Digestion Types: 4 Main Types of Digestion

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.