Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller
ones.
When food enters the mouth, its digestion starts by
the action of mastication, a form of mechanical digestion, and the contact
of saliva. Saliva, which is secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in
the food. After undergoing mastication and starch digestion, the food will now
be in the form of a small, round mass, called a bolus. It will then travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric
juice mainly contains hydrochloric acid and pepsin. As these two chemicals may damage the stomach
wall, mucus is secreted by the stomach, providing a slimy
layer that acts as a shield against the damaging effects of the chemicals. At
the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which are waves of muscular contractions that
move along the stomach wall. This allows the mass of food to further mix with
the digestive enzymes. After some time (typically an hour or two in humans, 4–6
hours in dogs, somewhat shorter duration in house cats), the resulting thick
liquid is called chyme. When the pyloric sphincter valve opens,
chyme enters the duodenum where it mixes with digestive enzymes from the pancreas, 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 absorption of nutrients
occurs in the small intestine. Water and minerals are reabsorbed back into the
blood in the colon (large intestine). Some vitamins, such as biotin and vitamin K (K2MK7) produced by bacteria in the
colon are also absorbed into the blood in the colon. Waste material is
eliminated during defecation.
DIGESTIVE SYSTEMS
Digestive systems take many forms. There is a
fundamental distinction between internal and external digestion. External
digestion is more primitive, and most fungi still rely on it. In this process, enzymes are secreted into the environment surrounding the organism,
where they break down an organic material, and some of the products diffuse back to the organism. Later, animals form a tube 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.
Some organisms, including nearly all spiders, simply 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 the organism, digestion can be conducted to a vesicle or a sac-like structure, through a tube, or
through several specialized organs aimed at making the absorption of nutrients
more efficient.
SECRETION SYSTEMS
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:
the ABC protein, membrane fusion protein (MFP), and outer membrane protein (OMP). This
secretion system transports various molecules, from ions, drugs, to proteins of
various sizes (20 - 900 kDa). The molecules secreted vary in size from the
small Escherichia coli peptide
colicin V, (10 kDa) to the Pseudomonas
fluorescens cell adhesion protein LapA of 900 kDa.
MOLECULAR SYRINGE
One molecular syringe is used through which a
bacterium (e.g. certain types of Salmonella,
Shigella, Yersinia) can inject nutrients into
protist cells. One such mechanism was first discovered in Y. pestis and showed that toxins
could be injected directly from the bacterial cytoplasm into the cytoplasm of
its host's cells rather than simply be secreted into the extracellular medium.
CONJUGATION MACHINERY
The conjugation machinery of some bacteria (and archaeal flagella)
is capable of transporting both DNA and proteins. It was discovered in Agrobacterium tumefaciens, which uses
this system to introduce the Ti plasmid and proteins into the host, which
develops the crown gall (tumor). The VirB complex of Agrobacterium tumefaciens is the prototypic system.
The nitrogen fixing Rhizobia are an interesting case, wherein conjugative
elements naturally engage in inter-kingdom conjugation. Such elements as the Agrobacterium 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 of opines, which the bacteria use as carbon and energy
sources. Infected plant cells form crown gall or root tumors. The Ti and Ri plasmids are thus endosymbionts 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 (the tra, or transfer, operon) from that
for inter-kingdom transfer (the vir,
or virulence, operon). Such transfer creates virulent strains
from previously avirulent Agrobacteria.
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 of outer membrane vesicles. 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.
Venus Flytrap (Dionaea muscipula) leaf
GASTROVASCULAR CAVITY
The gastrovascular 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 of epithelium. This cavity has only one opening to the outside
that functions as both a mouth and an anus: waste and undigested matter is
excreted through the mouth/anus, which can be described as an incomplete gut.
In a plant such as the Venus 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.
