Page 91 - Science Class 08

is bounded by its own membrane. The central
· In 1839, J. E. Purkinje discovered the
living fluid called protoplasm of the cell.
· In 1839-40, structure of the cell was
discovered by the two Germans.
· In 1855, Virchow proved that each cell
comes from pre-exis ng cells.
STRUCTURE OF A CELL
Cells are mostly round, spherical or elongated
in shape. Cells some mes are quite long. Some
are branched like the nerve cell or a neuron.
Components of the cell are enclosed in a
membrane which provides shape to the cells.
Cell wall is an addi onal covering over the cell
membrane to give shape and rigidity to plant
cells.
Cell Membrane
Every cell is bounded by a membrane that
separates it from its surroundings. It encloses a
liquid substance that is made up of liquids (fats
and oils) and protein. This membrane protects
the inner cell contents. Its main func on is to
regulate the movement of the molecules inside
and outside the cell. It encloses the other two
components– nucleus and cytoplasm of the
cell. These two components cons tute the
protoplasm.
(proto = first + plasma = liquid)
Ninety-nine per cent of the protoplasm is made
up of compounds of four elements– carbon,
hydrogen, oxygen, nitrogen. Some other
compounds like water, protein, fats, mineral
salts and nucleic acids are also there to provide
a living nature to the protoplasm.
Besides, the protoplasm has several smaller
components called organelles. Each organelle
Science-8
part of the cell is a denser region that is called
the nucleus of the cell.
mitochondrion
cell membrane
endoplasma c
cytoplasm nucleus re culum
DNA
lysosome
ribosome
Golgi
apparatus
Cell Structure
But, a plant-cell has an extra feature called cell-
wall, outside its membrane. It gives a lot of
structural strength and an extra safety to it.
NUCLEUS
As men oned above, the nucleus of a cell
forms its core. It directs all the ac vi es of the
cell. It has a double-layered membrane round
it. This membrane separates it from the
cytoplasm. But, it has some pores in it. These
pores allow the transfer of materials from
outside to inside and from inside to outside.
The nucleus contains two dis nct structures–
chromosomes and nucleoli.
Chromosomes are twining fibrous structures
made of a material called chroma n. They can
be normally seen when the cell is dividing itself.
Chromosome consists of two chief substances–
DNA and a par cular type of protein.
DNA stands for deoxyribonucleic acid. It
consists of several segments called genes that
are basic func onal units of heredity. They
store and transfer characters of parents to their
children.
Each gene has a DNA molecule. The chemical
structure of DNA determines the height, colour
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