Unicellular and Multicellular Organisms

The primary classification of living organisms on the basis of the number of cells comprising the living organism includes two classes: 

1. Unicellular organisms

2. Multicellular organisms

Although, the obvious difference between them is the number of cells constituting the organism. However, unicellular and multicellular organisms differ from each other in many ways. Lifespan, body complexity, function, nature, and habitat are differentiating factors to name. This article will discuss some important differentiating parameters between unicellular and multicellular organisms. 

Unicellular Organisms

Although, the cells are the common, basic structural, and functional unit of life for every organism. However, the cells have different characters and functions in unicellular and multicellular organisms. As obvious from the name, unicellular organisms are made up of single-cell and intracellular components such as Nucleus. The single cell is responsible for executing all the normal physiological functions in unicellular organisms. Examples of unicellular organisms include bacteria, protozoans such as Amoeba, etc. 

Multicellular Organisms

Multicellular organisms consist of multiple cells arranged in different functional units as a group of cells called tissues. Developed multicellular organisms have a group of tissues having dedicated specific physiological functions known as organs. Multicellular organisms include Plants, angiosperms, vertebrates, invertebrates, etc.

Unicellular and Multicellular Organisms: Differences

1. Body Organisation

In terms of body organisation, Multicellular organisms are more complex than unicellular organisms. Unicellular organisms are made up of single-cell and have simple body organisation. 

2. Functioning

The single cell in the unicellular organisms performs all the essential functions of life whereas, in multicellular organisms, cells are grouped into different functional units called tissues. 

3. Shape & Structure

Unicellular organisms lack a definite structure and are pleomorphic in nature, but multicellular organisms have a well-defined cellular outer structure and internal structure. For e.g., Amoeba is a free-living unicellular organism and is pleomorphic, i.e., it lacks a rigid, well-defined shape and structure. 

4. Size of Organism

Unicellular organisms are comparatively smaller in size than multicellular organisms. Unicellular organisms are microscopic, while multicellular organisms are macroscopic. 

5. Cellular Organisation

Unicellular organisms consist of both, Eukaryotes and Prokaryotes, whereas multicellular organisms only include Eukaryotes. 

Note: Prokaryotes have primitive cellular organisation and lack intracellular membrane-bound organelles. They also don’t have a well-defined nucleus. 

Eukaryotes have a well-defined nucleus as well as membrane-bound organelles such as chloroplast, mitochondria, etc. 

6. Mode of Nutrition

Unicellular organisms have heterotrophic modes of nutrition and depend on other forms for their survival. Whereas, Multicellular organisms include both heterotrophic as well as autotrophic nutritional modes. 

7. Evolutionary Stages

Unicellular organisms are primitive in nature and represent the oldest life forms on the earth, whereas multicellular organisms are advanced and evolved from primitive life forms in due course of evolution. 

Note: The process of evolution using natural selection enables the survival of the fittest. 

8. Mode of Reproduction

Unicellular and multicellular organisms also differ in the mode of reproduction. Unicellular organisms reproduce by Asexual mode of reproduction, whereas multicellular organisms reproduce by both sexual and asexual modes of reproduction. 

Note: Asexual reproduction does not involve the process of fertilization of male and female gametes. 

9. Rate of Multiplication

Unicellular organisms have a much higher rate of multiplication compared to that multicellular organisms because they reproduce by an asexual mode of reproduction. Sexual reproduction is a complex process, and thus, the rate of multiplication in sexually reproducing organisms is less. 

Note: A unicellular bacteria E.coli multiplies within 20minutes, whereas the birth of a human baby almost has a gestation period of 9months. 

10. Lifespan

Unicellular organisms have a rapid rate of multiplication but have a much shorter lifespan when compared with multicellular organisms. For example, a bacteria could die in minutes after it has been born, whereas humans live for about 60years on average. 

11. Consequences of Cellular Injury

A cellular injury in the case of unicellular organisms is always fatal because they only have one cell that acts as a basic structural and functional unit of life. But in the case of cellular injury to multicellular organisms, healing takes place through different regenerative mechanisms. 

Conclusion

There are plenty of parameters on which unicellular organisms can be differentiated from multicellular organisms. To summarise, let’s take a look at the tabular format of differences between the two types of organisms.