Encyclopedia
For the industrial process see anaerobic digestionDigestion 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 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 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 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: bile , pancreatic juice and enzymes, , and 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. 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 , 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 indicates a strong acid, while a high pH 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:
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 while birds and cockroaches may develop a
gizzard . 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 - 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 - 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 .
Carbon is stored within the plant along with water 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.
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 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 glycogon 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 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 , but there is some chain-branching 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 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
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