Carbohydrate Chemistry Basics

Carbohydrate Chemistry

• Carbohydrates are aldehyde or ketone derivatives of polyhydric alcohols, or any substances derived from them
• Carbohydrate is another name for carbon and water (C + H₂O)
• For every carbon atom, there is one molecule of water or two hydrogen atoms and one oxygen atom.
• They are also called saccharides.
• Carbohydrates make up approximately 60% of our diet.
• Carbohydrates provide energy.
• Carbohydrates combine with lipids to form glycolipids or with proteins to form glycoproteins. These combinations become part of the cell membrane.
• The suffix "-ose" means sugar.

Classification of Carbohydrates

• Carbohydrates are classified according to the number of sugar units.
• Monosaccharides: one sugar unit (e.g., glucose, fructose)
• Disaccharides: two sugar units (e.g., sucrose, lactose)
• Oligosaccharides: three to ten sugar units
• Polysaccharides: more than ten sugar units

Monosaccharides

• These are the simplest carbohydrate units.
• Their general formula is Cₙ(H₂O)ₙ.
• They can be divided into aldoses (aldehyde group) and ketoses (ketone group).
• Classification is based on the number of carbon atoms:
  • Triose: 3 carbon atoms (e.g., glyceraldehyde, dihydroxyacetone)
  • Tetrose: 4 carbon atoms (e.g., erythrose, erythrulose)
  • Pentose: 5 carbon atoms (e.g., ribose, arabinose, xylose, lyxose, ribulose, xylulose)
  • Hexose: 6 carbon atoms (e.g., glucose, galactose, mannose, fructose)
  • Heptose: 7 carbon atoms (e.g., sedoheptulose)

Importance of Pentoses

• Ribose and deoxyribose are components of RNA and DNA
• Ribose is part of high-energy phosphate compounds (ATP, GTP)
• Ribose is also found in coenzymes like NAD, NADP, and flavoproteins.
• Arabinose and xylose are components of glycoproteins in plants and animals.
• Lyxose is part of lyxoflavin, found in human heart muscle.

Importance of Hexoses

• Glucose: major energy source in mammals
• Fructose: main sugar in semen
• Galactose: essential for synthesizing lactose (milk sugar) in lactating mammary glands.
• Mannose: component of many glycoproteins.

Disaccharides

• Formed by condensation of two monosaccharides linked by a glycosidic bond
• General formula: Cₙ(H₂O)ₙ₋₁
• Key disaccharides:
  • Maltose: two α-glucose molecules linked by an α(1→4) glycosidic bond found in malt and produced by starch digestion.
  • Isomaltose: two α-glucose molecules linked by an α(1→6) glycosidic bond formed during starch and glycogen digestion by amylase.
  • Cellobiose: two β-glucose molecules linked by a β(1→4) glycosidic bond obtained by cellulose hydrolysis in plants.
  • Lactose: β-glucose and β-galactose linked by a β(1→4) glycosidic bond. Found in milk.
  • Sucrose: α-glucose and β-fructose linked by an α(1→2)β glycosidic bond. Found in cane and beet sugar, pineapple, and carrots.

Polysaccharides

• Composed of more than 10 monosaccharides linked by glycosidic bonds.
• Homopolysaccharides: consist of one type of monosaccharide:
  • Starch: storage carbohydrate in plants. Consists of amylose (15-20% of the granule) and amylopectin.
  • Glycogen: storage carbohydrate in animals. Branched structure.
  • Cellulose: structural component of plant cell walls. Cannot be digested by most mammals.