The Human Digestive System — AQA GCSE Biology
The digestive system breaks down food into small, soluble molecules that can be absorbed into the blood.
Why digestion is needed
Large food molecules (starch, proteins, lipids) are insoluble and too big to be absorbed. Digestive enzymes break them down into small, soluble molecules.
The organs
- Mouth — teeth break food up; salivary glands release amylase.
- Stomach — produces hydrochloric acid (kills microbes and gives the right pH for protease) and protease; churns food.
- Small intestine — receives enzymes from the pancreas and bile from the liver; digestion is completed and soluble products are absorbed into the blood.
- Large intestine — absorbs water, leaving faeces.
The role of bile
Bile is made in the liver, stored in the gall bladder, and released into the small intestine. It:
- neutralises stomach acid to provide alkaline conditions for enzymes, and
- emulsifies fats — breaking large fat droplets into smaller ones to increase the surface area for lipase.
Adaptations for absorption
The small intestine is lined with villi, which give a large surface area, have thin walls and a good blood supply — ideal for absorbing digested food efficiently.
Exam tips
- Match each organ to its role in digestion.
- Know that bile is made in the liver, stored in the gall bladder, and neutralises acid and emulsifies fats.
- Explain how villi are adapted for absorption (surface area, thin walls, blood supply).
- Remember the stomach is acidic; the small intestine is alkaline.