The cell is the basic structural unit of life. It is the smallest unit capable of independent exitence that can reproduce itself exactly. All living organisms (except viruses) are composed of one or more cells. Single-cell organisms such as bacteria, protozoa, and other microorganisms, are called unicellular, while plants and animals that contain many cells are called multicellular. Highly complex organisms such as humans consist of billions of cells, each of which is adapted to carry out specific functions - e.g. groups of specialised cells may be organised into tissues and organs.

Although cells may vary greatly in size, appearance, and function, their essential features are similar. Each is composed of a jelly-like substance called cytoplasm, surrounded by a membrane. The cytoplasm contains ribosomes, which carry out protein synthesis, and DNA, the coded instructions for the behaviour and reporduction of the cell

Living cells are divided into two types - prokaryotic and eukaryotic (or procaryotic and eucaryotic). This division is based on internal complexity.

Prokaryotic

Bacteria, cyanobacteria, and blue-green algae. These cells are simple in structure, with no recognizable organelles. The DNA forms a simple loop and there is no nucleus. They have an outer cell wall that gives them shape. Just under the rigid cell wall is the more fluid cell membrane. The cytoplasm enclosed within the cell membrane does not exhibit much structure when viewed by electron microscopy.

Eukaryotic

The cells of protozoa, fungi, plants and animals are highly structured. The DNA is organised into chromosomes and is contained within a clearly defined nucleus, which is surrounded by a double membrane. The cytoplasm also contains other membrane-bound structures called organelles, such as mitochondria and chloroplasts, which carry out specific functions. These cells tend to be larger than the cells of bacteria, and have developed specialized packaging and transport mechanisms that may be necessary to support their larger size.