Trophozoites of Entamoeba
histolytica with ingested erythrocytes
PHAGOSOME
A phagosome is a vacuole formed around a particle absorbed by phagocytosis. The vacuole is formed by the fusion of the cell membrane around the particle. A phagosome is a cellular compartment in which pathogenic microorganisms can be killed and digested.
Phagosomes fuse with lysosomes in their maturation process, forming phagolysosomes. In humans, Entamoeba histolytica can phagocytose red blood cells.
Specialised organs and behaviours
To aid in the digestion of their food animals
evolved organs such as beaks, tongues, teeth, a crop, gizzard, and others.
BEAKS
Birds have beaks that are specialised according to the bird's ecological niche. For example, macaws primarily eat seeds, nuts, and fruit, using their
impressive 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 the squid is equipped with a sharp horny beak mainly made of
cross-linked proteins. 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. The beak is
the only indigestible part of the squid.
TONGUE
The tongue
is skeletal muscle on the floor of the mouth that manipulates food for chewing (mastication) and swallowing (deglutition). It is sensitive and kept moist by saliva. The underside of the tongue is covered with a
smooth mucous membrane. The tongue also has a touch sense for locating
and positioning food particles that require further chewing. The tongue is
utilized to roll food particles into a bolus before being transported down the esophagus through peristalsis.
The sublingual region underneath the front of the tongue is a
location where the oral 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 highly vascular quality of the oral cavity, and allows for the
speedy application of medication into the cardiovascular system, bypassing the
gastrointestinal tract.
TEETH
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. The shape of an animal's teeth is related to
its diet. For example, plant matter is hard to digest, so herbivores have many
molars for chewing.
The teeth of carnivores are shaped to kill and tear meat, using specially
shaped canine teeth. Herbivores' teeth are made for grinding food
materials, in this case, plant parts.
CROP
A crop, or croup, is a thin-walled expanded portion of
the alimentary tract used for the storage of food prior to digestion.
In some birds it is an expanded, muscular pouch near the gullet or throat. In adult doves and pigeons, the crop
can produce crop milk to feed newly hatched birds.
ABOMASUM
Herbivores have evolved cecums (or an abomasum in the case of ruminants). Ruminants have a fore-stomach with four
chambers. These are the rumen, 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 (or bolus). The cud is then regurgitated, chewed slowly to
completely mix it with saliva and to break down the particle size.
Fibre, especially cellulose and hemi-cellulose, is primarily broken down into the volatile fatty acids, acetic acid, propionic acid and butyric acid in these chambers (the reticulo-rumen) by
microbes: (bacteria, protozoa, and fungi). In the omasum water and many of the inorganic
mineral elements are absorbed into the blood stream.
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.
SPECIALISED BEHAVIOURS
Regurgitation has been mentioned above under abomasum and crop,
referring to crop milk, a secretion from the lining of the crop of pigeons and doves with which the parents feed their young by
regurgitation.
Many sharks 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 as rabbits and rodents, practise coprophagia 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 more nutrition from grass by giving
their food a second pass through the gut. 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
An earthworm's digestive system consists of a mouth, pharynx, esophagus, crop, gizzard, and intestine. 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. By peristalsis, 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 most vertebrates, digestion is a multi-stage 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:
2.
Mechanical and chemical breakdown: mastication and
the mixing of the resulting bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex
molecules into simple structures,
3.
Absorption: of nutrients from the digestive system
to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, and
4.
Egestion (Excretion): Removal of undigested
materials from the digestive tract through defecation.
Underlying the process is muscle movement
throughout the system through swallowing and peristalsis. 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 by gut bacteria and for resorption of water
from digesta before excretion.
In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the
mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches. The stomach continues to break food down
mechanically and chemically through churning and mixing with both acids and
enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the
process finishes with defecation.
HUMAN DIGESTION PROCESS
Upper and Lower human gastrointestinal tract
The whole digestive system is around 12 meters
long. In a healthy human adult this process can take between 24 and 72 hours. Food digestion physiology varies between individuals
and upon other factors such as the characteristics of the food and size of the
meal.
