GLOBAL INDIAN INTERNATIONAL SCHOOL,

UPPAL, HYDERABAD
BIOLOGY NOTES-CLASS IX

UNIT 5: THE FUNDAMENTAL UNIT OF LIFE: CELL

Introduction

• The body of all organisms is made up of tiny microscopic units called cell.
• The cell is a fundamental, structural and functional unit of living organisms and basic unit of life.
• All living organisms are made up of cells. Cells make tissues, tissues together make organ, organs
make organ system and organ systems make body of organisms. So cell is called structural unit of
life.
• All the basic functions of the body like respiration, excretion etc. are carried out by cell through
its cell organelles, so cell is called functional unit of life.
• Cell biology is the study of cells in all aspects of structure and functions.

Discovery of cell:

• Cell was first discovered by Robert Hook in 1665. He observed the cell in a cork slice with the
help of a primitive microscope.
• Anton Von Leeuwenhoek (1674) was the first to observe free cells, like bacteria, protozoa, red
blood cells and sperms in his home made microscope.

Cell theory:

• The cell theory, that all the plants and animals are composed of cells and that the cell is basic unit
of life, was presented by two biologists, M. Schleiden (1838) and T. Schwann (1839).
• The cell theory was further expanded by a German physiologist, Rudolf Virchow (1855). He
gave the phrase Omnis cellula-e-cellula, i.e., all cells arises from pre-existing cells.

• Modified cell theory is termed as cell principle or modern cell theory which postulates that:
• All living organisms are composed of cells or cell products.
• All living cells arise from pre-existing cells.
• All cells are basically alike in chemical composition and metabolic processes.
• The function of an organism as a whole is the result of the activities and interaction of the
constituent cells.

Shape, size and number of cells:

Cell shape

• The shape of cells is related to the specific function they perform.

• Some cells like Amoeba and WBCs 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.

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BIOLOGY NOTES-CLASS IX

Cell size:

• The size of the cell also varies considerably in different animals and plants.
• The average cell size varies from 0.5 to 20µ (µ = micrometer).
• In human body, the smallest cell is RBC and the longest one is the nerve cells.
• An ostrich’s egg about 18cm is the largest cell

Cell number:

• The number of cells in living beings differs from the one in unicellular organisms to many in
multicellular forms.
• The number of cells in not definite in multicellular organisms, and may increase along with the
growth and volume of organism.
Unicellular organism Multicellular organisms

They have single cell They consist of many cells

A single cell is capable of performing all life A single cell carries out one or few processes
processes
Division of labour doesn’t exist Division of labour exist
Reproduction is by single cell Only some cells of the body called germ cells
take part in reproduction
Life span of an individual is short Life span of an individual is long

Ex: amoeba, bacteria etc Ex: man, hen, lion etc

Differences between Prokaryotic and Eukaryotic cells:

S. No. Prokaryotic cell Eukaryotic cell

1 Primitive and incomplete cells Advance and complete

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BIOLOGY NOTES-CLASS IX

Prokaryotes are always unicellular

2 organisms. Unicellular and multicellular

Nucleus is not well defined and known

3 Well defined nucleus is present
as nucleoid
Membrane bound organelles, such as Membrane bound organelles, such
Mitochondria, Golgi complex etc. are as Mitochondria, Golgi complex
4
absent. etc. are also present.

Ribosomes (80s) are bigger. They

Ribosomes ( 70s) are smaller and
are either attached to endoplasmic
5 scattered randomly in the cytoplasm.
reticulum or are found free.

The prokaryotes include archaebacteria,

Eukaryotes include all living
bacteria and cyanobacteria (blue green
6 organisms, except bacteria and blue-
algae).
green algae

Structure of cell:

 Structurally the cell is formed of three major parts:

1. Plasma membrane or cell membrane

2. Cytoplasm and its contents
3. Nucleus

Plasma membrane (cell membrane):

• Plasma membrane is the outermost covering of the cell that separates the contents of the cell from
its external environment.
• Plasma membrane is living, thin, delicate, elastic, selectively permeable membrane.
• The plasma membrane is flexible and made up of organic molecules called lipids and proteins.

Functions:

• Plasma membrane permits the entry and exit of some materials in the cells. It also prevents
movement of some other material. Therefore, the plasma membrane is called a selectively
permeable membrane.
• Substances can pass across a membrane by two processes- diffusion and osmosis.

