Sugar Structures
Monosaccharides
Monosaccharides represent the most fundamental form of carbohydrates in sugar structures and cannot be further hydrolyzed into simpler sugar units. Common examples include glucose and fructose.
1. Glucose
(1) Molecular formula: C₆H₁₂O₆
(2) Structural characteristics: A six-carbon sugar (hexose) that typically exists in a cyclic form, most often as a six-membered ring (pyranose structure). The configuration of the hydroxyl group (-OH) attached to the first carbon atom determines whether the molecule is α-glucose or β-glucose.
2. Fructose
(1) Molecular formula: C₆H₁₂O₆
(2) Structural characteristics: Although it is also a six-carbon sugar, fructose generally adopts a five-membered ring structure (furanose form). It naturally occurs in fruits and honey and is recognized as the most sweet-tasting naturally occurring sugar.
Disaccharides
As essential components of sugar structures, disaccharides are formed by the condensation of two monosaccharide units through glycosidic linkages. Representative disaccharides include sucrose and lactose.
1. Sucrose
(1) Molecular formula: C₁₂H₂₂O₁₁
(2) Structural characteristics: Composed of one glucose unit and one fructose unit, linked via an α-1,β-2 glycosidic bond.
2. Lactose
(1) Molecular formula: C₁₂H₂₂O₁₁
(2) Structural characteristics: Formed from one glucose unit and one galactose unit, connected through a β-1,4 glycosidic bond.
Polysaccharides
Polysaccharides are high-molecular-weight carbohydrates composed of numerous monosaccharide units joined by glycosidic bonds, playing a crucial role in complex sugar structures. Common examples include starch and cellulose.
1. Starch
Structural characteristics: Predominantly composed of α-glucose residues connected via α-1,4 and α-1,6 glycosidic linkages, resulting in both linear (amylose) and branched (amylopectin) structures. Starch is widely present in plants and serves as a major dietary source of carbohydrates for humans.
2. Cellulose
Structural characteristics: Constructed from β-glucose residues linked by β-1,4 glycosidic bonds, forming an extended linear chain. It is the principal component of the plant cell wall and cannot be digested by human enzymes, yet it remains a critical structural element within natural sugar structures.
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