PHASES OF GASTRIC SECRETION
- Cephalic phase - This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus 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) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.
- Gastric phase - This phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes. This activates the release of acetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid. Gastric acid is about 0.5% hydrochloric acid (HCl), which lowers the pH to the desired pH of 1-3. Acid release is also triggered by acetylcholine and histamine.
- Intestinal phase - This phase has 2 parts, the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.
ORAL CAVITY
In humans,
digestion begins in the Mouth, otherwise known as the "Buccal
Cavity", where food is chewed. Saliva is secreted in large amounts (1-1.5 litres/day) by
three pairs of exocrine salivary glands (parotid, submandibular, and sublingual)
in the oral cavity, and is mixed with the chewed food by the tongue. Saliva
cleans the oral cavity, moistens the food, and contains digestive enzymes such as salivary amylase, which aids in the chemical breakdown of polysaccharides such as starch into disaccharides such as maltose. It also contains mucus, a glycoprotein that helps soften the food and form it into a bolus. An additional enzyme, lingual lipase, hydrolyzes long-chain triglycerides into partial
glycerides and free fatty acids.
Swallowing transports the chewed food into the esophagus, passing through the oropharynx and hypopharynx. The mechanism for swallowing is coordinated by
the swallowing center in the medulla oblongata and pons. The reflex is initiated by touch receptors in the
pharynx as the bolus of food is pushed to the back of the mouth.
PHARYNX
The pharynx is the part of the neck and throat
situated immediately behind the mouth and nasal cavity, and cranial, or
superior, to the esophagus. It is part of the digestive system and respiratory system. Because both food and air pass through the
pharynx, a flap of connective tissue, the epiglottis closes over the trachea when food is swallowed to
prevent choking or asphyxiation.
The oropharynx is that part of the pharynx behind the oral
cavity. It is lined with stratified squamous epithelium. The nasopharynx lies behind the nasal cavity and like the nasal
passages is lined with ciliated columnar pseudostratified epithelium.
Like the oropharynx above it the hypopharynx (laryngopharynx) serves as a passageway for food and air and is
lined with a stratified squamous epithelium. It lies inferior to the upright
epiglottis and extends to the larynx, where the respiratory and digestive
pathways diverge. At that point, the laryngopharynx is continuous with the
esophagus. During swallowing, food has the "right of way", and air
passage temporarily stops.
ESOPHAGUS
The esophagus is a narrow muscular tube about 20-30
centimeters long, which starts at the pharynx at the back of the mouth, passes through the thoracic diaphragm, and ends at the cardiac orifice of the stomach. The wall of the esophagus is made up of two
layers of smooth muscles, which form a continuous layer from the esophagus
to the colon and contract slowly, over long periods of time. The inner layer of
muscles is arranged circularly in a series of descending rings, while the outer
layer is arranged longitudinally. At the top of the esophagus, is a flap of
tissue called the epiglottis that closes during swallowing to prevent food from
entering the trachea (windpipe). The chewed food is pushed down the
esophagus to the stomach through peristaltic contraction of these muscles. It takes only about
seven seconds for food to pass through the esophagus and now digestion takes
place.
STOMACH
The stomach is a small, 'J'-shaped pouch with walls
made of thick, distensible muscles, which stores and helps break down food. Food
reduced to very small particles is more likely to be fully digested in the
small intestine, and stomach churning has the effect of assisting the physical
disassembly begun in the mouth. Ruminants, who are able to digest fibrous material
(primarily cellulose), use fore-stomachs and repeated chewing to
further the disassembly. Rabbits and some other animals pass some material through their entire digestive systems twice. Most
birds ingest small stones to assist in mechanical processing in gizzards.
