Cell : The Fundamental unit of life
Definition of Cell :-
A cell is a mass of protoplasm , containing a nucleus and
surrounded by a cell membrane.
Each cell have DNA which stores information for some
reactions and process inside the cell.
Cells are called the structural and functional unit of all living
beings because :-
Cells are the smallest structural units of living organisms. All
the cells have similar basic structure with similar kind of cells
organelles. They are the building blocks from which tissues,
organs, and organ systems are formed. Every living organism,
whether it's a plant, animal, fungus, or bacterium, is
composed of one or more cells.
So cell is called the structural unit of all living beings.
A cell is capable of carrying out all life processes, such as
nutrition, excretion, respiration, etc. Hence it is called as the
functional unit of life.
Discovery of Cell :-
(1) Robert Hooke (1665) :-
First time discovered dead cell.
He examined thin section of cork
under a his self made
primitive microscope and
noticed honey comb like
compartments.
He coined them as cell.
He explained his observation in his book “Micrographia”.
Term cell was derived from a latin word “cellula meaning a
hollow space”.
Cork- A substance which comes from the bark of the tree.
(2) Anton von Leewenhoek (1674) :-
First time discovered living cells like Bacteria ,RBC, Sperm
and Protozoa.
(3) Robert Brown (1831):-
Reported the presence of nucleus in the root cells of orchids.
(4) Purkinje (1839) :-
Named jelly like material in a cell as protoplasm.
Protoplasm was called the physical basis of life by Huxley.
(5) Schleiden and Schwann (1839) :-
Proposed cell theory
(6) Rudolf Virchow (1858) :-
Extended the cell theory.
Cell Theory :-
Cell theory was proposed by Schleiden (German botanist) and
Schwann (British zoologist).
According to the cell theory :-
All the living organism are made up of cells and cell is the
basic unit of life.
Cells contain DNA which are responsible for transmission of
characters.
Rudolf Virchow (1858) expanded cell theory by suggesting that
“Omnis cellula a cellula” which means “new cells are formed
from the pre-existing cells”
Exception of Cell theory :-
(a) Viruses :-
Viruses are non-cellular in structure .
It cannot be classified either a living or non-living.
It is made of DNA or RNA .
Viruses can be crystallized.
(b) Bacteria and Blue-green algae which are prokaryotes lack well
organized nucleus.
Instrument for Studying Cells :-
Microscope is the most common optical instrument used to
observe cellular organization in living organism.
It provides enlarged image of cells and structures within the
cells.
Types of Microscope :-
Microscope can be classified into two categories :-
(1) Light Microscope
(2) Electron microscope
Light Microscope :-
Commonly used in the laboratory.
It uses glass lenses and visible light that illuminates and
passes through the object/specimen.
Types of Light Microscope :-
(a) Simple microscope :-
Simple microscopes are just magnifying glasses.
(b) Compound Microscope :-
It uses light (generally sunlight) to illuminate the object.
It has magnification of 100-1500 times.
Consist of two lenses
Objective lense – Produces a primary magnified image
Eye piece /Ocular – magnifies the first image.
Working of compound microscope :-
A glass slide containing the object /specimen under study is
kept on a stage under an objective lens.
Light is then passed through the object with the help of a
mirror (reflector) and the condenser from below the stage.
A magnified ,sharp image of the object can thus seen through
the ocular lens by focusing the coarse adjustment and fine
adjustment knobs properly.
We can increase or decrease the magnification of the image by
changing the objective lens of high or low power.
Electron Microscope :-
Discovered by Knoll and Ruska in 1940.
Very large instrument .
Helps in observing sub-cellular structures.
It uses very high voltage of electricity.
It uses electromagnets instead of glass
lenses and beam of electrons instead
of light for illumination.
Its magnification is –100000 to 500000 times.
nm= 10-9 m µm = 10-6 m
Ao= 10-10 m mm = 10-3 m
Types of Organism :-
A. On the basis of number of cells , organism are of two types
(i) Unicellular organism :-
Those organism which are made up of single cells are called
unicellular organism.
A single cells performs all the life activities.
There is no division of labour
Only single cell takes part in reproduction
e.g. Bacteria (Cyanobacteria , Mycoplasma)
Protista ( Amoeba , Paramoecium , Euglena )
Fungi ( Yeast )
(ii) Multicellular Organism :-
Those organism which are composed of many cells are called
multicellular organism.
