Digestion
Digestion is the process whereby a biological entity processes a substance, in order to chemically convert the substance into nutrients. Digestion occurs at the multicellular, cellular, and sub-cellular levels, usually in animals.
Digestion is usually divided into mechanical manipulation and chemical action. In most vertebrates, digestion is a multi-stage process in the digestive system, following ingestion of the raw materials, most often other organisms. The process of ingestion usually involves some type of mechanical manipulation.
Human digestion
In humans, digestion begins in the oral cavity where food is chewed (mastication) with the teeth. The process stimulates exocrine glands in the mouth to release digestive enzymes such as salivary amylase, which aid in the breakdown of food, particularly carbohydrates. Chewing (mechanical) also causes the release of saliva, which helps condense food into a bolus that can be easily passed through the esophagus. The esophagus is about 20 centimeters long. The food is pushed down by a movement called peristalsis, which is the wave-like contraction of smooth muscle tissue, characteristic of the digestive system. The uvula is a very sensitive organ that hangs from the roof of the mouth. Its main job is to close the nasopharynx, which prevents the food from entering the trachea. When swallowed, the food enters the pharynx, which makes special adaptations to prevent choking or aspiration when food is swallowed.
The food enters the stomach upon passage through the cardiac sphincter. In the stomach, food is churned and thoroughly mixed with a digestive fluid, composed chiefly of hydrochloric acid, and other digestive enzymes to further decompose it chemically for a few hours. As the acidic level changes in the stomach and later parts of the digestive tract, more enzymes are activated or deactivated to extract and process various nutrients.
After being processed in the stomach, food is passed to the small intestine via peristalsis. This is where most of the digestive process occurs. It passes through the pyloric sphincter and enters the first 10 inches of the small intestine, the duodenum, where it is further mixed with 3 different liquids, which are bile (which helps aid in fat digestion, otherwise known as emulsification), pancreatic juice (made by the pancreas), and intestinal juice. They also add several enzymes: maltase, lactase and sucrase, to process sugars. Trypsin and chymotrypsin are other enzymes added in the small intestine. (Bile also contains pigments that are by-products of red blood cell destruction in the liver; these bile pigments are eliminated from the body with the feces.) Most nutrient absorption takes place in the small intestine. The nutrients pass through the small intestine's wall, containing small, finger-like structures called villi. The blood, which has absorbed nutrients, is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.
After going through the small intestine, the food then goes to the large intestine. The large intestine has 3 parts: the cecum (or pouch that forms the T-junction with the small intestine), the colon, and the rectum. In the large intestine, water is reabsorbed, and the foods that cannot go thorugh the villi such as fiber, can be stored in large intestine. Fiber helps to keep the food moving through the G.I. tract. The food that cannot be broken down is called feces. Feces is stored in the rectum until it is expelled through the anus.
Specialized organs
Organisms develop specialized organs to aid in the digestion of their food, for example different types of tongues or teeth. Insects may have a crop (or the enlargement of oesophagus) while birds and cockroaches may develop a gizzard (or a stomach that acts as teeth and mechanically digests food). A herbivore may have a cecum that contains bacteria which can produce cellulase that helps break down the cellulose in plants. Ruminants, for example cows and sheep, have a specialised fore-stomach called a rumen where microbes help to break down cellulose before the food passes into the "true" stomach or abomasum.
Digestive hormones
There are at least four hormones that aid and regulate the digestive system:
- Gastrin - is in the stomach and stimulates the gastric glands to secrete pepsinogen and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH .
- Secretin - is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver. This hormone responds to the acidity of the chyme.
- Cholecystokinin (CCK) - is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.
- Gastric inhibitory peptide (GIP) - is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach.
