Digestion is the breakdown of large insolublefood compounds into small water-soluble components so that they can be absorbed into theblood plasma. In certain organisms, these smaller substances are absorbed through thesmall intestine into theblood stream. Digestion is a form ofcatabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The termmechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed bydigestive enzymes. Mechanical digestion takes place in themouth throughmastication and in the small intestine throughsegmentation contractions. Inchemical digestion,enzymes break down food into the small compounds that the body can use.
In thehuman digestive system, food enters the mouth and mechanical digestion of the food starts by the action of mastication (chewing), a form of mechanical digestion, and the wetting contact ofsaliva. Saliva, a liquid secreted by thesalivary glands, containssalivary amylase, an enzyme which starts the digestion ofstarch in the food.[1] The saliva also containsmucus, which lubricates the food; theelectrolyte hydrogencarbonate (HCO−3), which provides the ideal conditions of pH for amylase to work; and other electrolytes (Na+,K+,Cl−).[2] About 30% of starch ishydrolyzed intodisaccharide in the oral cavity (mouth). After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called abolus. It will then travel down theesophagus and into thestomach by the action ofperistalsis.Gastric juice in the stomach startsprotein digestion. Gastric juice mainly containshydrochloric acid andpepsin. Ininfants andtoddlers, gastric juice also containsrennin to digest milk proteins. As the first two chemicals may damage the stomach wall, mucus and bicarbonates are secreted by the stomach. They provide a slimy layer that acts as a shield against the damaging effects of chemicals like concentrated hydrochloric acid while also aiding lubrication.[3] Hydrochloric acid provides acidic pH for pepsin. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which is waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. Pepsin breaks down proteins intopeptides orproteoses, which is further broken down into dipeptides andamino acids by enzymes in the small intestine. Studies suggest that increasing the number of chews per bite increases relevant gut hormones and may decrease self-reported hunger and food intake.[4]
When thepyloric sphincter valve opens, partially digested food (chyme) enters theduodenum where it mixes with digestive enzymes from thepancreas and bile juice from theliver and then passes through the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of nutrient absorption occurs in the small intestine. Water and minerals are reabsorbed back into the blood in thecolon (large intestine) where the pH is slightly acidic (about 5.6 ~ 6.9). Some vitamins, such asbiotin andvitamin K (K2MK7) produced by bacteria in the colon are also absorbed into the blood in the colon. Absorption of water, simple sugar and alcohol also takes place in stomach. Waste material (feces) is eliminated from therectum duringdefecation.[5]
Digestive system
Digestive systems take many forms. There is a fundamental distinction between internal and external digestion. External digestion developed earlier in evolutionary history, and mostfungi still rely on it.[6] In this process,enzymes aresecreted into the environment surrounding the organism, where they break down an organic material, and some of the productsdiffuse back to the organism.Animals have a tube (gastrointestinal tract) in which internal digestion occurs, which is more efficient because more of the broken down products can be captured, and the internal chemical environment can be more efficiently controlled.[7]
Some organisms, including nearly allspiders, secrete biotoxins and digestive chemicals (e.g., enzymes) into the extracellular environment prior to ingestion of the consequent "soup". In others, once potential nutrients or food is inside theorganism, digestion can be conducted to avesicle or a sac-like structure, through a tube, or through several specialized organs aimed at making the absorption of nutrients more efficient.
Schematic drawing of bacterial conjugation.1- Donor cell producespilus.2- Pilus attaches to recipient cell, bringing the two cells together.3- The mobile plasmid is nicked and a single strand of DNA is transferred to the recipient cell.4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.
Bacteria use several systems to obtain nutrients from other organisms in the environments.
Channel transport system
In a channel transport system, several proteins form a contiguous channel traversing the inner and outer membranes of the bacteria. It is a simple system, which consists of only three protein subunits: theABC protein,membrane fusion protein (MFP), andouter membrane protein.[specify] This secretion system transports various chemical species, from ions, drugs, to proteins of various sizes (20–900 kDa). The chemical species secreted vary in size from the smallEscherichia coli peptide colicin V, (10 kDa) to thePseudomonas fluorescens cell adhesion protein LapA of 900 kDa.[8]
Molecular syringe
Atype III secretion system means that a molecular syringe is used through which a bacterium (e.g. certain types ofSalmonella,Shigella,Yersinia) can inject nutrients into protist cells. One such mechanism was first discovered inY. pestis and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host's cells rather than be secreted into the extracellular medium.[9]
Conjugation machinery
Theconjugation machinery of some bacteria (and archaeal flagella) is capable of transporting both DNA and proteins. It was discovered inAgrobacterium tumefaciens, which uses this system to introduce the Ti plasmid and proteins into the host, which develops the crown gall (tumor).[10] The VirB complex ofAgrobacterium tumefaciens is the prototypic system.[11]
In thenitrogen-fixingRhizobia, conjugative elements naturally engage in inter-kingdom conjugation. Such elements as theAgrobacterium Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and effectively transform the plant cells into factories for the production ofopines, which the bacteria use as carbon and energy sources. Infected plant cells formcrown gall orroot tumors. The Ti and Ri plasmids are thusendosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.
