How is digestion a series of chemical reactions?
Foods are complex materials. Reducing them to useful nutrients requires several stages of chemical processing, called digestion.
From the mechanical breakdown by chewing and the introduction of enzymes from saliva to begin the chemical transformation of foods, digestion is a continuous process of chemical reactions and processes up to the final water reclamation and waste disposal.
The stomach uses hydrochloric acid to break down structural membranes in foods, which then go into the small intestine for further decomposition by enzymes. The useful materials - proteins, carbohydrates and vitamins - are extracted primarily in the small intestine. The large intestine extracts additional nutrients while re-adsorbing as much moisture from the food and associated liquids as possible.