Makindo Medical Notes

Related Subjects: | Carbohydrates | Lipids | Proteins

🧾 Introduction

Carbohydrates are a fundamental class of biomolecules essential for energy supply, structural integrity, cellular communication, and metabolic regulation. They are composed of carbon (C), hydrogen (H), and oxygen (O) — often following the general formula C_nH_2nO_n. Their structural diversity — from simple sugars to large polymers — underpins their varied biological functions.

🔬 Chemical Structure and Classification

• Monosaccharides (simple sugars): The smallest carbohydrate units. Examples:
  • Glucose: primary energy substrate for most cells.
  • Fructose: abundant in fruit, metabolised mainly in the liver.
  • Galactose: component of lactose, crucial in infant nutrition.
  Cyclic forms predominate in vivo (pyranose and furanose rings).


• Disaccharides: Formed by glycosidic bonds between two monosaccharides. Examples:
  • Sucrose: glucose + fructose.
  • Lactose: glucose + galactose (requires lactase for digestion).
  • Maltose: glucose + glucose.


• Oligosaccharides & Polysaccharides:
  • Oligosaccharides: 3–10 sugar units, often important in glycoproteins (e.g., ABO blood group antigens).
  • Polysaccharides: long chains (hundreds to thousands of units).
    • Starch: plant energy storage (amylose + amylopectin).
    • Glycogen: animal storage form, highly branched, stored in liver and muscle.
    • Cellulose: plant cell wall component; indigestible in humans (dietary fibre).

⚡ Energy Metabolism

Carbohydrates provide the most immediate source of ATP.

• Glycolysis: glucose → pyruvate → 2 ATP + NADH.
• Aerobic metabolism: pyruvate enters mitochondria → TCA cycle + oxidative phosphorylation → up to 36–38 ATP per glucose.
• Anaerobic metabolism: pyruvate → lactate (in hypoxia, e.g., exercising muscle).
• Glycogenolysis & Gluconeogenesis: regulate glucose supply during fasting/exertion.

🧠 The brain relies almost entirely on glucose (except in prolonged fasting, when ketones are used).

🏗️ Structural & Biosynthetic Roles

• Glycoproteins & Glycolipids: Form the glycocalyx on cell surfaces → crucial for immune recognition, adhesion, and receptor function.
• Nucleic Acids: Ribose (RNA) & deoxyribose (DNA) are sugars that form the genetic backbone.
• Connective Tissue: Glycosaminoglycans (e.g., hyaluronic acid, chondroitin sulfate) provide tensile strength and hydration.

💧 Water Solubility & Molecular Interactions

Carbohydrates contain multiple hydroxyl (-OH) groups → highly polar, soluble in water, and able to form hydrogen bonds. This property is key for:

• Transport in blood (e.g., glucose).
• Hydration shells around glycogen and starch granules.
• Hydrogen-bond networks that stabilise protein–carbohydrate interactions.

🌍 Biological & Clinical Importance

• ⚡ Energy: Immediate ATP source and storage (glycogen).
• 🧬 Genetics: Sugar-phosphate backbone of DNA/RNA.
• 🛡️ Immunity: Carbohydrate antigens define blood groups; pathogens often exploit glycoproteins for entry.
• 🥗 Nutrition: Carbohydrates provide 45–65% of daily energy intake (WHO recommendation).
• 🧪 Clinical relevance:
  • Diabetes mellitus: impaired glucose handling.
  • Lactose intolerance: deficiency of lactase → bloating, diarrhoea.
  • Glycogen storage diseases: inherited enzyme defects (e.g., Von Gierke’s, McArdle’s).
  • Metabolic syndrome: excessive refined carbohydrate intake contributes to insulin resistance.

📊 Quick Comparison Table

Type	Examples	Main Role	Clinical Relevance
Monosaccharide	Glucose, Fructose	Energy substrate	Hypoglycaemia, Diabetes
Disaccharide	Sucrose, Lactose	Dietary energy	Lactose intolerance
Polysaccharide	Glycogen, Starch	Storage	Glycogen storage disorders
Structural	Cellulose, Glycosaminoglycans	Support & extracellular matrix	Joint/cartilage degeneration

✅ Conclusion

Carbohydrates are far more than “sugars” — they underpin metabolism, structure, signalling, and immunity. From glucose fuelling neurons to glycoproteins shaping immune recognition, they form an indispensable part of health and disease. 👉 A deep understanding of carbohydrate biology is essential for managing diabetes, nutrition, infection, and metabolic disorders.