Chapter 1: The Fundamental Unit of Life

Introduction -

• Cell is the structural and functional unit of life. It is the basic unit of life.
• It is discovered by Robert Hook in 1831 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 preexisting
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

Multicellular Organism: Many cells grouped together to perform different function in the body
and also form various body parts. Example: fungi, plants, animals.

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
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.

Difference between Animal cell and Plant cell

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 organelle
• 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
 

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.