Carbohydrates

Introduction to Carbohydrates

Carbohydrates are a major class of biological macromolecules composed of sugars and polymers of sugars. They play essential roles in energy storage, structural support, and cell recognition in living organisms.

• Definition: Organic molecules consisting of carbon, hydrogen, and oxygen, typically with the empirical formula (CH2O)n.

• Classification: Based on the number of sugar units:

  • Monosaccharides (single sugar units)

  • Disaccharides (two sugar units)

  • Oligosaccharides (few sugar units)

  • Polysaccharides (many sugar units)

Monosaccharides

Properties and Classification

Monosaccharides are the simplest carbohydrates, often referred to as simple sugars. They are water-soluble and typically taste sweet. Their chemical formulas are usually multiples of CH2O.

• Most common monosaccharide: Glucose ()

• Classification criteria:

  • Location of carbonyl group: Aldoses (aldehyde group) vs. Ketoses (ketone group)

  • Length of carbon skeleton: Trioses (3C), Pentoses (5C), Hexoses (6C)

  • Arrangement around asymmetric carbons: Isomers such as α-glucose and β-glucose

• Structure = Function: The specific arrangement of atoms determines the properties and biological roles of each monosaccharide.

Disaccharides

Formation and Examples

Disaccharides are carbohydrates composed of two monosaccharides joined by a covalent bond known as a glycosidic linkage. This bond forms through a dehydration reaction, which releases a molecule of water.

• Common disaccharides:

  • Sucrose (glucose + fructose)

  • Lactose (glucose + galactose)

  • Maltose (glucose + glucose)

• Glycosidic linkage: The bond between the two monosaccharides, e.g., 1-2 glycosidic linkage in sucrose.

• Lactose intolerance: The inability to digest lactose due to insufficient lactase enzyme, leading to digestive symptoms.

Equation for dehydration synthesis:

Polysaccharides

Structure and Functions

Polysaccharides are large carbohydrates formed by the polymerization of hundreds to thousands of monosaccharides via glycosidic linkages. They serve two primary functions: energy storage and structural support.

• Storage polysaccharides: Starch (plants) and glycogen (animals)

• Structural polysaccharides: Cellulose (plants), chitin (fungi and arthropods), peptidoglycan (bacteria)

Storage Polysaccharides

Starch and Glycogen

Storage polysaccharides allow organisms to store excess glucose for later use. Their structure determines their storage efficiency and accessibility.

• Starch: Polymer of α-glucose molecules joined by (mostly) 1-4 glycosidic linkages. Found in plants as amylose (unbranched) and amylopectin (branched).

• Glycogen: Polymer of α-glucose molecules joined by 1-4 linkages, highly branched. Stored mainly in the liver and muscle of animals.

• Carb loading: Athletes increase glycogen stores before endurance events.

Structural Polysaccharides

Cellulose, Chitin, and Peptidoglycan

Structural polysaccharides provide rigidity and support to cells and tissues. Their unique structures make them resistant to enzymatic breakdown.

• Cellulose: Polymer of β-glucose molecules joined by 1-4 linkages. Major component of plant cell walls.

• Chitin: Polymer of modified glucose molecules (N-acetylglucosamine, NAG). Found in fungal cell walls and arthropod exoskeletons.

• Peptidoglycan: Polymer of sugars and amino acids. Forms the cell wall of bacteria.

Starch vs. Cellulose

Structural Differences and Nutritional Implications

Although both starch and cellulose are polymers of glucose, their glycosidic linkages differ, resulting in distinct properties and functions.

• Starch: α-1,4 glycosidic linkages; easily digested by humans.

• Cellulose: β-1,4 glycosidic linkages; not digestible by humans due to lack of cellulase enzyme.

• Nutritional equivalence: Starch provides energy, while cellulose acts as dietary fiber.

Functions of Carbohydrates in Cells

Major Biological Roles

Carbohydrates perform several essential functions in living cells:

• Provide carbon skeletons: Serve as precursors for amino acids and nucleic acids.

• Structural support: Cellulose, chitin, and peptidoglycan contribute to cell wall and exoskeleton integrity.

• Energy storage: Glucose produced by photosynthesis is used to generate ATP.

• Cell-cell recognition: Carbohydrates on cell surfaces help distinguish cell types and identify foreign invaders (important in immune response and organ transplantation).

Clinical and Biological Importance

Consequences of Carbohydrate Metabolism Disorders

Impaired ability to use, break down, or store carbohydrates can lead to metabolic disorders such as diabetes mellitus, glycogen storage diseases, and lactose intolerance.

• Diabetes: Inability to regulate blood glucose levels.

• Lactose intolerance: Inability to digest lactose due to lactase deficiency.

• Glycogen storage diseases: Genetic disorders affecting glycogen synthesis or breakdown.

Additional info: The notes have been expanded to include definitions, examples, and tables for clarity and completeness. All major types of carbohydrates and their biological roles are covered for exam preparation.