Digestion Absorption And Metabolism Of Carbohydrates Notes

Carbohydrates In Diet

The human diet contains three types of carbohydrates.

1. Polysaccharides: The large polysaccharides are glycogen, amylose, and amylopectin which are in the form of starch (glucose polymers). Glycogen is available in nonvegetarian diets Amylose and amylopectin are available in vegetarian diets because of their plant origin.

2. Disaccharides:

Two types of disaccharides are available in the diet.

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1. Sucrose (Glucose + Fructose) which is called table sugar or cane sugar
2. Lactose (Glucose + Galactose) is the sugar available in milk.

3. Monosaccharides: The monosaccharides consumed in the human diet are mostly glucose and fructose.

• Other carbohydrates in the diet include
  • Alcohol
  • Lactic acid
  • Pyruvic acid
  • Pectins
  • Dextrins
  • Carbohydrates in meats.

The diet also contains a large amount of cellulose which cannot be digested in the human GI tract so, it is not considered as food for human beings.

Digestion Of Carbohydrates

• In The Mouth: The enzymes involved in the digestion of carbohydrates are known as anxiolytic enzymes. The only anxiolytic enzyme present in saliva is salivary amylase or ptyalin.
• In The Stomach: Gastric juice contains a weak amylase, which plays a minor role in the digestion of carbohydrates.
• In The Intestine: The carbohydrate-splitting enzymes present in the small intestine are derived from the pancreatic juice and the intestinal secretion, succus entericus.
  • Amylolytic Enzyme in Pancreatic Juice: Pancreatic juice contains pancreatic amylase.
  • Amylolytic Enzymes In Succus Entericus: Carbohydrate-splitting enzymes present in succus entericus are maltase, sucrase, lactase, dextrinase, and trehalase.

Final Products Of Carbohydrate Digestion: The final products of carbohydrate digestion are mono-saccharides, which are glucose, fructose, and galactose. Glucose represents 80% of the final product of carbohydrate digestion. Galactose and fructose represent the remaining 20%.

Absorption Of Carbohydrates

The carbohydrates are absorbed from the small intestine mainly as monosaccharides, viz. glucose, galactose, and fructose.

• Absorption Of Glucose:
  • Glucose is transported from the lumen of the small intestine into the epithelial cells in the mucus membrane of the small intestine by means of sodium co-transport. The energy for this is obtained by the binding process of sodium ion and the glucose molecule to the carrier protein.
  • From the epithelial cell, glucose is absorbed into the portal vein by facilitated diffusion. However, sodium ion moves laterally into the intercellular space. From here, it is transported into the blood by active transport, utilizing the anarchy liberated by breakdown of ATP.
• Absorption Of Galactose: Gaiectoss is also absorbed from the small intestine in the same mechanism as that of glucose.
• Absorption Of Fructose: Fructose is absorbed into the blood by means of facilitated diffusion. Some molecules of fructose are converted into glucose. The glucose is absorbed as described above.

Metabolism Of Carbohydrates

• Metabolism is the process in which food substances undergo chemical and energy transformation. After digestion and absorption, the food substances must be utilized by the body. The utilization occurs mainly by an oxidative process in which the carbohydrates, proteins, and lipids are burnt slowly to release energy. This process is known as catabolism.
• The part of the released energy is utilized by the tissues for physiological actions and the rest of the energy is stored as rich energy phosphate bonds and in the form of proteins, carbohydrates and lipids in the tissues. This process is called anabolism.
• The metabolism of carbohydrates is given in the form of a schematic diagram.