Digestion of carbohydrates

Carbohydrates of foods are carbohydrate source of the organism. Usage rate is 400-500 grams per day. Carbohydrates are the major source of energy needed to man. Carbohydrates as a part of glycoproteins also performe protective function. They are used for synthesis of NA.

Starch is accumulated in plants, lactose is present in breast milk, glucose and fructose are in honey and fruits, and maltose comes from the foods in which starch is partially hydrolyzed, such as malt.

Annual consumptionof carbohydrates is 400-500 g/day, with them comes the principal amount of calories needed to man.

Food poly- and disaccharides undergo enzymatic digestion in the digestive tract, where the enzymatic hydrolysis of glycosidic bonds goes. Monosaccharides are formed, then they are absorbed, enter the blood and tissues.

a-amylase of saliva decomposes a-1,4-glycosidic bonds. Starch is partially digested in the mouth. Dextrins and maltose are formed.

Gastric juice does not contain enzymes that break down dietary carbohydrates. Saliva amylase is inactivated in the stomach, because pH of gastric juice is 2, and pH optimum of amylase of saliva is 6.7. But amylase acts any time inside the food bolus.

Pancreatic amylase hydrolyzes starch in the upper small intestine by sequential cleavage of disaccharide residues. Maltose and isomaltose are formed.

Maltose, isomaltose, sucrose, lactose are hydrolyzed by glycosidases on the cell surface of the small intestine to monomers (maltose is hydrolyzed by the enzyme maltase to glucose, sucrose is hydrolyzed by the enzyme sucrase to glucose and fructose).

b-Cellulose is not cleaved in the gastrointestinal tract: a person cannot produce an enzyme hydrolyzing 1,4-glycoside bonds. Undigested cellulose of plant foods promotes normal bowel movements.

Glucose transport across the membrane occurs by facilitated diffusion or active transport.

Na⁺-glucose co-transporter, or simporter, carries the secondary active transport of glucose. Concentration gradient of Na⁺ is maintained through the work of Na⁺, K⁺-pump. Galactose is transported by the same way.

Glucose can be transported across the membrane also by facilitated diffusion with the help of carrier proteins. There are five types of glucose transporters (GLUT).The rate of the transmembrane transport of glucose depends on its concentration gradient. Exceptions are muscle cells and adipose tissue, where there are insulin-dependent transporters. In the absence of insulin, the membrane is impermeable to glucose.

Fructose is also absorbed by facilitated diffusion.

Hereditary or acquired defects of enzymes that hydrolyze carbohydrates is one of the causes of digestive disorders. The accumulation of undigested carbohydrates increases the flow of water into the lumen of the intestine that causes spasms and diarrhea. The action of bacteria on non-hydrolyzed carbohydrates leads to flatulence.

Most of the glucose (90%) comes from the blood through the portal vein to the liver. In the cell, glucose is phosphorylated (the active form) and subjected to further transformations.