What are the main functions of carbohydrates in the human body and in biological systems?

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Functions of Carbohydrates

• 01 Energy Supply: Carbohydrates are the primary metabolic fuel. Glucose yields $\text{38 ATP}$ via glycolysis, TCA cycle, and oxidative phosphorylation. Brain and RBCs are obligate glucose users.
• 02 Energy Storage: Glycogen serves as a glucose reserve in the liver (blood glucose homeostasis) and muscle (local fuel). Starch stores carbohydrates readily by photosynthesis in plants.
• 03 Structural Role: Carbohydrates form cell walls (e.g., fungi/yeast walls), bacterial cell walls, and plant cell walls (cellulose). They are also components of DNA/RNA, glycoproteins, and extracellular matrix (ECM) of cartilage joints.
• 04 Cell Recognition & Signaling: Glycoproteins and glycolipids serve as cell surface markers, blood group antigens, and receptors for hormones and viruses.
• 05 Protein Sparing: Adequate dietary carbohydrate intake prevents catabolism of proteins for energy. This is clinically critical in starvation, diabetes, and protein-energy malnutrition.
• 06 Metabolic Intermediates: Carbohydrates are precursors for the Pentose Phosphate Pathway (NADH, NADPH, C-skeleton for fatty acid synthesis), glycolysis, and gluconeogenesis. They are also involved in drug detoxification and biotransformation.
• 07 Dietary Fibre: Soluble fiber (pectin, gum) slows glucose absorption, lowers LDL, and promotes gut motility. Insoluble fiber (cellulose, hemicellulose) is fermented to SCFA.
• 08 Lubrication: Hyaluronic acid in synovial fluid lubricates joints. Glycosaminoglycans (GAGs) and proteoglycans are important components of the extracellular matrix and act as lubricants in various systems.

Clinical Relevance of Carbohydrate Biochemistry

• Diabetes Mellitus:
  • Mechanism: Absolute or relative deficiency of insulin, impaired cellular glucose uptake, or impaired glucose utilization.
  • Features: Fasting blood glucose $\text{>126 mg/dL}$, glycosuria, and polyuria.
• Galactosemia:
  • Mechanism: Deficiency of galactose-1-phosphate uridyltransferase, leading to accumulation of galactose-1-phosphate in tissues (liver, brain, eye, kidney).
  • Features: Neonatal metabolic screening, cataracts, and mental retardation.
• Lactose Intolerance:
  • Mechanism: Lactase deficiency, leading to undigested lactose reaching the large intestine and bacterial fermentation.
  • Features: Hydrogen breath test, and abdominal pain/bloating.
• Glycogen Storage Diseases:
  • Mechanism: Deficiency of glycogen enzymes (e.g., Von Gierke's, Pompe's, Cori's, McArdle's diseases), leading to abnormal glycogen accumulation.
  • Features: Liver biopsy, enzyme assay, and genetic testing.
• Mucopolysaccharidoses:
  • Mechanism: Deficiency of lysosomal enzymes for glycosaminoglycan degradation, leading to accumulation of GAGs in tissues (e.g., Hurler's, Hunter's syndromes).
  • Features: Urine GAGs, enzyme assay, and genetic confirmation.
• Cancer Cell Glycosylation:
  • Mechanism: Altered glycosylation patterns on cancer cell surfaces. These increase invasion and abnormal cell adhesion (Warburg effect: tumor cells rely on anaerobic glycolysis).
  • Features: $\text{CA 125}$ ($\text{CEA}$) are cancer biomarkers.