• Diffusion: Some substances like carbon dioxide or oxygen can move across the cell membrane
by a process called diffusion.

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BIOLOGY NOTES-CLASS IX

“Diffusion is the spontaneous movement of molecules from a region of its higher concentration to
a region of its lower concentration.”
Example: movement of carbon dioxide and oxygen during respiration in organisms.

Osmosis:
The movement of water molecules through a selectively permeable membrane from a region of
higher water concentration to a region of lower water concentration is called osmosis.
1.
If the medium surrounding the cell has a higher water concentration than the cell, i.e., if
solution is a much diluted solution, the cell will gain water by osmosis. Such a dilute solution is
called hypotonic solution.

• Water molecule will pass across the plasma membrane in both directions, but more
water molecules will enter the cell than will leave.
• The cell will therefore, swell up and increase in volume. This process is
called endosmosis.
A plant cell becomes turgid when placed in hypotonic solution while animal cell
swells and burst.
2. If the medium surrounding the cell is exactly the same water concentration as the cell, there
will be no net movement of water across the plasma membrane resulting in no change in the size
of the cell. Such a solution is called isotonic solution.

3. If the medium has a lower concentration of water than the cell, i.e., if it is very concentrated
solution, the cell will lose water by osmosis. Such a concentrated solution is called hypertonic
solution.

• In this case too, water crosses the plasma membrane in both the directions, but this time more
water leaves the cell than enter it. The cell will therefore, shrink and reduce in volume. This
process is known as exosmosis.
• In case of plant cell it is called plasmolysed , in case of animal cell it is said to be
crenated.

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BIOLOGY NOTES-CLASS IX

Endocytosis and Exocytosis

Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with
the cell membrane.
There are two main kinds of endocytosis:
• Phagocytosis, or cellular eating,
• Pinocytosis, or cellular drinking,

Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their
contents to the outside of the cell

Cell wall:
• In plant cells, there occurs a rigid cell wall which lies outside the plasma membrane.
• Cell wall is thick, non-living and permeable covering made up of cellulose. Cellulose is a kind of
carbohydrate (polysaccharide) and it provides structural strength to the plant.
Functions:

1. Cell wall protects cell membrane and the internal structures of the cell.
2. It provides rigidity and determines the shape of the plant cell.
3. It prevents drying of the cell and helps it to bear unfavorable conditions.
4. It provides mechanical strength to the plant cells.

Protoplasm:

• All cells contain living substance called protoplasm.

• It is jelly-like, viscous, colourless semi-fluid substance in which various cell organelles and
inclusion remains in colloidal form.
• Protoplasm can be distinguished in two forms:
1. Cytoplasm: it is that part of protoplasm which surrounds the nucleus.
2. Nucleoplasm: it is that part of protoplasm which is located inside the nucleus.
Protoplasm = cytoplasm + nucleoplasm

Cytoplasm
The part of the cell which occurs between the plasma membrane and nucleus envelop is
called the cytoplasm

• Cytoplasm consists of an aqueous ground substance, the cytosol, containing a variety of cell
organelles and other inclusions such as insoluble waste and storage product (starch, lipid, etc.).
• Chemically cytoplasm contains about 90% water,, 7% proteins, 2% carbohydrates and lipids and
1% inorganic minerals, minerals vitamins, etc.

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BIOLOGY NOTES-CLASS IX

Cell organelles:
• A cell has to perform different functions with the help of its various membrane- bound cell
organelles.
• Cell organelles are “small organs” of the cell and are found embedded in the cytosol. They form
living part of the cell and each of them has a definite shape, structure and function.

Nucleus:
• The nucleus is a major, centrally located spherical cellular component.
• It is the centre from where all cellular activities are controlled.
• It is the carrier of hereditary material in the cell.

STRUCTURE:
• It is bounded by two membranes, both forming a nuclear envelope.
• The nuclear envelope contains many pores known as nuclear pores and encloses the liquid
ground substance, the nucleoplasm.
• The nucleopores allow transfer of materials between the nucleoplasm and the cytoplasm. Within
nucleoplasm are embedded two types of nuclear structures- nucleolus and chromatin material.

