Carbohydrates: Definition and Importance

Carbohydrates

• Definition: Carbohydrates are biomolecules consisting of carbon, hydrogen, and oxygen atoms in the ratio Cn(H2O)n, literally a "carbon hydrate".
• Importance: Carbohydrates constitute about 60% of our diet and have extensive roles in all forms of life, including:
  • Primary source of energy (e.g., glucose)
  • Storage form of energy (e.g., glycogen)
  • Biosynthesis of energy compounds, DNA, RNA, receptors, and vitamins
  • Combining with lipids (glycolipids) or protein (glycoproteins) to form cell membrane structures

Classification of Carbohydrates

• Classified according to the number of sugar units:
  • Monosaccharides: 1 sugar unit
  • Disaccharides: 2 sugar units
  • Oligosaccharides: 3-9 sugar units
  • Polysaccharides: ≥10 sugar units
  • Glucoconjugates: carbohydrate part + non-CHO part

Monosaccharides

• Definition: Simplest units of carbohydrates that cannot be broken down to smaller molecules by hydrolysis
• General formula: Cn(H2O)n
• Classified according to the number of carbon atoms:
  • Trioses: 3 carbon atoms (e.g., glyceraldehyde, dihydroxyacetone)
  • Tetroses: 4 carbon atoms (e.g., D-erythrose, erythrulose)
  • Pentoses: 5 carbon atoms (e.g., D-ribose, deoxyribose, D-ribulose)
  • Hexoses: 6 carbon atoms (e.g., D-glucose, D-mannose, D-galactose, fructose)
  • Heptoses: 7 carbon atoms (e.g., D-sedoheptulose)

Structure of Monosaccharides

• Haworth Projection: Aldehyde or ketone reacts with OH group of C4 or C5 to form hemiacetal or hemiketal, creating a new asymmetric carbon called anomeric carbon (C1 or C2)
• Formation of cyclic ring: Five-membered ring (furanose) or six-membered ring (pyranose)
• α- and β-sugars: Depending on the arrangement of H and OH groups around carbon atoms

Asymmetric Carbon Atom (Chiral Carbon)

• Definition: Carbon atom attached to 4 different groups or atoms
• Properties: Optical activity and stereoisomerism
• Optical Activity: Ability to rotate plane polarized light either to the right (dextrorotatory) or to the left (levorotatory)

Isomerism

• Definition: Compounds with identical molecular formula but different structures or configurations
• Number of isomers: 2n, where n is the number of asymmetric carbon atoms
• Types of isomerism:
  • Structural Isomers (Aldose-Ketose Isomerism)
  • Stereoisomers (Enantiomers, Epimers, and Anomers)

Stereoisomers

• Enantiomers: Mirror images of each other, non-superposable, and classified into D and L forms
• Epimers: Sugar molecules that differ in configuration at only one of the chiral centers other than anomeric carbon
• Anomers: Isomers obtained from the change of position of hydroxyl attached to the anomeric carbon

Learn about the definition, importance, and roles of carbohydrates in our diet and life processes, including energy source, storage, and biosynthesis.