1.5 Unicellular organisms
Summary: Not all living organisms are made up of many cells. In fact, many organisms exist as only one cell. Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe a unicellular organism
- describe the features of an amoeba
- describe the features of a euglena.
Think back
- Which process moves substances into a cell?
- Which parts are in an animal cell?
- Which chemical reaction takes place inside chloroplasts?
Key idea Amoebas and euglenas are unicellular organisms.
Key words unicellular organism, amoeba, binary fission, euglena, flagellum
Not all living organisms are made up of many Cells. In fact, many organisms exist as only one cell.
What is a unicellular organism?
A unicellular organism is an organism that is made up of just one cell. Unicellular organisms are not plants or animals because these are made up of many cells.
Amoebas
An amoeba is a unicellular organism that has no fixed shape. Figure 1 shows an amoeba. They look a bit like a blob of jelly. They can be found in fresh water, salt water, wet soil, and even inside animals.
Figure 1Parts of an amoeba. Full text alternative to the content
Just like an animal cell, an amoeba consists of a cell membrane filled with cytoplasm. They also contain a nucleus which controls growth and reproduction.
Amoebas move by changing the shape of their body. They do this by moving part of their body in the direction they want to travel. The rest of the cell then slowly follows.
A Name two parts in both animal cells and amoebas.
What do amoebas eat?
Amoebas eat algae, bacteria, and plant cells. They flow around tiny particles of food and form a food vacuole. This is called engulfing. The food vacuole then breaks down the food.
The contractile vacuole removes water and waste. It does this by contracting (getting smaller).
How do amoebas reproduce?
To reproduce, an amoeba divides in half to form two identical cells. This is called binary fission. Figure 2 shows binary fission.
Figure 2Amoebas divide by binary fission. Full text alternative to the content
Euglenas
A euglena is a microscopic unicellular organism found in fresh water. Figure 3 shows a euglena.
Figure 3Parts of a euglena. Full text alternative to the content
Like amoebas, euglenas have a cell membrane, cytoplasm, a contractile vacuole, and a nucleus.
Unlike amoebas, they also contain chloroplasts. These give the euglena a green colour. The chloroplasts take in Energy from the Sun. The euglena can then make food by Photosynthesis.
Euglenas have an eye spot to sense Light. They also have a flagellum. This is like a tail. By moving the flagellum, the euglena can ‘swim’ towards the light to photosynthesize. This allows the euglena to make as much food as possible.
B Give one way in which a euglena is different from an amoeba.
What do euglenas eat?
Sometimes, a euglena does not have enough light to make its own food. It then finds other things to eat, usually other microorganisms like bacteria and algae. Just like amoebas, they eat by flowing around and engulfing their food.
How do euglenas reproduce?
Like amoebas, euglenas reproduce by binary fission.
Stretch zone
- Copy and complete the sentences. Amoebas and euglenas are examples of blank organisms. This means that they are only made up of blank cell. Both organisms reproduce byblank.
- Amoebas contain two types of vacuole. Describe the function of the food vacuole.Describe the function of the contractile vacuole.
Stretch zone
- A student suggested that a euglena is a plant cell. Describe one piece of evidence to support the student’s suggestion.Give one reason why a euglena would not be a plant cell.