The Ti and Ri plasmids are themselves conjugative. Ti and Ri transfer between bacteria uses an independent system (thetra, or transfer, operon) from that for inter-kingdom transfer (thevir, orvirulence, operon). Such transfer creates virulent strains from previously avirulentAgrobacteria.
Release of outer membrane vesicles
In addition to the use of the multiprotein complexes listed above,gram-negative bacteria possess another method for release of material: the formation ofouter membrane vesicles.[12][13] Portions of the outer membrane pinch off, forming spherical structures made of a lipid bilayer enclosing periplasmic materials. Vesicles from a number of bacterial species have been found to contain virulence factors, some have immunomodulatory effects, and some can directly adhere to and intoxicate host cells. While release of vesicles has been demonstrated as a general response to stress conditions, the process of loading cargo proteins seems to be selective.[14]
Venus Flytrap (Dionaea muscipula) leaf
Gastrovascular cavity
Thegastrovascular cavity functions as a stomach in both digestion and the distribution of nutrients to all parts of the body. Extracellular digestion takes place within this central cavity, which is lined with the gastrodermis, the internal layer ofepithelium. This cavity has only one opening to the outside that functions as both amouth and ananus: waste and undigested matter is excreted through the mouth/anus, which can be described as an incompletegut.
In a plant such as theVenus flytrap that can make its own food through photosynthesis, it does not eat and digest its prey for the traditional objectives of harvesting energy and carbon, but mines prey primarily for essential nutrients (nitrogen and phosphorus in particular) that are in short supply in its boggy, acidic habitat.[15]
Trophozoites ofEntamoeba histolytica with ingested erythrocytes
Birds have bonybeaks that are specialised according to the bird'secological niche. For example,macaws primarily eat seeds, nuts, and fruit, using their beaks to open even the toughest seed. First they scratch a thin line with the sharp point of the beak, then they shear the seed open with the sides of the beak.
The mouth of thesquid is equipped with a sharp horny beak mainly made of cross-linkedproteins. It is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain any minerals, unlike the teeth and jaws of many other organisms, including marine species.[17] The beak is the only indigestible part of the squid.
Thetongue is skeletal muscle on the floor of themouth of most vertebrates, that manipulatesfood for chewing (mastication) andswallowing (deglutition). It is sensitive and kept moist bysaliva. The underside of the tongue is covered with a smoothmucous membrane. The tongue also has a touch sense for locating and positioning food particles that require further chewing. The tongue is used to roll food particles into abolus before being transported down theesophagus throughperistalsis.
Thesublingual region underneath the front of the tongue is a location where theoral mucosa is very thin, and underlain by a plexus of veins. This is an ideal location for introducing certain medications to the body. The sublingual route takes advantage of the highlyvascular quality of the oral cavity, and allows for the speedy application of medication into the cardiovascular system, bypassing the gastrointestinal tract.
Teeth (singular tooth) are small whitish structures found in the jaws (or mouths) of many vertebrates that are used to tear, scrape, milk and chew food. Teeth are not made of bone, but rather of tissues of varying density and hardness, such as enamel, dentine and cementum. Human teeth have a blood and nerve supply which enables proprioception. This is the ability of sensation when chewing, for example if we were to bite into something too hard for our teeth, such as a chipped plate mixed in food, our teeth send a message to our brain and we realise that it cannot be chewed, so we stop trying.
The shapes, sizes and numbers of types of animals' teeth are related to their diets. For example, herbivores have a number of molars which are used to grind plant matter, which is difficult to digest.Carnivores havecanine teeth which are used to kill and tear meat.
Crop
Acrop, or croup, is a thin-walled expanded portion of thealimentary tract used for the storage of food prior to digestion. In some birds it is an expanded, muscular pouch near thegullet or throat. In adult doves and pigeons, the crop can producecrop milk to feed newly hatched birds.[18]
Certain insects may have a crop or enlargedesophagus.
Herbivores have evolvedcecums (or anabomasum in the case ofruminants). Ruminants have a fore-stomach with four chambers. These are therumen,reticulum,omasum, and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material. Solids clump together to form the cud (orbolus). The cud is then regurgitated, chewed slowly to completely mix it with saliva and to break down the particle size.
The abomasum is the fourth and final stomach compartment in ruminants. It is a close equivalent of a monogastric stomach (e.g., those in humans or pigs), and digesta is processed here in much the same way. It serves primarily as a site for acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine. Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. Microbes produced in the reticulo-rumen are also digested in the small intestine.
