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MADE BY: KUSHAGRA SHARMA VIII ‘B’

STRUCTURE AND FUNCTIONS

Chapter-8

CELL

DISCOVERY OF 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.

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.

SHAPE OF CELLSCells are of different shapes.Some cells keep changing their shape. Eg : amoeba Some cells are spherical, round, elongated, spindle shaped, or branched.

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).

PARTS OF A CELLThe main parts of a cell are cell membrane (plasma membrane), cytoplasm and nucleus. 1.Cell Membrane 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.

2.CYTOPLASM The cytoplasm is a jelly like substance between the cell membrane and nucleus. It contains other components (organelles) like mitochondria, Golgi bodies, ribosomes, vacuoles etc.3.NUCLEUS 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.

4.VACUOLE Vacuoles are storage sacs for solid and liquid contents.They are small sized in animal cell and large sized inPlant cell. 5.PLASTIDS Plastids are present only in plant cells. There are 2 types of plastids. First is Chloroplast and second is Lucoplast. Green colored plastids are called chloroplasts because they contain green pigment called chlorophyll which helps in photosynthesis.

PLANT CELL(A), ANIMAL CELL(B)

(a)

DIFFERENCES BETWEEN PLANT AND ANIMAL CELLS

1 Plant Cell

Animal Cell

1. Cell-wall is present.

1. Cell-wall is absent.

2. Plastids are present.

2. Plastids are absent.

3. Vacuoles are larger in size.

3. Vacuoles are smaller in size.

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