Cell structure and function

CELL THEORY

Two biologists, “Schleiden and Schwann” gave the “Cell theory” which was later on

expanded by “Rudolf Virchow ”. Cell theory states that

• All plants and animals are composed of cells.
• Cell is the basic unit of life.
• All cells arise from pre-existing cells.
• Viruses are the exceptions of cell theory.

CHARACTERISTICS OF CELLS

Cell shape

Shape of cells varies not only in different organisms but also in different parts of same organism.

Cells may be spherical, oval, elliptical, spindle shaped, cuboidal, polygonal, columnar or flat plate like.

Shape of the cell mainly depends upon the function it performs.

Other factors which determine the cell shape are : surface tension, viscosity of the protoplasm; rigidity of cell covering ; mechanical pressure of adjoining cells, etc.

CELL SIZE

The size of cells widely vary in different organisms.

In prokaryotes, cell size range between 1 to 10 µm while in eukaryotes it range between 10 to 100 µm. Smallest cell (PPLO) – Pleuro pneumonia like organism. Mycoplasma gallisepticum (0.1 µm to 0.3 µm)Largest animal cell – Ostrich egg (170 × 135 mm).

Largest plant cell – Acetabularia (one celled organism - green alga) (6 – 10 cm).

Longest cell in plant – fibre of ramie 55 cm to 100 cm long.

Longest cell in animal – Nerve cell (1 metre).

CELL NUMBER

Some organisms are one-celled called acellular e.g. Amoeba, Paramecium (earlier were called unicellular) and are always microscopic, as can be seen only with the help of microscope but most of the organisms are multicellular and have indefinite number of cells e.g. a newly-born human infant has about 2 × 10 12 cells, while at maturity the average adult male has about 100 trillions (10 14 ).