Carbohydrates: Aldoses and Ketoses

Classification of Sugars

• Sugars are classified as aldoses (containing an aldehyde group) or ketoses (containing a ketone group) based on their functional groups.
• Common monosaccharides include:
  • Glyceraldehyde (aldotriose)
  • Dihydroxyacetone (ketotriose)
  • Ribose (aldopentose)
  • Fructose (ketohexose)
• The smallest monosaccharides are glyceraldehyde and dihydroxyacetone with three carbon atoms.

Structure of Monosaccharides

• Monosaccharides can exist in straight-chain or cyclic forms, with five and six-membered rings being the most stable.
• Cyclic structures arise from the formation of hemiacetals when a hydroxyl group reacts with a carbonyl group.

Chiral Centers and Isomerism

• All monosaccharides except dihydroxyacetone contain chiral carbons, leading to optically active isomers (enantiomers).
• Glyceraldehyde has one chiral center, while glucose has four, resulting in different stereoisomers.

D and L Isomerism

• D and L configurations are based on the orientation of the -OH group farthest from the carbonyl carbon.
• Sugars with the -OH group on the right are D-isomers; on the left, they are L-isomers. D-isomers are predominant in biological systems.

Anomers and Glycosidic Bonds

• Monosaccharides with five or more carbons typically form cyclic structures.
• The conversion to cyclic form creates an anomeric carbon, leading to reducing disaccharides like maltose and lactose due to the presence of a free anomeric carbon.

Polysaccharides

• Polysaccharides consist of ten or more monosaccharide units, linked by glycosidic bonds.
• Homopolysaccharides contain a single type of monosaccharide (e.g., starch, cellulose), while heteropolysaccharides comprise multiple types and serve various structural and functional roles.

Storage and Structural Polysaccharides

• Storage polysaccharides like starch (in plants) and glycogen (in animals) serve as energy reserves.
• Structural polysaccharides such as cellulose (plant cell walls) and chitin (exoskeleton of insects) provide support and protection.

Heteropolysaccharides

• Heteropolysaccharides yield various monosaccharides upon hydrolysis and provide extracellular support.
• Examples include agar, used in gels for culturing and DNA separation, and peptidoglycan, vital for bacterial cell wall structure.

Physical Properties of Carbohydrates

• Carbohydrates are typically odorless, colorless crystalline solids at room temperature and are soluble in water while being insoluble in organic solvents.

Chemical Properties of Carbohydrates

• Sugars can form osazones through reactions with phenyl hydrazine.
• Oxidation converts reducing sugars into carboxylic acids.
• Hydrolysis of glycosidic bonds leads to the formation of α and β isomers.
• Glycosides form from the condensation of monosaccharides.

Functions of Carbohydrates

• Primary energy source, providing 4 kcal/g.
• Serve as storage reserves in the form of glycogen.
• Non-digestible carbohydrates (e.g., cellulose) act as dietary fibers.
• Serve as precursors for nucleic acids and other organic compounds.

Analytical Tests for Carbohydrates

• Molisch’s Test detects carbohydrates using α-naphthol and concentrated H2SO4; a positive result is indicated by a violet ring.
• Benedict’s Test identifies reducing sugars by changing color from green to brick-red when heated with Benedict’s solution, indicating the presence of free aldehyde or ketone groups.

This quiz explores the classification of sugars based on their carbon atom count and functional groups. It includes details on trioses, tetroses, pentoses, hexoses, and heptoses, covering both aldoses and ketoses. Test your understanding of these carbohydrates and their structures.