Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma.In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream.Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion.igestion begins in the mouth with the secretion of saliva and its digestive enzymes. Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis.