Cell: The Unit of Life
Introduction
Unicellular organisms are capable of
(i) independent existence
(ii) performing the essential functions of life.
Anything less than a complete structure of cell does not
ensure independent living. Hence, cell is the fundamental structural and
functional unit of all living organisms.
Anton Von leeuwenhoek first saw and described a live cell.
Robert Brown later discovered the neucleus. The invention of the microscope and
its improvement leading to the electron microscope revealed all the structural
details of the cell.
Cell
theory
Study of form, structure, and composition of cell is called
cytology.
• Cell is the
structural and functional unit of life. In unicellular organism (amoeba,
paramecium, incentive bacteria), single cell perform the all essential
functions of life.
• In multicellular
organism, different kinds of tissues perform different function and have
division of labour.
• Melthias Schleiden
and Theodore Schwann (1938) proposed the cell proposition
a. All living organisms are composed of cells and products
of cells.
b.All cells arise from pre-existing cells. Prokaryotic
cells Eukaryotic cells.
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An overview of cell
Shape and size of
cells varies greatly according to their position and function. Mycoplasma is
the lowest cell and largest insulated cell is the poltroon egg. The shape of
cell may be blockish, columnar, polygonal, thread like or irregular.
Prokaryotic cells
Prokaryotic cells are represented by Bacteria, Blue green algae, Mycoplasma and PPLO. They multiply fleetly and vary in size greatly.
·
Bacterial cells may be Bacillus ( rod shaped),
Coccus ( globular), Vibrio (comma- shaped) and Spirillum ( curl).
·
All prokaryotic cells have cell wall girding
the cell membrane except in Mycoplasma. Inheritable material is naked.
·
The plasmid DNA, in some bacteria provides some
special features like resistance to antibiotic.
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Cell organelles like Mitochondria, Golgi bodies
etc. are absent in prokaryotes. A technical discerned cell membrane called
Mesosome is the specific of prokaryotes.
·
In bacterial cell a chemically complex cell
envelope is present, which correspond of three layers. The remotest is
Glycocalyx, middle one cell wall and inner inmost is the cell membrane.
·
Glycocalyax may be as loose jacket in some
bacteria called slime subcaste. In some other bacteria Glycocalyx may be thick
and tough called capsule.
·
Plasma membrane issemi-permeable having
mesosome in form of vesicles, tubules and plates. They help in cell wall
conformation, DNA replication and distribution to daughter cells.
·
Motile bacterial cell contain flagella, which
is composed of hair, hook and rudimentary body. Pili and fimbriae are the other
face structure that help the bacteria in attach with host and other substance.
·
In prokaryotes, ribosome are attached with cell
membrane having two sub-units – 50S and 30S to form together 70S prokaryotic
ribosomes.
·
Ribosomes are point of protein conflation.
Ribosomes attached with mRNA to form a chain called polyribosomes.
·
Reserved accoutrements in prokaryotic cells are
present in cytoplasm as cell addition bodies, which may contain phosphate,
grains, glycogen grains etc.
·
Gas vacuoles are plant in blue green algae and
grandiloquent and green photosynthetic bacteria.
Eukaryotic Cell
Eukaryotic cells are present in Protista, plants, Creatures and Fungi. Cytoplasm is divided into chambers due to presence of membrane bounded organelles.
·
The cells contain well systematized nucleus
with nuclear membrane. The inheritable accoutrements are arrangerd in
chromosomes.
·
Plants cells differ in having cell wall,
plastids and large central vacuole as compared to animal cells. Animal cells
have centrioles, which are absent in plants cells.
·
Cell
membrane is composed of lipids and that are arranged in bilayer. A
lipid element is substantially composed on phosphoglycerides. Laterly it was
plant that protein is also present in cell membrane. Rate of protein and lipids
varies in different cells.
·
Membrane protein may be integral or
supplemental. Integral protein remains buried in membrane but supplemental
protein falsehoods of face.
·
Songster
and Nicholson (1972) proposed fluid mosaic model. According
to this model thequasi-fluid nature of lipid enables side movement of within
the bilayer of lipids.
