Chemical Digestion, Absorption, and Transport

Abstract

Chemical digestion involves the catalytic processing of food in the gastrointestinal (GI) tract by digestive enzymes, aided by co-secreted substances, required to break down the food substances into simpler molecules for absorption. This process is necessary for transport and subsequent metabolic reactions that access the macroergic bonds in food molecules. The catalytic process of digestion starts from the mouth for lipids and carbohydrates. For proteins, the catalytic process begins from the stomach. The sources of the enzymes and co-secreted factors are the glands lining the GI tract, namely salivary, submucosal, gastric, and intestinal glands. This chapter provides up-to-date information on the course of discovery as well as the discoverers of major digestive enzymes in humans. The functions and mechanisms of action of all major digestive enzymes discovered up to the twenty-first century are discussed herein. This chapter is a useful reference source on the history and discovery of major digestive enzymes and the mechanisms of regulation of their functions. The clinical importance of the enzymes is systematically provided at strategic points of the discussion. This chapter provides cellular and molecular mechanisms on the absorption and transport of nutrients, ions, dietary elements, vitamins, toxic metals, and pharmacological drugs in the gut. Pathological implications of some of the components of GI metabolism are strategically outlined.