell
Leaf cell
Red blood cells
Muscle cell
Sperm cell
• Cubical or rectangular: The cells of the leaf are cubical
or rectangular.
The cells in plants may be round, oval, cubical, rectangular
and polygonal.
Fig. 1.4: Cells of different shapes
Size of Cells
Most cells are microscopic and can be seen only under a powerful microscope. The size of the cells
is measured in micrometres or micron (µ). One micron is equal to one-thousandth of a millimetre.
Cells show a great range in their sizes.
The smallest cells are those of bacteria, which range from 0.2 to 0.5 µ in diameter. The smallest cells
in the human body are the red blood cells.
An ostrich’s egg is the largest cell.
The longest cells are the nerve cells. In an elephant, the nerve cells are as long as 3 m.
Bacterial cell
Egg cell
Fig. 1.5: Cells of different sizes
Nerve cell
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Test Yourself
Write down the contribution of the following scientists.
1. Antonie van Leeuwenhoek
2. Robert Hooke
3. M.J. Schleiden
4. R.C. Virchow
Fill in the blanks.
1. One _______ is one-thousandth of a millimetre.
2. The smallest cells in the human body are the ___________ cells.
3. Muscle cells are _____________ and _____________.
4. ___________ and _____________ are unicellular organisms.
Structure of the Cell
Although cells differ in their shape and size, but their basic structure is same. A cell can be divided
into the following three parts.
1. Cell membrane or plasma membrane
2.Cytoplasm
3. Nucleus
Protoplasm: The nucleus and cytoplasm together make up the protoplasm (proto means first and
plasma means liquid). It is the living substance of the cell.
Cell Membrane
Each cell is bound by a thin, delicate and
flexible covering called the cell membrane
or plasma membrane.
endoplasmic
reticulum
nucleus
cell wall
cell membrane
Golgi apparatus
It allows only certain substances to pass
through it and prevents the entry of other
substances. Thus, it is a semi-permeable
membrane.
Functions
chloroplast
vacuole
mitochondrion
Fig. 1.6: Three-dimensional structure of a cell
• It separates the cell from its surroundings.
• It protects the internal components of the cell.
• It gives a definite shape to the cell.
• It controls the movement of materials in and out of the cell.
In plant cells there is an additional protective layer outside the plasma membrane called the cell wall.
It is non-living and made up of a complex carbohydrate called cellulose. It performs the following
functions.
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• It gives definite shape to the cell.
• It provides rigidity and strength to the cell.
• It limits the entry of only large molecules that may be toxic to the cell.
Brainwork
Cytoplasm
The cytoplasm is a jelly-like, colourless and translucent fluid. It occupies
the space between the cell membrane and the nucleus. A number of small
components or structures known as the cell organelles are present in the
cytoplasm. All life functions take place in the cytoplasm.
Nucleus
Give three differences
between the cell wall
and cell membrane.
Fluid
A liquid or a gas that
can flow and has no
fixed shape
nucleus
The nucleus is a small spherical body present in the cytoplasm. It
is the most important part of the cell. It is surrounded by a double
membrane called the nuclear membrane. Inside the nuclear
membrane is a colourless, dense fluid called nucleoplasm. It contains
one or more spherical bodies called nucleoli (singular–nucleolus).
A network of thread-like structures called chromatin is present in
the nucleoplasm. The chromatin network forms small structures
called chromosomes at the time of cell division. The chromosomes
carry genes which are responsible for transfer of characters from
parents to the offspring. The number of chromosomes is fixed for
a particular organism. Different organisms have different number
of chromosomes.
Functions
• It is called the control centre of the cell. It controls all the
activities that go on within the cell.
• It helps in the transfer of characters from parents to their
offspring.
nucleolus
chromatin
network
Fig. 1.7: Cell showing nucleus
Chromosome numbers of
some organisms
Mosquito
Garden pea
Frog
Mouse
Man
Potato
Pigeon
Gold fish
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14
26
40
46
48
80
100
Activity 1.1
To study onion peel cells
Take an onion. Cut it into pieces. Take a piece and peel off the inner surface
of the onion leaf using forceps. Place it on a glass slide in a drop of water.
Add a drop of iodine solution to it. Then put a drop of glycerine on it and
cover it with a coverslip with the help of forceps. Soak extra glycerine by
a filter paper. Observe the slide under the microscope, first under the low
onion peel cells
power and then under the high power.
