CELL BIOLOGY
&GENETICS
Robert Hooke G.J. Mendel
(Father
of cytology)
(Father
of genetics)
Born in
28 July 1635
Born in 20
July 1822
Cell is the basic unit that organizes a body. It is defined
as “the structural, functional and
hereditary unit of life.”
Cytology (cell biology) is the
specialized branch of biological science that deals with the the study of
different types of cells, its structure and its functions.
The concept that cell is basic (structural and functional) unit of life
is known as cell theory. Credit goes to Robert
Hook for the discovery of cell in 1665 in cork tissue.
HISTORICAL APPROACH IN CYTOLOGY
Aristotle (384-322B.C) put forward a concept that all
animals and plants consist of certain structural units, termed as cell (by Robert Hook). R.Hook
observed the cell in the form of cellula(empty space) in cork in 1665. Antony
Van Leeuwenhoek discover the modern compound micro scope in 1674.He also
discovered the bacteria in the form of wild animalcules(1675). In 1772 Corti
purposed that cells is field by a dense proteinous substance. Felix
Dujardin(1836), gives the name sarcode for dense substance of the cell. M.J
Shleiden and Theodor formulated cell theory in 1838-1839. This theory states
that, the animals and plants differs from each other superficially, but the
pattern of organization and construction is similar. According to this theory
animals and plants body composed of cell. Each cell acts independently and also
functions as an integral part of complete organism. Schwann and Schielden
considered the cell as functional and biological units.
Protoplasm theory:-
The term protoplasm (juicy living substance) was coined
by Purkinje in 1840.It is derived from Greek words i.e. Protos = First and
plasm = organization.
According to protoplasm theory, all living
matters out of which animals and plants are formed is protoplasm. Also cell is
the accumulation of protoplasm possesses a nucleus. The protoplasm occurs every
where in the cell i.e. in the plasma membrane, nucleous and the portion in
between the plasma membrane and nucleous.
The
portion of protoplasm which occurs between the plasma membrane and nucleous is
called cytoplasm. The term cytoplasm was given by E. Strasburger in 1882.
TYPES OF CELLS
On the basis
specific function of cell, it is of three type’s i.e.
(a)
Undifferentiated
or, stem cells.
(b)
Differentiated
or, post mitotic cells.
(c)
Dedifferentiated
cells.
Undifferentiated or,
stem cells
They are unspecialized and most important types of the cells
which usually possess the power of division. For example zygot cell, stem
apical meristem cells, Root apical merisem cells, bone marrow cells, etc.
Differentiated or, post
mitotic cells
These are specialized
cells and well known for their function. Cells are diffretiated in shape, size,
structure and function. Cell activity is associated with some particular genes
i.e. inducers and repressors.
Ex:-
Dedifferentiated cells
They are differentiated
cells which take over the function of division. Such types of cells help in
healing of wounds, regeneration in animals and vegetative propagation in
plants.
Shape, Size and Number of
cells
Cell
shape:-
Measurable variability is found in the shape of different
types of cells. The plants and the animal cells exhibit various forms and
shape. Cells may be circular, oven, irregular, triangular, polygonal,
cylindrical, rounded and elongated. Some cells like Amoebae and Leucocytes
change their shape and some others have a typical shape. Some bacteria’s are
rod shaped, some are spiral and even comas shaped. One important fact is the
correlation of cell shape with its function. For example: - Epithelial cells are
flat and macular cells are elongated. Cell shape also depends upon the external
and internal environment.
Cell
size:-
Cells are widely varies in the size. Mostly eukaryotic cells
are microscopic in size but comparatively larger than bacterial cells.
The
size of cells varies from 1 Micron to 175 mm. The smallest cells are my co
plasma having a size of 0.1 to 0.5 Micron Meter. The ostrich egg cell is usually considered largest
cell with a diameter of 175 mm. Muscles and nerve cells are comparatively very
large. Certain nerve cells are 3 – 3.5 feet in length.
Cell Number:-
Unicellular organisms consist of a single cell but in case of
multicellular organisms, the number of cell depend upon the size and biomass of
an organisms. Small sized organisms have less number of cells in comparison to
large sized organisms.
Cell Structure:-
A plant cell consist of cell wall (outer protective wall)
which is absent in animal cell. The protoplasmic concentration of a cell is
denoted by protoplast. Plasma membrane, cytoplasm, nucleolus and vacuoles are
the major components of the cell.
Cytoplasm
consists of Cytoplasmic matrix (hyaloplasm) and endoplasmic reticulum.
Cytoplasm divisible into ectoplasm(outer cytoplasm) and endoplasm (cytoplasm
which lies near nucleous). Cytoplasmic matrix are embedded in a large number of
cell organelles i.e., mitochondria, plastic, ribosome’s, Golgi bodies,
centrioles, lysosome, peroxisomes, microfilaments, microtubules, etc. These
cell organelles are regarded as protoplasmic subunits.
