Life on Earth evolved from a single cell billions of years ago. Some of these cells are known as unicellular organisms. In a unicellular organism, all life processes, including digestion, secretion, and respiration, are contained in a single cell.
These are invisible to the naked eye and are therefore called microorganisms. There are many types of single-celled organisms, including bacteria, protozoa, algae, and fungi.
Characteristics of Unicellular Organisms
- Unicellular organisms usually reproduce asexually.
- They can be eukaryotes or prokaryotes.
- They are found almost everywhere, from hot springs to frozen tundra.
- They have box-like structures for navigation.
- Nutrients enter or leave the cell through the process of diffusion.
Types of Unicellular Organisms
Prokaryotes
- Prokaryotes are unicellular organisms without a true nucleus.
- They range in size from 0.1 to 5.0 μm. This facilitates the diffusion of ions and molecules to different parts of the cell.
- They have a peptidoglycan cell wall.
- The cell wall helps maintain cell morphology and prevent dehydration.
- They use flagella to move.
- They have fimbriae to attach to the host cell and pili to exchange genetic material during fusion.
Eukaryotes
- Eukaryotes are larger than prokaryotes and have membrane-bound nuclei.
- DNA in the cell nucleus.
- Eukaryotes can be plant cells or animal cells.
- Plant cells have a vacuole, chloroplasts, and a large central vacuole. These are not found in animal cells.
- Lysosomes and centrosomes are present in animal cells but not in plant cells.
Nutrition in Unicellular Organisms
Single-celled organisms eat other organisms or water. The digestion is intracellular. Larger particles are engulfed by phagocytosis or pinocytosis. Small particles enter the cell by osmosis and diffusion.