• Nucleolus:
• The nucleolus may be one or more in number ad is not bounded by any membrane.
• Nucleolus consists of DNA, RNA and protein.
• It is the site of ribosome formation.
• Nucleolus is known as factory of ribosomes.
Chromatin material:

• The chromatin is a thin, thread-like intermingled mass of chromosome material and composed
of the genetic substance DNA (deoxyribonucleic acid) and proteins
• DNA stores all the information necessary for the cell to function (metabolism), to grow and
to reproduce further cells of the next generation.
• The chromatin is condensed into two or more thick ribbon-like chromosomes during the division
of cell.
• Functional segments of DNA are called genes.
• Genes carry information for protein synthesis, which in turn control the development of
characters. Since genes are present on chromosomes and chromosomes pass from one generation
to the next through, genes are called hereditary units.

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BIOLOGY NOTES-CLASS IX

Functions of nucleus:

1. The nucleus controls all metabolic activities of the cell. If the nucleus is removed from the cell,
the protoplasm ultimately dries up and dies.
2. It regulates the cell cycle.
3. It is the storehouse of genes which are concerned with the development and manifestation of all
the body characters.
4. Nucleus helps in the formation of ribosomes and RNA.
5. It helps in the transmission of characters from one generation to the next.

ENDOPLASMIC RETICULUM
• It is absent in prokaryotic cells and matured RBCs of mammals.
ER occurs in three forms:
Cisternae (i.e., closed, fluid-filled sacs), vesicles and tubules.

ER is of two types:
1.Rough endoplasmic reticulum (RER) with ribosomes attached on its surface. They are found
in abundant in pancreatic cells
2.Smooth endoplasmic reticulum (SER) which is without ribosomes. They are found in
abundant in liver cells

It forms intracellular transporting system.

FUNCTIONS
1. SER takes part in the synthesis of lipids.
2. RER is concerned with the transport of proteins which are synthesized by ribosomes on their
surface.
3. ER forms supporting skeletal framework
4. SER of vertebrates helps in detoxification
Some of the synthesized proteins and lipids of RER and SER help in building plasma
membrane. This process is called membrane biogenesis.

GOLGI COMPLEX (GOLGI APPARATUS OR GOLGI BODY)

• Golgi complex was discovered by Camillo Golgi (1898).
It occurs in almost all eukaryotic cells. It is not found in prokaryotic cells.

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BIOLOGY NOTES-CLASS IX

• Structure
Golgi apparatus consists of a set of membrane-bounded, fluid-filled vesicles, vacuoles and
flattened cisternae.
• In animals Golgi is near nucleus, while in plants it in the form of subunits and it is called
dictyosome.
• The cis face is golgi near the ER .The cis face is the side of the Golgi apparatus that receives
vesicles from the endoplasmic reticulum
• The trans face is where they exit in the form of smaller detached vesicles.

FUNCTIONS
1. It is known as packaging and dispatching unit of the cell.
2.It is involved in the synthesis of cell wall, plasma membrane and lysosomes.
3.It is involved in the formation of cell plate during cell division.
2.

RIBOSOMES
They are present both in prokaryotic and eukaryotic cells.

STRUCTURE
Ribosomes are dense, spherical and granular particles which occur freely in cytoplasm or remain
attached to the endoplasmic reticulum (RER).
Chemically ribosomes are made up of ribonucleic acid (RNA) and proteins.
Ribosomes are of two types: 70S and 80S

v
FUNCTION
Ribosome is the site of protein synthesis so it is known as protein factory of the cell.

MITOCHONDRIA
• They are present in eukaryotic cell but absent in prokaryotic cells.
Mitochondria exist in variable shapes.

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BIOLOGY NOTES-CLASS IX

Structure:

• It is a double membranous organelle.

• Outer membrane is smooth and inner membrane forms folds like structure called cristae.
The cristae consist of F1 particles on its surface.
The matrix inside it contains ribosome (70s) , respiratory enzymes and a circular DNA.

FUNCTIONS

1. Mitochondria generate energy in the form of ATP (Adenosine Tri Phosphate), that is why it
is called powerhouse of the cell.
ATP is called energy currency of the cell
2. They are site for cellular respiration
3. Mitochondria have their own ribosomes and DNA, they can synthesize their own proteins, and they
are self-duplicating units. So, they are regarded as semiautonomous organelles.

