Digestion means the physical or chemical breakdown of complex food into small particles with and without the help of enzymes, much smaller. Digestion is the form of catabolism and it is divided into two processes.
1.    Mechanical digestion            
2.    Chemical digestion
Mechanical digestion: Physical breakdown of large food particles into small particles. Various types of organs are involved in mechanical digestion.
Chemical digestion: Chemical breakdown of oligomers or polymers in monomers. In chemical digestion, various types of enzymes are involved.
3.1.    Human Digestive Systems

3.1.1.    Digestion in the mouth/Buccal cavity :

Digestive system starts from mouth and end at the anus. In mouth, mechanical as well as chemical digestion occurs. In saliva, different enzymes like lipase, ptyalin (salivary amylase) are present along with IgA. pH of saliva is 6.8 (slightly acidic). Mucus in saliva make food soft by lubricating and masticate food particle into a bolus. So, food item becomes smaller in size and easily enter into the stomach through the oesophagus. In mouth, for mechanical digestion, teeth and saliva play a very important role. Four different types of teeth are present in all mammals. (exception-rabbit canines are not present). These are incisors, canines, premolar & molar. In carnivorous animals, canines are well developed than herbivores. In the adult human, teeth are thecodont and heterodont in nature. The formula for incisors canine premolar and molar for the human is 2123/2123. In mammals, the palate is present. It is of two types of primary Palate and secondary Palate. Secondary palate is found in only mammals. It is a separator palate for the nasal chamber and buccal cavity. Now, food passes from the buccal cavity to the stomach through the oesophagus. In the oesophagus, bolus moves through peristaltic movement. This peristaltic movement always occurs from mouth to stomach but during vomiting reverse peristaltic movement occurs.
The buccal cavity has saliva which helps in digestion of carbohydrates. 1,200-1,500 ml of saliva is produced daily in the buccal cavity. At mealtime, the secretory rate is highest. Saliva is cloudy in appearance due to the presence of mucin and cells. Saliva is slightly acidic whose pH range from 6.2 - 7.05. It loses CO2 & becomes alkaline on boiling. The freezing point of saliva is 0.07 - 0.34°C. Saliva contains 99.5% water and 0.5% solids which includes yeast cells, bacteria, protozoa, epithelial cells and leucocytes and inorganic salts like 0.2% of NaCl, KCl, CaCO3, calcium phosphate, potassium thiocyanate. Organic enzyme includes ptyalin (Salivary amylase), lipase, carbonic anhydrase, phosphates and lysozyme, mucin, urea, amino acid, cholesterol & vitamins in small amounts. Saliva also contains gases like 1 ml of oxygen, 2.5 ml of nitrogen and 50 ml of CO2 per 100 ml. Enzyme kallikrein is present in saliva which acts on plasma protein to produce kallidin or bradykinin. This produces vasodilator of salivary gland during secretion.
3.1.2.    Functions: It keeps the mouth moist and helps in speech. The constant flow of saliva washes the food debris which does not allow bacteria to grow. Saliva contains two enzymes :
(1)    Ptyalin: Divides starch to maltose & isomaltose and a-limit dextrin.
(2)    Maltase: Convert maltose into glucose.

3.2.    Alimentary Canal :
It is the tubular path through which ingested food passes during digestion & finally expelled out of the body. 
The alimentary canal is divided into '4' major divisions on the basis of structure  & function:-

A.    Head gut = receives ingested material
B.    Fore gut = it conducts, stores
C.    Mid gut = Digest and absorbs nutrients
D.    Hind gut = Absorbs water and also participate in defecation
3.2.1.    Head Gut
It is the anterior most part of alimentary canal which acts like food entry gate and posses of organs like teeth, buccal cavity, tonge, salivary glands, uvula etc. In small particle feeders such as coelenterate, flatworms, sponges, the head-gut is devoid of glandular secretions. In other metazoans, salivary glands are present whose secretions aids in ingestion.
The main function of saliva is to lubricate the food inside the buccal cavity, therefore, make to swallowing of food easily. Saliva contains a main protein called mucin. Mucin proteins are heavily glycosylated protein produced by epithelial tissue 
Saliva also contains, digestive enzymes, toxins (in same animals), Anticoagulants (blood sucking animals such as vampire bats and (leeches). Tongue help the mechanical digestion of food & also in swallowing of food. Tongue bear gustatory receptors called taste buds. Animals like snakes use their forked tongue to take olfaction from air & substrates (rock, soil, etc.). Snakes retract their tongue to wipe samples in the vomeronasal organ which consist of a pair of innervated chemosensory pits located in the roof of the buccal cavity. Vomeronasal organs are also called a Jacobson's organ that is also found in many other vertebrates.
3.2.2.    Foregut: Food conductions, storage and digestion
In vertebrates, the foregut is consist of an oesophagus, i.e. a tube that starts from the oral cavity and ends to the stomach.    Oesophagus
Oesophagus conducts food from head gut to the stomach.    In chordates, oesophagus conducts a bolus (mass of showed food) mixed with saliva by peristaltic movement from the buccal cavity through pharynx and oesophagus to the stomach. In some animals, oesophagus contains a sac-like expanded region called crop. This crop is used for the storage of food before digestion. The crop is found mostly in those animals which feed infrequently and stored to digest in later time. For eg. : Leech feeds very infrequently from weeks to some months between feeding periods. Therefore, they ingest a large quantity of blood at a single meal and stores it for many weeks and digest it very slowly. Some birds like pigeon use their crop for preparation of milk (food) and regurgitate for their nestlings.    Stomach
In vertebrates, primarily digestion of food takes place in the stomach. Stomach serves as a storage site for food. Stomach starts the digestion of protein part of food by secreting the proenzyme pepsinogon. A pepsinogen is an inactive form of the enzyme and in presence of HCl, it quickly converts into enzyme pepsin. HCl (hydrochloric acid) provides highly acidic envirment required for pepsin activation. The muscular walls of the stomach also provide mechanical mixing of food, saliva and stomach secretions. The stomach is classified as monogastric or digastric on the basis of the number of chambers.
A monogastric stomach is made up of a single strong muscular tube or sac. Vertebrates that are carnivores and omnivores have the monogastric stomach 

