Microbes and food 2. Microbe facts
Amoeba photo
Picture 2.5a An amoeba.
2.5 Protozoa (singular: protozoan)
Protozoa are a large group of organisms. Their main characteristics are:
  • they are single celled
  • their cells have membrane-bound nuclei (we call them eukaryotic)
  • they lack a rigid cell wall
  • they usually lack chloroplasts.

They vary widely in size, cell structure and form. For example, Amoeba has a very fluid shape and simple internal organization with few specialised organelles, whereas Paramecium has a fixed shape, complex internal organisation and many specialised organelles.

Where are they found?

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. They can absorb soluble nutrients such as sugars directly through the cell envelope. Parasitic protozoa take nutrients from the body fluids of their hosts. Some protozoa exist in symbiosis with bacteria that live in the cytoplasm or macronucleus.

Protozoan photomicrograph
Picture 2.5b The cilia on this protozoan enable it to move.
How do they move?

Most protozoa are motile. They use different 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.

Dormant stages
Protozoa can form a cyst, which is a dormant stage. This occurs in response to adverse conditions such as a 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.