What is life? Studying life takes us into the world of  living organisms. We get to know the way living organisms work that keeps them alive.When organisms breath or move , we say that they are alive. Plants do not move still they are living. There are certain characteristics of life that makes living organisms distinct from non living things.Just as, 'fire' is an abstract term which is defined by its characteristics i.e. it is hot, burns and destroys but cannot be defined exactly, the term 'life' is best considered as an abstraction. Therefore, we know life because of its characteristics, organization and activity of chemical substances present in the living system.
 All living organisms perform various functions. All those activities that maintain their existence are called life processes.

This chapter deals with the study of four life processes - nutrition, respiration, transportation and excretion that maintains the life of an organism.