Digestion

} Digestion is the process of metabolism whereby a biological entity processes a substance, in order to chemically and mechanically 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 process
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 catabolism) also causes the release of saliva, which helps condense food into a bolus that can be easily passed through the oesophagus. The oesophagus is about 20 centimeters long. Saliva also begins the process of chemical catabolism, hydrolysis. Once food is chewed properly, the food is swallowed. The bolus is pushed down by the movement called peristalsis, which is an involuntary 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 nasal passages by triggering closure of the soft palate. When swallowed, the food enters the pharynx, which makes special adaptations to prevent choking or aspiration when food is swallowed. The epiglottis is a cartlidge structure that closes temporarily during swallowing, preventing food and liquids from entering the trachea.

The food enters the stomach upon passage through the cardiac sphincter. In the stomach, food is further broken apart through a process of heuristic churning 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, which contains 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. The primary activity here is regulation of blood glucose levels through a prosess of temporary storage of excess glucose that is converted in the liver to glycogen in direct response to the hormone insulin Between meals, when blood glucose levels begin to drop, the glycogen is converted back to glucose in response to the hormone glucagon.

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 through the villi, such as dietary 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). An herbivore may have a cecum that breaks down the cellulose in plants. Ruminants, for example, cows and sheep, have a fourth and final 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
Overview

Carbohydrates are formed in growing plants and are found in grains, leafy vegetables, and other edible plant foods. The molecular structure of these plants is complex, or a polysaccharide; poly is a prefix meaning many. Plants form carbohydrate chains during growth by trapping carbon from the atmosphere, initially carbon dioxide (CO2).

Carbon is stored within the plant along with water (H2O) to form a complex starch containing a combination of carbon-hydrogen-oxygen in a fixed ratio of 1:2:1 respectively.

Plants with a high sugar content and table sugar represent a less complex structure and are called disaccharides, or two sugar molecules bonded. Once digestion of either of these forms of carbohydrates are complete, the result is a single sugar structure, a monosaccharide. These monosaccharides can be absorbed into the blood and used by individual cells to produce the energy compound adenosine triphosphate(ATP).

The digestive system starts the process of breaking down polysaccharides in the mouth through the introduction of amylase, a digestive enzyme in saliva. The high acid content of the stomach inhibits the enzyme activity, so carbohydrate digestion is suspended in the stomach. Upon emptying into the small intestines, potential hydrogen (pH) changes dramatically from a strong acid to an alkaline content. The pancreas secretes bicarbonate to neutralize the acid from the stomach, and the mucus secreted in the tissue lining the intestines is alkaline which promotes digestive enzyme activity. Amalayse is present in the small intestines and works with other enzymes to complete the breakdown of carbohydrate into a monosaccharide which is absorbed into the surrounding capillaries of the villi.

Nutrients in the blood are transported to the liver via the hepatic portal circuit, or loop, where final carbohydrate digestion is accomplished in the liver.

Note: In the discussion of digestion of carbohydrates; nouns ending in the suffix -ose usually indicate a sugar, lactose. Nouns ending in the suffix -ase indicates the enzyme that will break down the sugar, lactase. For example: lactose, sugar found naturally in milk, is digested by lactase resulting on a less complex molecule, a monosaccharide.

Discussion

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 of the vili: 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 or absence 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. This condition causes lactose intolerance in some people.

Lactase is of interest because, in most mammals and in many human ethnicities, 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 Europeans is only about 15%. It is highier still in western Europeans. However, the incidence in most Africans, American Indians, Asians, 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.

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 is active in the stomach and can Most fat digestion begins in the duodenum, pancreatic lipase being one of the most important enzymes involved. Us enzyme) with relative helix that covers theUlipase, opening of the lid is facilitated. Colipase represents about 4% of the total cholesterol is in the form of FFacids. When the concentration of bile salts 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..

Reference:
 * Kimball Biology Pages