The number of cells in the body varies with different organisms. In some lower forms of life (Example: Euglena, Amoeba) The body is formed of a single cell; these organism are  described as unicellular. In a unicellular organism, all life activities are carried out by itself with its internal structures. The intracellular structures are called the organelles.

In the vast majority of plants and animals, the body is made up of numerous cells. They are called multicellular organisms. A multicellular organism is actually an aggregation of cells. Here a group of cells functions in a same way to form a tissue or an organ (Example: Xylem vessels, cork cells, cells in the skin). Multicellular organisms have a great capacity to survive than unicellular organisms.

Unicellular Organism vs Multicellular Organism
Unicellular Organism
Multicellular Organism
The body of the organism is composed of a single cell. The body of the organism is composed of numerous cells.
Division of labour is at the organelle level.
It gives a low level of operation efficiency.
Division of labour may be at cellular, tissue, organ and organ system level.
It gives high degree of operational efficiency.
A single cell carries out all the life processes.
Unicellular organism- Amoeba proteus
Different cells are specialized  to perform different functions.
Different types of cells in multicellular organisms
The cell body is exposed to the environment on all sides. Only outer cells are specialized to face the environment. Inner cells are devoted to other functions.
A cell body cannot attain a large size because of the limit imposed by surface area to volume ratio. A multicellular body can attain a large size increasing the number of small cells.
An injury of the cell can cause death of the organism. Injury or death of some cells does not affect the organism as the same can be replaced by new one.
Power of division is not lost. Certain specialised cells lose power of division.
A well marked capacity of regeneration is present. The capacity of regeneration decreases with increasing specialisation.
The cells has the same role for itself and the organism. Cells have double role. One for themselves and other for the organism.
Life span is short due to heavy load of work. Life span is ling due to limited load of work for each cell type.

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