You will see that onion peel is made up of small rectangular cells. An outer cell wall is also seen.
Under the high power of microscope you will be able to see a small nucleus in each cell. Draw
what you see in your practical notebook.
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Activity 1.2
To study cheek cells
Rinse your mouth well with water. Gently scrape the inner surface of
your cheek with a clean toothpick. Spread it on a clean slide in a drop
of water. Add a drop of iodine solution to it. Then put a coverslip and
observe the slide under the microscope. First see it under the low power
and then under the high power.
What do you see?
You will see that cheek cells are irregular in shape with a small nucleus
in the centre. Draw them in your practical notebook.
human cheek cells
Organelles
As mentioned earlier, cytoplasm contains many small structures called cell organelles or organelles.
There are many types of organelles in the cell and they perform different functions.
Mitochondria
inner membrane
Mitochondria are small rod-like structures. Their number may
vary from a few hundred to a few lakh per cell. Mitochondria are
bound by two membranes, the outer and the inner. The inner
membrane is folded to form finger-like structures called cristae.
Function
• Cellular respiration takes place inside the mitochondria in
the cell. During this, food is oxidized in the presence of
oxygen to release energy. They are, therefore, called the
‘powerhouse of the cell’.
outer
membrane
cristae
Fig. 1.8: Mitochondrion
Golgi apparatus
The Golgi apparatus consist of stacks of membrane-bound structures
called cisternae and some small vesicles. The vesicles arise from the
cisternae. In plant cells, the Golgi apparatus is smaller, unconnected
and more in number. It is called dictyosome.
Functions
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• It secretes various substances such as hormones, enzymes,
etc.
• It helps in the synthesis of cell wall in plant cells.
vesicles
cisternae
Fig. 1.9: Golgi apparatus
Endoplasmic reticulum
Endoplasmic reticulum is a network of tube-like structures present
in the entire cytoplasm. In some cells it is so big that one end of it
is connected to the cell membrane and the other to the nuclear
membrane. The endoplasmic reticulum is of two types, rough and
smooth. The rough endoplasmic reticulum has ribosomes on its
outer surface whereas the smooth endoplasmic reticulum does not
have ribosomes.
Functions
nuclear
membrane
ribosomes
ribosomes
Fig. 1.10: Endoplasmic reticulum
• It provides internal support to the cell.
• It forms a pathway for transport of material within the cell and often between the cells.
Ribosomes
Ribosomes are small, rounded structures that are present either freely in the cytoplasm or are found
attached to the outer surface of the endoplasmic reticulum.
Function
• They synthesize proteins in the cell.
Lysosomes
Lysosomes are found only in animal cells. They are small vesicles bound by a single membrane.
Functions
• They contain many enzymes which help in the digestion of food material. They engulf foreign
bodies like germs that enter the cell and digest them. Thus they protect the cell from foreign
bodies.
• The digestive enzymes secreted by lysosomes also digest the old and worn-out cell organelles
and sometimes even the cells themselves. Hence they are also called the ‘suicide bags of the
cell’.
Centrosomes
Centrosomes are also found only in animal cells. They are small, non-membranous organelles located
very close to the nucleus. Each centrosome consists of two tiny granules called centrioles.
Function
• They initiate and regulate cell division.
Plastids
Plastids are found only in plant cells. They are surrounded by a double membrane. There are mainly
three types of plastids.
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outer
membrane
1. Chloroplasts: These are also called green plastids because
they contain the green pigment chlorophyll. They are present
in the green parts of the plant. Chlorophyll traps the solar
energy to prepare food for the plants hence, chloroplasts are
also called the ‘kitchen of the cell’.
2.
Chromoplasts: These are coloured plastids. They are found
mostly in fruits and flowers. The yellow, orange, red, etc. colour
of fruits and flowers is due to the presence of chromoplasts.
inner
membrane
Fig. 1.11: Chloroplast
3.
Leucoplasts: These are colourless plastids. They are found in roots and underground stems
like potato and ginger. They store food prepared by the plant in the form of starch, proteins
and fats.
Vacuoles
Vacuoles are sac-like structures containing a fluid called cell sap. They are bound by a single membrane
called tonoplast. Plant cells have large vacuoles whereas animal cells have many small vacuoles.
Some plants have only one large vacuole and some animals do not have a vacuole.
Functions
• They maintain the turgidity of the cell, that is, the cell remains in shape.
