Unified Cell Theory

The present unified cell theory states that

1. All living things are composed of either a single Cell or of the cells and cell products. The cell is a mass of protoplasm limited by a cell membrane, and possessing a nucleus;
2. New cells are formed only by division of pre-existing cells;
3. There are fundamental similarities in the chemical constituents and metabolic activities of all cells;
4. The activity of an organism as a whole can be understood as the collective activities and interaction of its interdependent cellular units.

In view of this present cell theory the living organisms may be broadly divided into:

• Acellular forms with body not composed of cells, for example viruses. 
• Cellular forms with body composed of cells.

The cellular forms may further be classified into:

a) Unicellular organisms with only one cell in their body, for example bacteria. blue-green algae, protozoans, etc. and
b) Multicellular organisms having many cells in their body. In such forms the cells organise themselves into tissues, tissues into organs and organs into organ systems. Examples of these are animals ranging from sponges to man and plants ranging from most algae to all flowering plants.

Figure: Some unicellular organisms: (a) Bacterium (Escherichia coli), (b) Blue green alga,(c) Mycoplasma, (d) Amoeba (a protozoan), (e) Chhydomonus (an alga), and (f) Mucor (a fungus)