Such stomach has sphincters for guarding food entrance and exit. Stomach lining has small invagination of gastric mucosa that forms gastric pits, containing goblet cells that produce mucus. The bottom of the gastric pits contains gastric glands which include parietal or oxyntic cells that secrete HCl, chief or peptic cells secrete pepsinogen. In some invertebrates, instead of stomach, some outpouchings termed as gastric ceaia (singular; ceacum) which are lined with enzyme secreting cells and phagocytic cells that engulf partially digested food. Some birds have a tough muscular gizzard or a crop or both. Birds swallow small stones, pebbles, sand, which lodged in crop or gizzard where they are used for grinding seeds and grains.    Digastric stomach 

The digastric stomach is the characteristic of the mammalian suborder Ruminantia (Herbivores such as deer, giraffes, sheep, cattle). Similar digastric stomach occurs in other herbivores tylopod (camel, Llams, Alpacas). In the first division of Symbiotic organisms stomach carry out fermentation (anaerobic conversion of the organic compound to simple compounds, yields ATP). In rumination, the partially digested food is regurgitated (means the transport of food back to mouth) for remastication. This process of rumination allows the animal to swallow food hastily and then chew it throughly later, when at rest in a place of relative safety from predators. After the remastication, food is swallowed again. This remasticated food passes into the second division of digastric stomach and begins the 2nd (second) stage of digestion. In this second stage, Hydrolysis of food takes place with the help of digestive enzymes. The digastric stomach of ruminants has four chambers.
The abomasum is true stomach because it secrets the digestive enzymes and it is also considered as the homologous organ to vertebrate stomach (monogastric stomach). Rumen & reticulam act as fermentation chambars that receives grazed vegetation. Bacteria & protozoans present in this chamber acts on the vegetation, fermenting its carbohydrates into products such as butyrate, lactate, acetate & propionate as well as gases like CO2 and methane. Non-gaseous products of fermentation, along with same peptides, amino acids, short-chain fatty acids, are absorbed into the blood stream from the rumen fluid.
Microorganisms from the rumen, along with undigested particle, are passed into omasum (absent in tylopods) and then in the abomasum. Only the abomasum secrete digestive enzymes. CO2 produced by fermentation is absorbed, where as methane (CH4) is much less soluble. At the height of fermentation, both CO2 and CH4 must be expelled by Eructation, i.e. the release of gas from the stomach via oesophagus (burping). An animal's inability to get rid of the gaseous products of fermentation results in bloat, i.e. a potentially lethal situation that is often associated with the rapid consumption of large amounts of leguminous vegetation.
3.2.3.    Midgut: Chemical digestion and absorption
Partially digested food is passed from the stomach to midgut, through pyloric sphincter, which relaxes as the peristaltic movements of the stomach, squeezing the acidic contents into the dudenum.
General structure and function of the midgut
In vertebrates, carnivores have shorter and simpler intestine than herbivores, reflecting that shorter time required to digest meat than vegetation.
For example, Amphibian tadpole, which is herbivores have a much longer intestine relative to body size than the adult frog, which is carnivorous. (Relative to their size).
Typically intestine has '3' parts:-
1.    Duodenum, = lining to duodenum secrete mucus and fluids and receives secretions from the liver and pancreas by ducts.
2.    Jejunum = It also secretes fluids and involved in digestion and absorption.
3.    Ileum: It primarily absorbs nutrients, digested in the duodenum and jejunum, although some secretion also occurs.
The cells of the liver produce bile salts which are carried in the bile duct. This fluid is known as bile juice.
Bile fluid has two main function 
A.    It emulsifies fats
B.     It helps to neutralize acid introduced into the duodenum from the stomach.
The cells of the liver produce bile salts which are carried into the duodenum through the bile duct.
Pancreatic juice which containg many enzymes, is also secreted in the duodenum through the pancreatic duct. Pancreatic juice contains many proteases, lipases, carbohydrates for digestion of food in the intestine.  The intestine of most animals is a complex ecosystem, containing large no. of bacteria, protozoans, and fungi.
Intestinal symbionts are very useful because they synthesize essential vitamins.
Invertebrates which contain extensive caeca and diverticula in the intestine have no digestive function in the intestine.    Intestinal epithelium
In vertebrates, the small intestine is anatomically adapted at the organisation level to amplify the surface area available for the absorption of nutrients. The rate of absorption is generally proportional to the area of the apical surface membrane of the cells.

