What is Biological cell? Describe Chloroplast, Mitochondria and Ribosomes.
Cell is the
basic unit of life and is the smallest structure in this universe capable of
growth and reproduction. Each cell is a tiny microscopic structure whose inner
space is filled by semi transparent, viscous fluid known as Protoplasm. Inside
protoplasm is a special body, rounded structure called Nucleus. Protoplasm and
nucleus are living parts and are surrounded by a distinct membrane the cell
wall which is non living parts of plant cell. The shape and size of cells are
variable. They may be spherical, oval, polygonal, rectangular or elongated in
shape. Their size varies from 0.5mm to about 1.00mm.
(i) CHLOROPLASTS
These are green
coloured plastids which are most important ones and are scattered in the
cytoplasm. The colour is due to the presence of pigment chlorophyll which is
present only in parts exposed to light. The number of chloroplasts in a cell
varies from one to many and their size varies from 1 to 10μm in diameter. In
higher plants they are generally discoid, oval or spherical in shape but in
longer plants they may also be ribbon shaped (spirogyra), net shaped
(oedogonium), spindle shaped (Anthoceros) or H shaped (chlamydomonas). In some
cases it may be marked by other pigments. Each chloroplast has double layered
membrane surrounding he colourless matrix called stroma.
In the stroma
there are numerous double membraned lamellae. The lamellae at some places are
swollen and form sac like thylacoids. The stacks of thylacoids are known as
grauna and contain chlorophyll. The double layer membrane and grama are made up
of lipoproteins. Chloroplast may have pigment chlorophyll (blue green),
chlorophyll (dark green), carotin (orange red) and xanthophyll (yellow). In
some chloroplasts phyrenoids are present. These are proteins grains surrounded
by starch sheath.
The functions of
chloroplasts are (1) to absorb energy of sun and to convert it into chemical
energy, (2) amyloplastic or starch building
(ii) MITOCHONDRIA
They are also called
chondriosomes. They are very small, spherical or rod shaped bodies present in
the cytoplasm. The shape of mitochondria depends upon the method used for its
fixation, staining and isolation. Each mitochondria has double membrane
envelope. The space between two membranes is filled with a fluid rich in
coenzymes and is peri mitochondrial space. The inner space of mitochondria is
filled with a fluid called matrix. The inner membrane forms numerous folds
called cristae. Some of them completely bridge the cavity. The matrix usually
is homogenous and consists of DNA RNA and ribosomes and enzymes of respiration
on the outer membrane of mitochondria simple outgrowths of inner surface of
cristae have three parts a base, a stalk and the head. These particles bring
about oxidation, transfer of electrons and catalysing reactions.
Mitochondria are
said to be permanent bodies of cytoplasm and have DNA and arise from pre
existing mitochondria origin of mitochondria was also suggested to take place
from golgi complex or nucleus or from the cell membrane.
The mitochondria
are said to be power house of the cell as they are the seats of energy rich
compound ATP. Besides, there are also related to cellular oxidation, fat
metabolism, formation of certain pigments and accumulation of Proteins.
(iii) RIBOSOMES
They are very
minute granular bodies. They are composed of RNA and Proteins. They may be
present free in the cell or be found attached on ER. Their main function is
protein synthesis. In young cells they are present free in the cytoplasm. As
the reticulum grows it comes in contact with ribosome which are then attached
to it. Often the ribosomes remain in clusters and are called as Polyribosomes
or polysomes. Each polysome has 4-6 ribosomes. These are held together by a
sting of mRNA and are actively and collectively engages in protein synthesis.
A ribosome
consists of two subunits, a small and a big subunit. The sedimentation
coefficient of complete ribosome is 10s and that of subunits 30s and 50s in
case of bacterial cell, mitochondrion and chloroplast while in cells of higher
plants is 80s for complete ribosome and 40s and 60s for the small and big units
respectively.
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