Define cell. Describe various organs cells of it and give their functions
The cell is the
smallest functional unit composed of elements which counter balance one another
in a endogenous equilibrium and compliment one another to carry on the
activities of life metabolism reproduction and specific response to stimuli. In
brief the cell is the basic unit of life and is the smallest structure in this
universe capable of growth and reproduction.
Inner space of
cell is filled by viscous fluid the Protoplasm inside which is rounded body
nucleus. Protoplasm and Nucleus are the living parts of cell. Cell is
surrounded by cell wall which is non living part of the plant cell. The cell
may be spherical, oval, polygonal, rectangular or elongated.
Protoplasm
Protoplasm is
living unit of cell. It is colloidal, viscous, elastic, semitransparent, jelly
like fluid, granular and saturated with water. Chemically the protoplasm is a
complex mixture of various substances Proteins. Protoplasm in dead condition
contains Carbon, Oxygen, Hydrogen and Nitrogen as the main parts. Besides these,
Chlorine, Sulphur, Potassium, Phosphorus, Silicon, Calcium, Magnesium, Iron and
Sodium are also present in small quantities. Protoplasm internally is
differentiated into Cytoplasm and Nucleus.
Cytoplasm
It is viscous
colourless and transparent fluid which forms general mass of Protoplasm.
Cytoplasm bears vacuoles filled with water fluid call cell sap. Cytoplasm
includes plastids, endoplasmic reticulum, ribosomes, chondriosomes, golgi
bodies, centrosomes and ergastric substances.
(1) Plastids:
The presence of plastids is special feature of plant cells. These are small,
specialized, discoid or spherical bodies present in the cytoplasm. Plastids
multiply in number by divisions of pre-existing ones. These are living bodies.
When mature plastids move away from the nucleus. Plastids are of three types
Leucoplasts, chloroplasts and chromoplasts. Leucoplasts are colourless plastids
and are found in underground stems and roots. As they are capable of converting
sugar into starch, these are mainly found in storage organs.
Chloroplasts are
green coloured plastids scattered in the cytoplasm. The colour is due to the
presence of a pigment chlorophyll which is present only in parts exposed to
light. Each chloroplast has double layered membrane surrounding colourless
matrix called stroma.
Chromoplasts are
colourless plastids various shaped present in the cells of the petals. They are
helpal for making plant parts attractive.
(2) Endoplasmic
Reticulum: Cytoplasm bears channels which connect with a complex set of
vesicles and are named as endoplasmic reticulum (ER). It exists in two forms
rough or granular having closely spaced granules called ribosomes on its
cytoplasmic side and smooth or a granular form. Two forms are continuous with
each other. It is continuous with plasma membrane, golgi complex and also with
nuclear membrane. They are made up of lipoproteins. It can be broken down and
reconstituted. It is connected with Protein synthesis.
(3) Ribosomes:
They are very minute granular bodies are composed of RNA and proteins. They may
be found free in the cell or attached to ER. Their main function is protein
synthesis.
(4) Mitochondria
or Chondriosomes: They are very small spherical or rod shaped bodies present in
the cytoplasm. Each mitochondrion has double membrane envelope. Inner space of
mitochondrion is filled with a fluid called matrix. Mitochondria are said to be
power house of the cell as they are the seats of energy rich compound ATP. They
are related to cellular oxidation, fat metabolism, formation of certain
pigments and accumulation of proteins.
(5) Golgi
complex: It was named by Golgi in 1898 who first found in cell. It is found in
all kinds of cells. It has a series of concentrically bent double membranes
which are flattered membranes bound sacs called cirternae, saccules or lamellar
units. Golgi bodies are concerned with secretion of proteins and complex
polysaccharides.
(6) Ergastic
substances: Together with living substances there are also present some
non-living inclusion in the cytoplasm. Vacuole is important non-living part of
the cell. It has watery solutions of salts, organic substances and waste
products of metabolism.
Nucleus
Nucleus is
present in the centre of cytoplasm in young cells while in nature cells it lies
on one side. It is spherical or oval in shape. Except blue green algae and
bacteria it is present in all living plant cells. In higher plants one nucleus
is present in each cell.
Nucleus is sac
like structure having delicate double layered membrane enclosing a network delicate
thread. Nucleus consists of nuclear membrane which is double. Nucleus contains
proteins, DNA, RNA, one rounded nucleolus and chromatin network.
Nucleus controls
the activity the protoplasm of the cell. It also exerts influence on growth and
differentiation. It is this taken to be brain of a cell. Most hereditary
characters are transferred by the nucleus.
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