Carbohydrates in Biology

Carbohydrates: Overview

• Carbohydrates are compounds crucial for biological processes.
• They provide energy through oxidation.
• They supply carbon for synthesizing components like ribose in nucleic acids.
• They act as stored chemical energy in cells and tissues.
• Carbohydrates are part of the structure of some cells and tissues.
• Together with lipids, proteins, nucleic acids, and other compounds, they are known as biomolecules.
• The term "carbohydrate" originates from the observation that heating sugars produces carbon and water.
• Carbohydrates have the general formula (C H₂O)n, hence the name "hydrates of carbon".
• Carbohydrates include sugars and starches, essential components in many foods (e.g., bread, pasta, rice).

Carbohydrate Structure

• Carbohydrates are aldehyde or ketone derivatives of polyhydric alcohols.
• They are also categorized as polyhydroxy aldehydes or polyhydroxy ketones.
• These compounds are also referred to as saccharides.

Biological Importance of Carbohydrates

• Carbohydrates are widely distributed in plants and animals.
• They constitute approximately 60% of the human diet.
• They are primary energy sources, exemplified by glucose.
• They serve as storage forms of energy, such as glycogen.
• Carbohydrates participate in the biosynthesis of essential compounds like ATP, DNA, RNA, and receptors.
• Some carbohydrates combine with lipids (glycolipids) or proteins (glycoproteins) to form components of cell membranes.

Classification of Carbohydrates

• Classified by the number of sugar units:
  • Monosaccharides (1 sugar unit): Sweet and soluble sugars.
  • Disaccharides (2 sugar units): Sweet and soluble component.
  • Oligosaccharides (3-9 sugar units)
  • Polysaccharides (≥ 10 sugar units): Polymers; not sweet or soluble.
  • Glucoconjugates (carbohydrate + non-carbohydrate parts): Carbohydrate molecules with added non-carbohydrate components.

Monosaccharides

• Simple sugars; the fundamental unit of carbohydrates.
• Classified by the number of carbon atoms and the type of carbonyl group they contain (either aldehyde or ketone).
• Examples include trioses, tetroses, pentoses, hexoses, and heptoses.
• Specific types of monosaccharides: glucose, fructose, ribose, etc.

Isomerism

• Compounds with the same molecular formula but different structures.
  • Structural isomers: Different arrangement of atoms. Glucose and fructose are examples.
  • Stereoisomers: Same arrangement of atoms but different spatial arrangements. Enantiomers are mirror-image stereoisomers (D and L forms). Epimers differ in configuration at only one chiral center. Examples include D-glucose, D-mannose, and D-galactose. Anomers differ in configuration around an anomeric center
• The number of possible monosaccharide isomers depends on the number of asymmetric carbon atoms.

Sugar Derivatives

• A- Sugar Acids: Formed by oxidizing the carbonyl carbon or the last hydroxyl carbon. Examples include gluconic acid, glucuronic acid, and glucaric acid.
• B- Sugar Alcohols: Formed by reducing the carbonyl group. Examples include sorbitol and mannitol.
• C- Deoxysugars: One hydroxyl group replaced by a hydrogen. Deoxyribose is an example.
• D- Amino Sugars: Some hydroxyl groups replaced by amino groups. Glucosamine and galactosamine are examples.

1-4 Disaccharides

• Maltose: Two glucose units linked by an α(1→4) glycosidic bond. Found in malt.
• Isomaltose: Two glucose units linked by an α(1→6) glycosidic bond.
• Cellobiose: Two glucose units linked by a β(1→4) glycosidic bond.

1-4 Disaccharides

• Lactose: β-glucose and β-galactose linked by a β(1→4) glycosidic bond.
• Sucrose: α-glucose and β-fructose linked by an α(1→2) β(1→2) glycosidic bond. Known as table sugar.

Polysaccharides

• Polymers consisting of many monosaccharide units linked by glycosidic bonds.
• Homopolysaccharides: Composed of the same type of monosaccharides:
  • Starch: Glucose units linked by α(1→4) glycosidic bonds.
  • Glycogen: Branched glucose polymer; stored in animals.
  • Cellulose: Linear polymer of glucose units linked by β(1→4) glycosidic bonds.
  • Other homopolysaccharides: (e.g., dextrans)
• Heteropolysaccharides: Composed of several different types of monosaccharides:
  • Mucopolysaccharides: Group of heteropolysaccharides.