Digestion and Absorption

AQA spec ref: 3.3.3 - Digestion and absorption

Digestion is the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. It occurs in two stages: mechanical digestion (physical breakdown, increasing surface area) and chemical digestion (enzymatic hydrolysis of specific bonds). Absorption then transfers the products across the gut epithelium and into the circulatory system. The ileum is the primary site of absorption and is highly adapted for this function. See Carbohydrates, Proteins, Lipids, Transport Across Cell Membranes.

Digestion of Carbohydrates

Carbohydrates in the diet are primarily starch (from plants) and glycogen (from meat), along with disaccharides (sucrose, lactose) and some free glucose.

Salivary amylase (in the mouth): hydrolyses starch → maltose (a disaccharide). Optimal pH ~7. Denatured in the stomach.

Pancreatic amylase (in the small intestine lumen): continues starch → maltose hydrolysis.

Membrane-bound disaccharidases (on the brush border of ileum epithelium):

• Maltase→maltose→2 glucose
• Sucrase→sucrose→glucose+fructose
• Lactase→lactose→glucose+galactose

All carbohydrate digestion ultimately produces monosaccharides (glucose, fructose, galactose) - the only form small enough to be absorbed.

Lactase deficiency (lactose intolerance): insufficient lactase → lactose remains undigested → fermented by gut bacteria → gas and osmotic diarrhoea.

Digestion of Proteins

Dietary proteins must be hydrolysed to amino acids for absorption. This requires endopeptidases (cleave internal peptide bonds, producing polypeptides) and exopeptidases (cleave terminal amino acids from polypeptide ends).

Pepsin (in the stomach):

• Secreted as inactive pepsinogen by chief cells→activated by HCl (low pH)→pepsin
• Endopeptidase: hydrolyses internal peptide bonds→polypeptides
• Optimum pH ~2

Trypsin (in the small intestine):

• Secreted as inactive trypsinogen by the pancreas→activated by enterokinase (enteropeptidase) on the brush border
• Endopeptidase: cleaves peptide bonds at Arg/Lys residues→shorter polypeptides

Chymotrypsin (in the small intestine): endopeptidase secreted by pancreas; cleaves at aromatic amino acid residues.

Peptidases on the brush border: exopeptidases that cleave the terminal amino acids from polypeptides → final breakdown to free amino acids.

Proteins are secreted as inactive zymogens (pepsinogen, trypsinogen) to prevent self-digestion of secretory cells.

Digestion of Lipids

Dietary lipids are mainly triglycerides (triacylglycerols). They are insoluble in water, which creates a problem for enzymatic digestion.

Bile Salts and Emulsification

Bile is produced by the liver, stored in the gall bladder, and secreted into the duodenum via the bile duct. Bile contains bile salts (not enzymes), which are amphipathic molecules (hydrophilic head, hydrophobic tail) that act as emulsifiers:

• Bile salts surround fat globules → break them into tiny micelles (small fat droplets, ~4 - 6 nm)
• This emulsification massively increases the surface area of the lipid available for lipase activity
• Micelles also allow lipids to remain suspended in the aqueous environment of the gut

Pancreatic lipase hydrolyses triglycerides at the surface of micelles:

• Triglyceride→2 fatty acids+monoglyceride (lipase cleaves the ester bonds at positions sn-1 and sn-3)

Absorption in the Ileum

The ileum is the main site of nutrient absorption. Its lining is highly folded and specialised to maximise surface area:

• Circular folds (plicae circulares) - permanent folds of the gut wall
• Villi - finger-like projections of the mucosa (0.5 - 1.5 mm long); each villus contains a capillary network and a lacteal (lymphatic vessel)
• Microvilli (brush border) - tiny projections on the surface of each epithelial cell (enterocyte); contain membrane-bound enzymes (disaccharidases, peptidases) and transporter proteins

Together, circular folds + villi + microvilli increase the absorptive surface area of the ileum by approximately 600-fold compared to a simple tube.

Absorption of Glucose and Amino Acids

Glucose and amino acids are absorbed by secondary active transport (cotransport), using the Na⁺ gradient established by Na⁺/K⁺ ATPase on the basolateral membrane:

1. Na⁺/K⁺ ATPase on the basolateral membrane actively pumps Na⁺ out of the epithelial cell → low [Na⁺] inside cell
2. Na⁺-glucose cotransporter (SGLT1) on the apical (brush border) membrane: Na⁺ moves down its concentration gradient into the cell, dragging glucose in against its concentration gradient (cotransport / secondary active transport)
3. Glucose exits the cell across the basolateral membrane via GLUT2 (facilitated diffusion) → into capillaries → hepatic portal vein → liver
4. Amino acids enter via similar Na⁺-dependent cotransporters; exit via facilitated diffusion

Fructose enters via GLUT5 (facilitated diffusion) on the apical membrane - not via Na⁺ cotransport.

Absorption of Lipids

Fatty acids and monoglycerides from micelles are absorbed differently:

1. Fatty acids and monoglycerides are small and non-polar→diffuse across the apical membrane directly (no transporter needed)
2. Inside the enterocyte, they are reassembled into triglycerides in the smooth ER
3. Triglycerides are packaged with phospholipids, cholesterol, and proteins into chylomicrons (large lipoprotein particles) by the Golgi apparatus
4. Chylomicrons are too large for capillaries→secreted by exocytosis into the lacteals (lymphatic vessels in the villus)
5. Travel through the lymphatic system→thoracic duct→subclavian vein→bloodstream

Lipids therefore bypass the hepatic portal vein initially and enter the blood at the thoracic duct - this is important pharmacologically (drugs in fatty foods can be absorbed this way).

The Hepatic Portal Vein

The capillaries of the intestinal villi drain into the hepatic portal vein, which carries absorbed nutrients (glucose, amino acids, minerals, vitamins) directly to the liver. The liver:

• Regulates blood glucose (stores as glycogen, releases glucose)
• Deaminates excess amino acids→urea
• Detoxifies absorbed toxins before they reach systemic circulation

Summary

• Villi + microvilli increase SA ~600-fold
• Glucose/amino acids→capillaries→hepatic portal vein→liver
• Lipids→lacteals→lymph→blood (bypasses liver initially)

AQA Exam Tips

• Secondary active transport: AQA often asks to explain glucose absorption. The full chain: Na⁺/K⁺ ATPase establishes Na⁺ gradient (using ATP) → Na⁺ moves in via SGLT1 cotransporter → drags glucose in against its concentration gradient → glucose exits via GLUT2 by facilitated diffusion. ATP is used indirectly, not directly, to move glucose.
• Bile salts ≠ enzymes: bile salts emulsify lipids (increase surface area) but do not hydrolyse them. Lipase does the hydrolysis. Distinguish clearly.
• Zymogens: explain why pepsinogen/trypsinogen must be activated after secretion - if secreted as active enzymes, they would digest the secretory cells themselves.
• Lacteals for lipids: lipids are too large (as chylomicrons) to enter capillaries, so they enter lacteals. This is a common mark-scheme point.
• Microvilli functions: (1) increase SA for absorption, (2) contain membrane-bound digestive enzymes. State both.
• Fructose via GLUT5: unlike glucose, fructose uses GLUT5 (facilitated diffusion, not cotransport). Fructose absorption is therefore not Na⁺-dependent.