Explain the role of enzymes in digestion with reference to amylase, protease and lipase.

7.2 Human Nutrition – The Alimentary Canal

1. Main Organs and Their Functions

2. Mechanical (Physical) Digestion

• Teeth: incisors cut, canines tear, premolars and molars grind food, increasing surface area.
• Chewing (mastication): mixes food with saliva and reduces particle size.
• Stomach churning: muscular walls contract, turning the food into a semi‑liquid mass called chyme.
• Peristalsis: wave‑like contractions of the oesophagus and intestines move chyme along the tract.

3. Chemical Digestion

3.1 Gastric Acid

• Secreted by gastric glands as hydrochloric acid (HCl), pH ≈ 2.
• Denatures (unfolds) proteins, exposing peptide bonds to enzymes.
• Activates the precursor pepsinogen → pepsin.

3.2 Bile

• Produced by the liver, stored in the gall‑bladder.
• Alkaline (pH ≈ 7.5–8) – helps neutralise the acidic chyme entering the duodenum.
• Bile salts emulsify large fat globules into tiny droplets, vastly increasing the surface area for lipase action.

3.3 Digestive Enzymes

4. Role of Enzymes in Digestion (Amylase, Protease, Lipase)

• Amylase – a carbohydrate‑digesting enzyme (α‑amylase).
  • Salivary amylase begins starch hydrolysis in the mouth (optimal pH ≈ 7).
  • Pancreatic amylase continues the reaction in the duodenum, producing maltose and maltotriose.
  • Products are later split into glucose by brush‑border maltase.
• Proteases – enzymes that cleave peptide bonds.
  • Pepsin (stomach) works best at pH ≈ 2, breaking proteins into large peptides.
  • Trypsin and chymotrypsin (pancreas) operate at pH ≈ 7.5–8, producing shorter peptides.
  • Brush‑border peptidases finish the job, releasing free amino acids ready for absorption.
• Lipase – a fat‑digesting enzyme.
  • Pancreatic lipase hydrolyses triglycerides into monoglycerides and free fatty acids.
  • Effective only after bile salts have emulsified the fat droplets.
  • The resulting monoglycerides and fatty acids form micelles that transport them to the intestinal brush border.

5. Cellular Basis of Nutrient Absorption

Absorption occurs across the apical membrane of the intestinal epithelial (enterocyte) cell and into the bloodstream across the basolateral membrane.

• Diffusion – lipid‑soluble molecules (e.g., fatty acids, some vitamins) pass directly through the phospholipid bilayer down a concentration gradient.
• Facilitated diffusion – carrier proteins help polar molecules (e.g., glucose via GLUT2) move down their concentration gradient.
• Active transport – requires ATP; important for uptake of glucose (SGLT1) and amino acids against a concentration gradient.
• Osmosis – water follows the osmotic gradient created by solute absorption, moving through aquaporins.

Diagram (suggested)

A cross‑section of an enterocyte showing microvilli, brush‑border enzymes, Na⁺/K⁺‑ATPase, GLUT2, SGLT1, amino‑acid transporters, aquaporins, and the movement of nutrients into the capillary and lacteal.

6. Step‑by‑Step Overview of Digestion

1. Ingestion & Mechanical Breakdown (Mouth)
   • Teeth chew; saliva moistens food and adds salivary α‑amylase.
2. Swallowing & Transport (Oesophagus)
   • Peristaltic waves move the bolus to the stomach.
3. Stomach Digestion
   • Gastric glands secrete HCl and pepsinogen → pepsin.
   • Acid denatures proteins; pepsin hydrolyses peptide bonds.
   • Churning mixes contents into chyme.
4. Duodenum – Chemical Digestion Continues
   • Pancreatic juice (bicarbonate + enzymes) neutralises acid and adds amylase, trypsin, chymotrypsin and lipase.
   • Bile emulsifies fats.
   • Brush‑border enzymes complete carbohydrate and protein digestion.
5. Small‑Intestine Absorption (Jejunum & Ileum)
   • Monosaccharides and amino acids enter enterocytes via active transport or diffusion.
   • Fatty acids and monoglycerides form micelles, diffuse into enterocytes, are re‑esterified into triglycerides, and packaged into chylomicrons for lymphatic transport.
6. Large‑Intestine
   • Absorption of water, electrolytes and some vitamins (e.g., vitamin K).
   • Fermentation of undigested carbohydrate by gut bacteria.
   • Formation and storage of solid faeces; eventual egestion.

7. Factors Influencing Enzyme Activity (IGCSE level)

• Temperature – Optimum ≈ 37 °C; higher temperatures denature the protein, lower temperatures slow molecular motion.
• pH – Each enzyme has a narrow optimum (e.g., pepsin ≈ 2, pancreatic enzymes ≈ 7.5–8). Deviations reduce the rate of catalysis.
• Inhibitors
  • Competitive – resemble the substrate and bind the active site.
  • Non‑competitive – bind elsewhere, altering enzyme shape.
• Substrate concentration – Reaction rate rises with more substrate until all active sites are occupied (V_max).

8. Simple Enzyme Kinetics (optional)

The Michaelis–Menten equation describes how reaction rate varies with substrate concentration:

$$ v = \frac{V_{\max}[S]}{K_m + [S]} $$

• v – observed rate.
• V_max – maximum rate when the enzyme is saturated.
• [S] – substrate concentration.
• K_m – substrate concentration at which the rate is half of V_max (low K_m = high affinity).

9. Balanced Diet & Nutrient Sources

10. Vitamins and Minerals Relevant to Digestion

• Vitamin C – antioxidant; essential for collagen synthesis and enhances iron absorption.
• Vitamin D – promotes calcium absorption in the gut; deficiency leads to rickets/osteomalacia.
• Calcium (Ca) – required for muscle contraction (including gut motility) and enzyme activation.
• Iron (Fe) – component of haemoglobin; absorbed mainly as Fe²⁺ in the duodenum; vitamin C improves its uptake.

11. Transport of Digested Nutrients

• Blood circulation – nutrients (glucose, amino acids, water‑soluble vitamins, minerals) enter the portal vein, travel to the liver for processing, then are distributed to body cells via the systemic circulation.
• Lymphatic system – long‑chain fatty acids and monoglycerides are re‑esterified into triglycerides, packed into chylomicrons, and enter lacteals → thoracic duct → bloodstream.
• Link to plant transport (curriculum reminder) – Just as xylem transports water and minerals from roots to leaves, the human circulatory system transports nutrients from the gut to tissues.

12. Common Digestive Disorders & Immunity

• Helicobacter pylori infection – damages gastric mucosa, can lead to ulcers; stomach acid and mucus provide primary defence.
• Food‑borne pathogens (e.g., Salmonella, E. coli) – cause gastroenteritis; gut‑associated lymphoid tissue (GALT) and secretory IgA help protect.
• Coeliac disease – autoimmune reaction to gluten; damages villi, reducing absorption of nutrients.
• Lactose intolerance – deficiency of brush‑border lactase; undigested lactose draws water into the lumen (osmotic diarrhoea).

13. Brief Link to Respiration (IGCSE integration)

• Glucose produced from carbohydrate digestion is oxidised in cellular respiration (aerobic) to generate ATP, the energy currency for active transport of nutrients.
• Oxygen is taken up by the lungs, transported by the blood to cells, and carbon dioxide (a waste product of respiration) is carried back to the lungs for exhalation.

14. Suggested Diagram for Revision