Unicellular Organisms

Animals and plants are made up of many cells, which is why they are called multicellular organisms. However, there are also many living things which are made up of only one cell. We call these unicellular organisms (or single-celled organisms). Unicellular organisms have adaptations to help them survive in their environment.

We cannot see unicellular organisms with our naked eye; they can only be observed using a microscope.

The three different unicellular organisms we will look at are:

  • Euglena
  • Amoeba
  • Bacteria

Each organism has adaptations to help it survive and function in its environment.


Euglena have:

  • A nucleus
  • Chloroplasts
  • Mitochondria
  • A cytoplasm

It also has an eyespot to detect light.

Euglena are unicellular organisms that live in water and use a flagellum, a tail-like structure, to swim. Many other unicellular organisms also have a flagellum or flagella to help them move around.

Similar to plants, euglena make their own food during photosynthesis. This is why they have chloroplasts.


An amoeba has features similar to animal and plant cells. It has:

  • A nucleus
  • A cell membrane
  • A cytoplasm

As they live in water, it is easy to take in too much water from the surroundings, which can be a problem. So to counteract this, they contain a contractile vacuole. This moves around inside them and collects any excess water then squeezes it out at the cell membrane. It also allows them to remove waste material.


A bacterium is a tiny unicellular organism, and ‘bacteria’ is its plural form.

All bacterial cells have many features similar to animal and plant cells, which include:

  • A cell membrane
  • A cytoplasm
  • Ribosomes
  • A cell wall

However, bacteria do not have a nucleus. Instead, their genetic information is found loose in the cytoplasm, in the form of chromosomal DNA. Bacteria also have small rings of DNA called plasmids.

Some bacteria, but not all, have a tail-like structure called a flagellum, which helps with movement.

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