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For the industrial process see anaerobic 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.
Overview
Digestion occurs at the multicellular, cellular, and sub-cellular levels, usually in animals. This process takes place in the digestive system, gastrointestinal tract, or alimentary canal.
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. Digestion is separated into five separate processes: 1) Ingestion: Placing food into the mouth, 2) Mechanical digestion: Mastication, the use of teeth to tear and crush food, and churning of the stomach. 3) Chemical digestion: Addition of chemicals (acid, bile, enzymes, and water) to break down complex molecules into simple structures, 4) Absorption: Movement of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, 5) Elimination: Removal of undigested materials from the digestive tract through defecation. Underlying the process is muscle movement throughout the system, deglutition and peristalsis.
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 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 mechanism for swallowing is co-ordinated by the swallowing centre in the medulla oblongata and pons. The reflex is initiated by touch receptors in the pharynx as a bolus of food is pushed to the back of the mouth by the tongue. The uvula is a small flap that hangs from the roof of the mouth. During swallowing it and the soft palate retract upward and to the rear 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 cartilage 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 is thoroughly mixed with a digestive fluid, composed chiefly of hydrochloric acid, and other digestive enzymes to further denature proteins. The parietal cells of the stomach also secrete a compound, intrinsic factor which is essential in the absorption of vitamin B-12. As the acidic level changes in the small intestines, more enzymes are activated to split apart the molecular structure of the various nutrients so they may be absorbed into the circulatory or lymphatic systems.
After being processed in the stomach, food is passed to the small intestine via the pyloric sphincter. This is where most of the digestive process occurs as chyme enters the first 10 inches of the small intestine, the duodenum. Here it is further mixed with 3 different liquids: (1)bile (which helps aid in fat digestion, otherwise known as emulsification), (2) pancreatic juice and enzymes, (made by the pancreas), and (3) intestinal enzymes of the alkaline mucosal membranes. The enzymes include: 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 process 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 fibre, can be stored in large intestine. Fibre helps to keep the food moving through the G.I. tract. The food that cannot be broken down is called faeces. Faeces are stored in the rectum until they are expelled through the anus.
Significance of pH in Digestion
Digestion is a complex process which is controlled by several factors. pH plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx, and esophagus, pH is typically about 6.8, a very weak acid. Saliva controls pH in this region of the digestive tract. Salivary amylase is contained in saliva and starts the breakdown of carbohydrates into monosaccharides. Most digestive enzymes are sensitive to pH and will not function in a low-pH environment like the stomach. Low pH (below 5) indicates a strong acid, while a high pH (above 8) indicates a strong base.
pH in the stomach is very low and inhibits the breakdown of carbohydrates while there. The strong acid content of the stomach provides two benefits, both serving to denature proteins for further digestion in the small intestines, as well as providing non-specific immunity, retarding or eliminating various pathogens.
In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The pancreatic duct empties into the stomach, adding bicarbonate to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline, creating a pH of about 8.5, thus enabling absorption in a mild alkaline environment.
References:
- Kimball's Biology Pages, Digestion
- Chemistry lecture
- American Journal of Physiology, article
- Journal article on pH in digestion
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 breaks down the cellulose in plants. Ruminants, for example, bovines 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.
Digestion Chemistry
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. The liver accomplishes carbohydrate digestion in response to the hormones insulin and glucagon. As blood glucose levels increase following digestion of a meal, the pancreas secretes insulin causing the liver to transform glucose to glycogen, which is stored in the liver, adipose tissue, and in muscle cells, preventing hyperglycemia. A few hours following a meal, blood glucose will drop due to muscle activity, and the pancreas will now secrete glucagon which causes glycogen to be converted into glucose to prevent hypoglycemia .
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
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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 the Ulipase, opening of the lid is facilitated. Colipase represents about 4% of the total cholesterol is in the form of free fatty (FF) acids. The breakdown of complex fat globules occurs in the duodenum as the contents of the pancreatic duct empty into the lumen. Bile acts as an emulsifier, eroding the edges of the larger globules into smaller globules for further digestion. The introduction of lipase, along with the concentration of bile salts, in contact with the brush border of the mucosal cells, creates the correct environment for final stage breakdown of fats. Final absorption of fat into the body occurs in the villi. Specialized lymphatic capillaries, lacteals, transport the FFs, chyle, to the lymph system for filtering and then are combined with the blood as lymph joins blood at the right and left subclavian veins.
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..
References
See also
External links
- Gastric Digestion (Animation)
- Human Physiology - Digestion
- NIH guide to digestive system
- Canadian Society of Intestinal Research - Registered charity that provides free information on digestive diseases and disorders