Organisms can be of various types and complexities. An organism is made up of tiny divisions called cells. One cell can make up the whole organism, or many cells can arrange to form complex organisms. The more the cells, the more complex is the organism. Less the cells of organisms more the functions of the cells.
Unicellular vs Multicellular Organism
The main difference between unicellular and multicellular organisms is that one is made up of one cell while the other is made up of multiple cells. Unicellular organisms are composed of single cells, while multicellular organisms are composed of more than one or multiple cells, each with a specific function.
The term unicellular means “uni- one” and “cellular-cells”. Thus unicellular organism means an organism made up of only one cell. The one cell makes up the whole organism, and this cell in itself performs all the bodily functions of the cell. This there is no division of labour between cells.
The term multicellular means “multi- many” and “cellular-cells”. This multicellular organism means an organism with more than one cell. The number can be a few to over millions. These cells interact with each other to form the entire organism. Thus in these organisms, division of labour between cells is present.
Comparison Table Between Unicellular and Multicellular Organism
| Parameters of Comparison | Unicellular Organism | Multicellular Organism |
| No of Cells | Only one cell makes up the whole organism in unicellular organisms. | Few to many cells make up the whole organism in multicellular organisms. |
| Functions | One cell performs all the bodily functions in unicellular organisms. | Different cells are specialized to perform specific functions in multicellular organisms. |
| Tissue formation | There is no tissue formation in unicellular organisms. | Tissue formation takes place in multicellular organisms. |
| Differentiation | Cells do not differentiate in unicellular organisms. | In multicellular organisms, cell differentiation takes place. |
| Labour Division | There is no possibility of division of labour in unicellular organisms. | Division of labour is present in between the different cells of the whole organism. |
| Reproduction | Unicellular organisms usually reproduce in a simple manner. | Multicellular organisms usually have complex reproduction. |
What is Unicellular Organism?
An organism made up of only one cell is called a unicellular organism. These are very simple organisms in which one cell carries out all the required functions of the entire organism. Hence as the name already suggests, these organisms are extremely small, almost microscopic as they are merely one cell big.
This cell making up their entire body is not arranged into tissues as it cannot. The cell is not differentiated into specialized cells for individual functions; however, each cell possesses all the required cell organelles in the cytoplasm required to perform all the bodily functions for the survival of the cell or the entire organism.
As there is no tissue formation or specialization, the entire body of the organism looks the same, and the body organization is simple compared to complex multicellular organisms. There is a division of labour at the organelle level in unicellular organisms.
These organisms include short-lived prokaryotes mainly and some eukaryotes as well like yeast, bacteria, protistans, some protozoans and so on. Reproduction is mostly asexual, but conjugation may exist in some. In short, simple organisms with one cell and simple reproduction are called unicellular organisms.
What is Multicellular Organism?
An organism made up of multiple cells is called a multicellular organism. These are organisms that can be simple (made of few cells) or complex (made up of millions of cells). These cells of the organism rearrange themselves to form associations called tissues, which in turn associate to form organs, organs form organ systems, and organ systems make up the whole organism.
Each cell is specialized to perform a specific function. During the developmental stages of the organism, the cells undergo differentiation and specialization. The complex interaction between the cells exists through cell junctions and gaps. Thus a highly interactive organism with specialized cells makes up the organism.
Thus specific cells like muscle cells, nerve cells, bone cells etc., are present, which are different from each other both in structure and in function. Usually, these cells are also short-lived, but the organism altogether is long-lived. Thus multicellular organisms have a long life span.
Multicellular organisms usually include eukaryotes only like man, dogs, corals, plants and so on. Each of these organisms is complex and interacts with the environment with its outermost cell layer. Division of labour in these organisms exists at cellular, organ and organ system levels.
Main Differences Between Unicellular and Multicellular Organism
- Unicellular organisms include organisms with only one cell. Whole multicellular organisms include organisms with multiple cells.
- Unicellular organisms are usually short-lived, while multicellular organisms are usually long-lived organisms.
- Unicellular organisms usually have no tissue organization, while multicellular organisms have tissue organization.
- Injury to a cell causes death in a unicellular organism, while in multicellular organisms, repair and regrowth are initiated in case of injury. The individual cell may die, but the organism lives.
- Cell differentiation and specialization is absent in unicellular organisms whereas the following is present in all multicellular organisms in varying degree.
- Unicellular organisms usually die easily, whereas multicellular organisms are harder.
Conclusion
Thus with the number of cells increasing, the complexity of the organism increases; thus, with many cells, the organism achieves its bodily functions like breathing, digestion, reproduction, etc., with differentiated cells which perform specific functions. Thus the difference between the two is evident.
Thus unicellular organisms are simple and complex organisms are usually multicellular as more no of cells contribute to more functions achieved. Usually, in a unicellular organism, the whole-cell interacts with the environment, whereas in multicellular organisms, only the outer layer of cells interacts with the environment keeping the inner layers of tissues and organs safe and protected.
References
- https://www.sciencedirect.com/science/article/pii/0022519374902094
- https://link.springer.com/article/10.1007/s004420000492