HBS' Writing

Unicellular organisms (also known as ‘single-celled organisms’) are organisms that contain only one cell. Because of this, we can’t see them with our naked eyes; we would need to use a microscope. There are a lot of different types of unicellular organisms (they are one of the oldest types of organisms on earth); the ones below are the amoeba, the euglena and the bacterium.

Amoeba

Amoeba can be found in fresh and salt water, wet soil and in animals. They eat algae, bacteria and plant cells. Just like animal cells, amoeba contain a nucleus, a cell membrane and cytoplasm. In addition, they also have a pseudopod, a food vacuole and a contractile vacuole.

Since amoebas do not have a cell wall, they do not have a fixed shape. This allows them to move freely in any direction they wish, which is done by extending their pseudopods.

Amoebas also use their pseudopods to eat. This is done in a process called ‘engulfing’. The amoeba will extend its pseudopod which will surround the food particles and form a food vacuole. The food vacuole will then digest the food.

To reproduce, Amoeba use binary fission, or splitting itself into two cells. The nucleus divides first, followed by the cytoplasm, to produce two identical, ‘daughter’ cells’.

Euglena

Euglenas can be found in fresh water. Like amoebas, they have a nucleus and cytoplasm. They also contain a flagellum, eye spot, contractile vacuole and chloroplasts.

The chloroplasts in the euglena make it appear green, and also give it the ability to carry out photosynthesis. The eye spot detects sunlight, and the flagellum acts like a propeller to get the euglena towards the sunlight. Being in the sun means that the euglena can maximise the amount of food it makes.

If the euglena cannot find enough sunlight to carry out photosynthesis, it will eat other microorganisms- like algae and bacteria. They eat in the same way amoebas do- by engulfing.

Euglenas also reproduce using binary fission.

Bacterium

Bacterium (plural: bacteria) also contain cytoplasm, a cell wall and a cell membrane. However, they do not have a nucleus. It stores its DNA in two places: a DNA plasmid and a loose DNA, which is called chromosomal DNA. Unlike chromosomal DNA, plasmid DNA can move from one bacterium to another. This gives variation between bacteria.

Some bacterium can have one or more flagellum, like the ones on euglenas.