Food digestion is completed in the small intestine, but the pancreas is the organ that secretes the enzymes that finish the job. Different types of enzymes digest different types of food molecules. The pancreas secretes proteases to break down protein, amylases to break down carbohydrates, lipases to break down fat, and nucleases to break down DNA and RNA. Some of these enzymes are so effective that the pancreas must secrete them as inactive enzymes that need to be activated, either by another protein or by their own activated siblings. This is a safeguard against accidental, uncontrolled enzyme activity and also allows for a sudden domino effect that turns on many enzymes by activating just a few. Pancreatic enzymes are secreted into the beginning of the small intestine, which has the neutral pH required for these enzymes to function.
The Snowball Effect
The pancreas secretes two main protease enzymes to digest protein: trypsin and chymotrypsin. Trypsin is secreted as an inactive form called trypsinogen; likewise, chymotrypsin is secreted as chymotrypsinogen. The cells that line the walls of the small intestine, into which the pancreas secretes these enzymes, contain a protein called enterokinase, which activates trypsinogen into trypsin. Once activated, trypsin then activates other trypsinogen pre-enzymes into trypsin, and also activates chymotrypsinogen into chymotrypsin. The effect is an activation cascade that results in a sudden burst of active enzymes that digest protein..
Partially digested food is passed from the stomach to the small intestine. The environment of the stomach is very acidic, having a low pH level between 2 and 3. But when the stomach contents enter the small intestine, buffer chemicals secreted by the pancreas and liver neutralize the acid. These chemicals are called buffers. The main one secreted by the pancreas is bicarbonate. Bicarbonate keeps the pH of the small intestine around 7, which is neutral. Pancreatic enzymes only function in this neutral pH.
Both proteins and carbohydrates (starches) are complex molecules that can form bulky structures like chains and branches. In order to digest them, the pancreas secretes several amylases to digest the starch and several proteases to digest the proteins. These enzymes work together to digest bulky molecules. Some amylases break the starch branches into single strands that are easier for other amylases to digest. Proteases secreted by the pancreas also exhibit this teamwork. Some proteases, called enteropeptidases, break a protein in the middle regions. Others, called exopeptidases, break pieces from the head and tail of the protein.
In addition to producing enzymes that work together, the pancreas also produces enzymes that need partners made from other organs in order to function. The pancreas secretes the enzyme lipase to digest fats. Similar to the theme of secreting inactive proteases as a safeguard against uncontrolled digestion, lipases do not function unless they are bound to partner colipase protein. Furthermore, pancreatic lipases need the help of salts from bile, a liquid produced by the liver that separates fat into tiny droplets.
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