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
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.
Different cells are specialized to perform different functions.