Difference Between Unicellular and Multicellular Organisms

Main Difference – Unicellular vs Multicellular Organisms

Unicellular and multicellular organisms are the two types of organisms found on earth. Unicellular organisms are often prokaryotes, who are simple in organization and small in size. Hence, they are usually microscopic. Most eukaryotes are multicellular, containing differentiated cell types in the body in order to perform various functions separately. The main difference between unicellular and multicellular organisms is that unicellular organisms contain a single cell in their body whereas multicellular organisms contain numerous cells in their body, differentiating into several types.

This article explains,

1. What are Unicellular Organisms
      – Definition, Structure, Characteristics, Examples
2. What are Multicellular Organisms
      – Definition, Structure, Characteristics, Examples
3. What is the difference between Unicellular and Multicellular Organisms

What are Unicellular Organisms

Single-celled organisms are known as unicellular organisms. Unicellular organisms are microscopic and contain simple organization in their body cell. Since a single cell works as the body, all the cellular processes occur inside the single cell. Most of the unicellular organisms are prokaryotes. Therefore, they membrane-bound organelles like nucleus or mitochondria. That means, there are no specialized compartments, concentrating each of the cellular function. Thereby, all the cellular functions occur in the cytoplasm itself. Asexual reproduction is prominent among unicellular organisms. Sexual reproduction mechanisms like conjugation are shown by bacteria. Some animals, plants, fungi and protists contain unicellular organisms as well in their lower organization levels. Paramecium and Euglena are unicellular animals. Some algae are also unicellular organisms. Protozoans like amoeba and fungi like baker’s yeast are also unicellular organisms. Most unicellular organisms take things in by simple diffusion. But, amoeba is capable of engulfing food particles by surrounding the food particles by forming pseudopodia. A group of Paramecium is shown in figure 1.

Figure 1: A group of Paramecium

What are Multicellular Organisms

Organisms which have multiple cells are known as multicellular organisms. Most eukaryotic organisms are multicellular, containing a higher organization compared to unicellular organisms. Since multicellular organisms contain numerous cells in the body, their cells are differentiated into several types, specialized to perform different functions in the body. These differentiated cells are organized into organs, increasing the efficiency of the functions they perform. Multicellular organisms can increase their body size by increasing the number of cells as well. Since most of them are eukaryotes, their cells consist of membrane-bound organelles, which work as specialized compartments for unique function in the cell. Therefore, most cellular processes occur inside organelles rather than cytoplasm. Cells of the multicellular organisms are connected with each other by cell junctions like tight junctions and desmosomes. Cells communicate with each other by extracellular signaling.

Simple diffusion, as well as the active and passive diffusion mechanisms, are involved in taking things into the cell. Multicellular organisms reproduce both sexually and asexually. Asexual reproduction occurs by mitosis. Multicellular organisms sexually reproduce by producing gametes via meiosis. Higher organisms of animals, plants, and fungi are examples of multicellular organisms. A mushroom of the fungi Psilocybe semilanceata, which is a multicellular fungi is shown in figure 2.   

Figure 2: Psilocybe semilanceata mushrooms

Difference Between Unicellular and Multicellular Organisms

Number of Cells

Unicellular Organisms: Unicellular organisms contain a single cell in their body.

Multicellular Organisms: Multicellular organisms contain numerous cells in their body.

Membrane-bound Organelles

Unicellular Organisms: Most unicellular organisms lack of membrane-bound organelles.

Multicellular Organisms: Most multicellular organisms contain membrane-bound organelles like nucleus and mitochondria.

Membrane Transport Mechanism

Unicellular Organisms: Simple diffusion is used as the transport mechanism in unicellular organisms.

Multicellular Organisms: Simple diffusion, as well as the active and passive transport mechanisms, are used by multicellular organisms.

Cellular Processes/Differentiation

Unicellular Organisms: All the cellular processes are carried out by the single cell.

Multicellular Organisms: Cells in the body are differentiated in order to perform specialized functions.

Cell Junctions

Unicellular Organisms: No cell junctions are formed between cells of the unicellular organisms.

Multicellular Organisms: Cell junctions like desmosomes and tight junctions are formed between the cells in a multicellular organism.

Organs

Unicellular Organisms: Unicellular organisms do not have organs.

Multicellular Organisms: Multicellular organisms have different organs like lungs, kidney and heart.

Exposure to Environment

Unicellular Organisms: The cell body is directly exposed to the environment.

Multicellular Organisms: The outer cells in the body are specialized for the exposure to environment.

Large Size

Unicellular Organisms: Since the organism is composed of a single cell, unicellular organisms are unable to attain a large body size

Multicellular Organisms: A large size is attained by increasing the number of cells in the body of multicellular organisms.

Visibility

Unicellular Organisms: Unicellular organisms are only visible under the microscope.

Multicellular Organisms: Some of the multicellular organisms are visible under the light microscope but others are visible to naked eye.

Injury to Cells

Unicellular Organisms: Injury in the cell leads to the death of the organism.

Multicellular Organisms: Injury to a cell in multicellular organisms does not let the cell die.

Role

Unicellular Organisms: The roles of both cell and organisms are same in unicellular organisms.

Multicellular Organisms: Cells have a dual role, one for itself and the other for the whole organism.

Asexual Reproduction

Unicellular Organisms: Unicellular organisms mostly exhibit asexual reproduction like binary fission.

Multicellular Organisms: Multicellular organisms asexually reproduce by mitosis.

Sexual Reproduction

Unicellular Organisms: Unicellular organisms sexually reproduce by conjugation.

Multicellular Organisms: Multicellular organisms sexually reproduce by producing gametes.

Lifespan

Unicellular Organisms: Lifespan is too short in unicellular organisms.

Multicellular Organisms: Lifetime is long in multicellular organisms compared to unicellular organisms.

Regeneration Capacity

Unicellular Organisms: Unicellular organisms have a high regeneration capacity.

Multicellular Organisms: Multicellular organisms have a low regeneration capacity.

Examples

Unicellular Organisms: Prokaryotes like bacteria, cyanobacteria are unicellular organisms. Some of the protists like amoeba are unicellular. Eukaryotes like Paramecium and Euglena are unicellular organisms as well.

Multicellular Organisms: Most of the organisms on earth are multicellular like animals, plants, and fungi.

Conclusion

Unicellular and multicellular organisms are the two types of organisms found on earth. All prokaryotes are unicellular organisms, containing a single cell in their body. They are simple in organization and microscopic. All cellular processes occur inside the same call. In contrast, multicellular organisms consist of higher organization and are capable of growing large by increasing the number of cells in the body. Cells in the multicellular organisms are differentiated in order to perform specific functions inside the body. Differentiated cells for a particular function are concentrated into organs in multicellular organisms. The lifespan of multicellular organisms is much longer when compared to unicellular organisms. Therefore, the main difference between unicellular and multicellular organisms is their cellular organization.

Reference:
Lodish, Harvey. “The Architecture of Cells.” Molecular Cell Biology. 4th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 04 Apr. 2017.
Gilbert, Scott F. “The Evolution of Developmental Patterns in Unicellular Protists.” Developmental Biology. 6th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 04 Apr. 2017.
Gilbert, Scott F. “Multicellularity: The Evolution of Differentiation.” Developmental Biology. 6th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 04 Apr. 2017.

Image Courtesy:
1. “Grupo de Paramecium caudatum” By HernanToro – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Psilocybe semilanceata 6514” By Arp – Image Number 6514 at Mushroom Observer (CC BY-SA 3.0) via Commons Wikimedia