Introduction

• Cell is the structural and functional unit of life. It is the basic

unit of life.

• It is discovered by Robert Hook in 1665 in cork slice with the

help of primitive microscope.

• Leeuwenhoek (1674), discovered the free living cells in pond

water with the improved microscope.

• Robert Brown discovered the nucleus in the cell in 1831.

• Purkinje coined the term ‘protoplasm’ for the fluid substance

of the cell in 1839.

The cell theory

• The theory that all the plants and animals are composed of
cells and the cell is the basic unit of life, was presented by two
biologists, Schleiden and Schwann.

• The cell theory was further expanded by Virchow by

suggesting that all cells arise from pre-existing cells.

→ Types of organisms

• On the basis of no. of cells, organisms are of two types:

(i) Unicellular Organism
(ii) Multicellular Organism

(i) Unicellular Organism: These organisms are single celled

which perform all the functions. Example: Amoeba,
paramecium, bacteria.

(ii) Multicellular Organism: Many cells grouped together to

perform different function in the body and also form various
body parts. Example: fungi, plants, animals.

• The shape and size of cell are different according to the kind
of function they perform. There is division of labour in cells.

• Each cell has certain kind of cell organelles to perform

different type of function like mitochondria for respiration.

→ Types of cells

• There are two types of cells:

(i) Prokaryotes
(ii) Eukaryotes

Prokaryotes Eukaryotes

Cells of organism lacks Cells of organism have nuclear

nuclear membrane. membrane.

Nucleolus is absent. Nucleolus is present.

 Single chromosomes. Single or multi chromosomes

Reproduction is always Reproduction is both sexual and

asexual. asexual.

Always unicellular. Often multicellular.

Membrane bound cell Membrane bound organelles are

organelles are absent. present like mitochondria.

Centriole is present only in

Centriole is absent. animals cell.

Cell division is by binary Cell division is by mitosis or

fission. meiosis.

Example: Bacteria, Blue Example: Fungi, Plant cell, Animal

green algae, etc. cell etc.

Difference between Animal cell and Plant cell

 Animal Cell Plant Cell

Cell wall is absent. Cell wall is present.

Plastids are absent. Plastids are present.

Centrioles are present. Centrioles are absent.

Golgi bodies are present and

Golgi bodies are present. called dictyosome.

Vacuoles are absent. If Vacuoles are present and large

present, they are small. in size.

Chloroplasts are absent. Chloroplasts are present.

Diffusion

• The spontaneous movement of a substance from a region of

high concentration to the region of low concentration is called
diffusion.

• Some substances like carbon dioxide or oxygen can move

across the cell membrane by a process called diffusion. Cell also
obtains nutrition from the environment.

Osmosis

• The movement of water molecules through selectively

permeable membrane along the concentration gradient is
called osmosis.

• Plant cell tend to obtain water through osmosis.

Hypotonic or Hypertonic or Isotonic solution

What happened to cell in sugar or salt solution?

Name of the
solution Condition Result

Medium surrounding
cell has higher water Cell will gain water by
Hypotonic concentration than osmosis and likely to
solution cell. swell up.

Water crosses the cell

Medium has exactly membrane in both
same water directions.
Isotonic concentration as the Cell will stay the same
solution cell. size.

Water crosses the cell

Medium has lower in both directions, but
Hypertonic
concentration of more water leaves the
solution
water than the cell. cell than enters it.

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 is called selectively permeable membrane.

• It is made up of lipid and protein.

→ Properties of Plasma membrane

• It is flexible (made up of organic molecules called lipids and

proteins).

• Its flexibility enables cell to engulf in food and other from the
external environment. This process is called endocytosis.
Amoeba acquire food through this process.

→ Functions of Plasma membrane

• It permits the entry and exit of some materials in and out of

the cell.

• It prevents movement of some other materials not required

for the cell as it acts like selectively permeable membrane.
Cell Wall

• Cell wall is another rigid outer covering in addition to the

plasma membrane found in plant cell. The cell wall lies outside
the plasma membrane.

• The plant cell wall is mainly composed of cellulose. Cellulose

is a complex substance which provides structural strength to
plants.

→ Function of Cell Wall

• 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 cell wall, cells can withstand much greater
changes in the surrounding medium than animal cells.

Plasmolysis

• 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.

Nucleus
• It is called the brain of the cell as it controls all the activities of
cell.

→ Composition of Nucleus

• 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.

→ Functions of chromosomes

• Chromosomes contain information for inheritance of features

from parents to next generation in the form of DNA (Deoxyribo
Nucleic Acid) molecules. Chromosomes are composed of DNA
and protein.

• DNA molecules contain the information necessary for

constructing and organising cells.

• Functional segments of DNA are called genes.

• In non-dividing cell, 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 and perform cell
division.

→ Functions of Nucleus

• The nucleus plays a central role in cellular reproduction. It is

the process by which a single cell divides and forms two new
cells.

• It also plays a crucial part, along with the environment, in

determining the way the cell will develop and what form it will
exhibit at maturity, by directing the chemical activities of the
cell.

