SRIMATHI SUNDARAVALLI MEMORIAL SCHOOL

CHENNAI
2020-21
BIOLOGY
THE FUNDAMENTAL UNIT OF LIFE
CLASS: IX
GIST OF THE CHAPTER

The cell is the fundamental structural unit of living organisms. It is also the basic
functional unit of life.

DISCOVERY OF CELLS
Cells were first discovered by Robert Hooke in 1665. While examining a thin slice of cork,
with the help of a primitive microscope, he saw that the cork resembled the structure of a
honeycomb consisting of many little compartments. Cell is a Latin word for „a little room‟.

Robert Hooke, the microscope used by him and the cork cells that he observed

Leeuwenhoek (1674), with the improved microscope, discovered the free-living cells in pond
water for the first time.

Leeuwenhoek and his microscopic observations

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Each cell acquires its structure and the ability to function because of the organisation of its
membrane and organelles in specific ways. The cell thus has a basic structural
organisation. This helps the cells to perform functions like respiration, obtaining nutrition,
and clearing of waste material, or forming new proteins.

UNICELLULAR AND MULTICELLULAR ORGANISMS

All organisms that we observe around are made up of cells. However, there are also single
cells that live on their own. A single cell may constitute a whole organism as in Yeast,
Amoeba, Euglena, Chlamydomonas, Paramoecium and bacteria.

Paramoecium Bacteria

Euglena Amoeba
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.

Fungi and Plants Jellyfish Butterfly

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THE CELL THEORY
Two biologists, Schleiden (1838) and Schwann (1839) proposed the cell theory as follows:
 All the plants and animals are composed of cells.
 The cell is the basic unit of life.
The cell theory was further expanded by Virchow (1855) who suggested that all cells arise
from pre-existing cells.

THE SHAPE AND THE SIZE OF CELLS

The shape and the size of the cells are related to the specific function they perform. Cells
are mostly round, spherical or elongated in shape. Some cells are quite long. Some are
branched, e.g. the neuron. Some cells like Amoeba have changing shapes.

THE STRUCTURAL ORGANISATION OF A CELL

If we study a cell under a microscope, we would come across three features in almost every
cell:
 Plasma membrane
 Nucleus
 Cytoplasm
All the activities inside the cell and the interactions of the cell with its environment are
possible due to these features.

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 STRUCTURE OF A PLANT CELL

STRUCTURE OF AN ANIMAL CELL

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PLASMA MEMBRANE (OR) CELL MEMBRANE

 This is the outermost covering of the cell.

 It 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 the movement of some other materials. Therefore, it is
called a selectively permeable membrane.
 It is flexible and is made up of organic molecules called lipids and proteins.
 The flexibility of the cell membrane also enables the cell to engulf food from its external
environment. This is known as endocytosis. Amoeba acquires its food through
endocytosis.

Transport / Movement of Molecules

The molecules move from the region of higher concentration to the region of lower
concentration by diffusion.

Carbon dioxide, a cellular waste, accumulates in high concentration inside the cells. In the
external environment, the concentration of carbon dioxide is less.
So, carbon dioxide easily diffuses through the cell membrane and moves out of the cell.
Thus, it is eliminated from the cells.

Osmosis is the movement of water from the region of its higher concentration to the region
of its lower concentration, through a selectively permeable membrane, till equilibrium is
reached.

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The given figure shows the process of osmosis. The yellow dotted lines indicate the semi-
permeable membrane. The net movement of water molecules takes place from the region of
higher water concentration (i.e. low solute concentration) to the region of lower water
concentration (i.e. high solute concentration).

 Unicellular freshwater organisms tend to gain water through osmosis.

 Absorption of water by plant roots is also an example of osmosis.
 Only living cells are able to absorb water by osmosis.
There are three types of solutions that can occur in our body based on solute
concentration:
 A hypotonic solution is the one in which the concentration of solutes is greater
inside the cell than outside of it.
 A hypertonic solution is the one where the concentration of solutes is greater
outside the cell than inside it.
 An isotonic solution is the one in which the concentration of solutes is the same
both inside and outside of the cell. For the cells in our body, the ideal solution is an
isotonic solution.

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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:

Some molecules also move in and out of the cell against concentration gradient, using
energy. It is called active transport.

CELL WALL
 The bacterial, plant and fungal 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.
 The fungal cell wall is composed of chitin.
 Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute
(hypotonic) external media without bursting.

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NUCLEUS
When a cell is stained with dyes such as methylene blue and observed under the
microscope, a darkly coloured, spherical or oval, dot-like structure can be seen near the
centre of each cell. This structure is called the nucleus. Robert Brown discovered the
nucleus in 1831.

 The nucleus has a double layered covering called nuclear membrane.

 The nuclear membrane has pores which allow the transfer of material from inside
the nucleus to the cytoplasm.
 The nucleus contains chromosomes, which are visible as rod-shaped structures
only when the cell is about to divide.

 Chromosomes contain the information for the inheritance of characters from

parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.
 Chromosomes are composed of DNA and proteins.
 DNA molecules contain the information necessary for constructing and organising
cells.
 The functional segments of DNA are called „GENES‟.