The outermost layer is serosa which is the same tissue that covers the other visceral organs of the abdomen. The serosa overlies the outer layer of longitudinal smooth muscle. An inner layer of circular smooth muscles surrounds the epithelial layer which consists of the submucosa (a layer of fibrous connective tissue) and the mucosa (mucus membrane). Several folds are found in mucosa called as folds of kerckring or the circular folds. In addition, to increase surface area, these folds slow the progress of food through the intestine, allowing more time for digestion.
At next anatomic level is the finger like Villi which line the folds and stand about 1 mm tall.
Each villus present in a circular depression known as the crypt of liberkuhn 
Within each villus, is a network of blood vessels formed by arterioles, capillaries, and venules.
The villus also contains a network of lymph vessels, the largest of which is the central lacteal
Central lacteal absorbs large particles.
The villi are covered by the actual absorptive surface of the small intestine, the cells of the digestive system.
The epithelium consists of goblet cells interspersed among columnar absorptive cells 

The absorptive cells proliferate at the base of the villus and migrate towards it's tip.
In humans, absorptive cells are sloughed off at the rate of about 2 x 1010 cells per day. means that the entire midgut lining is replaced every few days.
The apical surface of each absorptive cell, where finger like structures called microvilli, collectively forms a brush border.

There are up to several thousand microvilli per cell 2 x 105 per square meter: each is 0.5 - 1.5 nm tall and about 0.1 nm wide.
The membrane of microvillus is continuous with the plasma membrane of the epithelium and surrounds actin filaments that form cross-bridge links with myosin filaments present at the base of each microvillus.
Intermittent actin-myosin interactions produce rhythmic motions of microvilli which are helping to mix & exchange the intestinal chyme near the absorptive surface.
The surfaces of microvilli are covered by glycocalyx i.e. a meshwork upto 0.3 nm thick made up of acidic mucopolyssaccharides and glycoproteins.
Water and mucus are trapped with in the interstices of glycocalyx.
The mucus is secreted by goblet cells that are found among the absorptive cells.
The absorptive cells are tightly connected with the help of desmosomes.
Near the apex, the tight junction with neighbouring cells form zonula occludens that encircles each cell 
Tight junctions are so tight that the apical membranes of absorptive cells form a continuous sheet of the membrane without breaks between cells.
Because of the impermeability of tight junctions, all nutrients must pass across the apical membrane and through the cell cytoplasm to get from the lumen to the blood & lymph vessels with in the villi.
In humans, the lumen of the small intestine has a gross cylindrical surface area of only about (0.4) m2., Actual area across which absorption can occur is increased at least 500 times, to a total of 200-300 m2, or about the size of a doubles tennis court.
3.2.4.    Hindgut: - Water and ion absorption and defeation
The main function of hindgut is to serve as the store of remnants of digested food. In hindgut, same inorgnic ions and excess water are absorbed from this material and return to the blood. This function is carried out in hind portion of the small intestine and in large intestine or colon invertebrates. The hindgut consolidates undigested material. The faeces pass into cloaca or rectum and then exit out from the anus in the process of defecation. The hindgut is a major site for bacterial digestion in herbivores mammals, reptiles, birds. Colon acts as a modified plug-flow reactor in most large animals that are hindgut fermenters (eg. :-horse, Zebra, tapirs, sirenians, marsupial wambats). In small hindgut fermenters (Rabbits, Rodents, hyraxes, howler monkeys, koala, opossums), the tremendously enlarged caecum, i.e. an outpocketing of the small intestine, acts as continuous-flow, stirred tank reactor. Hindgut fermenter in a cloaca is found in many vertebrates, including cyclostomes, elasmobranchs, adult amphibions, reptiles, birds and a few memmals (monotremes, marsupials, insectivores, a few rodents). Cloaca reabsorbs urinary ions and water.
Dynamics of Gut structure and the influcence of diet
The Gut's size structure, enzymatic activity, and absorptive capabilities are quite dynamic in response to changes in both energy demand and food quality in most animals. To check out this, some mouse were induced to increase their food intake through exposure to combinations of low ambient temperature and enforced excercise over several months. They responded by increasing the overall length of the small intestine by about one-fifth; efficiency of nutrient uptake increased as a result.
The mass of the empty stomach of thirteen-lined ground squirrel (spermophilus tridecemlineatus) increses three folds to four folds within a few months after the animal wake up from hibernation. Reptiles have a much lower rate of metabolism than birds and mammals. Some reptiles appear to remodel the gut in response to food intake much more rapidly than birds and mammals do, some times within a few hours or days.
For eg. : - In burmese python (Phython molurus), small intestine mass increases by '40%' over fasting levels within six hours of a large meal (defied as 25% of body mass).
These changes are due to largely proliferation of mucosal layer, rather than the serosal layer. Also, the capacity of absorption of amino acid uptake increases 10 - 24 times. Increasing substrate level stimulate an increase in concentration or activity of transporters of glucose, fructose, some nonessential aminoacids, and peptides. 
3.5.    Motility of the alimentary canal
The ability of alimentary canal to contract i.e. a characteristic called motility is important to digestive function in several ways:
1.    Propulsion of food along the entire length of the alimentry canal and the final expulsion of faecal material.
2.    Mechanical treatment of food by grinding and kneading to mix it with digestive juices and convert it into a soluble form.
3.    Stirring of the gut contents so that there is the continued renewal of material in contact with the absorbing and secreting surfaces of the epithelial lining.
3.5.1.    Muscular and ciliary motility :
The motality of alimentry canal is regulated by two different mechanisms. 
(1)    Muscular motality                
(2)    Ciliary motality
In muscular motality, transport of food is done by muscular activity contractions in the walls of the alimentary canal. 
In chordates, smooth muscle fibres are found to perform muscular motality but in many arthropods, motality is achieved by striated muscle fibre contractions.
Cilia lining the digestive tract generate currents of fluid within alimentry canal.
The muscular is a layer of alimentary canal bears outer longitudinal muscles & inner circular smooth muscles 
Contraction of circular layer co-ordinated with the relaxation of the longitudinal layer produces an active constriction with an elongation of Gut.
An active shortening of longitudinal layer co-ordinated with relaxation of circular layer produces distension. Peristalsis consists of a travelling wave of constriction produced by contraction of circular muscle that is proceeded along it's length by a simultaneous contraction of longitudinal muscle and relaxation of the circular muscle.
This pattern of constriction pushes the luminal content in direction of peristaltic wave. Mixing & than packing of luminal contents is achieved primarily by a process called segmentation. Swallowing propels a bolus from the buccal cavity to the stomach. In vertebrates, swallowing involves the integrated movements of muscles in the tongue and pharynx as well as peristaltic movements of the oesophagus. 

