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Carbohydrate Structure

Monosaccharides are simple sugars
Many saccharide structures differ only in the orientation of the hydroxyl groups (-OH). This slight structural difference makes a big difference in the biochemical properties, organoleptic properties (e.g., taste), and in the physical properties such as melting point and Specific Rotation (how polarized light is distorted). A chain-form monosaccharide that has a carbonyl group (C=O) on an end carbon forming an aldehyde group (-CHO) is classified as an aldose. When the carbonyl group is on an inner atom forming a ketone, it is classified as a ketose.

Monosaccharide classifications based on the number of carbons

Number of
Carbons
Category Name Examples
4 Tetrose Erythrose, Threose
5 Pentose Arabinose, Ribose, Ribulose, Xylose, Xylulose, Lyxose
6 Hexose Allose, Altrose, Fructose, Galactose, Glucose, Gulose, Idose, Mannose, Sorbose, Talose, Tagatose
7 Heptose Sedoheptulose, Mannoheptulose

Disaccharides consist of two simple sugars
Sucrose, also called saccharose, is ordinary table sugar refined from sugar cane or sugar beets. It is the main ingredient in turbinado sugar, evaporated or dried cane juice, brown sugar, and confectioner’s sugar. Lactose has a molecular structure consisting of galactose and glucose. It is of interest because it is associated withlactose intolerance which is the intestinal distress caused by a deficiency of lactase, an intestinal enzyme needed to absorb and digest lactose in milk. Undigested lactose ferments in the colon and causes abdominal pain, bloating, gas, and diarrhea. Yogurt does not cause these problems because lactose is consumed by the bacteria that transform milk into yogurt.

Disaccharide descriptions and components

Disaccharide Description Component monosaccharides
sucrose common table sugar glucose 1α→2 fructose
maltose product of starch hydrolysis glucose 1α→4 glucose
trehalose found in fungi glucose 1α→1 glucose
lactose main sugar in milk galactose 1β→4 glucose
melibiose found in legumes galactose 1α→6 glucose

Polysaccharides are polymers of simple sugars
Many polysaccharides, unlike sugars, are insoluble in water. Dietary fiber includes polysaccharides and oligosaccharides that are resistant to digestion and absorption in the human small intestine but which are completely or partially fermented by microorganisms in the large intestine. The polysaccharides described below play important roles in nutrition, biology, or food preparation.

Starch
Starch is the major form of stored carbohydrate in plants. Starch is composed of a mixture of two substances: amylose, an essentially linear polysaccharide, andamylopectin, a highly branched polysaccharide. Both forms of starch are polymers of α-D-Glucose. Natural starches contain 10-20% amylose and 80-90% amylopectin. Amylose forms a colloidal dispersion in hot water (which helps to thicken gravies) whereas amylopectin is completely insoluble.
Amylose molecules consist typically of 200 to 20,000 glucose units which form a helix as a result of the bond angles between the glucose units.
Amylopectin differs from amylose in being highly branched. Short side chains of about 30 glucose units are attached with 1α→6 linkages approximately every twenty to thirty glucose units along the chain. Amylopectin molecules may contain up to two million glucose units.

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