- Digestion is defined as the conversion of non diffusible food elements into diffusible constituents.
- Digestion is the conversion of large molecules into small ones.
- In Digestive system the process by which food substances are broken down by mechanical and chemical means.
- Mechanical digestion comprises mastication or chewing, liquefaction of food by digestive juices, swallowing and peristalsis.
- Major utility of breaking up of food into small bits during chewing is to increase the surface area of food. It helps in the enzymatic action.
- Chemical digestion includes the enzymatic action on food.
- All enzymes are chemically proteins.
- All digestive enzymes are hydrolytic. They catalyse hydrolysis of nutrients.
- In hydrolysis of nutrients, a small amount of energy is released as heat.
- Digestive glands secretes digestive enzymes.Four main types of digestive enzymes are; Carbohydrases, Proteinases, Lipases and Nucleases.
- Carbohydrases include amylase (polysaccharides to disaccharides) and disaccharidases (Maltase, sucrase and lactase).
- Proteinases can be endopeptidase and exopeptidase.
- Lipase (steapsin) acts on ester bonds of fats or triglycerides.
- Nuclease hydrolyses nucleic acid into nucleotides and finally into nitrogenous bases, pentose sugar and phosphate group, e.g., D Nase and R Nase.
Digestive Juices in Alimentary canal
Digestive juice-- Location
- Saliva----Mouth cavity
- Gastric juice----Stomach
- Pancreatic juice----Duodenum
- Intestinal juice----(succus entericus) Small intestine
Digestion in mouth cavity
- In rabbit, the digestion starts from mouth. Masticated food in mouth is mixed with saliva secreted by salivary glands (3 pairs in man, 4 pairs in rabbit)
- pH of saliva is about 6.8 (slightly acidic, almost neutral)
- Food mixed with saliva in the buccal cavity is called 'bolus'.
- Saliva contains a starch splitting enzyme ptyalin (salivary amylase) acts on cooked starch changing them into a sugar maltose, isomaltose and limit' dextrins.
- Saliva contains an antibacterial enzyme lysozyme. It dissolves the cell wall of Gram positive bacteria and kills them.
- There is no digestion in oesophagus. It conducts the food from mouth into stomach.
Digestion in STOMACH
- Stomach secretes gastric juice (pH 1-3.5)
- Protein digestion starts in the stomach.
- Stomach is the chief site of protein digestion.
- The columnar epithelium of stomach form many gastric pits with gastric glands.
- Gastric glands are located on mucosa of stomach.
- Gastric glands are lined with three kinds of secreting cells: Zymogenic (main, peptic or chief) cells, parietal (oxyntic) cells and mucous cells.
- The main, peptic or zymogen cells secrete digestive proenzymes namely pepsinogen and prorennin.
- Oxyntic cells secrete HCI.
- Pepsinogen is activated into pepsin by HCI. It is a protein splitting endopeptidase.
- Protein -------> Pepsin -------> Proteoses + Peptones
- Pepsin can digest even collagen of connective tissue fibers, but not keratin of nail or hair.
- The mucus secreted by stomach protects its wall from the action of pepsin.
- Rennin is found in calf gastric juice. It is a milk coagulating proteinase.
- Inactive pro rennin is converted into active rennin again by HCI
- Rennin acts on casein, a protein in milk, changing it into calcium paracaseinate. It is known as curdling of milk.
- Rennin + Milk protein (Casein) -------> Ca++ -------> Cal.paracaseinate
- But adult cows or human infants do not secrete rennin, The function of rennin is taken over then by pepsin.
- Another enzyme of the stomach is gastric lipase. It splits the butterfat molecules found in milk.
- Lipase + fats -------> Fatty acids and glycerol
- The presence of gastric lipase in rabbit is doubtful.
- A carbohydrate splitting enzyme gastric amylase is present in both rabbit and man in very minute quantity. Their role in digestion is negligible.
- Thus the semi digested thick pasty solution is formed. This is called chyme. It passes into duodenum.
DIGESTION IN SMALL INTESTINE
- The food in the small intestine is mixed with three digestive fluids namely bile, pancreatic juice and intestinal juice.
- Bile is secreted by liver and is stored in the gallbladder.
- Bile is alkaline, viscous, yellow to green in colour, pH 7.8-8.6. There is no digestive enzyme in bile, its main role in digestion is emulsification of fat.
- In bile there are salts like sodium bicarbonate, sodium taurocholate and sodium glycocholate. Emulsification of fats takes place through these bile salts.
- The action of bile on fat is non enzymatic.
- The presence of bile in the small intestine is also necessary for the absorption of the fat-soluble vitamins A, D, E and K.
- If gallbladder is removed, the digestion of fat is affected. Acidity may continue in the duodenum.
- The bile pigments present in bile juice are bileverdin and bile rubin. They are formed by the break down of haemoglobin of worn out RBC in liver.
