Carbohydrates in the diet provide the major exogenous source for glucose, which is the primary energy source for cells.
Carbohydrates
are hydrophilic and require a series of reactions to digest them to
monosaccharides which are absorbed in the small intestine. Carbohydrates
consist of three main groups, simple carbohydrates (monosaccharides),
disaccharides and complex carbohydrates (starch, glycogen, and
fiber).The common monosaccharides include glucose, fructose, galactose,
xylose and ribose.
Starch, the major food polysaccharide,
consists of 85% amylopectin and 15% amylose. Amylose is composed of
straight chains of glucose molecules linked through1:4-α bond where as
amylopectin, in addition to chains of 1:4- α linked glucose molecules,
also has 1:6- α links between glucose molecules in adjacent chains
forming bridges.
When the person eat carbohydrates, such as a
bowl of pasta or some vegetables, the digestive system breaks the
carbohydrates down into simple sugars such as glucose, which travel into
and through the bloodstream to nourish and energize cell.
The
digestion process of polysaccharides such as starch will begin in the
mouth where it is hydrolysed by salivary amylase. Chewing, also known as
mastication, crumbles the carbohydrate foods into smaller and smaller
pieces. The salivary glands in the oral cavity secrete saliva that coats
the food particles. Salivary amylase breaks the bonds between the
monomeric sugar units of disaccharides, oligosaccharides, and starches.
The salivary amylase breaks down amylose and amylopectin into smaller
chains of glucose, called dextrins and maltose.
The goal of
carbohydrate digestion is to break down all disaccharides and complex
carbohydrates into monosaccharides for absorption, although not all are
completely absorbed in the small intestine (e.g.,fiber).
Fructose
is absorbed by facilitated diffusion while glucose and galactose are
actively transported. Glucose, at low concentrations is transported
through the mucosal lining into the epithelial cells of the intestine by
active transport, via a sodium dependant transporter. The first organ
to receive glucose, fructose, and galactose is the liver. The liver
takes them up and converts galactose to glucose, breaks fructose into
even smaller carbon-containing units, and either stores glucose as
glycogen or exports it back to the blood.
Carbohydrate: Process of digestion
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