7. Human Nutrition (Part 2) (2/2) (Cambridge IGCSE Biology 0610 for exams in 2026, 2027 and 2028)

Main Ideas / Lessons (Topic 7: Human Nutrition – Part 2, Chemical Digestion & Absorption)

Chemical digestion (core concept)

• Chemical digestion is the breakdown of large, insoluble molecules into small, soluble molecules.
• This process produces molecules that can be absorbed into the blood/lymph.

Enzymes and what they do (enzyme → substrate → products → location)

Amylase

• Breaks down: starch
• Products (stepwise): starch → maltose → glucose
• Where it acts:
  • Mouth (from salivary glands)
  • Duodenum (from the pancreas)

Proteases (two main types)

Pepsin

• Conditions: acidic (stomach environment)
• Breaks down: protein → amino acids
• Location: stomach

Trypsin

• Conditions: alkaline (small intestine environment)
• Breaks down: protein → amino acids
• Where it acts: duodenum (secreted by the pancreas)

Lipase

• Breaks down: fats and oils
• Products: fats/oils → fatty acids + glycerol
• Where it acts: duodenum (secreted by the pancreas)

Hydrochloric acid (HCl) in gastric juice

• Kills harmful microorganisms in food.
• Provides an acidic pH needed for pepsin to work at its optimum (fastest) rate.

Bile’s role in digestion (especially for fats and pH)

• Emulsifies fats (as introduced in the previous video).
• Bile is an alkaline mixture that neutralizes stomach acid and gastric juice entering the duodenum.
• This creates a suitable pH for small intestine enzymes, which have a higher alkaline optimum pH than stomach enzymes.

Absorption (core concept + where/why)

• Absorption = movement of digested food molecules from the digestive system into the blood (and fats into lymph).
• The small intestine is where most nutrients are absorbed.
• Water is mainly absorbed from the small intestine, but some is also absorbed from the colon.

Ilium adaptation for absorption

• The ilium has:
  • a highly folded surface
  • millions of villi and microvilli
• This increases surface area, making absorption faster and more efficient.

Structure involved in absorption

Villi (finger-like projections)

• Contain capillaries (blood vessels).
• The covering layer plus microvilli increase surface area further.

Lactials (in villi)

• Absorb fatty acids and glycerol.
• Transport these via lymph.

Capillaries

• Absorb:
  • glucose and amino acids into the blood

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Methodology / Instruction-like Steps (Enzyme Digestion + Digestion Environment)

1. Start with starch digestion (carbohydrates)

   • Amylase breaks down starch → maltose
   • Maltase converts maltose → glucose (implied)
   • Locations:
     • Mouth (amylase from salivary glands)
     • Duodenum (amylase from pancreas)

2. Digest proteins (protein → amino acids)

   • In the stomach (acidic pH):
     • Pepsin breaks protein → amino acids
     • Hydrochloric acid (HCl) provides the acidic pH for pepsin to work optimally
   • In the duodenum/small intestine (alkaline pH):
     • Trypsin breaks protein → amino acids
     • Bile helps neutralize acidic food entering the duodenum to reach a suitable alkaline pH

3. Digest fats (fats/oils → fatty acids + glycerol)

   • Lipase breaks fats and oils → fatty acids + glycerol
   • Location: duodenum (secreted by the pancreas)
   • Bile also emulsifies fats, increasing effective surface area for digestion.

4. Absorb digestion products

   • The small intestine absorbs most nutrients.
   • Absorption in the ilium is efficient due to:
     • villi + microvilli (very large surface area)
   • Transport paths:
     • Capillaries absorb glucose + amino acids into the blood
     • Lactials absorb fatty acids + glycerol into lymph

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Speakers / Sources Featured

• “IGCSE Study Buddy” (narrator/host speaking in the video)