CELL-THE UNIT OF LIFE -NOTES
CELLL-THE UNIT OF LIFE
- 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, yeast, bacteria) single cell
performs all the essential functions of life.
- In multicellular organism, different kinds of tissues
perform different function and have division of labour.
- Anton Von Leeuwenhoek first saw and described a live
cell. Robert Brown later discovered the nucleus.
- Metthias Schleiden and Theodore Schwann( 1938) proposed
the cell theory which was later modified by Rudolf Virchow(1855)-
1.
All living organisms are composed of
cells and products of cells.
2.
All cells arise from pre-existing
cells.
Prokaryotic cells |
Eukaryotic cells |
1.
Membrane-bound
nucleus is absent. 2.
Cells are
smaller in size. 3.
Single
chromosome is present. 4.
Membrane-bound
organelles are absent. |
1.
Membrane-bound
nucleus is present. 2.
Cells are
larger in size. 3.
More than
one chromosome is present. 4.
Membrane-bound
organelles are present. |
Shape and size of cells varies
greatly according to their position and function. Mycoplasma is the smallest
cell and largest isolated cell is the ostrich egg. The shape of cell may be
cuboid, columnar, polygonal, thread-like or irregular.
Prokaryotic
Cells
- Prokaryotic cells are represented by Bacteria,
Blue-green algae, Mycoplasma and PPLO. They multiply rapidly and vary in
size greatly.
- Bacterial cells may be Bacillus (rod-shaped), Coccus
(spherical), Vibrio (comma-shaped) and Spirillum (spiral).
- All prokaryotic cells have cell wall surrounding the
cell membrane except in Mycoplasma. Genetic material is naked.
- The plasmid DNA, in some bacteria, provides some
special features like resistance to antibiotics.
- Cell organelles like Mitochondria, Golgi bodies etc. are absent in prokaryotes. A specialized differentiated cell membrane called Mesosome is the characteristic of prokaryotes
- In bacterial cell, a chemically complex cell envelope
is present, which consist of three layers. The outermost is Glycocalyx,
middle one cell wall and inner innermost is the cell membrane.
- Glycocalyax may
be as loose sheath in some bacteria called slime
layer. In some other bacteria, Glycocalyx may be thick and
tough called capsule.
- Plasma membrane is semi-permeable having mesosome in
the form of vesicles, tubules, and lamellae. They help in cell wall
formation, DNA replication and distribution to daughter cells.
- Motile bacterial cell contains flagella, which is
composed of filament, hook and basal body. Pili and fimbriae are the other
surface structures that help the bacteria to attach with host and other
substances.
- In prokaryotes, ribosome are attached with cell
membrane having two sub-units – 50S and 30S to form together 70S
prokaryotic ribosomes.
- Ribosomes are site of protein synthesis. Ribosomes
attached with mRNA to form a chain are called polyribosomes.
- Reserved materials in prokaryotic cells are present in
cytoplasm as cell inclusion bodies, which may contain phosphate, granules,
glycogen granules etc.
- Gas vacuoles are found in blue-green algae and purple
and green photosynthetic bacteria.
Eukaryotic Cell
-
- Eukaryotic cells are present in Protista, Plants,
Animals, and Fungi. Cytoplasm is divided into compartments due to presence
of membrane-bounded organelles.
- The cells contain well-organized nucleus with nuclear
membrane. The genetic materials are arranged 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 plant cells.
- Eukaryotic cells are present in Protista, Plants,
Animals, and Fungi. Cytoplasm is divided into compartments due to presence
of membrane-bounded organelles.
Animal cell
- Cell membrane is
composed of lipids that are arranged in bilayer. The lipid component is
mainly composed of phosphorglycerides. Later it was found that protein is
also present in cell membrane. Ratio of protein and lipids varies in
different cells.
- Membrane protein may be integral or peripheral.
Integral protein remains buried in membrane but peripheral protein lies on
the surface.
- Singer and Nicholson (1972) proposed fluid mosaic model. According to this model,
the quasi-fluid nature of lipid enables lateral movement of protein within
the bilayer of lipids.
- The main function of plasma membrane is the transport
of molecules across it.
Active Transport
Passive Transport
1. The transport involves an expenditure of energy by the cells.
2. It occurs against the concentration gradient.
3. It is a rapid process.
1. The cells do not spend energy in passive transport.
2. This transport is always along the concentration gradient.
3. It is comparatively slow process.
- The movement of water from higher concentration to
lower concentration by diffusion is called osmosis.
