An organism can be compared to machines used for various purpose. Each part of a machine carries out a specific function and a collection of parts performs diverse functions. The total structure and co-ordinate functions, both in organisms and in machines, make them an organized identity.

Similar to machines, organisms take raw materials in form of food. Energy is obtained from organic nutrients through the process of respiration. This energy utilized in their growth. The sum of all chemical reactions taking place within the living unit is called metabolism. The structure of a machine or an organism is not simple but rather complex. Some organisms are simple and unicellular (single-celled) like Amoeba, Paramoecium, algae, etc.

The single cell performs all the necessary life functions. Complex multicellular organisms (many-celled) like trees, animals and man have many specialized functions which are performed by special structures. Thus organization is present in every individual, simple or complex, single-celled or multi celled.