Download CHAPTER - 8 CELL * STRUCTURE AND FUNCTIONS

CHAPTER - 8
CELL – STRUCTURE AND
FUNCTIONS
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1) Discovery of the cell
In 1965 Robert Hooke observed slices of cork
from the bark of a tree under a simple magnifying
glass. He noticed small boxes or compartments in
the cork slice. The boxes were separated from
each other by a wall or partition. He named each
box as a Cell.The cells which Hooke observed
were actually dead cells of plants.
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2) The Cell
Cells are the basic structural units of living organisms.
Some organisms are made up of a single cell and some are
made up of many cells.
The number of cells in a tall tree or an animal like an
elephant or human being are billions and trillions of cells.
Organisms made up of many cells are called multicellular
organisms. Eg: a tree, an elephant.
Organisms made up of only one cell are called unicellular
organisms. Eg: amoeba, paramaecium.
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3) Shape of cells
Cells are of different shapes.
Some cells keep changing their shape. Eg : amoeba,
white blood cells in human beings.
Some cells are spherical, round, elongated, spindle
shaped, or branched.
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4) Size of cells
Cells in living organisms are of different sizes.They may be as
small as millionth of a meter (micrometre or micron) or large as a
few centimetres.
The smallest cell is the cell of bacteria (0.1 to 0.5 micrometre).
The largest cell is the egg of an ostrich (17cm x 13cm).
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Onion peel cells as seen under a microscope
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Human cheek cells as seen under a microscope
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Structure of a plant cell
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Structure of an animal cell
vacuole
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Structure of an animal cell
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5) Parts of a cell :The main parts of a cell are cell membrane (plasma
membrane), cytoplasm and nucleus.
The cell membrane gives shape to the cell, protects the inner
parts of the cell and allows movement of materials in and out of
the cell. In plant cells the cell membrane is covered by another
thick covering called cell wall.
The cytoplasm is a jelly like substance between the cell
membrane ad nucleus. It contains other components (organells)
like mitochondria, golgi bodies, ribosomes, vacuoles etc.
The nucleus is a spherical body generally found in the centre of
the cell. It has a membrane called nuclear membrane. It has a
smaller spherical body called nucleolus and thread like structures
called chromosomes.The chromosomes carry genes which
transfer characters from the parents to the off springs.
Vacuoles are blank looking structures in the cytoplasm.
Vacuoles are large in size in plant cells and small in size in animal
cells.
Plant cells have several small coloured bodies in the cytoplasm
called plastids.The green coloured plastids are called chloroplasts.
They contain chlorophyll which helps in photosynthesis.
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Structure of an animal cell and a plant cell
Vacoule
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Comparison of Plant cell and Animal cell
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Comparison of Plant cell and Animal
SN.
Part
cell
Plant cell
Animal cell
1
Cell membrane
Present
Present
2
Cell wall
Present
Absent
3
Nucleus
Present
Present
4
Nuclear membrane
Present
Present
5
Cytoplasm
Present
Present
6
Plastids
Present
Absent
7
Vacuole
Present
(smaller in size
Present
(larger in size)
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You don't always get what you wish for;
you get what you work for.
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