·
The main function of plasma membrane is the
transport of motes across it.
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The movement of water from advanced attention
to lower attention by prolixity is called osmosis.
·
Cell wall is present in plants cells and fungi.
Algae have cell wall made up of cellulose, galactans and minerals like calcium
carbonate. In other plants it consists of cellulose, hemicellulose, pectin and
proteins.
·
Primary cell wall of youthful plants is able of
growth, which dwindle in mature cells. Secondary cell wall is formed on inner
side of the cells.
·
Plasmodesmata connect the cytoplasm of
bordering cells. Endomembrane system of cell includes endoplasmic reticulum,
golgi complex, lysosomes and vacuoles.
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Endoplasmic Reticulum are the tubular haphazard structure scattered
in the cytoplasm.
(i) Rough endoplasmic reticulum bears ribosomes on
its face. RER is involved in protein conflation and stashing.
(ii) Smooth
endoplasmic reticulum doesn't bear ribosomes on its face. SER is involved in
lipid conflation and steroidal hormones.
·
Golgi apparatus was
first observed by Camillo Golgi in 1898 near nexus. They consists of numerous
flat, slice- shaped sacs or cisternae staked resemblant to each other.
·
Golgi apparatus performs the function of
packaging of accoutrements and its transportation. A number of protein
synthesized by ribosomes are modified in cisternae of golgi apparatus. Golgi apparatus
is the point for conflation of Glycoprotiens and glycolipids.
·
Lysosomes are
membrane bound vesicular structure formed by process of packaging in the Golgi apparatus.
They're rich in hydrolytic enzyme-lipase, protease, carbohydrases active at
acidic PH. These enzymes are able of digesting carbohydrates, proteins, lipids
and nucleic acids.
·
Vacuoles are
membrane bound space plant in cytoplasm water, tire and excretory product
bounded by single membrane. They form contractile vacuole and food vacuole in
numerous organisms.
·
Mitochondria
double membrane bound structure with the external membrane and inner membrane
dividing its lumen in two chambers membrane forms a number of infoldings called
cristae towards the matrix.
Two membranes have
own specific enzyme.
·
Mitochondria are
spots for aerobic respiration. They produce cellular energy in form of ATP so,
they're called power house of the cells. The matrix of mitochondria also
contain indirect DNA motes, a many RNA motes, ribosomes and factors of protein
conflation.
·
Plastids are
plant only in plants cells and Eugleoids having specific colors to give colours
toplants corridor.
·
Chloroplast
contains
chlorophyll that traps solar energy for photosynthesis. Chromoplast provides
unheroic, orange and red colours to different corridor of plants.
·
Leucoplasts are
colourless plastids that store food, amyloplasts (carbohydrates), elaioplasts (canvases)
and aleuroplasts (proteins).
·
Chloroplasts are
double membrane structure. The space limited by inner membrane is called
stroma. Thylakoids are present in stroma as heaps like the piles of coins
called grana.
·
Stroma
contain enzyme for conflation of protein and carbohydrates. Double beachfront
indirect DNA and ribosomes are also present in stroma.
·
Ribosomes are
grainy structure of 80S.
·
Centrosomes is an organelles containing two
spherical structures called centrioles. Each centrioles is made up of 9 fibrils
of tubulin protein. Central corridor of centriole is called mecca and
supplemental fibrils are called spokes.
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Nucleus
has
largely extended, elaborate nucleoprotein fibres called chromatin, nuclear
matrix and nucleoli. The external membrane is nonstop with endoplasmic
reticulum and bears ribosomes.
Plastids Chloroplast (contain chlorophyll and caratenoids) Chromoplast ( contain carotene and xanthophylls) Leucoplast (colourless plastids).
·
The chromatin accoutrements change into
chromosome during active cell division consists of DNA and histone proteins.
Every chromosome has a primary condensation or the centromere, on the sides of which slice shaped kinetochores are present.
·
On the base of position of centromere
chromosomes are of following types-
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Some chromosomes have non-staining secondary
compression at certain position. This give a small scrap called satellite.