Solution

Digestion: The process of conversion of non-diffusible form of food into the simple and diffusible form by chemical and mechanical processes in the alimentary canal, is called digestion. Digestion is necessary because: (i) Large molecules must be broken down into molecules small enough to permeate living membranes. (ii) Insoluble substances must be changed into substances which can be carried in solution around body. (iii) Complex molecules must be converted into molecules simple enough to be used by cells in the assimilation.