A flesh fly "blowing a bubble", possibly to concentrate its food by evaporating water
Specialised behaviours
Regurgitation has been mentioned above under abomasum and crop, referring to crop milk, a secretion from the lining of the crop ofpigeons and doves with which the parents feed their young by regurgitation.[19]
Manysharks have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of unwanted contents (perhaps developed as a way to reduce exposure to toxins).
Other animals, such asrabbits androdents, practisecoprophagia behaviours – eating specialised faeces in order to re-digest food, especially in the case of roughage. Capybara, rabbits, hamsters and other related species do not have a complex digestive system as do, for example, ruminants. Instead they extract morenutrition from grass by giving their food a second pass through thegut. Soft faecal pellets of partially digested food are excreted and generally consumed immediately. They also produce normal droppings, which are not eaten.
Young elephants, pandas, koalas, and hippos eat the faeces of their mother, probably to obtain the bacteria required to properly digest vegetation. When they are born, their intestines do not contain these bacteria (they are completely sterile). Without them, they would be unable to get any nutritional value from many plant components.
In earthworms
Anearthworm's digestive system consists of a mouth,pharynx,esophagus, crop,gizzard, andintestine. The mouth is surrounded by strong lips, which act like a hand to grab pieces of dead grass, leaves, and weeds, with bits of soil to help chew. The lips break the food down into smaller pieces. In the pharynx, the food is lubricated by mucus secretions for easier passage. The esophagus adds calcium carbonate to neutralize the acids formed by food matter decay. Temporary storage occurs in the crop where food and calcium carbonate are mixed. The powerful muscles of the gizzard churn and mix the mass of food and dirt. When the churning is complete, the glands in the walls of the gizzard add enzymes to the thick paste, which helps chemically breakdown the organic matter. Byperistalsis, the mixture is sent to the intestine where friendly bacteria continue chemical breakdown. This releases carbohydrates, protein, fat, and various vitamins and minerals for absorption into the body.
Overview of vertebrate digestion
In mostvertebrates, digestion is a multistage process in the digestive system, starting from ingestion of raw materials, most often other organisms. Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:
Ingestion: placing food into the mouth (entry of food in the digestive system),
Mechanical and chemical breakdown: mastication and the mixing of the resultingbolus with water,acids,bile and enzymes in the stomach and intestine to break down complex chemical species into simple structures,
Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries throughosmosis,active transport, anddiffusion, and
Egestion (Excretion): Removal of undigested materials from the digestive tract throughdefecation.
Underlying the process is muscle movement throughout the system through swallowing andperistalsis. Each step in digestion requires energy, and thus imposes an "overhead charge" on the energy made available from absorbed substances. Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids (lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc.).
The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter bygut bacteria and for resorption of water from digests before excretion.
Inmammals, preparation for digestion begins with thecephalic phase in which saliva is produced in the mouth anddigestive enzymes are produced in thestomach. Mechanical and chemical digestion begin in the mouth where food ischewed, and mixed with saliva to begin enzymatic processing ofstarches. The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes.Absorption occurs in the stomach andgastrointestinal tract, and the process finishes with defecation.[5]
Thehuman gastrointestinal tract is around 9 metres (30 feet) long. Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours.[20]
Digestion begins in themouth with the secretion of saliva and its digestive enzymes. Food is formed into abolus by the mechanicalmastication and swallowed into theesophagus from where it enters the stomach through the action ofperistalsis.Gastric juice containshydrochloric acid andpepsin which could damage the stomach lining, butmucus and bicarbonates are secreted for protection. In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach. Mainly proteins are digested in stomach. The partially digested food enters theduodenum as a thick semi-liquidchyme. In the small intestine, the larger part of digestion takes place and this is helped by the secretions ofbile,pancreatic juice andintestinal juice. The intestinal walls are lined withvilli, and theirepithelial cells are covered with numerousmicrovilli to improve the absorption of nutrients by increasing thesurface area of the intestine. Bile helps in emulsification of fats and also activates lipases.
In the large intestine, the passage of food is slower to enable fermentation by thegut flora to take place. Here, water is absorbed and waste material stored asfeces to be removed by defecation via theanal canal andanus.
The cephalic phase occurs at the sight, thought and smell of food, which stimulate thecerebral cortex. Taste and smell stimuli are sent to thehypothalamus andmedulla oblongata. After this it is routed through thevagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) andG cell (secretes gastrin) activity viaD cell secretion ofsomatostatin.
