Revision sheet: Digestive System: Enzymes and Absorption

📋 Course Outline

1. Hydrochloric acid and pepsin activity
2. Enzymes, digestion types, and digestive system organs
3. Bile, fat emulsification, and duodenum neutralisation
4. Small intestine absorption and villi microvilli

📖 1. Hydrochloric acid and pepsin activity

🔑 Key Concepts & Definitions

• Hydrochloric acid : Hydrochloric acid is a gastric juice component that creates a very low pH in the stomach.
• Pepsin activity : Pepsin activity is the effectiveness of the enzyme pepsin, which depends on the surrounding pH.

📝 Essential Points

• Hydrochloric acid provides a low pH that denatures enzymes in harmful microorganisms from food.
• Hydrochloric acid helps set the optimum pH needed for pepsin to work effectively.
• Pepsin works best only when the stomach environment reaches its optimum pH range.

💡 Memory Hook

Low pH = “kill + prime”: denature microbes and prime pepsin.

📖 2. Enzymes, digestion types, and digestive system organs

🔑 Key Concepts & Definitions

• Enzymes : Enzymes are proteins that act as biological catalysts, speeding up chemical reactions without being consumed.
• Chemical digestion : Chemical digestion is digestion where enzymes break food molecules into smaller chemical products.
• Mechanical digestion : Mechanical digestion is digestion where physical actions break food into smaller pieces without chemical change.
• Digestive system organs : Digestive system organs are the body parts that carry out digestion and absorption, from mouth to anus.

📝 Essential Points

• Enzymes increase the rate of a chemical reaction but are not changed by the reaction.
• Water diffuses through partially permeable membranes by osmosis.
• The digestive system regions include mouth, salivary glands, oesophagus, stomach, small intestine (duodenum and ileum), pancreas, liver, gallbladder, and large intestine (colon, rectum, anus).
• Chemical digestion relies on enzymes, while mechanical digestion relies on physical breakdown.
• The small intestine includes duodenum and ileum as distinct regions within the absorption zone.

💡 Memory Hook

Enzymes are “protein catalysts”: faster reactions, unchanged enzyme.

📖 3. Bile, fat emulsification, and duodenum neutralisation

🔑 Key Concepts & Definitions

• Bile : Bile is a digestive fluid that helps neutralise stomach acid entering the duodenum and supports fat digestion.
• Fat emulsification : Fat emulsification is the breakdown of large fat droplets into smaller ones to increase surface area for enzyme action.
• Duodenum neutralisation : Duodenum neutralisation is the process of reducing the acidity of the food and gastric juice mixture entering the duodenum.

📝 Essential Points

• Bile neutralises the acidic mixture of food and gastric juices that enters the duodenum from the stomach.
• Bile emulsifies fats to increase surface area for chemical digestion of fat.
• Lipase digests fat into fatty acids and glycerol.
• Neutralisation by bile makes the duodenum environment more suitable for fat digestion processes.
• Bile’s two roles are neutralising acidity and enabling fat digestion via emulsification.

💡 Memory Hook

Bile does two jobs: neutralise acid + emulsify fat for lipase.

📖 4. Small intestine absorption and villi microvilli

🔑 Key Concepts & Definitions

• Small intestine absorption : Small intestine absorption is the uptake of digested food molecules from the gut into the body.
• Villi : Villi are finger-like projections in the small intestine that increase the internal surface area for absorption.
• Microvilli : Microvilli are tiny projections on villi that further increase surface area for absorption.

📝 Essential Points

• The small intestine is the region where absorption of digested food occurs.
• Water is absorbed in both the small intestine and the colon.
• Most water absorption happens in the small intestine.
• Villi and microvilli increase the internal surface area of the small intestine.
• Greater surface area supports more efficient absorption of digested substances.

💡 Memory Hook

Surface area boost: villi then microvilli = more absorption space.

⚠️ Common Pitfalls & Confusions

1. Confusing chemical digestion with mechanical digestion: only chemical digestion uses enzymes to break molecules.
2. Thinking enzymes are used up in reactions: enzymes speed reactions but are not changed by them.
3. Assuming water absorption happens mainly in the colon: most water absorption occurs in the small intestine.
4. Mixing up bile’s roles: bile neutralises duodenal acidity and emulsifies fats for lipase action.
5. Believing pepsin works at any pH: pepsin activity depends on the optimum pH created by hydrochloric acid.

✅ Exam Checklist

1. State the functions of hydrochloric acid in gastric juice limited to low pH.
2. Explain how low pH affects harmful microorganisms and pepsin activity.
3. Define enzymes as proteins that act as biological catalysts and are not changed by the reaction.
4. Distinguish chemical digestion from mechanical digestion.
5. List the main digestive system regions and associated organs: mouth, salivary glands, oesophagus, stomach, small intestine (duodenum and ileum), pancreas, liver, gallbladder, large intestine (colon, rectum, anus).
6. Describe bile’s role in neutralising the acidic mixture entering the duodenum.
7. Explain how bile emulsifies fats and how this increases fat digestion by lipase into fatty acids and glycerol.
8. Identify the small intestine as the region for absorption of digested food.
9. Compare water absorption in small intestine vs colon and state where most water absorption happens.
10. Explain why villi and microvilli increase internal surface area and improve absorption.