The next step of carbohydrate digestion takes place in the duodenum. The chyme from the stomach enters the duodenum and mixes with the digestive secretions from the pancreas, liver, and gallbladder. Pancreatic juices also contain an amylase enzyme, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in cells lining the small intestine. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and lactase breaks down lactose (or “milk sugar”) into glucose and galactose. The monosaccharides (e.g., glucose and fructose) thus produced are absorbed by the intestinal cells and transported into the bloodstream. The steps in carbohydrate digestion are summarized in [link] and [link] .
| Digestion of Carbohydrates | ||||
|---|---|---|---|---|
| Enzyme | Produced By | Site of Action | Substrate Acting On | End Products |
| Salivary amylase | Salivary glands | Mouth | Polysaccharides (Starch) | Disaccharides (maltose), oligosaccharides |
| Pancreatic amylase | Pancreas | Small intestine | Polysaccharides (starch) | Disaccharides (maltose), monosaccharides |
| Oligosaccharidases | Lining of the intestine; brush border membrane | Small intestine | Disaccharides | Monosaccharides (e.g., glucose, fructose, galactose) |
Protein
A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes—trypsin, elastase, and chymotrypsin—act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are summarized in [link] and [link] .
| Digestion of Protein | ||||
|---|---|---|---|---|
| Enzyme | Produced By | Site of Action | Substrate Acting On | End Products |
| Pepsin | Stomach chief cells | Stomach | Proteins | Peptides |
|
Pancreas | Small intestine | Proteins | Peptides |
| Carboxypeptidase | Pancreas | Small intestine | Peptides | Amino acids and peptides |
|
Lining of intestine | Small intestine | Peptides | Amino acids |
Lipids
The bulk of lipid digestion occurs in the small intestine, via the action of pancreatic lipase. When chyme enters the duodenum, it triggers a hormonal response resulting in the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides, by emulsification. Emulsification is a physical process in which large lipid globules are dispersed into several small lipid globules. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form large globules to minimize exposure to water. These small globules have a larger surface-to-volume ratio and thus an increased surface area for the lipases to interact with. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.
By forming an emulsion, bile salts increase the available surface area of the lipid particles significantly. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in [link] . Lipases break down the dietary triglycerides into fatty acids and monoglycerides (one fatty acid attached to a glycerol molecule). These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. Lipase products (fatty acids and monoglycerides) pass through the intestinal cells where they are reassembled into triglycerides, and then are combined with proteins to form large fatty complexes called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. The surface is also composed of the hydrophilic phosphate "heads" of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood via the thoracic duct on their way to the liver.
Summary of digestion
Elimination
The final step in digestion is the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed. Recall that the colonis also home to the microflora called “intestinal flora” that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. Asthe rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of therectum.