The following points highlight the four main types of digestion. The types are: 1. Digestion in the Mouth 2. Digestion in the Stomach 3. Pancreatic Digestion 4. Digestion in Intestine.
Type # 1. Digestion in the Mouth:
Constituents of Saliva:
a. Saliva is colourless, slightly acid (pH 6.4- 7.1) rather than alkaline, viscous fluid secreted mainly by the parotid, sub-maxillary and sublingual salivary glands. The sub-maxillary secretion contains most of the glycoproteins (mucin) but the parotid contains none.
b. The amount of secretion from each gland varies with the stimulus and nature of the food. The amount is stated to be 1,000 – 1,500 ml each day.
c. A saliva of 0.6% solids will contain 0.4% organic and 0.2% inorganic material.
d. The chief organic substances are glycoprotein (mucin), salivary amylase (ptyalin), small amounts of albumin and globulin, urea and uric acid, traces of thiocyanic acid.
e. The inorganic substances are CI, Na+, K+, Ca++, Mg++, HCO1 and HPO–4. Chloride ion is an important activator of amylase. Calcium ion helps to stabilise the amylase.
Salivary Digestion:
a. The salivary amylase or ptyalin brings about the hydrolysis of starch and glycogen to maltose at the optimum pH 6.8. Chloride ion activates the enzyme.
b. The enzyme can act on the food for a short time (5 to 6 minutes) which is of little significance.
c. Salivary amylase is readily inactivated at pH 4.0 or less. So the digestive action on food in the mouth is soon ceased in the acid environment of the stomach.
d. In many animals, salivary amylase is entirely absent.
Functions of Saliva:
a. Saliva moistens dry food and facilitates swallowing by the lubricating action of the glycoprotein.
b. The digestion of starch begins in the mouth.
c. Saliva contains buffering substances e.g., bicarbonate, phosphate and mucin.
d. It keeps the mouth at a neutral pH and thus protects the teeth from decalcification and also keeps the mouth and teeth clean.
Type # 2. Digestion in the Stomach:
Gastric Secretion:
a. Nervous or reflex mechanisms cause the initiation of gastric secretion.
b. The continued gastric secretion is regulated by the hormone gastrin (gastric secretion). The chemical stimulant is produced by the gastric glands and absorbed into the blood, which carries back to the stomach where it stimulates gastric secretion.
c. Histamine, the decarboxylated product of the amino acid histidine, also stimulates the secretion of gastric juice.
Constituents of Gastric Juice:
a. Normal gastric juice is a thin, light coloured fluid which is strongly acidic.
b. Hydrochloric acid secreted by the oxyntic cells (parietal cells) is 0.55 per cent which is equivalent to pH 0.9. The high acidity is neutralized by the high water content of saliva, the glycoprotein of saliva, the proteins of the food functioning as weak acid and the mucus of the stomach.
c. The gastric juice contains 97-99% water and about 0.55% solids of which about 0.4 per cent is organic.
d. The organic substances are the enzyme pepsin, glycoprotein, traces of lipase, rennin in infants.
e. The inorganic substances are mainly CI–, K+, Na+, traces of Ca++, Mg++, phosphate and sulfate.
f. On ordinary diet, the amount of secretion is 2-3 litres daily by an adult.
g. The intrinsic factor (HCl and mucoproteins), present in the gastric juice, helps in the absorption of vitamin B12. The mucopolysaccharide present in the intrinsic factor contains fucose, hexosamine and neuraminic acid.
Gastric Digestion:
An ordinary meal is evacuated from the stomach in about four hours. The time may be more if the food is not properly masticated or if there is an excess of fat or gastric secretion is disturbed by emotion.
A. Hydrochloric acid:
Hydrochloric acid is formed by parietal cells and the process is similar to that of the ‘chloride shift’.
Carbonic anhydrase catalyzes the formation of H2CO3 from H2O and CO2 as shown in Fig. 16.1.
Production of no hydrochloric acid leads to the condition achlorhydria.
Functions of hydrochloric acid:
a. It maintains the optimal pH (1.2-1.5) for digestion of proteins by pepsin.
b. It converts inactive pepsinogen into active pepsin.
c. It stimulates duodenum to liberate secretin.
d. It helps the absorption of iron by converting ferric hydroxide of the food into ferric form which is changed to the ferrous form by reduction.
e. It denatures food proteins making them more readily digestible.
f. It has a germicidal effect on microorganisms and hence prevents the growth of microorganisms in the stomach.
B. Pepsin:
a. It is secreted in the inactive form pepsinogens which is activated to pepsin by the action of HC1 and a small amount of pepsin can cause the activation of the remaining pepsinogen.
b. It converts native protein into proteoses and peptones.
c. It has a molecular weight of 32,700.
d. It contains large amount of acidic amino acids.
C. Rennin (Chymosin, Rennet):
a. This enzyme occurs in the stomach of infants and is absent from the stomach of adults.
b. It causes the coagulation of milk and prevents the rapid passage of milk from the stomach.
c. It changes the casein of milk to para-casein in presence of calcium at pH 5.0. Para-casein is then acted on by pepsin:
D. Lipase:
a. The gastric lipase has mild lipolytic action which is of no significance.
b. It can act only at the pH 5 to 7. Hence it is not so active at pH 1.3-1.5.
Type # 3. Pancreatic Digestion:
Pancreatic Secretion:
a. The secretion of pancreatic juice is controlled by both nervous and hormonal means.
b. Hydrochloric acid, fats, proteins, carbohydrates and partially digested foodstuffs entering the duodenum and the upper jejunum stimulate the secretion of hormone secretin and pancreozymin.
