Digestion is the process that breaks food down into particles that the body can absorb and use. The break-down uses both mechanical force --- like chewing and the muscle action that moves food through the digestive tract --- as well as enzymes that chemically degrade foods into their constituent molecules. The process begins in the mouth, continues in the stomach and ends in the small intestines, with contributions from the pancreas and liver along the way.
Amylase
Salivary amylase is secreted into the mouth as the first enzyme to begin the process of digestion. Amylase, also known as ptyalin, degrades starches, which are complex sugars known as polysaccharides, into simpler sugar molecules called disaccharides.
Pepsin
Pepsin is the main digestive enzyme in the stomach. It acts on proteins, breaking them into smaller pieces called peptides. Pepsin is made from a precursor called pepsinogen. In the presence of stomach acid, pepsinogen becomes pepsin. Pepsinogen and stomach acid are released together in a mixture known as gastric juice. The stomach has a protective lining of mucus to thwart the corrosive effects of gastric juice which prevents it from digesting itself.
Trypsin and Chymotrypsin
When food enters the small intestines, proteins and starches are only partially digested, and digestion of fats has not started. The enzymes trypsin and chymotrypsin enter the small intestines as components of pancreatic juice. These enzymes acts as proteases, breaking proteins into peptides.
Other Pancreatic Enzymes
Pancreatic juice also contains pancreatic amylase and lipase. Like salivary amylase, pancreatic amylase digests starches into simpler sugars, primarily maltose. Lipase begins the digestion of fats or lipids, converting them into fatty acids and glycerol.
Role of the Liver and Gallbladder
Although the liver and gallbladder do not produce digestive enzymes, their activity is essential to digestion. The liver produces a digestive juice called bile that is stored in the gall bladder and released into the small intestines as needed. Bile emulsifies fats, scattering them into individual droplets to allow the fat-digesting enzymes of the small intestines to do their job.
