Carbohydrate Digestion and Absorption


Write a Short Essay on Carbohydrates Digestion and Absorption


In the Mouth: Carbohydrate digestion begins in the mouth by saliva. Saliva begins digestion of carbohydrate in the mouth through action of salivary amylase, an enzyme that breaks polysaccharides down into maltose, a disaccharide consisting of two glucose molecules. Digestion in the mouth involves the hydrolysis of polysaccharides into disaccharides by amylase. However, most of the digestion by this enzyme is accomplished in the body of the stomach after the food mass and saliva have been swallowed. Acid inactivates amylase, but in the center of the food mass, where stomach acid has not yet reached, this salivary enzyme continues to function for several more hours.


In the stomach: because food is not mixed with gastric secretions in the body of the stomach, very little protein digestion, occurs here. Acid and pepsin are able to attack only the surface of the food mass. In the interior of the mass, however, carbohydrate digestion continues under the influence of salivary amylase. Even though acid inactivates salivary amylase, the unmixed interior of the food mass is free acid.


In the Small Intestine: When gastric contents are emptied into the duodenum, they are mixed they are mixed not only by the juice secreted by the small intestine mucosa but also with the secretions of the exocrine pancreas. Pancreatic amylase, like salivary amylase, contributes to carbohydrate digestion by converting polysaccharides into disaccharides.

Carbohydrate digestion in the small intestine finishes converting digestible polysaccharides and disaccharides into monosaccharides into monosaccharides that can be absorbed.  Pancreatic amylase continues to digest starch into maltose.

Because of pancreatic enzymatic activity, carbohydrates are reduced to disaccharides and some monosaccharides. It is important to note that whereas fat digestion is completed within the small intestine lumen, carbohydrate and protien digestions have not been brought to completion. Special actin-stiffened, hair-like projections on the luminal surface of the small intestine epithelial cells form the brush border. The brush-border plasma membrane contains, in addition to other enzymes, the disaccharidases (maltase, sucrase, and lactase), which complete carbohydrate digestion by hydrolyzing the remaining disaccharides (maltose, sucrose, and lactose, respectively) into their constituent monosaccharides. Thus, carbohydrate digestion is competed intracellularly within the confines of the brush border.


Carbohydrate Absorption: Dietary carbohydrate is presented to the small intestine for absorption mainly in the forms of the disaccharides maltose (the product of polysaccharide digestion), sucrose and lactose.

The disaccharides located in the brush borders of the small intestine cells further reduce these disaccharides into the absorbable monosaccharides units of glucose, galactose, and fructose.

Glucose and galactose are both absorbed by secondary active transport, in which cotransport carriers on the luminal border transport both the monosaccharide and Na+ from the lumen into the interior of the intestinal cell. The operation of these cotransport carriers, which do not directly use energy themselves, depends o the Na+ concentration gradient established by the energy-consuming basolateral Na+-K+ pump. Glucose (or galactose), having been concentrated in the cell by the cotransport carriers, leaves the cell down its concentration gradient by means of a passive carrier in the basolateral border to enter the blood within the villus. In addition to glucose being absorbed through the cells by means of a cotransport carrier. Fructose is absorbed into the blood solely by facilitated diffusion (passive carrier-mediated transport).

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