Digestion

Digestion

Digestion is the process of breaking down food into molecules that cells can absorb. Carbohydrates, proteins, nucleic acids, and fats are broken down into their smallest units (monomers) by digestive enzymes. These hydrolytic enzymes break chemical bonds through a reaction that requires water. Each hydrolytic enzyme is named after the substances it hydrolyzes. For example, carbohydrases break carbohydrates into single sugars (monosaccharides), proteases break proteins into amino acids, nucleases break nucleic acids into nucleotides, and lipases hydrolyze fats to fatty acids.

The Digestive System

The digestive system of humans consists of a one-way digestive tract with several specialized chambers along the way—mouth, stomach, small intestine, and large intestine. Each chamber has a specific function. In the human digestive system, digestion starts in the mouth. There food is physically digested by the action of our jaws and teeth, which break it down into smaller pieces, increasing the surface area for the digestive enzymes to work on. While being chewed, the food is mixed with saliva. Saliva makes the food slippery for swallowing, and also contains the enzyme amylase, a carbohydrase that breaks down starch into smaller polysaccharides. Once food is swallowed it passes along the digestive system through the activity of peristalsis (wavelike smooth muscle contractions). The passage of food from one chamber to the next is regulated by sphincters, or ringlike muscles.

Once the food arrives in the stomach, it is mixed with gastric juice, which contains hydrochloric acid (HCL) and pepsin. HCL kills most swallowed bacteria, and breaks down most food into individual cells, further increasing the surface area for enzyme attack. Pepsin (a protease) begins the hydrolysis of proteins into smaller polypeptides. The main site of human digestion and absorption of nutrients is the small intestine. There accessory glands of the digestive system, such as the pancreas and the liver, secrete their products into the digestive tract. The pancreas secretes bicarbonate ions that neutralize the acid from the stomach, protecting the digestive enzymes of the small intestine. The pancreas also releases a carbohydrase (pancreatic amylase), which continues the carbohydrate digestion started by salivary amylase in the mouth. The resulting disaccharides (e.g., maltose) are further hydrolyzed into monosaccharides (e.g., glucose) by enzymes (e.g., maltase), which are built into the membranes of cells lining the small intestine. This is also where sugar is absorbed from the lumen of the digestive tract into the cells lining the digestive tract.

Other enzymes that are produced and released by the pancreas are pro-teases (e.g., trypsin), nucleases, and lipases. Proteases attached to the membrane of cells lining the walls of the digestive tract are responsible for hydrolysis of small peptides to amino acids (which are then absorbed), and attached nucleases process nucleotides whose components are also taken up.

For the pancreatic lipases to be efficient, the lipid clumps that form in the watery environment of the digestive tract have to be broken into tiny droplets, which will increase the surface area for the lipases to work on. This is the function of bile salts, which are produced by the liver and stored in the gall bladder before being released into the small intestine. The fatty acids resulting from the action of the lipases are then absorbed.

While digestion and absorption of nutrients are taking place in the small intestine, peristalsis slowly pushes the content of the small intestine into the large intestine, where water and ion absorption are taking place. In the large intestine, populations of bacteria live on material that is not digestible by humans. As a byproduct of their metabolism, they produce gas as well as vitamins such as vitamin K that can be absorbed.

Control and Digestion

How is it possible to digest food molecules and not the cells of the digestive system itself, which is made of the same molecules? Most enzymes are produced in an inactive form called zymogens, that do not affect the cells that produce them. These zymogens are then released into the digestive system where they are activated. The cells lining the digestive tract are protected from the active enzymes by a thick layer of mucus. However, the mucus lining is constantly eroded, and if the lining is eroded faster (e.g., by acid-resistant bacteria) than it is regenerated, stomach ulcers may occur.

To make efficient use of all the nutrients contained in food, control and coordination of the digestion process is crucial. This is accomplished by several negative feedback systems. For example, when we see, smell or taste food, our brain signals the stomach to secrete gastric juice. When food proteins are actually present in the stomach, they trigger the release of the hormone gastrin into the blood stream. This hormone causes the cells in the stomach wall to release even more gastric juice, ensuring that proteins in the stomach are properly predigested. If the stomach becomes too acidic, the release of gastrin—and thus the production of gastric juice—ceases. When the acidic content of the stomach enters the small intestine, another hormone, secretin, is released into the blood stream from the cells lining the small intestine. Secretin causes the pancreas to dump bicarbonate from the pancreas into the small intestine, buffering the acid. When amino acids or fatty acids are detected in the small intestine, another hormone, choleocystokinin (CCK), is released from the intestinal cells into the blood. CCK triggers the gall bladder to release bile and the pancreas to release its digestive enzymes. At the same time, CCK inhibits the peristalsis of the stomach thus slowing down food transport. This allows enough time for the digestion of the food already present in the small intestine before more food from the stomach arrives.

see also Digestive System; Homeostasis.

Kathrin F. Stanger-Hall

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