Carbohydrate Digestion
The principal dietary carbohydrates are polysaccharides, disaccharides, and monosaccharides. Starches (glucose polymers) and their derivatives are the only polysaccharides that are digested to any degree in the human gastrointestinal tract. In glycogen, the glucose molecules are mostly in long chains (glucose molecules in 1:4a linkage), but there is some chain-branching (produced by 1:6a linkages. Amylopectin, which constitutes 80-90% of dietary starch, is similar but less branched, whereas amylose is a straight chain with only 1:4a linkages. Glycogen is found in animals, whereas amylose and amylopectin are of plant origin. The disaccharides lactose (milk sugar) and sucrose (table sugar) are also ingested, along with the monosaccharides fructose and glucose.
In the mouth, starch is attacked by salivary a-amylase. However, the optimal pH for this enzyme is 6.7, and its action is inhibited by the acidic gastric juice when food enters the stomach. In the small intestine, both the salivary and the pancreatic a-amylase also acts on the ingested polysaccharides. Both the salivary and the pancreatic a-amylases hydrolyze 1:4a linkages but spare 1:6a linkages, terminal 1:4a linkages, and the 1:4a linkages next to branching points. Consequently, the end products of a-amylase digestion are oligosaccharides: the disaccharide maltose; the trisaccharide maltotriose; some slightly larger polymers with glucose in 1:4a linkage; and a-dextrins, polymers of glucose containing an average of about eight glucose molecules with 1:6a linkages .
The oligosaccharidases responsible for the further digestion of the starch derivatives are located in the outer portion of the brush border, the membrane of the microvilli of the small intestine . Some of these enzymes have more than one substrate. a-Dextrinase, which is also known as isomaltase, is mainly responsible for hydrolysis of 1:6a linkages. Along with maltase and sucrase, it also breaks down maltotriose and maltose. Sucrase and a-dextrinase are initially synthesized as a single glycoprotein chain which is inserted into the brush border membrane. It is then hydrolyzed by pancreatic proteases into sucrase and isomaltase subunits, but the subunits reassociate noncovalently at the intestinal surface.
Sucrase hydrolyzes sucrose into a molecule of glucose and a molecule of fructose. In addition, there are two disaccharidases in the brush border: lactase, which hydrolyzes lactose to glucose and galactose, and trehalase, which hydrolyzes trehalose, a 1:1a-linked dimer of glucose, into two glucose molecules.
Deficiency of one or more of the brush border oligosaccharidases may cause diarrhea, bloating, and flatulence after ingestion of sugar. The diarrhea is due to the increased number of osmotically active oligosaccharide molecules that remain in the intestinal lumen, causing the volume of the intestinal contents to increase. In the colon, bacteria break down some of the oligosaccharides, further increasing the number of osmotically active particles. The bloating and flatulence are due to the production of gas (CO2and H+) from disaccharide residues in the lower small intestine and colon.
Lactase is of interest because, in most mammals and in many races of humans, intestinal lactase activity is high at birth, then declines to low levels during childhood and adulthood. The low lactase levels are associated with intolerance to milk (lactose intolerance). Most Europeans and their American descendants retain their intestinal lactase activity in adulthood; the incidence of lactase deficiency in northern and western Europeans is only about 15%. However, the incidence in blacks, American Indians, Orientals, and Mediterranean populations is 70- 100%. Milk intolerance can be ameliorated by administration of commercial lactase preparations, but this is expensive. Yogurt is better tolerated than milk in intolerant individuals because it contains its own bacterial lactase.
Protein Digestion
Protein digestion begins in the stomach, where pepsins cleave some of the peptide linkages. Like many of the other enzymes concerned with protein digestion, pepsins are secreted in the form of inactive precursors (proenzymes) and activated in the gastrointestinal tract. The pepsin precursors are called pepsinogens and are activated by gastric hydrochloric acid. Human gastric mucosa contains a number of related pepsinogens, which can be divided into two immunohistochemically distinct groups, pepsinogen I and pepsinogen II. Pepsinogen I is found only in acid-secreting regions, whereas pepsinogen II is also found in the pyloric region. Maximal acid secretion correlates with pepsinogen I levels.
Pepsins hydrolyze the bonds between aromatic amino acids such as phenylalanine or tyrosine and a second amino acid, so the products of peptic digestion are polypeptides of very diverse sizes. A gelatinase that liquefies gelatin is also found in the stomach. Chymosin, a milk-clotting gastric enzyme also known as rennin, is found in the stomachs of young animals but is probably absent in humans.