Both actions are under direct neuronal control centres in the medulla oblongata of the brain. Regurgitation occurs when peristalsis takes place in the reverse direction, moving luminal contents back into the buccal cavity.
3.5.2.    Control of motility
1.    Intrinsic control: We know that in alimentary canal smooth muscles are abundant and the contraction in the smooth muscles tissue in alimentary canal is myogenic.  So an intrinsic cycle of electrical activity leads to muscle castration without any external stimulation. In mammals, pacemaker cells of the alimentary canal generate rhythmic depolarization called BER (Basic electricity rythm). Voltage clamp studies revealed that

The intrinsic patterns of BER are modulated by locally released gastrointestinal peptide hormones.
Thus, a chemical stimulant in chyme can cause the release of a paracrine hormone and this hormone can modulate the motility of muscle tissue.
2.    Extrinsic Constrol :
In addition to local stimulation, alimentary canal's motility is also influenced by diffused innervation from ANS.
Sympathetic and para sympathetic postganglionic neurons from networks that are dispersed throughout the smooth muscle layer.

The parasympathetic network is made up of cholinergic neurons and is divided into 
(a)    The myenteric plexus
(b)    Submucosal plexus
Those neuron networks, which receive their input primarily via branches of the vagus nerve, control the excitatory actions (i.e. increasing the motility and gastrointestinal secretion) of the digestive tract. In contrast, the innervation of sympathetic neurons is primarily associated with inhibition. Post ganglianic neurons of the sympathetic division directly innervate all the tissues of the gut wall as well as neurons of the myenteric and sub-mucosal plexus. The activity of the sympathetic efferents inhibits the motility of stomach and intestine. The development of action potentials in smooth muscle cells of the intestine is inhibited by nor-epinephrine which is released by sympathetic nerve endings and is promoted by acetylcholine released by parasympathetic nerve endings.
Each impulse is associated with excitation produces an increment of tension, which subsides with cessation of impulses.

Smooth muscles of the alimentary canal of vertebrates are also regulated by non-adrenergic and Non-cholinergic neurons that release a variety of peptide and purine nucleotides.
Aminergic neurons release ATP, serotonin, dopamine, GABA while peptidergic neurons release enkephalins, VIP (Vasoactive intestinal peptide), substance 'P', bombesin/gastrin releasing peptide, neurotensin, cholecystokinin (cck) and neuropeptide-Y. These above mentioned whole facts play a very fine control over the numerous integrating functions of the alimentary canal. The combined effect of intrinsic pacemaker activity and neuronal and endocrine control result in different rates of contractile activity in different regions of the alimentary canal