• They store food (sugars, proteins, amino acids and minerals), water and wastes.
Cell inclusions
As a result of the biochemical reactions taking place in the plant cells, some substances are formed.
They are found in the cytoplasm or in the vacuoles. These substances are known as cell inclusions.
These may be food products like starch, proteins and oils or waste materials like gums, resins, tannins
and latex.
Differences between Plant and Animal Cells
The basic structure is same in all plant and animal cells. Both have plasma membrane, cytoplasm,
nucleus and cell organelles like mitochondria, Golgi apparatus, endoplasmic reticulum and ribosomes.
However, they show some differences like plant cells have a rigid cell wall outside the plasma
membrane which animal cells do not have. On the other hand, centrosomes and lysosomes which
are present in animal cells are not found in plant cells. The differences between plant and animal
cells are given in the following table.
Differences between a plant cell and an animal cell
Plant cell
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Cell wall made of cellulose is present.
Plastids are present.
Large vacuole is present.
Centrosome is absent.
Lysosomes are absent.
Animal cell
Cell wall is absent.
Plastids are absent.
Vacuoles are absent. If present, they are very small.
Centrosome is present.
Lysosomes are present.
dictyosome
cell wall
plasma
membrane
vacuole
chromatin
nucleus
nuclear envelope
endoplasmic
reticulum
chloroplast
free ribosome
mitochondrion
cell membrane
ribosomes
centrosome
Golgi
apparatus
nucleus
nucleolus
nuclear
membrane
vacuole
rough ER
smooth ER
cytoplasm
lysosome
mitochondrion
Plant cell
Animal cell
Fig. 1.12: Structure of a plant cell and an animal cell
Test Yourself
Give one word for the following.
1. Responsible for transfer of characters from parents to progeny
2. The control centre of the cell
3. The powerhouse of the cell
4. Network of tube-like structures present in cytoplasm
5. Suicide bags of the cell
6. Kitchen of the cell
Cell Division
We have learnt that growth and reproduction are two of the characteristics which all living things
show. Both these characteristics are possible because of division of cells. Cells are small when they
are formed. But they grow and become big. After reaching a certain critical size, they divide to form
two cells. The division of cells to form two new cells is called cell division. The new cells formed in
this way are known as daughter cells and the original cell is known as the parent cell.
chromatid
Importance of Cell Division
Cell division is important for the following reasons:
For growth: Unicellular organisms may grow by enlargement of
their cell but in multicellular organisms growth takes place both
by enlargement and division of cells. Multicellular organisms
start their life with a single cell (fertilized egg). This single cell
divides and redivides and forms the full-fledged organism.
For replacement of old cells: In an adult living organism, there is
always some wear and tear of cells in the course of normal body
functions. These old cells are replaced by new cells through the
parent
cell
daughter
cells
Fig. 1.13: Cell division
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division of their parent cells. For example, in a human body about 20 million red blood cells become
old and die every minute. But they are constantly replaced by new cells through cell division. The skin
cells also keep dividing. However, nerve cells of the brain cannot be replaced once they die.
For repair: Apart from the normal wear and tear of the tissues in the body, there may be accidental
injuries. For example, cut in the skin or fracture in the bone. They are healed by formation of new
cells which fill the gaps and also join the broken ends.
For reproduction: Cell division is also essential for reproduction and birth of young ones.
QUICK Review
1. All living organisms are made up of one or more cells.
2. Cell is the basic structural and functional unit of all living organisms.
3. Cell was discovered by the research and observation of many scientists like Antonie van
Leeuwenhoek, Robert Hooke, Matthias Schleiden, Theodor Schwann and Rudolph Virchow.
Their work also led to the formulation of cell theory.
4. Cells show variation in their number, shape and size.
5. In unicellular organisms, a single cell performs all life functions. In multicellular organisms,
cells are specialized to perform certain functions.
6. Every cell has three basic parts—cell membrane, cytoplasm and nucleus.
7. Cell membrane gives shape to the cell and also controls the movement of substances in and
out of the cell.
8. Cytoplasm is a jelly-like, living substance inside the cell membrane.
9. Nucleus is present in the cytoplasm. It is the control centre of the cell and contains chromosomes
which are responsible for inheritance of characters from one generation to the other.
10. Cytoplasm also contains various cell organelles—mitochondria, Golgi apparatus, endoplasmic
reticulum, ribosomes, plastids, lysosomes and centrosomes.