Prokaryotic and Eukaryotic cells:-
Cells are classified into two class’s i.e. prokaryotic and
eukaryotic cell. Prokaryotic (Gr. Pro = primitive, karyon = nucleus) cells
present in bacteria, cyanobacteria(blue green algae), E.coli, protozoa, etc.
Eukaryotic (Gr. Eu = good or, true; karyon = nucleolus) cells are found in
plants and animals. The major difference between prokaryotic and eukaryotic
cells are listed below as:-
Prokaryotic cell
|
Ekaryotic cells
|
Primitive or, false
nucleus carrying cell
|
Good or, true nucleus
carrying cell
|
Prokaryotic cells are
generally present in uni-cellular organisms
|
Eukaryotic cells are
generally present in multi-cellular
organisms
|
DNA is circular and
lies in the cytoplasm
|
DNA is linear and
bonded in a nucleus
|
RNA and proteins
synthesis occurs in same compartments
|
RNA synthesized in
the nucleus and protein synthesized in the cytoplasm
|
Cell division occurs
by amitosis
|
Cell division occurs
by mitosis and meiosis
|
In prokaryotic cells
nucleolus and spindle fibers are absent
|
In eukaryotic cells
nucleolus and spindle fibers are present
|
Cell wall is made up
of polysaccharides and muramic acid
|
Cell wall is made up
of cellulose and pectin
|
Metabolic activity
occurs by both aerobic and anaerobic activity
|
Metabolic activity is
occurs by only aerobic activity
|
Eg:- Bacteria,
cyanobacteria, E.coli, etc
|
Eg:- protista, fungi,
plants and animals.
|
Difference between plant cell and animal cell.
Plant cell
|
Animal cell
|
Plant cell has a regide cell wall of
cellulose
|
animal cell having lack of cell wall
|
Cell wall is the outer most boundary of plant cell
|
plasma membrane is the outer most of animal cell
|
Usually large in size
|
Comparatively smaller
than plant cell
|
Having fix shape
|
can change their shape
|
A plant cell in general is autotrophic as it Contains
chloroplast
|
An animal cell is heterotrophic in general as it
lack of chloroplast
|
Nucleus lies towards the peripheral region of
cytoplasm
|
Nucleus lies in the centre
|
Centriole, basal body and flagella are absent In the
plant cell.
|
Cntriole(except invertebrate), basal Body and
flagella are present in the animal cell
|
Mitochondria are comparatively fewer more
|
Mitochondria are generally numerous
|
Mitochondrial cristae are tubular
|
Mitochondrial cristae are plate like
|
A plant cell capable to synthesized all the Amino
acids, coenzymes and vitamins
|
An animal cell can not able to synthesized the amino
acid coenzyme and vitamins
|
CELL WALL
(Discovered
by Robert Hook in 1865)
Cell wall is
a nonliving, thick and rigid wall of cellulose. It is present in plant cell
(including bacterial cell) and absent in animal cells. In case of plant cell,
cell wall is the outer most layer but in animal cell, the outer most layer is
plasma lemma which is also called plasma membrane or, cell membrane. Cell wall
is secreted by cytoplasm (living protoplasm) of the cell.
The
main function of cell wall is to protect the cell from mechanical injury and
maintenance of cell shape.
Cell
wall is mainly consisting of cellulose. However it may contain certain amounts
of hemicelluloses, pectin, lignin, chitin, and chutin. Cell wall is composed of
the following four layers.
(i)
Middle lamella:-
It is mainly composed of calcium and magnesium pectate and
having lack of cellulose. It is the outer most layer of the plant cell wall and
connects the two adjacent cells. There is a minute pore like structure
possessed by the middle lamella and adjacent primary cell wall which extend
fine Cytoplasmic strands that link the content of another cells. These are
called plasmodesmata that help in the exchange of materials between the
cells.
(ii)
Primary cell wall:-
Primary cell wall is mainly composed of cellulose and it is a
thin elastic membrane lies between middle lamella and secondary cell wall. It
develops after middle lamella by the deposition of hemicellulose, cellulose and
pectin. Cells of the most leaves, fruits, and pits have only primary cell wall.
(iii) Secondary cell wall:-
The composition of secondary cell wall is basically similar to the
primary cell wall i.e. it is primarily composed of cellulose microfibrils. In
some tissues, besides cellulose, lignin and suberin are also found in the
secondary cell wall. Secondary cell wall is multilayered in it self and
relatively thick (5-10micron meter) and it is the inner most
layer of the cell wall which lies between primary cell wall and plasma
membrane.
(iv)
Tertiary cell wall:-
It is also known as the lamella. Comparatively it is thin and formed of
cellulose and xylan (polymer of xylose).
Function of cell wall:-
(i)
protect the protoplasm from mechanical
injury.
(ii)
Provide the shaoe and rigidity to the
cell.
(iii) Gives
strength to the tree.
(iv)
Protect the cell from attack of
pathogens.
(v)
Helps in transport of substance between
two cells.
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