In mammals, stomach contractions are slow but powerful for additional work of churning movement (Pendular motions) for mixing of food after a meal.
While in contrast, the contractions in the small intestine are more frequent and are of shorter duration.
Colon provides stronger contractions of greater duration to ensure that the forming faecal matter is compacted and ejected.
3.6.    Gastrointestinal Sectetions 
The alimentary canal is a large exocrine that secrete gastrointestinal secretions consisting of a mixture of substances in aqueous form.
Exocrine tissue of alimentary canal - 
1.    Salivary gland
2.    Secretory cells in the stomach and intestinal
3.    Epithelium 
4.    Secretory cells of liver and pancreas
These exocrine secretions enter in gland's secretory duct and then secondarily modified. This secondary modified secretion is further modified involving transport of water and electrolyte and finally converting into secretory juice.
3.6.1.    Exocrine secretions of the alimentary canal :
Exocrine secretion of alimentary canal consists of some combination of water, ions, mucus and enzymes.
Formation of saliva in mammalian salivary gland
Water and electrolytes :
The exocrine secretion of the alimentary canal is a water-based fluid bearing digestive enzymes and other chemicals that are secreted into the lumen.
Most of the water is reabsorbed in the distal portion of the gut.
The goblet cells of stomach and intestine secrete an aqueous mucus solution that provides a slippery, thick lubricant the prevent mechanical injury to the lining of the gut.
The salivary gland and pancreas also secrete a mucoid solution but that is thinner.
Secretion of Inorganic constituents of the digestive fluid occurs in two step-
1.    Water and ions are secreted into the lumen of gland either by passive ultrafiltration or by active transport. Passive ultratiltration accurs due to hydrostatic pressure across the luminal epithelium.
2.    Active transport of ions is followed by the osmotic flow of water into the acinus.
This fluid undergoes secondary modification by active or passive transport across the epithelium, lining the exocrine ducts as it passes along the ducts toward the alimentary canal.

