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