The following describes the main chemical interactions that happen, at a cell and molecular level, during digestion. Like all other descriptions in this site, my descriptions are limited by my knowledge at the time of writing. Also, I have only picked the main steps in digestion. Many non-cell-enzyme-type interactions are also happening, as well as other processes I don't mention (yet). Plus, to-date even our collective knowledge only covers so much of the whole picture.
The digestive system follows food from when it enters the mouth, is passed on to the stomach, duodenum, and small and large intestines, and then out of the body. The system is composed of many types of cells specific to digestion, as well as such cells as nerves, blood, epithelial, and so on.
Mouth: amylase and starches
The main components of the saliva are:
Ptyalin – converts starch to simple soluble sugars.
Amylase – converts starch to soluble sugars.
Betaine – maintains cell fluid balance as osmolytes.
The main enzyme, amylase in the saliva, breaks down the starches, also called polysaccharides (chains comprising many complex sugar molecules linked together), into the individual sugar molecules.
Stomach: begins breaking down protein molecule chains
The main enzymes doing work in the stomach are:
Pepsin is the main gastric enzyme - is activated by hydrocholric acid and breaks proteins into smaller peptide fragments.
Gelatinase - degrades type I and type V gelatin and type IV and V collagen, which are proteoglycans in meat.
Gastric amylase - degrades starch, but is of minor significance at this level.
Gastric lipase - a tributyrase by its biochemical activity, as it acts almost exclusively on tributyrin, a butter fat enzyme.
Renin - changes liquid milk to solid.
Duodenum: breaks food molecules into smaller sizes
The duodenum provides a space where juices from the pancreas and liver can mix with the liquified food and break up the large food molecules.
The enzymes found in pancreatic juice break down all of the major nutrients, including carbohydrates, proteins and fats:
Pancreatic lipase – degrades triglycerides into fatty acids and glycerol.
Chymotrypsin – converts proteins to aromatic amino acids.
Carboxypeptidase – degrades proteins into fragments and amino acids.
Pancreatic amylase – degrades carbohydrates to simple sugars.
Elastase – degrades the protein elastin.
Pancreatic nucleases – converts nucleic acids to nucleotides and nucleosides.
Trypsin – converts proteins to basic amino acids.
Steapsin – breaks down of triglycerides to glycerol and fatty acids.
Phospholipase – hydrolyzes phospholipids into fatty acids and lipophilic substances.
The pancreas also produces icarbonate, which reduces the acidity of the fluid coming from the stomach.
The liver secretes bile. Bile is not an enzyme. It contains bile salts that break large fat droplets into smaller ones. This increases the surface area of the fat and makes it easier for lipase to reach and degrade more fat molecules.
Small intestine: absorbes food molecules into the blood and lymphatic systems
The main enzymes that work in the small intestine are:
Cholecystokinin – stimulates digestion of proteins and fats.
Secretin – controls secretion of duodenum and osmoregulation.
Sucrase – converts sucrose to disaccharides and monosaccharides.
Maltase – converts maltose to glucose.
Lactase – converts lactose to glucose and galactose.
Isomaltase – converts maltose to isomaltose.
Large intestine
The large intestine produces no enzymes and does no digestion. Its main function is to absorb water from the waste and expel it from the body.