Bile and bile salts
The vertebrate liver does not produce digestive enzymes. Instead of this, it secretes bile i.e. a fluid that is essential for digestion of fat by their emulsification and provide a basic medium for properly working of many enzymes that act on lipid.
The bile is composed of + Basic mixture in water.
Basic mixture posses cholesterol, lecithin, inorganic salts, bile salts and bile pigments.
The bile salts are the organic salts composed of cholic acid, synthesized by the liver from cholesterol and conjugated with amino acids.
The bile pigments derived from biliverdin and bilirubin which are the product of the breakdown of haemoglobin.
The bile produced in the liver is transported via the hepatic duct to the gall bladder. Where it gets concentrated and stored.
Water is removed osmotically, following active transport of Na+ and Cl from the bile across the gall bladder epithelium.
Bile serves numerous function important to digestion-
1.    Provide a high alkalinity medium : 
The bile duct is consist of basic mixtures of cholesterol and lecithin. Due to this, the pH of bile juice is very high which neutralize the acidic medium of gastric secretion in the duodenum and facilitate well functioning of the various digestive enzyme upon fat.
2.    Emulsification of fat droplets : 
Bile salts break down large fat droplets into multiple microscopic droplets and dispersing them in aqueous solution for more effective attack by digestive enzymes.
Bile salts are removed from the large intestine by active transport and returned to the bloodstream where they bound to a plasma carrier protein and returned to the liver to be recycled.
Bile salts also disperse lipid soluble vitamins for transport in blood.
3.    Bile-contains waste substances 
Bile-contains waste substances removed from the blood by the liver such as haemoglobbin pigments, cholesterols, steroids and hydrophobic drugs. These substances either digested or excreted in the faces.
3.6.2.    Digestive enzyme :
Digestion is a complex chemical process in which special digestive enzymes catalyze the hydrolysis of large food molecules into simpler compounds that are small enough to cross the cell membrane of the intestinal barrier.
Digestive enzyme carries out hydrolysis by breaking the peptide bond that links amino acids in proteins or the glycosidic bond that link sugar monomers in polysaccharides.
Digestive enzymes making these large molecules small enough for absorption from the alimentary canal into the circulating body fluids and for subsequent entry into cells to be metabolized.
Digestive enzymes like all enzymes, exhibit substrate specificity and are sensitive to temprature, PH and certain ions.
There are three major groups of digestive enzyme.
1.    Proteases: These are proteolytic enzymes that break the peptide bond.
Exopeptidases - Break peptide bond at terminal site produced free amino acid or tri or dipeptides.
Endopeptidases: Break peptide bond within a long polypeptide chain producing small polypeptide chain and provide more site for attacking exopeptidases.
Some proteases have specificity for particular amino acid residues located on either side of the bond they attack.
Ex. - Endopeptidase trypsin: Attacks those peptides bond in which –COOH group is provided by arginine or lysine regardless of where they occur in the peptide chain. 
In mammals, protein digestion usually begins in the stomach with the action of the gastric protease pepsin.
The most powerful form of pepsin functions best in acidic enviorment, with a pH of only 2. This function is aided by secretion of HCl in the stomach.
Carbohydrases: Divided functionally into polysaccharides and glycosidases. 
Poly saccharideses hydrolyze the glycosidic bonds of long chain carbohydrate such as cellulose, glycogen and starch. 
Ex. - Amylases - It causes the terminal glycosidic bond breakdown in starch and glycogen. Amylases are secreted invertebrates by - salivary gland, pancreas and in a small amount from the stomach and in most invertebrates, by the salivary gland and intestinal epithelium.
The glycosidases, which occurs in the glycocalyx, are attached to the surface of the absorptive cells and act on disaccharides such as sucrose and fructose by hydrolyzing the remaining alpha-1, 6 and alpha - 1, 4 glycosidic bonds.
Many herbivores consume large amounts of plant cell walls, containing hemicellulose and lignin. Cellulose, which is in greatest abundance, consists of glucose molecules polymerized via beta - 1, 4 bonds. Whereas cellulose, a poly saccharide that digests cellulose and hemicellulose, is produced by a symbiotic microorganism in the guts of host animals as diverse as termites and cattle, which themselves are incapable of producing this enzyme.
In cattle, the symbiotic microbes take up cellulose molecules, digest them intracellularly, and pass some digested fragments into the surrounding fluid. These symbiotic gut bacteria, in turn, multiply and are themselves subsequently digested. Only a few animals, such as the shipworm, teredo and silverfish can secrete cellulase without the help of symbionts.
Lipases: Fats are water-insoluble which presents a special problem for their digestion. First, fats must be emulsified by the chemical action of bile salts and phospholipid lecithin. Bile salts have a fat-soluble hydrophobic and a water-soluble hydrophilic end. Fat attaches at one end of bile salt whereas water attaches at another end. This reaction disperses fat into the water-based fluid in the digestive tract. The second step in vertebrates is the formation of micelles aided by bile salts.
Lipids are degraded into fatty acid and monoglycerides in sufficient bile salt and in fatty acid and diglycerides in the insufficient bile salt. Fat digestion undergoes incomplete digestion and undigested fat is allowed to enter the colon and be eliminated without being absorbed.
Proenzyme: Certain digestive enzyme - (the proteolytic enzyme in particular) are synthesized, stored and released in an inactive molecular form known as a proenzyme or zymogen. Proenzyme requires activation, usually by hydrochloric acid in the lumen of the gastric gland, before they can carry out their degradative functions. The pro-enzyme is activated by the removal of a portion of the molecule, either by the action of another enzyme specific for this purpose or through a rise in ambient acidity.
Other digestive enzymes: There are also other enzymes that play a less important role in digestion. Nucleases, nucleotidases, and nucleosidases hydrolyze nucleic acid and their residues.
Estrases hydrolyze esters which include the fruity smelling compound.
3.6.3.    Control of gastrointestinal secretions 
The primary stimulus for secretion of digestive juice is the presence of food in a given part of the digestive tract.
The chemoreceptors present in the alimentary canal stimulates from the food molecules. This activates autonomic efferent neuron that activates or inhibit motility and exocrine secretion.
Appropriate food molecules also stimulate epithelial endocrine cells by contact with their membrane receptor. This causing secretion of gastrointestinal hormon into the local circulation.
These all reflexes causing co-ordination of secretory organ situate outside the alimentary canal Eg. liver and pancreas.
Cephalic influence such as mental images of food as well as learned behaviour also stimulates digestive secretion.
Gastro intestinal peptide hormone secreted by endocrine cells of gastric and intestinal sub-mucosa largely centrol gastrointestinal secretion.
Several of these gastrointestinal peptide hormones are identical to neuro-peptide that act as a transmitter in the central nervous system. This finding indicates that the genetic machinery for producing these biologically active peptides has been put to use by cells of both the central nervous system and alimentary canal.
Characteristics of digestive secretion: Depends on several interacting features as-
Whether the secretion is neuronally or hormonally controlled 
Where in the alimentary canal secretion occurs.
How long the food is normally present in the region being stimulated.
For e.g.: Salivary secretion is very rapid and is entirely under control of involuntary neuronal control. Gastric secretions are under hormonal as well as neuronal control and intestinal secretions are slower and are primarily under harmonal control.    Salivary secretions :
Mammalian saliva contains water, electrolytes, (Na+, K+, Cl–, HCO3 and in ruminates PO4–3) mucin, amylase and antimicrobial agents such as lysozyme and thiocynate. The water and electrolytes are derived from blood plasma while the HCO3 is provided by secretory cells. The ionic content of saliva varies with species and with the rate of flow.
In the absence of food, the salivary gland produces a slow flow of watery saliva.
Secretion of saliva is stimulated by the presence of food in the mouth or by any mechanical stimulation of tissues within the mouth via cholinergic parasympathetic nerves projecting to the salivary glands.
Salivary secretion is decreased by the release of Nor-epinephrine 
Many sympathetic nerves innervate salivary gland that secretes this hormone.
The amylase in salvia digest starch during chewing.    Gastric secretion: A major secretion of the stomach is HCl gastric pits are located in the gastric mucosal layer.
In these gastric pits, there are many oxyntic or parietal cells which secrete HCl.
Vagus Nerve: Control the secretion of HCl. Parasympathetic activity in the vagus nerve stimulates the secretion of HCl.
The hormono gastrin + histamin → stimulate this parasympatnetic activity.
Many food substances such as caffein are active ingredients of spices.
Secreted HCl activates some gastric enzyme for e.g. pepsinogen in pepsin and kills microorganism that enters with food.
In some cases, the amount of H+ that used to produce HCl is so large that blood and other extracellular fluid become alkalotic for hours or days after ingestion of the large meal. This is called alkaline tide that results in increasing blood pH by 0.5 or even 1.0 pH unit in crocodiles or snakes and other predators that have a large and infrequent meal. The parietal cells produce a concentration of the gastric ion in the gastric juice that is 1020 times greater than blood plasma. They do this by carbonic anhydrase which catalyzes the reaction of H2O with CO2.

