SANT SUJAN SINGH JI INTERNATIONAL SCHOOL

CLASS – IX SCIENCE

CHAPTER 5 – THE FUNDAMENTAL UNIT OF LIFE

 CELL- All living organisms are made up of small microscopic units called
cells. It is basic structural and functional unit of life.
 CYTOLOGY- Branch of science deals with the structure and composition
of cells.
 DISCOVERY OF CELL – Robert Hooke discovered the cells in 1665. He
examined a thin slice of cork or bark of trees under a self designed
microscope and observed that the cork resembles like the structure of honey
comb. Anton Von Leeuwenhock discovered living cell.
 DIFFERENCES BETWEEN UNICELLULAR AND
MULTICELLULAR ORGANISMS

Unicellular Multicellular
1. Organisms made up of single cell. 1. Organisms made up of many cells.
2. Single cell carry out different 2. Different cells carry out different
functions. Ex. Amoeba, Paramecium, functions. Ex. Plants, animals etc.
Euglena etc.
 SHAPE OF CELL
1. Not definite shape- Amoeba, WBC
2. Fixed shape- Paramecium
3. Circular shape- RBC
4. Star shape- platelets
5. Elongated shape- Muscle cell

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 MEASUREMENT OF CELL- Cells are measured in micrometer(µm) or
micron(µ)
1µm = 1/ 1000nm or 10-3nm
 SIZE OF CELL-
Largest animal cell in the world- Ostrich egg
Smallest animal cell – PPLO (Pleuro Pneumonia Like Organism)
Largest cell in human body- Nerve cell
Smallest cell in human body- RBC
 DIFFERENCES BETWEEN PROKARYOTIC AND EUKARYOTIC
CELLS-

Prokaryotic cell Eukaryotic cell

1. Nucleus is absent. 1. Nucleus is present.
2.Nuclear region is not well defined 2. Nuclear region is surrounded by a
due to the absence of nuclear nuclear membrane.
membrane.
3. Single chromosome is present. 3. More than one chromosome is
present.
4. Smaller in size. 4.Larger in size.

 COMPONENTS OF CELL- Three components of cells are-

1.Plasma membrane 2. Nucleus 3. Cytoplasm
I. PLASMA MEMBRANE OR CELL MEMBRANE: It separates
cytoplasm from its surroundings.
ii. It is partially permeable, allows only certain materials to pass in and out
of the cell. That’s why it is called selectively permeable membrane.

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 iii. It is found in both plants and animals.
iv. It is made up of proteins and lipids.
Function: It allows movement of materials in and out of the cell.
TRANSPORTATION OF MOLECULE ACROSS THE PLASMA
MEMBRANE:
1. DIFFUSION: Movement of solute or ions or gases from a region of
higher concentration to a lower concentration is called diffusion.
2. OSMOSIS: The movement of water or solvent from a region of higher
concentration to a region of lower concentration through a semipermeable
membrane or vice versa is called osmosis. Its types are-
i. Endomosis: Movement of solvent molecules into a cell when placed into
a hypotonic solution is called endomosis.
ii. Exosmosis: Movement of solvent molecules outside the cell when placed
in a hypertonic solution is called exosmosis.

TYPES OF SOLUTION ON THE BASIS OF CONCENTRATION:

1. ISOTONIC SOLUTION: The two solutions which have same concentration as

body fluid are called isotonic solution.
2. HYPERTONIC SOLUTION: When the concentration of solution outside the
cell is more than the inside cell is called hypertonic solution. A cell will shrink
when placed in hypertonic solution.
3. HYPOTONIC SOLUTION: When the concentration of the solution outside
the cell is less than the inside cell is called hypotonic solution. A cell will swell in
hypotonic solution.
ENDOCYTOSIS: The process by which cell membrane gets food and other
materials from the environment. Ex- amoeba gets food by this process.
CELL WALL: It is the outermost covering of the plant cell. It is absent in animal
cell. It is made up of cellulose. It is rigid, strong, porous and non-living structure. It
lies outside the plasma membrane.
Functions: It provides definite shape to the cell.
ii. It provides strength to the cell.
iii. It is permeable and allows entry of molecules into the cell of different sizes.

II. NUCLEUS: It is called headquarter of the cell because it controls and directs
all cellular activities.

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ii. It was discovered by Robert Brown.
iii. It has a double layer covering called nuclear membrane. It has pores which
allow the transfer of material from the inside of the nucleus to cytoplasm.
iv. It has a thread like structure called chromosomes. These contain DNA which
store and trasnsmit genetic information for a cell to grow, function and reproduce.
Function:
i. It controls ;all metabolic activities and regulates cell cycle.
ii. It helps in transfer of hereditary traits from the parents to the offsprings.
iii. It plays a central role in cellular reproduction.