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  In a cell which is not dividing, this DNA is present as part of chromatin material.
 Chromatin material is visible as entangled mass of thread like structures.
 Whenever the cell is about to divide, the chromatin material gets organised into
chromosomes.
 The nucleus plays a central role in cellular reproduction directing the chemical
activities of the cell.
 Centrioles, found only in animal cells, are present near the nucleus and they help
in cell division.
PROKARYOTES AND EUKARYOTES
 In some organisms like bacteria, the nuclear region of the cell may be poorly defined
due to the absence of a nuclear membrane.
 Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

 Organisms which lack a membrane-bound nucleus are called „prokaryotes‟.

 Organisms that have a membrane-bound nucleus are called „eukaryotes‟.
CYTOPLASM
 The cytoplasm is the fluid content inside the plasma membrane.
 Apart from the nucleus, it contains many specialised cell organelles.

Purkinje, in 1839,
coined the term
‘protoplasm’ for the
fluid substance of
the cell

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CELL ORGANELLES
 Large and complex cells, including cells from multicellular organisms, need a lot of
chemical activities to support their complicated structure and functions. To keep these
activities of different kinds, separate from each other, these cells use membrane-bound
little structures or „organelles‟ within themselves.
 Organelles like endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and
plastids carry out some very crucial functions in cells.
 The prokaryotes lack the cell organelles.

ENDOPLASMIC RETICULUM (ER)

 It is a large network of membrane-bound tubes and sheets.
 It looks like long tubules or round or oblong bags (vesicles).
 There are two types of ER:
 Rough endoplasmic reticulum (RER)
 Smooth endoplasmic reticulum (SER)

 RER looks rough under a microscope because it has particles called ribosomes
attached to its surface.
 The ribosomes are the sites of protein manufacture.
 The manufactured proteins are then sent to various places in the cell depending on
need.
 The SER helps in the manufacture of fat molecules, or lipids, important for cell
function.
Functions of ER
 ER serves as the channel for the transport of materials (especially proteins)
between various regions of the cytoplasm or between the cytoplasm and the nucleus.

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  The proteins and lipids synthesised by the ER help in building the cell membrane.
This process is known as membrane biogenesis.
 Some other proteins and lipids function as enzymes and hormones.
 In the liver cells of the vertebrates, SER helps in detoxifying many poisons and
drugs.
GOLGI APPARATUS / GOLGI BODIES
 The Golgi apparatus, first described by Camillo Golgi, consists of a system of
membrane-bound vesicles (flattened sacs) arranged approximately parallel to each
other in stacks called cisterns.
 These membranes have connections with the membranes of ER.
 The proteins and lipids synthesised near the ER are received, packaged and
dispatched to various targets inside and outside the cell through the
Golgi apparatus.

Functions of Golgi Apparatus

 Its main function is the storage, modification and packaging of products in vesicles.
 It is involved in the formation of lysosomes.
 Complex sugars are made from the simple sugars in the Golgi apparatus.

LYSOSOMES
 Lysosomes are membrane-bound sacs filled with powerful digestive enzymes made by
the RER.
 These help to keep the cell clean by digesting any foreign material as well as
worn-out cell organelles.
 Foreign materials entering the cell, such as bacteria or food, as well as old organelles
end up in the lysosomes, which break complex substances into simpler substances.

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  During the disturbance in cellular metabolism, for example, when the cell gets
damaged, lysosomes may burst and the enzymes digest their own cell.
 Therefore, lysosomes are also known as the „suicide bags‟ of a cell.

MITOCHONDRIA
 Mitochondria have two membrane coverings.
 The outer membrane is porous.
 The inner membrane is deeply folded.
 These folds increase surface area for ATP generating chemical reactions.

 The energy required for various chemical activities needed for life is released by
mitochondria in the form of ATP (Adenosine tri phosphate) molecules. Hence, they
are known as the “powerhouses of the cell”.
 The body uses energy stored in ATP for making new chemical compounds and for
mechanical work. ATP is known as the energy currency of the cell.
 Mitochondria are semi-autonomous organelles. They have their own DNA and
ribosomes. Therefore, they make some of their own proteins.

PLASTIDS

 Plastids are present only in plant cells. They are semi-autonomous as well.
 There are two types of plastids:
 Chromoplasts (coloured plastids)
 Leucoplasts (white or colourless plastids)
 Chromoplasts containing the pigment chlorophyll are known as chloroplasts.
 Chloroplasts are important for photosynthesis in plants. They also contain various
yellow or orange pigments in addition to chlorophyll.
 Starch, oils and protein granules are stored in the leucoplasts.
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VACUOLES
 Vacuoles are the storage sacs for solid or liquid contents.
 Vacuoles are small sized in animal cells while plant cells have very large vacuoles.
 In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to
the cell. They maintain the osmotic pressure. Many important substances like
amino acids, sugars, organic acids and some proteins are also stored in the
vacuoles of plant cells.

