One cell, all life processes
A unicellular organism is made of only one cell, yet that single cell must carry out every life process: nutrition, respiration, excretion, movement, response and reproduction. Common examples are Amoeba, Paramecium and Euglena.
How the single cell copes
- Movement — Amoeba uses pseudopodia (false feet); Paramecium uses tiny hairs called cilia; Euglena uses a whip-like flagellum.
- Nutrition — Amoeba engulfs food; Euglena has chloroplasts and can make food by photosynthesis.
- Gaseous exchange and excretion — happen by diffusion across the plasma membrane, because the cell is very small.
- Osmoregulation — a contractile vacuole pumps out excess water that enters by osmosis.
- Reproduction — usually by binary fission, one cell dividing into two.
Example
An Amoeba surrounds a food particle with its pseudopodia to form a food vacuole, where the food is digested inside the single cell.
Key idea
A unicellular organism has a large surface-area-to-volume ratio, so gases and wastes can move fast enough by diffusion alone — no transport system is needed.
Responding and surviving
A unicellular organism can also respond to its surroundings. Euglena swims towards light so it can photosynthesise, and Amoeba moves away from harmful chemicals or bright light. Some, such as certain bacteria, form a tough resting stage called a cyst or spore when conditions become dry or too hot, then become active again when things improve and enough water and warmth return. In this way a single cell manages to sense danger, avoid harm, find food and stay alive in a changing environment.
Because everything happens in one cell, unicellular organisms are simple but complete living things. They remind us that the cell is truly the basic unit of life: a single one can perform all the functions that, in a large animal, are shared out among many specialised cells, tissues and organs.