Food enters the stomach through the cardiac orifice
where it is further broken apart and thoroughly mixed with gastric acid, pepsin and other digestive enzymes to break down proteins. The enzymes in the stomach
also have an optimum conditions, meaning that they work at a specific pH and
temperature better than any others. The acid itself does not break down food
molecules, rather it provides an optimum pH for the reaction of the enzyme pepsin and kills many microorganisms that are ingested
with the food. It can also denature proteins. This is the process of reducing
polypeptide bonds and disrupting salt bridges, which in turn causes a loss of
secondary, tertiary, or quaternary protein structure. The parietal cells of the stomach also secrete a glycoprotein called intrinsic factor, which enables the absorption of vitamin B-12. Mucus neck cells are present in the gastric
glands of the stomach. They secrete mucus, which along with gastric juice plays an important role in lubrication and
protection of the mucosal epithelium from excoriation by the highly
concentrated hydrochloric acid. Other small molecules such as alcohol are absorbed in the stomach, passing through the membrane of
the stomach and entering the circulatory system directly. Food in the stomach is in semi-liquid
form, which upon completion is known as chyme.
After consumption of food, digestive
"tonic" and peristaltic contractions begin, which helps break down the
food and move it through. When the chyme reaches the opening to the duodenum known as the pylorus, contractions "squirt" the food back
into the stomach through a process called retropulsion, which exerts additional
force and further grinds down food into smaller particles. Gastric emptying is
the release of food from the stomach into the duodenum; the process is tightly
controlled with liquids being emptied much more quickly than solids. Gastric
emptying has attracted medical interest as rapid gastric emptying is related
to obesity and delayed gastric emptying syndrome is
associated with diabetes mellitus, aging, and gastroesophageal reflux.
The transverse section of the alimentary canal reveals
four (or five, see description under mucosa) distinct and well developed layers
within the stomach:
- Serous membrane, a thin layer of mesothelial cells that is the outermost wall of the stomach.
- Muscular coat, a well-developed layer of muscles used to mix ingested foo`d, composed of three sets running in three different alignments. The outermost layer runs parallel to the vertical axis of the stomach (from top to bottom), the middle is concentric to the axis (horizontally circling the stomach cavity) and the innermost oblique layer, which is responsible for mixing and breaking down ingested food, runs diagonal to the longitudinal axis. The inner layer is unique to the stomach, all other parts of the digestive tract have only the first two layers.
- Submucosa, composed of connective tissue that links the inner muscular layer to the mucosa and contains the nerves, blood and lymph vessels.
- Mucosa is the extensively folded innermost layer. It can be divided into the epithelium, lamina propria, and the muscularis mucosae, though some consider the outermost muscularis mucosae to be a distinct layer, as it develops from the mesoderm rather than the endoderm (thus making a total of five layers). The epithelium and lamina are filled with connective tissue and covered in gastric glands that may be simple or branched tubular, and secrete mucus, hydrochloric acid, pepsinogen and rennin. The mucus lubricates the food and also prevents hydrochloric acid from acting on the walls of the stomach.
SMALL INTESTINE
After being processed in the stomach, food is
passed to the small intestine via the pyloric sphincter. The majority of digestion and absorption occurs
here after the milky chyme enters the duodenum. Here it is further mixed with three different
liquids:
- Bile, which emulsifies fats to allow absorption, neutralizes the chyme and is used to excrete waste products such as bilin and bile acids. Bile is produced by the liver and then stored in the gallbladder where it will be released to the small intestine via the bile duct. The bile in the gallbladder is much more concentrated.
- Pancreatic juice made by the pancreas. It secrete enzymes such as pancreatic amylase, pancreatic lipase, and trypsinogen (inactive form of protease).
- Intestinal juice secreted by the intestinal glands in the small intestine. It contains enzymes such as enteropeptidase, erepsin, trypsin, chymotrypsin, maltase, lactase and sucrase (all three of which process only sugars).