CELL DIVISION
The process by which new cells are made is called cell division.
There are two main types of cell division:
 Mitosis
 Meiosis

Mitosis takes place in all body cells and helps in growth, repair and replacement of cells.
Meiosis takes place in reproductive cells only. The daughter cells formed by meiosis are
called “gametes” (egg / sperm). The fertilisation of gametes will restore the original
chromosome number in the zygote. Thus, the new organism will also have the exact
number of chromosomes as its parents.

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 SRIMATHI SUNDARAVALLI MEMORIAL SCHOOL
CHENNAI
2020-21
BIOLOGY
ASSIGNMENT - 1
CLASS: IX
TOPIC: THE FUNDAMENTAL UNIT OF LIFE

1. SOLVE THE CROSS-WORD PUZZLE:

1
c e l l w a l l
2
c h
e l
l 3
g o l g i b o d i e s
l r
4
mi t o c h o n d r i a
e p 5
v
m h a
b y 6
c
r l u
a l o
n l
7 8
e n d o p l a s m i c r e t i c u l u m
e s
9
r i b o s o me s n
t
r
i
n u c e u e s
10
o
l
e

ACROSS DOWN
1 Protects and supports the plant cell 1 Produces food in plant cell
3 Receives the proteins from ER and Controls what goes in and out of the
packs them out to other parts of the 2
cell
cell
4 Produces energy in a cell 5 Stores food and water
7 Transports proteins and other
materials from one part of the cell to 6 Made up of DNA
another
9 Found in the cytoplasm and rough
8 Helps in cellular division
ER; also help in producing proteins
10 The cell‟s control centre that directs
the cell‟s activity
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 2. MATCH THE FOLLOWING:

COLUMN I COLUMN II
A Unit of inheritance Cytoplasm
B Power house of the cell Active transport
C Jelly-like substance in the cell Meiosis
D Suicidal bag of the cell Gene
E Movement of molecules that require ATP Ribosomes
F Group of similar cells performing a specific function Chromatin
G Cell division in reproductive cells Cell wall
H The sites of protein manufacture Tissue
I Outer thick layer in plant cell Mitochondria
J Thread-like structures in the nucleus Lysosome

CHOOSE THE CORRECT ANSWER:

3. Cell is the structural and functional unit of life. The word cell is derived from the
Latin word „cellula’ which means “a little room”. Can you name the scientist who
coined the term cell?
(a) Robert Hooke (b) Leeuwenhoek
(c) Robert Brown (d) Ernst Haeckel

4. Which of the following statements is the difference between plant cell and animal
cell?
(a) Plant cells have cell wall which animal cells do not.
(b) Plant cells do not have vacuole while animal cells do have.
(c) Plant cells have only cell membrane while animal cells have both cell wall as well
as cell membrane.
(d) Plant cells have more plastids while animal cells have few plastids.

5. Which of the following statements is not related to the endoplasmic reticulum?

(a) It behaves as transport channel for proteins between nucleus and cytoplasm.
(b) It transports materials between various regions in cytoplasm.
(c) It can be the site of energy generation.
(d) It can be the site of some biochemical activities of the cell.

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 6. Here is shown a figure of nucleus with some of its components labelled as A, B, C
and D. Can you name these components correctly?

(a) A – Nucleolus ; B – Chromatin; C – Nuclear pore ; D – Nucleoplasm

(b) A – Nucleus ; B – Chromatin; C – Nuclear membrane ; D – Nucleoplasm
(c) A – Nucleolus ; B – Chromatin; C – Nuclear membrane ; D – Nucleoplasm
(d) A – Nucleolus ; B – Chromatin; C – Nuclear membrane ; D – Nuclear wall

7. Which among the following is not a function of the vacuoles?

(a) They help to store the toxic metabolic by-products of the plant cell.
(b) They provide turgidity and rigidity to the plant cell.
(c) They help to maintain the osmotic pressure in the cell.
(d) They help the plant in its growth by the process of cell division.

ANSWER THE FOLLOWING

8. Draw a neat diagram of the plant cell and label any three parts which differentiate it
from the animal cell.
9. Give one point of difference between the following:
a) Prokaryotic and Eukaryotic cell.
b) Cell wall and Cell membrane.
c) Smooth ER and Rough ER.
d) Plant cell and Animal cell.
e) Unicellular and Multicellular organisms.
10. What is the plasma membrane made up of? Mention how the flexibility of this
membrane is significant to some living organisms.
11. Define membrane biogenesis.

12. Why is the chloroplast called a semi-autonomous cell organelle?

13. Write the postulates of the cell theory. Name the scientists who proposed them.

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ASSERTION AND REASON

DIRECTIONS:
In the following questions (14 – 16), a statement of assertion (A) is followed by a
statement of reason (R). Mark the correct choice as:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct
explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct
explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.
(e) Both Assertion (A) and (R) are false.

14. Assertion: The endoplasmic reticulum which lacks ribosomes is called smooth
endoplasmic reticulum(SER).
Reason: SER is mainly involved in protein synthesis.

15. Assertion: Mitochondria are called the „power houses‟ of the cell.
Reason: Mitochondria produce cellular energy in the form of ATP.

16. Assertion: Cell wall is a living part of the cell.

Reason: It offers protection, definite shape and support.

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