Unicellular Organisms

Life on earth evolved from a single cell millions of years ago. These single cells are known as unicellular organisms. All the life processes in a unicellular organism, including digestion, excretion, respiration, occur within a single cell. These cannot be seen by naked eyes and are hence called microorganisms. There are several kinds of unicellular organisms such as bacteria, protozoa, algae, fungi, etc.

Let us have a detailed look at the characteristics, types, and examples of unicellular organisms.

Characteristics of Unicellular Organisms

The characteristics of unicellular organisms are as follows:

1. The unicellular organisms usually reproduce by asexual means.
2. They can be eukaryotes or prokaryotes.
3. They are found in almost all habitats, from hot springs to frozen tundra.
4. They possess whip-like structures for movement.
5. The nutrients enter or leave the cell by the process of diffusion.

Also Read: Multicellular Organisms

Types of Unicellular Organisms

There are two types of unicellular organisms:

• Prokaryotes
• Eukaryotes

Prokaryotes

• Prokaryotes are unicellular organisms without a true nucleus.
• They are very small 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 to maintain the shape of the cell and prevents dehydration.
• They use flagella for locomotion.
• They possess fimbriae for attachment to the host cell, and pili to exchange genetic material during conjugation.
• Archaebacteria, eubacteria are a few examples of prokaryotes.

Eukaryotes

• Eukaryotes are larger than prokaryotes, with a membrane-bound nucleus.
• The DNA is present in the nucleus of the cell.
• Eukaryotes can be plant cells or animal cells.
• The plant cells contain vacuoles, chloroplast and a large central vacuole. These cannot be found in animal cells.
• Lysosomes and centrosomes are present in animal cells and not in plant cells.
• Eukaryotes include Protozoa and Protista.

Nutrition in Unicellular Organisms

Unicellular organisms feed on other organisms or liquid matter. The digestion is intracellular. Larger particles are ingested by phagocytosis or pinocytosis. The smaller sized particles enter into the cell through osmosis and diffusion.

Reproduction in Unicellular Organisms

Unicellular organisms reproduce by the following ways:

• The unicellular organisms reproduce by binary fission. In this, a single cell divides, giving rise to two daughter cells. This can be seen in bacteria and amoeba.
• The yeast cells reproduce by the process called budding.
• Sometimes amoeba reproduces by the process called encysting. During unfavourable conditions, it forms a protective covering around itself called a cyst. This cyst contains chitin that helps it to reproduce.

Respiration in Unicellular Organisms

The unicellular organisms respire aerobically or anaerobically. They do not have any respiratory structures for respiration. They respire by the process of diffusion through the skin surface.

Unicellular Organisms Examples

Following are some of the examples of unicellular organisms:

• Escherichia coli
• Diatoms
• Protozoa
• Protista
• Streptococcus
• Pneumococci
• Dinoflagellates

Also Read: Difference between Unicellular and Multicellular Organisms

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