- Pancreatic juice, secreted by exocrine pancreatic acini, is a complete digestive juice.
- Pancreatic juice takes part in the digestion of proteins, carbohydrates and fats.
- Pancreatic juice is colourless watery fluid, slightly alkaline, pH 7.5 - 8 due to presence of sodium bicarbonate.
- Pancreatic amylase is a starch splitting enzyme similar to ptyalin hydrolysing starch and glycogen to maltose, isomaltose and 'Limit' dextrins.
- Trypsin and chymotrypsin are proteolytic endopeptidases.
- They are initially inactive trypsinogen and chymotrypsinogen.
- The enterokinase secreted by intestine stimulates trypsinogen to active trypsin.
- The trypsin changes chymotrypsinogen into active chymotrypsin.
- Both trypsin and chymotrypsin hydrolyse proteins, proteoses and peptones to polypeptides.
- Trypsin cannot coagulate milk.
- In predator animals drinking the blood of their prey, trypsin hydrolyses, fibrinogen of blood into fibrin leading to blood coagulation.
- Like pepsin, trypsin cannot hydrolyse keratin.
- Pancreatic juice contains the enzyme lipase.
- Pancreatic lipase hydrolyses fat into fatty acids and glycerol.
- Intestinal juice or succus entericus is mainly secreted by crypts of Lieberkuhn.
- Intestinal juice is a clear yellow fluid with slightly alkaline nature pH of 7.6, contains water, mucus and enzymes that complete the digestion of carbohydrates, proteins, fats and nucleic acids.
- The important enzymes in intestinal juice are.
- Disaccharidases-----Intestinal lipase
i) Enterokinase activates trypsinogen to trypsin.
ii) Erepsin contains exopeptidase, changes polypeptides to amino acids
iii) Disaccharidases act as follows:
- Sucrose invertase -------> Glucose + Fructose
- Maltose Maltase -------> Glucose + Glucose
- Lactose Lactase -------> Glucose + Galactose
iv) Intestinal lipase act as follows:
- Emulsified fat -------> lipase -------> Fatty acids + Glycerol
v) Polynucleotidase and nucleosidase act as follows :
- Nucleic acid -------> Potynucleotaidse -------> Nucleotides
- Nucleotides -------> nucleotidase -------> Nucleosides + Phosphate
- Nucleosides -------> Nucleosidase -------> Nitrogenous bases + Pentose sugar
- Digestion of all major nutrients of food is completed in the small intestine.
- The end products of carbohydrate digestion are monosaccharides.
- The end products of protein digestion are amino acids.
- The end products of fat digestion are fatty acids and glycerol.
- The end products of nucleic acid digestion are nitrogenous bases, pentose sugars and phosphoric acid.
Thus the digestion of various food stuffs is completed and as the food passes through the intestine it receives more water and form fluid emulsion called chyle. The enzymatic action remains continuous throughout the process and finally proteins, carbohydrates, fats are hydrolysed into amino acids, glucose, fatty acids and glycerol.
The digestion of cellulose occurs in caecum which is well developed in Rabbit, as cellulose decomposing bacteria and protozoans are present in caecum.
- Absorption of digested food mainly occurs in ileum of small intestine.
- Fatty acids and glycerol are absorbed by lacteals in villi. They are taken in lymph vessels.
- Amino acids and glucose are carried by hepatic portal vein.
- Some nutrients such as fructose and maltose are absorbed from intestine by simple process of diffusion and finally carried to liver through portal circulation.
- Water is absorbed by osmosis from the intestinal lumen to intestinal cells.
Hormonal control of digestion
- Gastrin: G-cells of pyloric gland secretes a hormone called gastrin which stimulates secretion of gastric juice.
- Enterogastrone: Enterogastrone produced by small intestine slows down the secretion of gastric juice.
- Cholecystokinin: Release of bile into duodenum is promoted by a hormone CCK, which induces rhythmic contraction of the gallbladder.
- Secretin: Secreted by duodenum stimulates pancreas and controls the volume of pancreatic juice including water and electrolytes.
- Pancreozymin: Secreted by duodenum, controls the amount of enzymes in pancreatic juice.
- Enterocrinin: Secreted by small intestine stimulates intestine to secrete the intestinal juice.
The transport of digested food materials from blood into tissues
takes place where they undergo oxidation releasing energy. This process is called assimilation.
The digested food materials involved in the synthesis of new substances help for the growth of the animal.
The indigestible food passes through colon and is pushed outside therectum in the form of semi-solid faeces. In the colon and rectum, the water is reabsorbed.
The faeces passed out side are soft and moist due to the presence of large quantities of plant cell walls, which are digested by the enzymes. This faecal matter is eaten up by rabbit and thus passed through gut once more for digestion and absorption of simplified cellulose. By this habbit rabbit gets maximum amount of nutrients from the food and it is called coprophagy or pseudorumination or refection.