It is an essential physiological activity in all animals, whether they feed on minute food particles (microphagus) or on large plants and animals (macrophsgus).
Some of the internal body parasites, such as the tapeworms, can dispense with a digestive system and absorb food pre- digested by their hosts.
Whatsoever the food animals take that cannot be used directly by the cells for the metabolism c f their body.
It is because of two reasons; first, many substances of the food are insoluble in water and could not enter the plasma membranes of the cells even if they reached them.
Second, these foods are too complex chemically for growth and repair by protein synthesis.
Digestion brings about changes in both these conditions, with the result that substances of the food are converted into a state where they can be used directly in the cells of the body for the resynthesis of new cell constituents or as substrate in the machinery that converts the energy in the food into available energy for the organism.
Thus, in digestion, complex food molecules are broken down both mechanically and chemically, into smaller molecules of water soluble substances that can be used by the cells.
The first part of the change that occurs during digestion is mechanical, it involves the chewing of the food in the mouth (particularly in higher animals like man) and the constant churning and mixing actions brought about by the muscular movements of the walls of the digestive organs.
The second part of the change includes the breakdown of the large food molecules into smaller molecules or the component parts and the thorough mixing with various digestive juices secreted by different digestive organs.
Digestive juices contain many enzymes which help in digesting the various food substances. Digestion, in higher animals, occurs in digestive tract.