Because pepsins have a pH optimum of 1.6-3.2, their action is terminated when the gastric contents are mixed with the alkaline pancreatic juice in the duodenum and jejunum. The pH of the intestinal contents in the duodenal cap is 2.0-4.0, but in the rest of the duodenum it is about 6.5.
In the small intestine, the polypeptides formed by digestion in the stomach are further digested by the powerful proteolytic enzymes of the pancreas and intestinal mucosa. Trypsin, the chymotrypsins, and elastase act at interior peptide bonds in the peptide molecules and are called endopeptidases. The carboxypeptidases of the pancreas are exopeptidases that hydrolyze the amino acids at the carboxyl and amino ends of the polypeptides. Some free amino acids are liberated in the intestinal lumen, but others are liberated at the cell surface by the aminopeptidases, carboxypeptidases, endopeptidases, and dipeptidases in the brush border of the mucosal cells. Some di- and tripeptides are actively transported into the intestinal cells and hydrolyzed by intracellular peptidases, with the amino acids entering the bloodstream. Thus, the final digestion to amino acids occurs in three locations: the intestinal lumen, the brush border, and the cytoplasm of the mucosal cells.
Fat Digestion
A lingual lipase is secreted by Ebner's glands on the dorsal surface of the tongue, and the stomach also secretes a lipase . The gastric lipase is of little importance except in pancreatic insufficiency, but lingual lipase is active in the stomach and can digest as much as 30% of dietary triglyceride.
Most fat digestion begins in the duodenum, pancreatic lipase being one of the most important enzymes involved. This enzyme hydrolyzes the 1- and 3-bonds of the triglycerides (triacylglycerols) with relative ease but acts on the 2-bonds at a very low rate, so the principal products of its action are free fatty acids and 2-monoglycerides (2-monoacylglycerols). It acts on fats that have been emulsified. Its activity is facilitated when an amphipathic helix that covers the active site like a lid is bent back. Colipase, a protein with a molecular weight of about 11,000, is also secreted in the pancreatic juice, and when this molecule binds to the -COOH-terminal domain of the pancreatic lipase, opening of the lid is facilitated. Colipase is secreted in an inactive proform and is activated in the intestinal lumen by trypsin.
Another pancreatic lipase that is activated by bile salts has been characterized. This 100,000-kDa bile salt-activated lipase represents about 4% of the total protein in pancreatic juice. In adults, pancreatic lipase is 10-60 times more active, but unlike pancreatic lipase, bile salt-activated lipase catalyzes the hydrolysis of cholesterol esters, esters of fat-soluble vitamins, and phospholipids, as well as triglycerides. A very similar enzyme is found in human milk.
Most of the dietary cholesterol is in the form of cholesteryl esters, and cholesteryl ester hydrolase also hydrolyzes these esters in the intestinal lumen.
Fats are finely emulsified in the small intestine by the detergent action of bile salts, lecithin, and monoglycerides. When the concentration of bile salts in the intestine is high, as it is after contraction of the gallbladder, lipids and bile salts interact spontaneously to form micelles . These cylindrical aggregates take up lipids and although their lipid concentration varies, they generally contain fatty acids, monoglycerides, and cholesterol in their hydrophobic centers. Micellar formation further solubilizes the lipids and provides a mechanism for their transport to the enterocytes. Thus, the micelles move down their concentration gradient through the unstirred layer to the brush border of the mucosal cells. The lipids diffuse out of the micelles, and a saturated aqueous solution of the lipids is maintained in contact with the brush border of the mucosal cells.
Nucleic Acids Digestion
Nucleic acids are split into nucleotides in the intestine by the pancreatic nucleases, and the nucleotides are split into the nucleosides and phosphoric acid by enzymes that appear to be located on the luminal surfaces of the mucosal cells. The nucleosides are then split into their constituent sugars and purine and pyrimidine bases. The bases are absorbed by active transport.