Nutrition and Digestion
What you must understand
You need to know the nutrients needed by humans - carbohydrates, proteins, lipids, vitamins A, C and D, minerals calcium and iron, water and fibre - with a source, a function and a deficiency disease for each. Energy requirements vary with age, sex, activity and pregnancy.
Follow the digestive tract: mouth (teeth, amylase), oesophagus, stomach (acid, pepsin), small intestine (duodenum and ileum; pancreatic and intestinal enzymes; villi for absorption), large intestine (water reabsorption), rectum and anus. Know the enzymes: amylase (starch to maltose), maltase (maltose to glucose), protease (protein to amino acids), lipase (lipids to glycerol and fatty acids). Bile from the liver, stored in the gall bladder, emulsifies fats and neutralises stomach acid.
Learn the villus adaptations: large surface area, thin wall, rich blood supply, lacteal, moist surface. Plants need light, carbon dioxide, water and minerals; nitrate for proteins, magnesium for chlorophyll, phosphate for roots.
Traps examiners set
- Saying bile digests fat - it emulsifies it; lipase digests it.
- Placing absorption mainly in the stomach - most absorption is in the small intestine.
- Confusing peristalsis with absorption.
- Forgetting that fibre is not digested but aids movement of food.
Worked rule of thumb
For any "explain the adaptation" question, name the feature, then link it to rate of absorption: a large surface area and thin wall increase the diffusion gradient and shorten diffusion distance, so absorption is faster.