Carbonic anhydrase
H2CO3 dissociates into HCO3 and H+ in the parietal cell. Then a HCO3 / Cl antiporter work and do the exchange of HCO3 and Cl– between the parietal cell and blood plasma. This transporter located on the basolateral membrane of the parietal cell.

In the apical membrane, there is a Cl channel. This Import Clinto the lumen of gastric gland via this channel.
On the other hand, the H+ ion is also transported via apical cell membrane into the lumen of gastric gland along with electrochemical gradient with K+.
K+ have property to transport from the apical and basolateral membrane. This property of importing and exporting ions allow the parietal cells to maintain a constant pH whereas providing an acidic condition to stomach solution.
Pepsin Secretion :
Pepsin is a major enzyme secreted by the stomach. This is a proteolytic enzyme secreted in the form of pepsinogen. This Enzyme secreted by zygomatic cells (Exocrine cells) also called chief cells. Vagus nerve controls the secretion of chief cells by the gastric branch of it. Also, controlled by the enzyme gastrin. Gastrin secreted by the pyloric region of gastric mucosa. Pepsinogen converted in pepsin that acts as an endopeptidase by a low PH dependent cleavage of a peptide chain.
Goblet cells
The stomach secretes gastric mucus containing various mucopolysaccharides. This mucus coats the gastric epithelium protecting it from digestion by pepsin and HCl.
Phases of gastric secretion: The secretion of HCl, pepsin occurs in three distinct phases:-
1.    Cephalic phase: Gastric secretion occurs in response to the sight, smell or taste of food.
Mediated by the brain via the vagus nerve. This phase abolished when the vagus nerve leading to stomach is cut.
2.    Gastric phase: Secretion of HCl and pepsin stimulated by the presence of food in the stomach.
Food in the stomach stimulated both chemoreceptor and mechano-receptor.
Gastrin and histamin is major polypeptide hormone through which this phase is mediated.
Gastrin is secreted from the endocrine cells of pyloric mucosa and histamine is synthesized in the mast cells of the gastric mucosa. Gastrin secreted in response to the gastric chyme containing protein. Gastrin increases stomach motility by binding to smooth muscle of the stomach wall.
Gastrin binds to the secretory cell of the stomach lining and induces a strong secretion of HCl and moderate secretion of pepsin. When chyme pH drops to 3.5, gastrin secretion slow and on the pH 1.5, it stops. Secretion of histamine by gastric mucosa also stimulates the secretion of HCl. 

3.    Intestinal phase: Intestinal phase is more complex than others. This phase is not only controlled by the enzyme gastrin it is also controlled by secretin, vasoactive intestinal peptide (VIP) and gastric inhibitory peptide (GIP). GIP is released by the endocrine cells in the mucosa of the upper small intestine in response to the entry of fat and sugars in the duodenum.
As food enters the duodenum, partially digested food stimulate the duodenal mucosa to secrete enteric gastrin. Enteric gastrin has the same activity as stomach gastrin, i.e. to stimulate the gastric gland to increase their rate of secretion. The secretion of gastric juice can be reduced by the absence of stimulating factors and by reflex inhibition. 
The enterogastric reflex. Which inhibits gastric secretion, is triggered when partially digested protein containing food at low pH is food pumped to duodenum from stomach.
Gastric secretion also inhibited by strong activation of the sympathetic nervous system.
Sympathetic neurons released neurotransmitter nor-epinephrine which inhibit both gastric secretion and gastric emptying.    Intestinal and pancreatic secretions: The epithelium of mammalian small intestine secrets a mixture of fluids called intestinal juice or succuss entericus.
Secretion of Brunner's gland: Located between the pyloric sphincter and pancreatic duct in the first part of the duodenum and secrete a viscous, enzyme-free, alkaline, mucoid fluid. It enables duodenum to withstand acidic chyme coming from the stomach Until it gets neutralized by the pancreatic and biliary secretion coming from pancreatic duct.
Secretion of crypts of Liberkuhn: A thinner enzyme rich alkaline fluid arise in crypts of liberkuhn and mixes with duodenal secretions.
Regulation of Intestinal juice: Regulated by several hormones including secretin, gastric inhibitory peptide (GIP), gastrin and other neuronal control.
Distension of the wall of the small intestine elicits a local secretory reflex. Vagal innervation also stimulates secretion of Intestinal juice.
Pancreatic Secretion: The exocrine tissue of pancreas produce several digestive enzymes that enter the small intestine through the pancreatic duct. The pancreatic enzyme including a - amylase, trypsin, chymotripsin, carboxypeptidases, lipases and nucleases.
Control: Exocrine secretion of the pancreas is controlled by the peptide hormones produced in the upper small intestine. Secretin and VIP both produced in the endocrine cells of the upper small intestine. These peptides reach the duct of pancreas though circulation and stimulate them to produce their secretion.
CCK (Chole cystokinin) also have an effect on pancreatic secretion which is synthesized and secreted from the epithelial endocrine cells of the small intestine. In response to the fatty acid and amino acids in the intestinal chyme. CCK also stimulates pancreatic secretions of the enzyme as well as the contraction of smooth muscle of Gall bladder wall.
The neuropeptide somatostatin identified in endocrine cells of upper intestinal mucosa in the vertebrate gut inhibits gastric acid secretion, pancreatic secretion and intestinal motility as well as blood flow.
Endocrinologists have recently discovered in rats and pigeons that oesophagus, stomach, duodenum and colon, all synthesize melatonin. Removal of pineal gland has no effect on alimentary melatonin level.
Melatonin has several effects on the alimentary canal:-
Inhibition of epithelial growth in Jejunum.
Reduced Na + transport in the colon.
Inhibition of Serotonin induced smooth muscle contraction
Melatonin is related to seasonal changes. It is considered that in many vertebrates, there are many structural changes in the gut that is due to melatonin.
3.6.4.    Nutrient uptake in the intestine
Carbohydrate - rich filaments made up of glycocalyx arise from the plasma-membrane of microvillus itself.
The brush bordered (microvilli + Glycocalyx) has been found to contain digestive enzymes.
These enzymes are also membrane-associated glycoproteins having carbohydrate side chains protruding into the lumen. 
The enzymes associated with the brushbordered include 
1.    Disaccharidases        
2.    Amino peptidases        
3.    Phosphatases
In the important process of absorption of nutrients from the lumen of the intestine to the wall of the alimentary canal includes 
*    Simple (passive diffusion)                
*    Faciliated diffusion co-transport
*    Active transport Endocytosis
Simple diffusion
Simple diffusion takes place across the lipid bilayer (provide the ability to pass) or through water-filled pores. Fatty acids, monoglycerides, cholesterol and other fat soluble substances cross a lipid bilayer. Substances that pass through water filled pores include water, certain sugars, alcohol and other water soluble molecules. For non-electrolytes, the net diffusion rate is proportional to their chemical concentration gradient. For electrolytes, It is propartional to their electrochemical gradient. In passive diffusion, net transfer is always 'downhill' using the energy of concentration gradient.
Faciliated diffusion :
Absorption of monosaccharides and amino acids presents the problem that, these molecules are hydrophillic because of their-OH groups. These problems are solved by faciliated diffusion with the help of transporter proteins in the absorptive cells.

