The small intestine is a long tube-like network connecting the stomach and large intestine. This organ plays a role in both digestion and absorption. Enzymes secreted by the pancreas and intestinal wall and bile from the gall bladder aid in breaking down macronutrients and food particles, while ATPases and other cellular enzymes help molecules pass across the mucosa to the blood.
Secretions into the small intestine
Other organs affect digestion in the small intestine. One is the pancreas. Pancreatic enzymes secreted into the small intestine include amylase, which breaks down sugars, lipases that break down lipids, and proteases, which break down proteins into smaller peptides. The pancreatic proteases include trypsin, chymotrypsin, and carboxypeptidases. Another enzyme involved in the digestion of proteins is pepsin. The stomach secretes pepsinogen, which is converted to pepsin by the acidity of the stomach environment and passes into the small intestine with the food.
Another important secretion is bile. Bile acids are produced in the liver from cholesterol and stored in the gall bladder. After secretion into the small intestine, the bile acids break down lipids and cholesterol in a process known as emulsification.
Intestinal digestive enzymes
Brush border hydrolases are attached to intestinal wall cells (the absorptive enterocytes) and conduct the final breakdown of dietary sugars into monosaccharides. This family of enzymes widely varies with specific members for specific sugars. Examples are sucrase, maltase, and lactase, which break down sucrose, maltose, and lactose, respectively. Brush border peptidases are similarly attached to the absorptive enterocytes but break down peptides into amino acids. Aminopeptidases specifically attack the N-terminal of peptides.
Enzymes involved in absorption
ATPases are enzymes that catalyze the removal of a phosphate from ATP (adenosine triphosphate), a common form of energy storage and usage in human cells. The energy released from breaking that chemical bond is used to transport molecules across the cell membrane. For the cells lining the small intestine, these enzymes create an electrochemical gradient for sodium transport across the mucosa from the intestinal lumen (i.e. the interior of the intestine where food is digested). Thousands of these sodium pumps are found in each intestinal cell.
This gradient also aids in the absorption of water from the intestines into the blood stream via an osmotic gradient, as well as glucose and galactose via co-transport with sodium and the sodium-dependent transport of amino acids. Some amino acids are transported via other membrane proteins and mechanisms. Water and ions are also transported in the other direction by crypt cells as part of an absorption-reabsorption cycle in the small intestine. An enzyme involved in this opposite flow is adenylyl cyclase.
For lipid absorption, fatty acids and monoglycerides obtained by the digestion of lipids are transported into the intestinal cells (i.e. enterocytes), where they are processed to produce triglycerides, lipoproteins, and chylomicrons, which involves the various cellular enzymes in the fatty acid metabolism pathway. One required enzyme is acetyl coenzyme A synthetase.
For visual representations of some of the processes mentioned, see the New Human Physiology online textbook.