Carbohydrates

Carbohydrates Overview

• Carbohydrates play crucial roles in nutrition, energy production, and structure in living organisms.

• Main categories:

  • Monosaccharides: Simple sugars, e.g., glucose.

  • Disaccharides: Composed of two monosaccharides, e.g., sucrose.

  • Polysaccharides: Long chains of monosaccharides, e.g., starch, cellulose.

Structure and Classification

20.1 An Introduction to Carbohydrates

• Learning Objective: Classify carbohydrates by functional group and carbon number.

• Monosaccharides: Can be classified as aldoses (with an aldehyde group) or ketoses (with a ketone group).

• Structural classification based on carbon atoms:

  • Triose (3 C)

  • Tetrose (4 C)

  • Pentose (5 C)

  • Hexose (6 C)

  • Heptose (7 C)

Molecular Structure of Monosaccharides

• General Formula: C_nH_(2n)O_n.

• Key functional groups:

  • Aldehyde Group: Makes the sugar an aldose.

  • Ketone Group: Makes the sugar a ketose.

• Example:

  • Glucose: C6H12O6, an aldohexose.

Properties of Carbohydrates

Common Reactions

• Monosaccharides can react to form disaccharides and polysaccharides via glycosidic bonds.

• Reactivity influenced by:

  • Functional groups (hydroxyl, carbonyl).

  • Anomeric carbon’s orientation (α and β forms).

Special Cases

• Chirality: Monosaccharides exhibit chirality and can exist in two enantiomeric forms (D and L).

• Mutarotation: The process where cyclic forms interconvert, altering optical activity.

Key Monosaccharides

Important Monosaccharides

• Glucose: Primary energy source in organisms; involved in cellular respiration.

• Fructose: Present in fruits; sweeter than glucose.

• Galactose: Part of lactose; metabolized into glucose.

• Ribose and Deoxyribose: Components of RNA and DNA respectively.

Reactions of Monosaccharides

Oxidation-Reduction

• Oxidation: Aldoses can be oxidized to carboxylic acids.

• Reduction: Carbonyl groups reduce to alcohols, forming sugar alcohols (alditols) such as sorbitol and mannitol.

Glycosidic Bond Formation

• Hemiacetals react with alcohols to form glycosides (acetals).

• Glycosidic Bond: The bond between a monosaccharide and an alcohol through the anomeric carbon.

Common Disaccharides

Major Disaccharides

• Sucrose: Composed of glucose and fructose linked via a glycosidic bond; commonly known as table sugar.

• Lactose: Composed of galactose and glucose; found in milk.

• Maltose: Composed of two glucose units; found in malted foods.

Important Polysaccharides

Polysaccharides Overview

• Starch: Energy storage in plants.

  • Amylose: Unbranched helical structure (α-1,4 linkages).

  • Amylopectin: Branched structure (α-1,4 and α-1,6 linkages).

• Glycogen: Energy storage in animals, more extensively branched than amylopectin.

• Cellulose: Structural component of plant cell walls; composed of β-1,4 linkages, indigestible for humans.

Modified Polysaccharides

• Hyaluronate: Provides lubrication in joints.

• Chondroitin Sulfate: Structural component in cartilage.

• Heparin: Anticoagulant found in blood.

Dietary Considerations

• Adequate carbohydrate intake is essential for energy provision and metabolic health.

• Dietary Fiber: Includes non-digestible carbohydrates like cellulose which contribute to digestive health.

• Recommended guidelines focus on increasing complex carbohydrates and fiber intake while managing simple sugars.

Summary of Key Concepts

• Monosaccharides (simple sugars), disaccharides, and polysaccharides form the backbone of dietary carbohydrates.

• Knowledge of carbohydrate structure, classifications, and reactions aids in understanding their role in biology and nutrition.