Hydrophilic & lipid insoluble sugars such as fructose are carried down their concentration gradient by faciliated diffusion via transporter proteins, in a process by coupling sugar transport to the electrochemical gradient for Na+ across the plasma membrane. The Na+/Glucose cotransporter, SGLT 1 is the integral membrane protein that couples the transport of Na+ with Glucose across the brush border. GLUT 5 is the brush border fructose transporter and GLUT 2 is basolateral membrane transporter for fructose as well as glucose and galactose.
Same monosaccharides are taken up into the absorptive cells by a related mechanism. Hydrolase Transport in which a glycosidase attached to the apical membrane hydrolyzes the parent disaccharide (eg: - Sucrose, maltose) and also acts as or is coupled to, the mechanism that transfers the resulting monosacchardes in the absorptive cells.
Active transport
Sodium driven transport of amino acids into absorptive cells takes place via four separate co-transport systems.
1.    Three dibasic amino acids having two basic amino groups each (Lysine, Arginine, Histidine)
2.    Two diacidic amino acid having two carboxyl groups each (glutamate and aspartate).
3.    A special amino acid class consisting of Glycine, Proline, and Hydroxyproline.
4.    Remaining neutral Amino acids.
Dipeptides and tripeptides are transported into absorptive cells by a separate transport system.
Inside the cell, dipeptides and tripeptides are cleaved into their constitutive amino acids by intracellular peptidases. 
This system has the advantage of preventing a build-up of oligopeptides within the cell. So that there is always a large inwardly directed concentration gradient promoting their inward transport.
Sugar and amino acids present in intestinal epithelium enter the blood by diffusion into the capillaries within the villi.
After reaching to other tissues of the body, the sugars and amino acids are transferred into cells by the same type of active transport and facilitated diffusion mechanisms.
Special handling of lipids
Digestion products of fats i.e. monoglycerides, fatty acids, and Glycerol diffuse through the brush border membrane and reconstructed within the absorptive cell into triglycerides.
The breakdown parts of fats are then collected together with phospholipids and cholesterol, into tiny droplets called chylomicrons. Which are about 150 nm in diameter?

Chylomicrons are coated with a layer of protein and are loosely contained in vesicles formed by Golgi complex. 
They are subsequently expelled by exocytosis through the fusion of these vesicles with the basolateral membrane of the absorptive cell.
Chylomicrons then enter the central lacteal, from which they are sent to lymph and then to blood for broad distribution throughout the body.
Transport of Nutrients in Blood
Once after the digestion of products in the absorptive cells of the intestine, products are pass-through basolateral membrane into the interior of the villus. From the interstitial fluid of the villus, they enter the blood or lymphatic circulation. In humans, 80% of chylomicrons enter the bloodstream via the lymphatic system, while 20% enters blood directly. The pathway into the lymphatic system begins with the blind central lacteal of the villus. Sugars and amino acids primarily enter the capillary of the villus. which are drained by venules that lead into the hepatic portal vein. This vein takes the blood from the intestine directly to the liver. Under the influence of insulin, the glucose is taken up by hepatocytes and converted into glycogen.

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