<P> Food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter . </P> <P> The small intestine is where most chemical digestion takes place . Many of the digestive enzymes that act in the small intestine are secreted by the pancreas and liver and enter the small intestine via the pancreatic duct . Pancreatic enzymes and bile from the gallbladder enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients . Secretin, another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach . </P> <P> The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates: </P> <Ul> <Li> Proteins are degraded into small peptides and amino acids before absorption . Chemical breakdown begins in the stomach and continues in the small intestine . Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides . Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time . Aminopeptidase and dipeptidase free the end amino acid products . </Li> <Li> Lipids (fats) are degraded into fatty acids and glycerol . Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides . Pancreatic lipase works with the help of the salts from the bile secreted by the liver and stored in the gall bladder . Bile salts attach to triglycerides to help emulsify them, which aids access by pancreatic lipase . This occurs because the lipase is water - soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings . The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption . </Li> <Li> Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose). Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides . Other carbohydrates pass undigested into the large intestine and further handling by intestinal bacteria . Brush border enzymes take over from there . The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides . Other brush border enzymes are maltase, sucrase and lactase . Lactase is absent in some adult humans and, for them, lactose, like most poly - saccharides, is not digested in the small intestine . Some carbohydrates, such as cellulose, are not digested at all, despite being made of multiple glucose units . This is because the cellulose is made out of beta - glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha - glucose . Humans lack the enzyme for splitting the beta - glucose - bonds, something reserved for herbivores and bacteria from the large intestine . </Li> </Ul>

Types of food digested in the small intestine
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