CHAPTER - 8
CELL – STRUCTURE AND FUNCTIONS
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.
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.
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.
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).
Onion peel cells as seen under a microscope
Human cheek cells as seen under a microscope
Structure of a plant cell
Structure of an animal cell
vacuole
Structure of an animal cell
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.
Structure of an animal cell and a plant cell
Vacoule
Comparison of Plant cell and Animal cell
Comparison of Plant cell and Animal cell
Sl.No.
Part
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)