Multicellular organisms are composed of more than one cell, while unicellular organisms are a single living cell. This difference determines the size and complexity of an organism, as well as the division of labor within it. Each kind of organism has its own set of survival benefits.
Multicellular organisms are naturally larger and more complex than unicellular organisms. They obtain a high degree of operational efficiency because they distribute subsistence tasks at the organelle, cell, tissue, organ and organ system levels. Moreover, different cells are designed to perform specialized functions. Unicellular organisms, on the other hand, achieve a low level of operational efficiency because a single cell performs all life functions, and the only division of labor occurs at the organelle level.
The cell body of a unicellular organism is exposed to the environment. This places the organism at greater risk, because injury to the cell can cause the death of the organism. With multicellular organisms, only outer cells designed to face the environment are exposed to the outside world. The death of some cells does not necessarily put the entire organism at risk because it simply replaces them with new ones. This process is quite common in some organisms; humans, for example, are constantly replacing skin and hair cells.