Unicellular organisms

A unicellular organism is a living thing that is just one cell. There are different types of unicellular organism, including:

  • bacteria
  • protozoa
  • unicellular fungi

You might be tempted to think that these organisms are very simple, but in fact they can be very complex. They have adaptations that make them very well suited for life in their environment.

Bacteria

Bacteria are tiny. A typical bacterial cell is just a few micrometres across (a few thousandths of a millimetre). The structure of a bacterial cell is different to an animal or plant cell. For example, they do not have a nucleus but they may have a flagellum. This is a tail-like part of the cell that can spin, moving the cell along.

Bacteria cell, labelling chromosomal DNA, plasmid DNA, flagellum, cell membrane and cell wall.The main parts of bacteria

Protozoa

Protozoa are unicellular organisms that live in water or in damp places. The amoeba is an example of one. Although it is just one cell, it has adaptations that let it behave a bit like an animal:

  • it produces pseudopodia (“false feet”) that let it move about
  • its pseudopodia can surround food and take it inside the cell
  • contractile vacuoles appear inside the cell, then merge with the surface to remove waste
Change animalecule, (amoeba proteus) formerly known as chaos diffluens
A light microscope image of an amoeba, showing several pseudopodia

Yeast

You may be familiar with fungi from seeing mushrooms and toadstools. Yeast are unicellular fungi. They are used by brewers and wine-makers because they convert sugar into alcohol, and by bakers because they can produce carbon dioxide to make bread to rise.

Yeast have a cell wall, like plant cells, but no chloroplasts. This means they have to absorb sugars for their nutrition, rather than being able to make their own food by photosynthesis

Yeast can reproduce by producing a bud. The bud grows until it is large enough to split from the parent cell as a new yeast cell.

Yeast cells. This fungus consists of single vegetative cells
Several yeast cells showing buds