<Tr> <Td> </Td> <Td> This section needs expansion with: digestion of other substances . You can help by adding to it . (August 2011) </Td> </Tr> <P> Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids . The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens . For example, trypsin is secreted by pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin . Trypsin then cleaves proteins to smaller polypeptides . </P> <P> Digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides . However fats are mainly digested in the small intestine . The presence of fat in the small intestine produces hormones that stimulate the release of pancreatic lipase from the pancreas and bile from the liver which helps in the emulsification of fats for absorption of fatty acids . Complete digestion of one molecule of fat (a triglyceride) results a mixture of fatty acids, mono - and di - glycerides, as well as some undigested triglycerides, but no free glycerol molecules . </P> <P> In humans, dietary starches are composed of glucose units arranged in long chains called amylose, a polysaccharide . During digestion, bonds between glucose molecules are broken by salivary and pancreatic amylase, resulting in progressively smaller chains of glucose . This results in simple sugars glucose and maltose (2 glucose molecules) that can be absorbed by the small intestine . </P>

Where does chemical digestion first occur to fats