It shows division of labour ( many cells grouped together to
perform different functions in the body parts )
Generally germ cells take part in reproduction.
e.g. Fungi (Rhizopus , Mushroom , penicillium)
Plants and Animals
B. On the basis of complexity of organization ( nucleus ) , cells can
be divided into two types :-
(i) Prokaryotic cells :-
(Pro= before ; Karyon= Nucleus)
Prokaryotic cells are a type of cells
that lacks membrane bound nucleus.
Organelles present but membrane
bound cell organelles are absent
like ER , Mitochondria , Golgi body.
Naked DNA present i.e. DNA lies in
direct contact with the cytoplasm.
Histone protein absent
Cytoskeletal elements absent
Size – 2 – 5 µm
e.g. Monera Kingdom ( Bacteria)
(ii) Eukaryotic Cells :-
Eukaryotic cells are a type of cells
that have membrane bound nucleus.
DNA(linear) is present inside the nucleus.
Histone protein present
Generally larger in size ( 5-100µm)
Membrane bound cells organelles
are present.
Cytoskeletal elements are present.
e.g. Protista ,Fungi ,Plantae and Animalia.
Prokaryotic Cells (Bacteria)
Habitat –
Air, land ,water
Some unusual area – Deep sea , Desert, Intestine
Structure of Bacteria :-
1. Glycocalyx :-
Outermost layer
Made up of carbohydrates ( Mucopolysaccharides)
It is of two types :-
(a) Slime layer :-
Loose and Slimy
Prevent from dessication
(b) Capsule :-
Thick and Tough layer
Helps to cause infections.
2. Cell wall :-
Outer to the cell membrane.
Bacterial cell wall is made up of peptidoglycan
(muramic acid)
In Mycoplasma Cell wall is absent.
Role :-
Rigid and provide structural support.
It protect cell from bursting
Protect cell from collapsing.
Gram staining :-
On the basis of thickness of cell wall ,bacteria are of two types :-
(a) Gram Positive :-
Cell wall thick
Retain stain
Pilli and porins absent
e.g. Lactobacillus
Clostridium
Frankia
Mycobacterium
(b) Gram negative :-
Cell wall thin
Loose stain
Pilli and porins present
e.g. Mycoplasma
cyanobacteria
Rhizobium
E.coli
Cell membrane :-
It is made up of Lipid (Phospholipid Bilayer) .
Cell membrane is similar in structure and function in both
prokaryotic and eukaryotic cell.
Mesosomes :-
Mesosomes are the infolding of plasma membrane.
It increases the surface area.
Helps in Respiration.
Mesosomes is similar in function to mitochondria.
Chromatophores :-
In some bacteria (cyanobacteria) ,mesosomes bears some pigments
known as chromatophores.
Helps in Photosynthesis.
Chromatophores is similar to the chloroplast.
Nucleoid :-
Bacterial cell have single,double stranded ,circular DNA known as
nucleoid which is naked.
Plasmid :-
Extra chromosomal DNA other than nucleoid is called plasmid.
It provide antibiotic resistance to bacteria.
Ribosomes :-
Small size organelle(15 – 20 nm)
Granular in structure.
Without membrane
Involved in protein synthesis.
Flagella :-
Thread like structure present on the surface of bacteria .
It helps in locomotion of the bacteria.
Flagella is made up of Flagellin protein.
Arrangement of flagella :-
Depending upon the number of flagella ,bacteria are of following
types :-
(a) Monotrichous :-
Single flagella at one end
(b) Amphitrichous :-
Two flagella ,one at each end
(c) Peritrichous :-
Several flagella distributed on whole of bacteria.
(d) Cephalotrichous :-
Two or more flagella at one end.
(e) Lophotrichous :-
Groups of flagella one at each end.
(f) Atrichous :-
Flagella absent
Pilli and Fimbriae :-
Pilli Fimbriae
Tubular in shape Bristle (brush) in shape.
Less in number Large in number
Special role in sexual It helps in attachement on
Reproduction. Rock or Host (Substratum)
Inclusion bodies :-
Reserve materials present in the cytoplasm of prokaryotic cells .
e.g. Glycogen granules , Phosphate granules etc.
Eukaryotic Cells :-
Types of Organelle :-
Without membrane
•Ribosomes Double Membrane
•centrioles
Single membrane
•Nucleoid Endoplasmic Reticulum Mitochondria
Golgi bodies Chloroplast
Lysosome Nucleus
Vesicles
vacuoles
peroxisomes
Cillia and flagella