The pH level increases in the small intestine as
all three fluids are alkaline. A more basic environment causes more helpful
enzymes to activate and begin to help in the breakdown of molecules such as fat globules. Small, finger-like structures called villi, and their epithelial cells is covered with numerous microvilli improve the absorption of nutrients by increasing
the surface area of the intestine and enhancing speed at which
nutrients are absorbed. Blood containing the 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 small intestine and remainder of the digestive
tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive
juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes. When
the circular muscles contract, the lumen becomes narrower and longer and the food is
squeezed and pushed forward. When the longitudinal muscles contract, the
circular muscles relax and the gut dilates to become wider and shorter to allow
food to enter.
LARGE INTESTINE
After the food has been passed through the small
intestine, the food enters the large intestine. Within it, digestion is retained long enough to
allow fermentation due to the action of gut bacteria, which breaks down some of
the substances that remain after processing in the small intestine; some of the
breakdown products are absorbed. In humans, these include most complex
saccharides (at most three disaccharides are digestible in humans). In
addition, in many vertebrates, the large intestine reabsorbs fluid; in a few,
with desert lifestyles, this reabsorbtion makes continued existence possible.
In humans, the large intestine is roughly 1.5
meters long, with three parts: the cecum at the junction with the small intestine, the colon, and the rectum. The colon itself has four parts: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The large intestine absorbs water from the chyme and stores feces until it can be egested. Food products that cannot go through the villi, such as cellulose (dietary fiber), are mixed with other waste products from the
body and become hard and concentrated feces. The feces is stored in the rectum for a certain period and then the stored feces is
eliminated from the body due to the contraction and relaxation through the anus. The exit of this waste material is regulated by
the anal sphincter.
BREAKDOWN INTO NUTRIENTS
PROTEIN DIGESTION
Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins
into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. The digestive enzymes however are mostly secreted
as their inactive precursors, the zymogens. For example, trypsin is secreted by pancreas in
the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleaves proteins to smaller polypeptides.
FAT DIGESTION
Digestion of some fats can begin in the mouth where
lingual lipase breaks down some short chain lipids into diglycerides. The presence of fat in the small intestine
produces hormones that stimulate the release of pancreatic lipase from the pancreas and bile from the liver for breakdown of fats into fatty acids. Complete digestion of one molecule of fat (a triglyceride) results in 3 fatty acid molecules and one glycerol molecule.
CARBOHYDRATE DIGESTION
In humans, dietary starches are composed of glucose units arranged in long chains called amylose, a polysaccharide. During digestion, bonds between glucose molecules
are broken by salivary and pancreatic amylase, resulting in progressively smaller chains of
glucose. This results in simple sugars glucose and maltose (2 glucose molecules) that can be absorbed by the
small intestine.
Lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose and galactose. Glucose and galactose can be absorbed by the
small intestine. Approximately half of the adult population produce only small
amounts of lactase and are unable to eat milk-based foods. This is commonly
known as lactose intolerance.
Sucrase is an enzyme that breaks down the disaccharide sucrose, commonly known as table sugar, cane sugar, or
beet sugar. Sucrose digestion yields the sugars fructose and glucose which are readily absorbed by the
small intestine.
DNA AND RNA DIGESTION
DNA and RNA are broken down into mononucleotides by the nucleases deoxyribonuclease and ribonuclease (DNase and RNase) from the pancreas.
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. For instance, more connections to metabolic
control (largely the glucose-insulin system) have been uncovered in recent
years.
- Gastrin - is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) 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. Another function is to induce insulin secretion.
- Motilin - is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.
SIGNIFICANCE OF PH IN DIGESTION
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 into monosaccharides. 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
of carbohydrates within it. This acidity confers two benefits: it denatures proteins for further digestion in the small
intestines, and provides non-specific immunity, damaging or eliminating various pathogens
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 the
pancreatic duct empties into the duodenum, adding bicarbonate to neutralize the acidic chyme, thus creating of neutral environment. The mucosal
tissue of the small intestines is alkaline with a pH of about 8.5.
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Dusenbery, David B. (1996). “Life at Small Scale”,
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