Protozoa

Protozoa:
singular protozoan

Oxytricha

Stentor

Tillina

Protozoa are a large group of eukaryotic, single celled organisms, which lack a rigid cell wall and usually chloroplasts. They vary widely in size, cell structure and form, ranging from Amoeba with its very fluid shape and simple internal organization and few specialised organelles through to Paramecium with its fixed shape, complex internal organisation and many specialised organelles.

Protozoa are usually free living and can be found in a variety of freshwater and marine environments and the soil; many are parasitic in other animals, including humans. They feed by taking in other organisms such as bacteria and algae or organic particles such as animal or plant debris. Soluble nutrients such as sugars can be absorbed directly through the cell envelope. Parasitic protozoa take nutrients from the body fluids of their hosts. Some protozoa live in symbiosis with bacteria which live in the cytoplasm or macronucleus.

Most protozoa are motile by various means such as:

Pseudopodia, where the protoplasm streams forward, changing the shape of the organism as it moves e.g. Amoeba
Flagella - long, whip-like appendages
Cilia - short appendages which are distributed all over the surface of the organism, which beat together.

The way by which protozoa move is so characteristic that it is used to classify them into groups.

Protozoa can form a cyst which is a dormant stage. This occurs in response to adverse conditions such as lack of food. The cyst is a protective stage and it also allows parasitic species to survive outside the host until it can infect another one.

Protozoa can reproduce asexually, usually by binary fission, or sexually.