Functions of Carbohydrates

Carbohydrates are one of the major macronutrients essential for life. They serve structural, energy, and metabolic roles in living organisms.

1. Energy Source and Storage

• Primary source of energy: Carbohydrates provide 4 kcal per gram of energy.
• Glucose (C₆H₁₂O₆) is the main fuel for cellular respiration.
• Glycogen (animals) and starch (plants) act as storage forms of carbohydrates for future energy needs.

Example:

• Glycogen is stored in the liver and muscles and broken down when energy is needed.
• Starch is stored in plant tissues and serves as an energy reserve.

2. Structural Functions

• Cellulose: Provides strength to plant cell walls (β(1→4) linked glucose units).
• Chitin: Forms the exoskeleton of insects and crustaceans and is present in fungal cell walls.
• Peptidoglycan: Forms the cell wall of bacteria and provides rigidity.

Example:

• Cotton fibers are made of pure cellulose.
• Chitin is used to make surgical threads and biodegradable plastics.

3. Role in Metabolism and Biochemical Pathways

• Carbohydrates participate in glycolysis, gluconeogenesis, and the citric acid cycle (Krebs cycle) to generate ATP.
• Glucose is precursor for synthesizing lipids, amino acids, and nucleotides.

Example:

• Pentose phosphate pathway provides NADPH and ribose sugars for nucleotide synthesis.

4. Cell Recognition and Communication

• Glycoproteins and glycolipids present in cell membranes help in cell-cell recognition and signaling.
• Play a role in immune response and blood group determination (ABO system).

Example:

• Glycoproteins on white blood cells help in immune recognition.
• Blood group antigens (A, B, O) are oligosaccharides attached to membrane proteins.

5. Role in Brain and Nervous System Function

• The brain primarily depends on glucose for energy (about 120g/day is used by the brain).
• Lack of glucose can lead to hypoglycemia, causing dizziness, confusion, and even unconsciousness.

Example: Glucose transporters (GLUT1, GLUT3) help in glucose uptake by brain cells.

6. Dietary Fiber and Digestive Health

• Dietary fiber (cellulose, hemicellulose, pectin) helps in proper digestion and prevents constipation.
• Soluble fiber (pectin, gums) lowers cholesterol and regulates blood sugar.
• Insoluble fiber (cellulose, lignin) promotes bowel movement and prevents colon diseases.

Example:

• Oatmeal, apples, and beans contain soluble fiber that helps reduce cholesterol.
• Wheat bran and vegetables provide insoluble fiber for gut health.

7. Protective and Lubricating Functions

• Mucopolysaccharides (e.g., hyaluronic acid) act as lubricants in joints.
• Mucin (glycoprotein) in saliva protects the digestive tract lining.

Example: Hyaluronic acid in synovial fluid prevents joint friction.

8. Detoxification

• Carbohydrates help in detoxification by forming glucuronic acid, which binds to toxins for excretion.

Example: Liver detoxification of drugs through glucuronidation.

9. Role in Nucleotide and Nucleic Acid Formation

• Ribose (C₅H₁₀O₅) is a component of RNA, ATP, NADH, and FADH₂.
• Deoxyribose (C₅H₁₀O₄) forms the backbone of DNA.

Example: ATP (adenosine triphosphate) has ribose sugar and acts as an energy carrier.

10. Prevention of Protein and Fat Breakdown

• When carbohydrates are insufficient, the body breaks down proteins and fats for energy.
• This leads to muscle loss and ketone body production, causing ketoacidosis.

Example: In starvation or diabetes, ketone bodies accumulate, leading to metabolic acidosis.

Summary Table