The gastric phase takes 3 to 4 hours. It is stimulated bydistension of the stomach, presence of food in stomach and decrease inpH. Distention activates long and myenteric reflexes. This activates the release ofacetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds tohydrogen ions, which raises the pH of the stomach. Inhibition of gastrin andgastric acid secretion is lifted. This triggers G cells to releasegastrin, which in turn stimulatesparietal cells to secrete gastric acid. Gastric acid is about 0.5%hydrochloric acid, which lowers the pH to the desired pH of 1–3. Acid release is also triggered byacetylcholine andhistamine.
The intestinal phase has two parts, the excitatory and the inhibitory. Partially digested food fills theduodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activatingsympathetic fibers causing thepyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.
Protein digestion occurs in the stomach andduodenum in which 3 main enzymes, pepsin secreted by the stomach andtrypsin andchymotrypsin secreted by the pancreas, break down food proteins intopolypeptides that are then broken down by variousexopeptidases anddipeptidases intoamino acids. The digestive enzymes however are mostly secreted as their inactive precursors, thezymogens. For example, trypsin is secreted by pancreas in the form oftrypsinogen, which is activated in the duodenum byenterokinase to form trypsin. Trypsin then cleavesproteins to smaller polypeptides.
Digestion of some fats can begin in the mouth wherelingual lipase breaks down some short chain lipids intodiglycerides. However fats are mainly digested in the small intestine.[21] The presence of fat in the small intestine produces hormones that stimulate the release ofpancreatic lipase from the pancreas andbile from the liver which helps in the emulsification of fats for absorption offatty acids.[21] Complete digestion of one molecule of fat (atriglyceride) results a mixture of fatty acids, mono- and di-glycerides, but noglycerol.[21]
In humans, dietary starches are composed ofglucose units arranged in long chains called amylose, apolysaccharide. During digestion, bonds between glucose molecules are broken by salivary and pancreaticamylase, resulting in progressively smaller chains of glucose. This results in simple sugars glucose andmaltose (2 glucose molecules) that can be absorbed by the small intestine.
Lactase is an enzyme that breaks down thedisaccharidelactose to its component parts, glucose andgalactose. Glucose and galactose can be absorbed by the small intestine. Approximately 65 percent of the adult population produce only small amounts of lactase and are unable to eatunfermented milk-based foods. This is commonly known aslactose intolerance. Lactose intolerance varies widely by genetic heritage; more than 90 percent of peoples of east Asian descent are lactose intolerant, in contrast to about 5 percent of people of northern European descent.[22]
Sucrase is an enzyme that breaks down the disaccharidesucrose, commonly known as table sugar, cane sugar, or beet sugar. Sucrose digestion yields the sugarsfructose and glucose which are readily absorbed by the small intestine.
Some nutrients are complex molecules (for examplevitamin B12) which would be destroyed if they were broken down into theirfunctional groups. To digest vitamin B12 non-destructively,haptocorrin in saliva strongly binds and protects the B12 molecules from stomach acid as they enter the stomach and are cleaved from their protein complexes.[23]
After the B12-haptocorrin complexes pass from the stomach via the pylorus to the duodenum, pancreatic proteases cleave haptocorrin from the B12 molecules which rebind tointrinsic factor (IF). These B12-IF complexes travel to the ileum portion of the small intestine wherecubilin receptors enableassimilation and circulation of B12-IF complexes in the blood.[24]
Digestive hormones
Action of the major digestive hormones
There are at least five hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds. Arrangements are complex and additional details are regularly discovered. Connections to metabolic control (largely the glucose-insulin system) have been uncovered.
Gastrin – is in the stomach and stimulates thegastric glands to secretepepsinogen (an inactive form of the enzyme pepsin) andhydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH.
Secretin – is in theduodenum and signals the secretion of sodium bicarbonate in thepancreas and it stimulates the bile secretion in theliver. 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 thegall 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. Another function is to induceinsulin secretion.
Motilin – is in the duodenum and increases themigrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.
Significance of pH
Digestion is a complex process 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, very weakly acidic. Saliva controls pH in this region of the digestive tract.Salivary amylase is contained in saliva and starts the breakdown of carbohydrates intomonosaccharides. Most digestive enzymes are sensitive to pH and will denature in a high or low pH environment.
The stomach's high acidity inhibits the breakdown ofcarbohydrates within it. This acidity confers two benefits: itdenatures proteins for further digestion in the small intestines, and providesnon-specific immunity, damaging or eliminating variouspathogens.[25]
In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralize the acidic conditions from the stomach, and thepancreatic duct empties into the duodenum, addingbicarbonate to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline with a pH of about 8.5.[citation needed]
^Viola-Villegas N, Rabideau AE, Bartholomä M, Zubieta J, Doyle RP (August 2009). "Targeting the cubilin receptor through the vitamin B(12) uptake pathway: cytotoxicity and mechanistic insight through fluorescent Re(I) delivery".J. Med. Chem.52 (16):5253–5261.doi:10.1021/jm900777v.PMID19627091.
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