Editing Small intestine (section)

===Digestion===
The small intestine is where most chemical digestion takes place. Many of the [[digestive enzyme]]s 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 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.

The three major classes of nutrients that undergo digestion are [[protein]]s, [[lipid]]s (fats) and [[carbohydrate]]s:
*  Proteins are degraded into small [[peptide]]s and [[amino acid]]s before absorption.<ref>{{cite journal |author=Silk DB |title=Progress report. Peptide absorption in man|journal=Gut |volume=15|issue=6|pages=494–501|year=1974|pmid= 4604970 |pmc=1413009 |doi=10.1136/gut.15.6.494}}</ref> 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.
* Lipids (fats) are degraded into [[fatty acid]]s and [[glycerol]]. Pancreatic lipase breaks down [[triglyceride]]s into free fatty acids and [[monoglyceride]]s. 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 [[emulsion|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.
* Some carbohydrates are degraded into simple sugars, or [[monosaccharide]]s (e.g., [[glucose]]). Pancreatic amylase breaks down some carbohydrates (notably [[starch]]) into oligosaccharides. Other carbohydrates pass undigested into the large intestine for 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 (a disaccharide), as well as most polysaccharides, 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.