Digestion breaks down foods into nutrient molecules that are small enough to be absorbed into an animal's circulatory system. Following digestion, nutrients are delivered to cells, where energy is extracted from their chemical bonds. Digestion often begins with a mechanical tearing apart of food into smaller pieces, which are then chemically dismantled in a stepwise fashion.
Because digestive chemicals are harsh, food processing in an animal's body takes place in compartments. Some single-celled organisms, such as protista, sequester food particles in food vacuoles, where enzymes break them down. Simple multicellular organisms, such as hydra and flatworms, have one-opening digestive systems. They must digest the nutrients and expel the waste before eating anew. Digestive systems of more complex animals have two openings, allowing simultaneous ingestion, digestion, and excretion. Roundworms have the simplest two-opening digestive system, which is little more than a tube. Enzymes in the tube break down the food, and nutrients are absorbed from there into the body fluids. More complex animals, such as vertebrates, have a gastrointestinal tract that is specialized into compartments where digestive enzymes and other substances process the food. Waves of muscular contraction called peristalsis help to move the food along.
Digestive system specializations reflect the lifestyles of their owners. Birds eat and digest nearly all the time. They can store food in an enlarged sac called a crop, and have a muscular organ called a gizzard that uses small pebbles to grind food. In some migratory species, the intestines actually enlarge before a long flight, enabling the animal to obtain energy throughout the journey. Ruminants such as cows have several stomachs, which contain cellulose -digesting bacteria, enabling them to digest grasses.
In humans, digestion begins at the mouth, where teeth tear food into small pieces, and the enzyme salivary amylase begins the breakdown of starch. During swallowing, food travels quickly through the esophagus , landing in the stomach. Here, the food is churned and further mechanically broken down as it mixes with gastric juice into a slurry called chyme. Each day, 40 million cells that line the stomach's interior release up to three quarts of gastric juice, which consists of water, mucus, salts, hydrochloric acid, and the enzyme pepsin, which breaks down protein into peptides. Hydrochloric acid unwinds proteins and kills many microorganisms.
After a length of time that reflects the components of the meal, a drawstringlike muscular structure called the pyloric sphincter at the stomach's exit opens, and chyme squirts into the duodenum, the first ten inches of the small intestine. The next two segments are the jejunum and the ileum. In addition to peristalsis, the small intestine undergoes localized muscle contractions that slosh the chyme back and forth, exposing it to several types of digestive enzymes. Trypsin and chymotrypsin continue the breakdown of peptides, and then peptidases break these down further into amino acids . Carbohydrases and pancreatic amylase continue the carbohydrate digestion that began in the mouth, and nucleases break down deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Bile, which is produced in the liver and stored in the gallbladder, emulsifies fats, which are then chemically digested by lipases into fatty acids and monoglycerides. The pancreas secretes trypsin, chymotrypsin, amylase, lipase, and nucleases.
Absorption of most of the products of digestion occurs in the small intestine. Water, electrolytes , and minerals are absorbed in the large intestine. The remaining material, which consists mostly of bacteria, bile, cellulose and shed intestinal lining cells, is compacted into feces in the rectum, and exits the body through the anus.
SEE ALSO Digestive System
Alexander, R. McNeill. "News of Chews: The Optimization of Mastication." Nature 391 (1998): 329–331.
Johnson, Leonard R., and Thomas A. Gerwin. Gastrointestinal Physiology. New York: Mosby, 2001.