Unicellular organisms

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Key points

  • Bacteria are among the smallest living things.
  • A single bacterium consists of just one cell.
  • Not all bacteria are harmful.
  • There are different unicellular organisms and types of bacteria.

What are unicellular organisms?

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

  • Unicellular fungi
  • Protozoa
  • Bacteria

These organisms have adaptations that make them well suited for life in their environment.

Unicellular fungi

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. Fungi can also form into mushrooms and toadstools.


Protozoa are unicellular organisms that live in water or in damp places, for example, the amoeba.


Even though a bacterium is just one cell, it can carry out all seven life processes - movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.

This toadstool is a fungus.


Video - How knowledge of bacteria helps a chef

Chef Gennaro describes how knowledge of bacteria helps with food hygiene in his kitchen

Can you answer these questions based on the video?

1. What is the width of a bacterium?

2. What is the transfer of bacteria from one area to another called?

  1. Two micrometres
  1. Cross contamination

What’s in a bacterium?

There are similarities between plant and animal cells and a bacterium. They all share a cell membrane and cytoplasm. Bacterium also have a cell wall around them, like plant cells. However unlike most plant and animal cells, it has no nucleus.

a bacteria cell labelled cell wall, cell membrane, plasmids, cytoplasm, dna molecule
A bacteria cell
Cell wallTough outer layer of the cell, which provides strength and support to the cell.
Large circular DNA moleculeBacteria do not have a nucleus and their genetic material (DNA) is not arranged into chromosomes.
Cell membranePermeable inner layer that surrounds the cell and controls which substances can pass into and out from the cell.
CytoplasmLiquid that makes up most of the cell in which chemical reactions happen. This is mainly water.
PlasmidsSmall circular section of DNA that can move from one bacterium to another.

There are millions of different types of bacteria. Different types of bacteria have different shapes.

Many types of bacteria have extra cell structures to help them to survive. For example, they may have:

  • A slime capsule, outside the cell wall, to protect them and stop the bacterium drying out.

  • Flagellum, a tail-like structure that allows bacteria to move through liquids.

diagram of a bacteria cell - the flagellum is like a tail which allows the cell to move
The main parts of bacteria

Why is it useful to know about bacteria?

Some bacteria cause diseases, including food poisoning, meningitis and scarlet fever. Inside the body, these bacteria reproduce quickly. They produce poisons that make you feel ill.

Anyone who cooks needs to know how to prevent bacteria spreading in their kitchens, to ensure all the food they produce is safe to eat.

Not all bacteria are harmful. For example, yoghurt and cheese makers use bacteria to make their products. Bacteria like intestinal flora can be helpful to our bodies whereas E.coli can be harmful and cause disease.

There are millions of bacteria in your digestive system. Having the right mix of bacteria here is vital. Gut bacteria help to digest food. They also help to prevent diabetes, obesity and some types of cancer.

E. coli as seen under a microscope

Quiz - Bacteria