Secretin stimulates the pancreas to produce thin, watery fluid, high in bicarbonate but low in enzyme content. Pancreozymin stimulates the pancreas to produce a viscous fluid low in bicarbonate but high in enzyme content.
Constituents of Pancreatic Juice:
a. It is a clear alkaline fluid with a pH about 8.0.
b. It contains 1.8% of solids (including HCO3−) of which 0.6% is organic substances.
c. The organic substances include proteins, the enzymes trypsinogen, chymotrypsinogen, carboxypeptidase, lipase, amylase, maltase, phospholipase, ribonuclease, de-oxy-ribonuclease, cholesteryl ester hydrolase and collagenase.
d. The inorganic substances are Na+, CI−, HCO3− with small amounts of K+, Ca++ and HPO4–.
e. The amount of juice secreted each day is 600-800 ml.
Pancreatic Digestion:
A. Trypsin:
a. It is secreted in the inactive form trypsinogen which is converted into trypsin by the enzyme enterokinase secreted by the duodenal mucosa.
b. It attacks the native protein, proteoses and peptones to produce polypeptides.
c. It attacks peptide linkages containing arginine or lysine residue.
B. Chymotrypsin:
a. It is also secreted in the inactive form chymotrypsinogen which is converted into chymotrypsin by the action of trypsin.
b. It also attacks the native protein, proteoses and peptones to produce polypeptides.
c. It attacks peptide linkages containing tyrosine and phenylalanine residues.
C. Carboxypeptidase:
a. It is a zinc containing enzyme.
b. Two carboxy-peptidases (A and B) occur in the pancreatic juice.
c. They are exopeptidase and hydrolyze only the terminal peptide linkage.
d. Carboxypeptidase A hydrolyses terminal peptide linkage containing tyrosine, phenylalanine and tryptophan while carboxypeptidase B hydrolyses terminal peptide linkages containing lysine and arginine.
D. Amylase:
a. Pancreatic amylase is similar in action to salivary amylase.
b. It is an α-amylase and an endo-amylase.
c. It hydrolyses starch and glycogen into maltose and a mixture of branched of (1:6) oligosaccharides into glucose.
d. In pancreatitis, serum and urine amylase level is increased showing diagnostic importance.
E. Lipase:
a. Pancreatic lipase is most important in the digestion of fats. It hydrolyses fats into diglyceride, monoglyceride, glycerol and fatty acids.
b. It is specific for the hydrolysis of primary ester linkages which occurs in position 1 and 3 of triglyceride.
c. The hydrolysis of fat by lipase is increased when the fat is emulsified by bile salts. This is due to the larger surface exposed owing to the reduction of surface tension to the action of lipase.
F. Amino-peptidase and Di-peptidase:
The amino-peptidase attacks the terminal peptide bond at the free amino end of the chain.
G. Phospholipses:
These enzymes hydrolyse phospholipids (lecithin and cephalin).
H. Cholesteryl ester hydrolase (Cholesterol esterase):
This enzyme hydrolyses the esterification of free cholesterol with fatty acids.
I. Collagenase:
This enzyme hydrolyses collagen present in meat and fish.
J. Ribonuclease (RNAase) and Deoxyribonuclease (DNAase):
a. These enzymes are specific for the hydrolysis of RNA and DNA, respectively.
b. These are endonucleases. Both the enzymes are capable of cleaving internal phosphodiester bonds to produce a 3′-hydroxyl and a 5′-phosphoryl or a 5′-hydroxy 1 and a 3′-phosphoryl terminus.
c. Some are capable of hydrolysing both strands of a double stranded molecule whereas others can only cleave single strand of nucleic acids.
d. Some nucleases are exonucleases. These are capable of hydrolysing a nucleotide when it is present at a terminus of a molecule.
Type # 4. Digestion in Intestine:
Intestinal Secretion:
Hydrochloric acid, fats, proteins, carbohydrates and partially digested foodstuffs entering the duodenum and the upper jejunum stimulates the secretion of enterocrinin which induces the flow of intestinal juice.
Constituents of Intestinal Juice:
a. It is an alkaline fluid of pH about 7.8.
b. The amount of the juice secreted each day by an adult is 2 to 3 litres.
c. It contains about 1.5 per cent solids of which nearly two-thirds is inorganic and the remainder is organic substances.
d. Half the inorganic part is NaHCO3 and half NaCl.
e. The organic substances are mainly enzymes and proteins. The enzymes include amino-peptidase and di-peptidase, lactase, maltase, sucrase, lipase, phosphatases, nucleases, nucleotidases, nucleosidases, phospholipases, enterokinase.
Intestinal Digestion:
a. Disaccharidases (lactase, maltase, sucrase):
They hydrolyse the corresponding dis-saccharides into monosaccharides for absorption:
b. Phosphatases:
They remove phosphate from certain organic phosphates such as hexophosphate, glycerophosphate and the nucleotides derived from the food.
c. Nucleases:
They cause the hydrolysis of nucleic acids to mononucleotides.
d. Nucleotidases:
They hydrolyse nucleotides to nucleosides removing phosphate.
e. Nucleosidases:
They hydrolyse the nucleosides into purine or pyrimidine base and pentose sugars (ribose or deoxyribose).
f. Phospholipases:
They hydrolyse phospholipids into glycerol, fatty acids, phosphoric acid and bases such as choline.
g. Enterokinase:
This is secreted by the duodenum. It converts inactive trypsinogen to active trypsin.
h. Lipase:
This converts triglycerides and diglycerides into monoglycerides and fatty acids.