DIAGRAM OF NUCLEUS

III. CYTOPLASM: It is a jelly like living substance of the cell which contains all
the cell organelles. The various cell organelles are:
1. ENDOPLASMIC RETICULUM: It is a network of membrane bound tubes. It
is absent in prokaryotic cells and mammalion RBCs. It has two types-
i. Smooth Endoplasmic Reticulum(SER) ii. Rough Endoplasmic Reticulum(RER)

Rough Endoplasmic Reticulum (RER) Smooth Endoplasmic Reticulum (SER)

1. It looks rough under microscope 1. It looks smooth because it doesn’t have
because it has ribosomes attached to its ribosomes.
surface.
2. It helps in protein synthesis. 2. It helps in synthesis of lipids and
steroids.

2. GOLGI BODIES OR GOLGI APPRATUS: It was discovered by Camilo

Golgi.
ii. It is a special protein of smooth endoplasmic reticulum.

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iii. It helps in synthesis of ribosomes.
iv. It arises from SER which in turn originates from RER.;
v. It is not found in bacteria, blue green algae, RBC of mammals and other
animals.
Function-
i. It helps in formation of lysosomes and complex sugars from simple sugars.
ii. It helps in the storage, modification and packaging of products.
iii. It helps in the formation of lipids.

3. MITOCHONDRIA: Energy which is needed for carrying out the various

chemical processes by the cell is released by mitochondria in the form of ATP
(Adenosine Tri-Phosphate) so it is called powerhouse of the cell.
ii. It is able to make some of its own proteins.
iii. The outer covering of mitochondria is very porous while inner membrane is
deeply folded which creates a large surface area.
Function: It provides energy to the cell in the form of ATP.

4. RIBOSOMES: They are the site of protein synthesis.

5. PLASTIDS: It is present only in plant cells. It has their own genetic materials.
They are of three types:
i. Leucoplast – They are colourless plastids without any pigments. They store
starch, protein and lipids.
ii. Chloroplast – Green in colour, found in arial parts of plants which contain a
green pigment called chlorophyll that is needed for making food through
photosynthesis. It has their own genetic material.
iii. Chromoplast – These contain yellow, orange and red pigments. They are
mainly present in petals of flowers and fruits. They are responsible for colours of
fruits and vegetables.
Function:
i. Leucoplasts store food as carbohydrates, proteins, or fats.
ii. Chloroplasts trap solar energy with the help of chlorophyll to prepare food in
photosynthesis therefore they are called kitchen of the plant.
iii. Chromoplasts provide colour to the fruits, flowers and vegetables.

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6. VACOULES: They are membrane bound regions of the cytoplasm containing
water and other substances.
ii. They are small in size in animal cell while larger in plant cell.
Function-
i. It helps in maintaining osmotic pressure.
ii. They store amino acids, sugars and proteins.

7. LYSOSOMES: They are tiny sac like structures found in cytoplasm.

ii. They are synthesized by RER.
iii. They are called suicide bags of the cell because if cell gets damaged lysosomes
burst and the enzymes present inside it digest their own cells.
Functions:
i. They provide a waste disposal system.
ii. They help in digesting foreign material and cells.

DIAGRAM OF PLANT CELL

DIAGRAM OF ANIMAL CELL

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TEXTBOOK QUESTION ANSWERS

Q1. Who discovered cells, and how?

Ans. In 1665, Robert Hooke discovered cells while examining a thin slice of cork
through a self-designed microscope. He observed that the cork resembled the
structure of a honeycomb consisting of numerous tiny compartments. The
minuscule boxes are referred to as cells.
Q2. Why is the cell called the structural and functional unit of life?
Ans. Cells form the structure of an organism. A group of cells form a tissue,
further an organ and ultimately an organ system. They perform fundamental
functions and life processes such as respiration, digestion, excretion etc in both
unicellular and multicellular organisms. They perform all the activities
independently. Hence, cells are referred to as structural and fundamental units of
life.
Q3. How do substances like CO2 and water move in and out of the cell?
Discuss.
Ans. CO2 moves by diffusion – These cellular waste accumulates in high
concentrations in the cell, whereas the concentration of CO2 in the external
surroundings is comparatively lower. This difference in the concentration level
inside and out of the cell causes the CO2 to diffuse from a region of higher(within
the cell) to a lower concentration.

