Carbohydrates: Definition and Classification

Carbohydrate Definition and Importance

• Carbohydrates are polyhydroxy-aldehydes, polyhydroxy-ketones, or derivatives thereof. They are organic compounds composed of carbon, hydrogen, and oxygen, also known as saccharides.
• Key functions: primary energy source, structural and protective roles (mucopolysaccharides), joint and tendon lubrication (glycoproteins & proteoglycans), cell adhesion and recognition, components of glycoconjugates (glycoproteins or glycolipids), and constituents of nucleic acids (DNA, RNA), coenzymes (NAD, FAD), regulatory molecules (cAMP, cGMP), and high-energy compounds (ATP, GTP).

Carbohydrate Classification

• By number of sugar units: monosaccharides (single unit), disaccharides (two units linked by a glycosidic bond), oligosaccharides (3-10 units), and polysaccharides (>10 units; can be linear or branched).
• Monosaccharide Classification:
  • Number of carbon atoms: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C). Examples include glyceraldehyde (aldotriose), dihydroxyacetone (ketotriose), erythrose, ribose, glucose, galactose, mannose, and fructose.
  • Active group: aldoses (aldehyde group -CHO) or ketoses (ketone group -C=O). Many monosaccharides are both aldoses and ketoses.

Monosaccharide Characteristics

• Physical Characteristics:
  • Stereoisomers: compounds with the same formula but different spatial arrangements. Types include:
    • Aldo-keto isomers: Differ in the position of the carbonyl group (aldehyde vs. ketone).
    • Epimers: Differ in the configuration around one carbon atom (e.g., glucose, galactose, mannose).
    • Enantiomers: Mirror images (D and L isomers; D-form is physiologically predominant). Anomers: α and β forms, differing in the configuration at the anomeric carbon.
  • Optical activity: Ability to rotate plane-polarized light (dextrorotatory or levorotatory). Most monosaccharides are optically active except dihydroxyacetone.
• Chemical Properties:
  • Reducing character: Due to free aldehyde or ketone groups, all monosaccharides exhibit reducing properties.
  • Derivatives: Modifications of the monosaccharide structure:
    • Amino sugars: Replacement of an -OH group with an amino group (e.g., glucosamine).
    • Deoxy sugars: Replacement of an -OH group with an -H (e.g., 2-deoxyribose in DNA).
    • Sugar acids: Oxidation of -CHO or -CH2OH groups to form carboxylic acid (-COOH) groups (e.g., aldonic acids, uronic acids, saccharic acids).
    • Sugar alcohols: Reduction of the aldehyde or ketone group to an alcohol (-CH2OH) (e.g., sorbitol, mannitol).

Clinical Applications of Monosaccharide Derivatives

• Sorbitol: Artificial sweetener.
• Mannitol: Treatment of brain edema and as an osmotic diuretic.

This quiz focuses on the definition and importance of carbohydrates, as well as their classification by sugar units. It covers key functions and types of carbohydrates, including monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Test your knowledge about these essential organic compounds and their roles in biological systems.