Carbohydrates in Digestion

Carbohydrates

The major carbohydrates present in our diet are starch, glycogen, sucrose, lactose , maltose and very little concentrations of fructose and pentose.

1. Digestion in mouth

Milk and other fluid items like juices escape digestion in the mouth as they do not reside in the mouth for a longer time, whereas, starch and glycogen containing solid foods are masticated with saliva thoroughly. Saliva contains ptyalin, an a amylase, which attacks the a 1-4 linkages resulting in the formation of monosaccharide glucose, disaccharide maltose and trisaccharide maltotriose. However, because of steric hindrance caused by the branches, some of the interior a 1-4 linkages are inaccessible for the enzyme. This results in the formation of a highly branched structure called as limit dextrin.

The optimum pH for salivary amylase is pH 6.7. Ptyalin needs chloride ions for their effective action.

       Glycogen, Starch  -------->  Glucose, Maltose, Maltotriose, Limit Dextrin

When food along with ptyalin reaches the stomach, ptyalin is inactivated due to low pH. There are no enzymes to act upon carbohydrates in the stomach. No change to polysaccharides occurs in the stomach. Dietary sucrose may be hydrolyzed to equimolar quantities of glucose and fructose by the HCl present.

2.  Digestion in duodenum

When the food bolus reaches the duodenum, it is mixed with the pancreatic juice, which contains α - amylase. Its action is similar to that of the ptyalin, but it is more powerful because

·              It can act upon raw starch.

·              It can hydrolyze the interior linkages of the starch, which were inaccessible for ptyalin.

The optimum pH of pancreatic amylase ranges between 6.9 – 7.1 and it needs chloride ions for its action

3.  Digestion in small intestine

Five enzymes are present in small intestine to hydrolyze the carbohydrates completely to monosaccharides.

a. Intestinal amylase :It hydrolyses the terminala1-4 linkages in polysaccharidesand oligosaccharides releasing free glucose molecules.

b. Lactase :It isb-galactosidase that hydrolyses lactose molecule to equimolar amounts of glucose and galactose. Its optimum pH is 5.4 – 6.0

c. Maltase : It is a glucosidase that acts ona1-4 linkages of maltose yieldingglucose molecules. Five different maltases have been identified in the intestinal epithelial cells. Its optimum pH ranges between 5.8 – 6.2. 

d. Sucrase : It hydrolyzes sucrose to equimolar amounts of glucose and fructose byhydrolyzing b 1-2 linkages.

e. Isomaltase : It hydrolyses thea1-6 branch points of limit dextrin and liberatesmaltose and glucose.

There are no enzymes present in our digestive system to hydrolyze b1,4 linkages in cellulose, so it cannot be digested.

4.  Absorption of carbohydrates

Only monosaccharides can be absorbed by the intestinal mucosa. A few disaccharides can be pinocytosed and hydrolyzed to monosaccharides by disaccharidases. The absorption rate of the monosaccharides is in the following order:

Galactose > Glucose > Fructose > Mannose > Xylose > Arabinose

Mechanism of absorption

1. Simple Diffusion: Initially, when the concentration of glucose in the intestinallumen is high, by simple diffusion it crosses the membrane.

2. Active Transport: To speed up the absorption process, active transportmechanisms are involved. Absorption of glucose is a secondary active transport process, which involves ATP hydrolysis indirectly.

The steps involved in the transport of glucose are :

·              One molecule of Na+ and glucose binds to the transporter.

·              Binding of sodium and glucose induces a conformational change in the transporter. 

·              The conformational change leads to the delivery of sodium and glucose molecules to the intestinal cells.

·              Sodium is pumped out of the cell resulting in the net absorption of glucose molecule alone.

Step (iv) needs energy that is derived by ATP hydrolysis.

5. Factors affecting rate of absorption

·              If the intestinal mucosa is damaged, absorption is reduced.

·              Hormones like thyroid hormone, adrenal cortex hormones and pituitary hormones enhance the absorption of carbohydrates.

·              Insulin has no effect in the absorption of glucose.

·              Pyrimidine and pantothenic acid deficiencies diminish the absorption.

·              Inherited enzyme deficiency states like lactose intolerance decreases the absorption.