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H2O diffuses by osmosis through the cell membrane. It moves from a region of
higher concentration to a lower concentrated region through a selectively
permeable membrane until equilibrium is reached.
Q4. Why is the plasma membrane called a selectively permeable membrane?
Ans. The plasma membrane is called as a selectively permeable membrane as it
permits the movement of only a certain molecules in and out of the cells. Not all
molecules are free to diffuse.
Q5. Can you name the two organelles we have studied that contain their own
genetic material?
Ans. The two organelles which have their own genetic material are:
1. Mitochondria
2. Plastids

Q6. If the organisation of a cell is destroyed due to some physical or chemical

influence, what will happen?
Ans. In the event of any damage to cells and when revival of cells is not possible,
Lysosomes burst and enzymes digest such cells. This is why lysosomes are often
referred to as ‘suicide bags’.
Q7. Why are lysosomes known as suicide bags?
Ans.When there is damage to the cell and when revival is not possible, lysosomes
may burst, and the enzymes digest their own cell. Consequently, lysosomes are
known as suicide bags.
Q8. Where are proteins synthesised inside the cell?
Ans. Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also
referred to as protein factories. Ribosomes are particles that are found attached to
the rough endoplasmic reticulum.
Q9. Make a comparison and write down ways in which plant cells are
different from animal cells.
Ans. The following table depicts the differences between plant cells and animal
cells.

Characteristic Plant Cell Animal Cell

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 Cell wall Present Absent

Shape of cell Distinct edges, shape is either Round and irregular shape
rectangular or square shaped.

Nucleus Present. Lies on one side of the cell Present. Lies in the center
of the cell

Lysosomes Rarely present Always present

Plastids Present Absent

Structure of Single or a few large vacuole Presence of numerous and

Vacuoles small vacuoles

Q10. What would happen if the plasma membrane ruptures or breaks down?
Ans. If plasma membrane ruptures or breaks down then molecules of some
substances will freely move in and out of the cells. As plasma membrane acts as a
mechanical barrier, exchange of material from its surroundings through osmosis or
diffusion in a cell won’t take place. Consequently, the cell would die due to the
disappearance of the protoplasmic material.
Q11. What would happen to the life of a cell if there was no Golgi apparatus?
Ans. The Golgi apparatus consists of stacks of membrane-bound vesicles whose
functions are as follows:

 storage of substances
 packaging of substances
 manufacture of substances
Without the golgi apparatus, the cells will be disabled from packing and
dispatching materials that were produced by the cells. The golgi apparatus is also
involved in the formation of cells. Hence, in the absence of golgi apparatus, cells
will not be produced.
Q12. Which organelle is known as the powerhouse of the cell? Why?
Ans. Mitochondria are known as the powerhouse of the cell. It is because it
releases the energy required for different activities of life. Mitochondria releases

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energy in the form of ATP(Adenosine triphosphate) molecules, essential for
numerous chemical activities of life. Hence ATP is often referred to as ‘energy
currency of the cell’.
Q13. Where do the lipids and proteins constituting the cell membrane get
synthesised?
Ans. Lipids and proteins are synthesised in the ER [Endoplasmic Reticulum].
Q14. How does an Amoeba obtain its food?
Ans. Through the process of endocytosis, an Amoeba obtains its food. As its cell
membrane is flexible enough, food particles are engulfed forming a food vacuole
girdling it which is assisted by the pseudopodia. Amoeba secretes digestive
enzymes to bring about digestion of the engulfed particle once food is trapped.

Q15. What is osmosis?

Ans. The process of movement of a water molecule from a region of higher
concentration to a region of lower concentration through a semipermeable
membrane is known as osmosis.
Q16. Which type of cell division is required for growth and repair of body and
which type is involved in formation of gametes?
Solution:
There are two ways in which a cell divides:

 Mitosis
 Meiosis
Mitosis is the type of cell division that is involved in the growth and repair of body
whereas meiosis is a type of cell division which results in the formation of
gametes.
ASSIGNMENT

Q1. Define osmosis.

Q2. Which is the outer most layer in- i) animal cell ii) plant cell
Q3. Where are proteins synthesized inside the cell?

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Q4. Where do the lipids and proteins constituting the cell membrane get
synthesized?
Q5. What are chromosomes?
Q6. State two differences between prokaryotic cell and eukaryotic cell.

Q7. a) Name two organelles which contain their own genetic material.

b) What type of cell wall is present in plant cell?

Q8. Why is plasma membrane called a selectively permeable membrane?

Q9. Give two differences between Rough Endoplasmic Reticulum (RER) and
Smooth Endoplasmic Reticulum (SER).
Q10. Which organelle is known as the powerhouse of the cell? Why?

X-X-X-X-X-X-X-X

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