A unicellular organism has a single cell carrying out all life process. This is unable to exhibit a wide range of different functions and easily susceptible to damage, that can lead to the death of the organism.
For example: Amoeba, Chlamydomonas etc. A multicellular organism has different cells carrying out different functions. They are more efficient and carries out no. of activities. They have a greater capacity of survival and they are continuously replaced.
For example: Cat, Dot, Human being etc.