- Cell wall is present in plant 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 young plant is capable of growth, which diminish in mature cells. Secondary cell wall is formed on inner side of the cells.
- Plasmodesmata connects the cytoplasm of neighboring cells.
- Endomembrane system of cell includes endoplasmic
reticulum, golgi complex, lysosomes and vacuoles.
- Endoplasmic Reticulum are
the tubular structure scattered in the cytoplasm.
1. Rough endoplasmic reticulum bears ribosomes on its surface. RER is involved in protein synthesis and secretion.
2. Smooth endoplasmic reticulum does not bear ribosomes on its surface. SER is involved in lipid synthesis and steroidal hormones.
- Golgi apparatus was
first observed by Camillo Golgi in 1898 near nucleus. They consist of many
flat, disc-shaped sacs or cisternae stacked parallel to each other.
- Golgi apparatus performs the function of packaging of
materials and its transportation. A number of protein synthesized by
ribosomes are modified in cisternae of Golgi apparatus. Golgi apparatus is
the site for synthesis of Glycoproteins and glycolipids.
- Lysosomes are membrane-bound vesicular structures
formed by the process of packaging in the Golgi apparatus. They are rich
in hydrolytic enzymes- lipase, protease, carbohydrases active at acidic
PH. These enzymes are capable of digesting carbohydrates, proteins,
lipids, and nucleic acids.
- Vacuoles are
membrane-bound space found in cytoplasm containing water, sap and
excretory product. They are bound by single membrane. They form
contractile vacuole and food vacuole in many organisms.
- Mitochondria is
double membrane-bound structure with the outer membrane and inner membrane
dividing its lumen in two compartments. The inner membrane forms a number
of infoldings called cristae towards
the matrix.
- Lysosomes are membrane-bound vesicular structures
formed by the process of packaging in the Golgi apparatus. They are rich
in hydrolytic enzymes- lipase, protease, carbohydrases active at acidic
PH. These enzymes are capable of digesting carbohydrates, proteins,
lipids, and nucleic acids.
- Golgi apparatus performs the function of packaging of
materials and its transportation. A number of protein synthesized by
ribosomes are modified in cisternae of Golgi apparatus. Golgi apparatus is
the site for synthesis of Glycoproteins and glycolipids.
- Endoplasmic Reticulum are
the tubular structure scattered in the cytoplasm.
- Two membranes have their own specific enzyme.
- Mitochondria are sites for aerobic respiration. They
produce cellular energy in form of ATP so, they are called power house of the cells. The matrix of
mitochondria also contain circular DNA molecules, a few RNA molecules,
ribosomes and components of protein synthesis.
- Plastids are
found in plant cells and in Euglenoids.
- Chloroplast contains chlorophyll that traps solar
energy for photosynthesis. Chromoplast provides yellow, orange and red
colours to different parts of plants.
- Leucoplasts are colourless plastids that store food,
amyloplasts (carbohydrates), elaioplasts (oils) and aleuroplasts
(proteins).
- Chloroplasts are double membrane structures. The space
limited by inner membrane is called stroma. Thylakoids are present in
stroma as stacks like the piles of coins called grana.
- Stroma contains enzymes for synthesis of protein and
carbohydrates. Double strand circular DNA and ribosomes are also present
in stroma.
- Eukaryotic cells have 80S ribosomes.
They have granuler structure with two subunits.
- Centrosome is
an organelles containing two cylindrical structures called centrioles.
Each centrioles is made up of 9 fibrils of tubulin protein. Central part
of centriole is called hub and peripheral fibrils are called spokes.
- Nucleus has
highly extended, elaborate nucleoprotein fibers called chromatin, nuclear
matrix, and nucleoli. The outer membrane is continuous with endoplasmic
reticulum and bears ribosomes.
- The chromatin materials change into chromosome during
active cell division. It consists of DNA and histone proteins.
- Every chromosome has a primary constriction or the
centromere, on the sides of which disc-shaped kinetochores are present.
On the basis of position of centromere, chromosomes are of following types-Some chromosomes have non-staining secondary constriction at certain location. This gives a small fragment called satellite.
- Stroma contains enzymes for synthesis of protein and
carbohydrates. Double strand circular DNA and ribosomes are also present
in stroma.
- Two membranes have their own specific enzyme.
- The main function of plasma membrane is the transport
of molecules across it.
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