Cell Theory

All organisms are composed of structural and functional units of life called cells. The body of some organisms like bacteria, protozoans and some algae is made up of a single cell whereas the body of higher fungi, plants and animals are composed of many cells. Human body is built of about one trillion cells.

In 1838, M.J. Schleiden and Theodore Schwann formulated the cell theory, which maintains that:

• All organisms are composed of cells.
• Cell is the structural and functional unit of life.
• Cells arise from pre-existing cells.

The cells vary considerably in shapes and sizes. There is also wide variation in the number of cells in different organisms.

The Cell

A cell is defined as a unit of protoplasm bound by a plasma or cell membrane and possessing a nucleus. Protoplasm is the life giving substance and includes the cytoplasm and the nucleus. The cytoplasm has in it organelles such as ribosomes, mitochondria, golgi bodies, plastids, lysosomes and endoplasmic reticulum.

Plant cells have in their cytoplasm, large vacuoles containing non-living inclusions like crystals, and pigments.

The bacteria have neither defined cell organelles nor a well formed nucleus.

Every cell has three major components:

1. Plasma or cell membrane
2. Cytoplasm
3. DNA (naked in bacteria) and enclosed by a nuclear membrane in all other organisms