Nucleoid
• 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.

Cytoplasm

• The cytoplasm is the fluid content inside the plasma

membrane.

• It also contains many specialised cell organelles. Each of these

organelles performs a specific function for the cell.

→ Function of Cytoplasm

• It helps in exchange of material between cell organelles.

• It act as store of vital chemicals such as amino acid, glucose,

vitamins and iron etc.

• It is the site of certain metabolic pathways such as glycolysis.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is a large network of

membrane-bound tubes and sheets.
• It looks like long tubules or round or oblong bags (vesicles).

• The ER membrane is similar in structure to the plasma

membrane. It is also made up of lipid and proteins.

→ Types of Endoplasmic Reticulum

(i) Rough endoplasmic reticulum (RER)

(ii) Smooth endoplasmic reticulum (SER)

→ Functions of Endoplasmic Reticulum

• RER looks rough under a microscope because it has particles

called ribosomes attached to its surface. The ribosomes, which
are present in all active cells, are the sites of protein
manufacture. The manufactured proteins are then sent to
various places in the cell depending on need, using the ER.

• The SER helps in the manufacture of fat molecules, or lipids,

important for cell function.

• Some of these proteins and lipids help in building the cell

membrane. This process is known as membrane biogenesis.

• Some other proteins and lipids function as enzymes and

hormones.

• Although the ER varies greatly in appearance in different cells,

it always forms a network system.

• One function of the ER is to serve as channels for the

transport of materials (especially proteins) between various
regions of the cytoplasm or between the cytoplasm and the
nucleus.

• The ER also functions as a cytoplasmic framework providing a

surface for some of the biochemical activities of the cell.

• In the liver cells of the group of animals called vertebrates,

SER plays a crucial role in detoxifying many poisons and drugs.
Golgi Apparatus

• The Golgi apparatus consists of a system of membrane-bound

vesicles arranged approximately parallel to each other in stacks
called cisterns.

• These membranes often have connections with the

membranes of ER and therefore constitute another portion of a
complex cellular membrane system.

→ Function of Golgi Body

• The material synthesised near the ER is packaged and

dispatched to various targets inside and outside the cell
through the Golgi apparatus.
• Its functions include the storage, modification and packaging
of products in vesicles. In some cases, complex sugars may be
made from simple sugars in the Golgi apparatus.

• The Golgi apparatus is also involved in the formation of

lysosomes.

Lysosomes

• Lysosomes are a kind of waste disposal system of the cell.

• It helps to keep the cell clean by digesting any foreign

material as well as worn-out cell organelles. • Lysosomes have
membrane-bounded structure whose sacs are filled with
digestive enzymes.

→ Functions of Lysosomes

• Lysosomes break foreign materials entering the cell, such as

bacteria or food as well as old organelles into small pieces.

• They contain powerful digestive enzymes which are made in

RER which is capable of breaking down all organic material
made in RER.

• During the disturbance in cellular metabolism such as 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 are known as the powerhouses of the cell.

→ Structure of mitochondria

• Mitochondria have two membrane coverings.

• The outer membrane is very porous while the inner

membrane is deeply folded.

• These folds create a large surface area for ATP-generating

chemical reactions.

→ Functions of mitochondria

• The energy required for various chemical activities needed for

life is released by mitochondria in the form of ATP (Adenosine
triphopshate) molecules.

• ATP is known as the energy currency of the cell. The body

uses energy stored in ATP for making new chemical compounds
and for mechanical work.

• Mitochondria have their own DNA and ribosomes. Therefore,

mitochondria are able to make some of their own proteins.

Plastids

• Plastids are present only in plant cells.

• There are three types of plastids:

(i) Chromoplasts (coloured plastids).
(ii) Leucoplasts (white or colourless plastids).
(iii) Chloroplasts (contains the pigment chlorophyll).

→ Structure of Plastids

• The internal organisation of the plastids consists of numerous

membrane layers embedded in a material called the stroma.

• Plastids also have their own DNA and ribosomes like

mitochondria and similar to its structure.

→ Function of Plastids
• Chloroplasts are important for photosynthesis in plants.

• Chloroplasts also contain various yellow or orange pigments

in addition to chlorophyll.

• Leucoplasts are primarily organelles in which materials such

as starch, oils and protein granules are stored.

Vacuoles

• Vacuoles are storage sacs for solid or liquid contents.

• They are small sized in animal cells while plant cells have very
large vacuoles.

→ Function of vacuoles

• The central vacuole of some plant cells may occupy 50-90% of

the cell volume.

• In plant cells vacuoles are full of cell sap and provide turgidity
and rigidity to the cell.

• Many important substance in the life of the plant cell are

stored in vacuoles which include amino acids, sugars, various
organic acids and some proteins.

• In single-celled organisms like Amoeba, the food vacuole

contains the food items that the Amoeba has consumed.

• In some unicellular organisms, specialised vacuoles also play

important roles in expelling excess water and some wastes
from the cell.