LYSOSOMES
Structure
• Lysosomes are dark, spherical, single membrane bound sacs containing several digestive
enzymes.
• These enzymes are capable of digesting or breaking down all organic materials.
Lysosomes are formed by Golgi complex.
• They are found in the cytoplasm of eukaryotic cell, mostly in animal cells.
Lysosomes are involved in intracellular digestion.

• FUNCTIONS
1. During starvation or aging, the cell digests its own organelles through lysosomal enzymes, a
process called autophagy.
2. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn out cell
organelles.
3. When a cell is destined to die, the lysosomal enzymes digest the whole cell, a process
called autolysis. Therefore lysosomes are also known as ‘suicide bags’ of the cell.

PLASTIDS
The term ‘plastid’ was given by Haeckel in 1866. Plastids occur in most plant cells and are absent
in animal cells.
On the basis of colour, plastids are of following three types:

1. Chloroplasts: Green-coloured plastids containing chlorophyll.

2. Leucoplasts: Colourless plastids.

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BIOLOGY NOTES-CLASS IX

3. Chromoplast: Coloured plastids (except green colour).

CHLOROPLAST
Structure
 They are spherical or discoidal in shape and are enclosed in double membrane.
 Inside a plastid, two definite regions are clearly visible- Grana and Stroma.
Grana are stacks of membrane-bounded, flatterened discoid sacs containing the molecules of
chlorophyll.
 Stacks of grana are called thylakoid which are connected to each other by stroma lamellae
Stroma is the homogeneous matrix in which grana are embedded.
Plastids contain their own DNA and ribosomes i.e., they have their own protein synthesizing
machinery. They are also self-replicating organelles.

Functions:

1. Chloroplasts are photosynthetic organelles. The chlorophyll present in them trap solar energy for
the purpose of synthesizing food for the plant. So chloroplasts are the ‘Kitchens of the cell’.
2. Grana are site of light reaction while stroma are sites of dark reaction
3. Leucoplasts help in storing food products like starch, protein and lipids.
4. Chromoplast provide colour to flower which in turn attracts insects for pollination.

CENTROSOMES
Centrosome is found only in animal cells.

Structure
It is not bounded by any membrane but consists of two granule-like centrioles. Centrioles are hollow
cylindrical structures which are made up of microtubules.

FUNCTION
1. Centrosome helps in cell division in animal cells. During cell division centrioles migrate to the
poles of animal cells and are involved in the formation of the spindle.
2. They produce basal bodies from which cilia and flagella arise.

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BIOLOGY NOTES-CLASS IX

VACUOLES

Vacuoles are fluid-filled or solid-filled and membrane bound spaces in the cytoplasm.

 Structure
Vacuoles are small sized in animal cells while plant cells have very large vacuoles.
The vacuole is bounded by a membrane, called tonoplast. The vacuole is filled with cell sap which
is watery solution rich in sugar, amino acids, proteins, minerals and metabolic wastes.

Function.
1. Vacuoles are meant for the storage of food, water and other substances.
2. Vacuoles help to maintain the osmotic pressure in a cell (osmoregulation).
3. Vacuoles provide turgidity and rigidity to the plant cells.

PEROXISOMES
 Peroxisomes are small and spherical organelles containing powerful oxidative enzymes.
 They are bounded by a single membrane.
 Peroxisomes are specialized for carrying out some oxidative reactions, such as detoxification or
removal of toxic substances from the cell.

CELL DIVISION

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BIOLOGY NOTES-CLASS IX

 Cell division happens when a parent cell divides into two or more cells called daughter cells.
 Cell division usually occurs as part of a larger cell cycle.
 All cells reproduce by splitting into two, where each parental cell gives rise to two daughter cells.
 These newly formed daughter cells could themselves divide and grow, giving rise to a new cell
population that is formed by the division and growth of a single parental cell and its descendant.
 In other words, such cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.

Types of Cell Division

There are two distinct types of cell division out of which the first one is vegetative division, wherein each
daughter cell duplicates the parent cell called mitosis. The second one is meiosis, which divides into four
haploid daughter cells.

Mitosis: The process cells use to make exact replicas of themselves. Mitosis is observed in almost all the
body’s cells, including eyes, skin, hair, and muscle cells.
Meiosis: In this type of cell division, sperm or egg cells are produced instead of identical daughter cells
as in mitosis.

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