CELL — STRUCTURE AND FUNCTIONS

All organisms are made of smaller parts called organs. Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.

CELL

The smallest structural and functional unit of an organism, which is typically microscopic and consists of cytoplasm and a nucleus enclosed in a membrane. Cells were first observed in cork by Robert Hooke in 1665.
Cells exhibit variety of shapes and sizes. Number of cells also varies from organism to organism. Some cells are big enough to be seen with the unaided eye. Ex : Hen’s egg Some organisms are single-celled, while others contain large number of cells- multi- cellular. The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.
The cell has three main parts: (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, (iii) the nucleus.

1. CELL MEMBRANE

The basic component of a cell. The cytoplasm and nucleus are enclosed within cell membrane. There is an outer thick layer in cells of plants called cell wall.

2. CYTOPLASM

The jelly-like substance present between the cell membrane and the nucleus. Various organelles present in the cytoplasm are:

(i) Mitochondria

(ii) Plastids (present only in plant cell)

(iii) Endoplasmic reticulum (ER)

(iv) Ribosome

(v) Lysosome

(vi) Vacuole

(vii) Golgi body

(viii) Centrosome (present only in animal cell).

Diagram Representing a Cell

3. NUCLEUS

Nucleus is separated from the cytoplasm by a nuclear membrane. It is generally spherical in the centre of the cell. Nuclear membrane: The nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. Nucleus contains a still smaller round body known as nucleolus. Vacuoles are empty or blank looking structures in the cytoplasm. The nucleus contains thread-like structures called chromosomes. These carry genes. Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring. Cells without a well-organised nucleus, i.e. lacking nuclear membrane, are called prokaryotic cells.

SIZE OF CELLS

The size may be as small as a millionth of a metre or maybe as large as a few centimetres. The size of the cells has no relation with the size of the body of the animal or plant. It is related to its function.

COMPARISON OF PLANT AND ANIMAL CELL

TABULAR COLUMN

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