11. Plastids are of three types—chloroplasts, chromoplasts and leucoplasts.
12. Plant cells are different from animal cells due to the presence of cell wall, plastids and large
vacuoles, and absence of centrosomes and lysosomes.
13. The division of a cell into two new cells is known as cell division.
14. New cells required for growth and repair of organisms are formed by cell division.
EXERCISES
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A. Answer in detail.
1. How were cells discovered? Why was the invention of microscope so important for the discovery
of cells?
2. What are the main points of cell theory?
3. What are the various types of shapes found in the cells?
4.
5.
6.
7.
8.
9.
Describe the structure and function of the nucleus.
Which organelles are called the ‘powerhouse of the cell’? Explain their structure and function.
What are plastids? Describe the structure and functions of different types of plastids.
What are vacuoles and what is their main role in the cells?
What is meant by the term cell division? Why is cell division important?
Draw well-labelled diagrams of plant and animal cells.
B.Write short answers.
1. What are basic structure of life? Why are they called so?
2. Name the three scientists whose work led to the formulation of cell theory.
3. What are ribosomes and what is their main role in the cell?
4. Where are genes located in the cell and what is their importance?
C. Differentiate between the following.
1. Unicellular organisms and multicellular organisms 2. Cell wall and cell membrane
3. Cytoplasm and protoplasm
4. Nucleus and nucleolus
5. Chromatin and chromosomes
6. Centrosomes and lysosomes
7. Chloroplast and chromoplast
8. Rough endoplasmic reticulum and smooth endoplasmic reticulum
D.Write True or False. Correct the false statements.
1. Cell is only the structural unit of life.
2. Discovery of cell was possible due to the discovery of telescope.
3. All cells are bound by a cell wall.
4. Centrosomes are the control centres of the cell.
5. Golgi apparatus helps in the synthesis of cell wall in plant cells.
6. Endoplasmic reticulum is present in a corner in the cytoplasm.
7. Chromoplasts are the kitchen of the cell.
E. Tick the most appropriate answer.
1. Who discovered and named the cells for the first time?
(a) Matthias Jakob Schleiden
(b) Theodor Schwann
(c) Robert Hooke
(d) Rudolph Carl Virchow
2. Size of a cell is measured in
(a) mm.
(b) cm.
(c) µ.
(d) none of these.
3. The longest cells in the body are
(a) egg cells.
(b) sperm cells.
(c) nerve cells.
(d) none of these.
4. Which of these is not present in an animal cell?
(a) Mitochondria
(b) Nucleus
(c) Cell membrane
(d) Plastids
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5. Which organelles are responsible for energy production in the cell?
(a) Mitochondria
(b) Golgi apparatus
(c) Endoplasmic reticulum
(d) Vacuoles
6. The organelle which is known as the suicide bag of the cell is
(a) nucleus. (b) endoplasmic reticulum.
(c) lysosome.
(d) centrosome.
7. The centrosome has
(a) one centriole.
(b) two centrioles.
(c) three centrioles.
(d) no centriole.
8. Which of the following cells cannot be replaced once they die?
(a) Red blood cells
(b) White blood cells
(c) Nerve cells
(d) Skin cells
F. Fill in the blanks.
1. All living organisms are made up of one or more ___________.
2. The word ‘cell’ is derived from the Latin word ________.
3. ________ membrane is semi-permeable.
4. The jelly-like substance present in the cells is called ____________.
5. A network of thread-like structures in the nucleus is called ___________.
6. _________ are located on the chromosomes.
G. Match the columns.
Column A
1. Mitochondria
(a)
2. Endoplasmic reticulum
(b)
3. Golgi apparatus
(c)
4. Nucleus
(d)
5. Chloroplast
(e)
6. Vacuoles
(f)
7. Centrosomes
(g)
8. Leucoplast
(h)
9. Chromoplast
(i)
10. Cell membrane
(j)
Column B
Transport of material
Helps in cell division
Kitchen of the cell
Makes the cell turgid
Storage of starch and proteins
Control centre of the cell
Powerhouse of the cell
Secretion of many types of substances
Controls movement in and out of cell
Responsible for colours of fruits and flowers
Project Activities
1. Visit the biology laboratory in your school. Request the teacher to provide you with the slides
of some organisms including both plants and animals. Observe them under the microscope.
Draw what you see in your notebook.
2. Make a chart or a model of plant and animal cells.
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