O1iii: Outline the process of digestion & absorption of dietary carbohydrate

Definition

Carbohydrate = simple sugar, the main energy source of humans

  • 3 major sources of carbs in diet:
    1. SUCROSE (disaccharide)
    2. LACTOSE (disaccharide)
    3. STARCHES (large polymer)

NB → large amount of cellulose (carb) but no enzyme to hydrolyse exists

Mouth + Stomach

  • Saliva contains PTYALINE (α-amylose), secreted by parotid glands
  • Hydrolyses starch into dissach. Maltose + smaller polymers 3 – 9 glucose long
  • Short transit time in mouth, only 5% starch hydrolysed
  • Digestion continues until salivary amylase is deactivated by gastric secretions (pH < 4.0)

SI

  • Chyme empties into duodenum → mixes with pancreatic juice
  • Pancreas secretes α-amylase (large quantity & more powerful)
  • Carbs totally digested to maltose & small glucose polymers
  • Enterocytes of SI villi have 4 enzymes
    1. LACTASE
    2. SUCRASE
    3. MALTASE
    4. α-DEXTRINASE
  • Splits dissachs → monosaccharides

Absorption

  • Absorbed are monosacchs
  • Glucose 80%, galactose, fructose
  • All monosacchs absorbed by ACTIVE TRANSPORT
  • Glucose: SODIUM COTRANSPORT MECHANISM

Step 1)

  • Na+ actively transported out of intestinal epithelial cells
  • ↓[Na+] intracellularly

Step 2)

    • Na+ moves with glucose via brush border SGLT-1 KA 2° ACTIVE TRANSPORT

Step 3)

  • GLUT2 transports glucose out of intestinal cell
Glut2