Monosaccharides
Monosaccharide are water soluble , white crystalline solids with a sweet taste. All contain at least three carbon and are named accordingly: triose, tetrose, pentose, hexose, heptose. They are also named according to the position of their carbonyl group: aldoses are aldehydes, ketoses are ketones.
All monosaccharide contain one or more chiral centers, and may be D or L enantiomers depending on the configuration about the chiral carbon most distant from the carbonyl. Glyceraldehyde, an aldotriose, contains one chiral carbon:
The arrangement of chiral carbons is unique for each monosaccharide. Molecules that differ in the configuration at only one of several chiral centers are epimers of each other. Glucose is an aldohexose with four chiral carbons and two epimers: galactose and manose:
Monosaccharide in aqueous solution exist as cyclic rings. The carbonyl carbon (C1) undergoes nucleophilic attack by the hydroxyl oxygen at the opposite end of the molecule. The carbonyl carbon is known as the numeric carbon and it symmetry results in two stereoisomers: alpha and beta.
An aldohexose will form a six atom pyranous ring, for example glucose.
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A ketohexose will form a five atom furanose ring, for example fructose.
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The six-atom pyranose ring is not planar and can assume a chair conformation or a boat conformation.
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Ribose is another important monosaccharide, an aldopentose that forms a furanose sing in aqueous solution.
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Important derivatives of monosaccharide include sugars phosphates, deoxy sugars, and amino sugars. Sugar phosphates have a phosphate group substituting for a hydroxyl. In a deoxy sugar, an hydroxyl group has been deoxygenated. Amino sugars have an amino group instead of an hydroxyl.
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Polysacharides
Disaccharides contain two monosaccharide joined covalently by O-glycosidic bonds. Glycosidic bonds are formed by the anomeric carbon of a sugar condensing with the hydroxyl from another molecule
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When the glycosidic bond occurs between glucose molecules, it is known as a glucosidic bond.
The naming of disaccharides specifies the linking atom: the molecule providing the numeric carbon ends in -syl. The name also contains the configuration of the glucosidic bond, for example alpha(1 to 4). Disaccharides also have common names, for example:
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beta-D-galactopyranosyl
(1 to 4) beta-D-glucopyranose
= beta-lactose
= one galactose joined by its numeric carbon to the 4th carbon of a glucose
Polysaccharides can be homoglycans or heteroglycans. Homoglycans (homopolysaccharides) are polymers containing one type of monoshaccharide residue. Heteroglycans (heteropolysaccharides) are polymers of more than one type of monosaccharide residue. They are all created without a template by the addition of a particular monosaccharide or oligosaccharide residue. The length of a particular polysaccharide molecule varies within a population.
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Carbohydrates
Carbohydrates are energy storage molecules with at least one carbonyl group. They are also found in protective and supportive structures as well as in some enzymes and nucleic acids. There are three major size classes: monosaccharide are simple sugars, oligosaccharides are chains of up to twenty monosaccharide, and polysaccharides are chains larger than twenty.
D-glucose is the main source of energy for many organisms and is stored intracellularly in polymeric form. The most common glucose storage homoglycans are starch and glycogen. Plants and fungi store glucose as starch, while animals store it as glycogen. Bacteria may store glucose as either starch or glycogen.
There are two forms of starch: amylose and amylopectin. Amylose is an unbranched polymer of 100 to 1000 D-glucose molecules attached to each other by alpha-1-to-4 bonds. Amylopectin is a branched version of amylose., were branches are attached by apha-1-to-6 glucosidic bonds to linear chains of residues linked by alpha-1-to-4 glucosidic bonds. Branches occur on average every 25 residues, with side chains containing about 15-25 glucose residues. Some chains are themselves branched. Amylopectin isolated from cells contains 300-600 glucose residues.
Glycogen is a branched polymer of glucose, with the same linkages as amylopectin but smaller branches occurring more frequently, about every 8-12 residues. Glycogen molecules are larger than starch molecules.
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