Nutrition and Digestion
What you must understand
Know the seven components of a balanced diet: carbohydrates, proteins, fats, vitamins, minerals, water and fibre (roughage). Learn the source, function and deficiency disease for each. Carbohydrates give energy; proteins are for growth and repair; fats give energy and insulation; vitamin A for vision (night blindness); vitamin C for healthy skin and gums (scurvy); vitamin D for calcium absorption (rickets); iron for haemoglobin (anaemia); calcium for bones and teeth.
Understand digestion as the breakdown of large insoluble molecules into small soluble ones that can be absorbed. Know the enzymes: amylase (starch to maltose), protease (protein to amino acids), lipase (fats to fatty acids and glycerol). State where each is produced: salivary glands, stomach, pancreas and small intestine. Bile is made in the liver, stored in the gall bladder and emulsifies fats; it is not an enzyme.
Traps examiners set
- Saying bile digests fat. It emulsifies it, increasing surface area for lipase.
- Confusing absorption (small intestine into blood) with assimilation (use by cells).
- Forgetting that the stomach produces acid and protease but absorbs little.
- Placing water absorption anywhere other than the large intestine.
Worked example
A question asks why the small intestine has villi. Answer in terms of surface area: villi increase the surface area for absorption, and each villus has a thin wall and a good blood supply to speed diffusion.
Rule of thumb: whenever asked "why is this structure folded, long or thin?", answer with surface area, thin walls and concentration gradient.
Practical link
Use the Benedict's test for reducing sugars (blue to brick red on heating), iodine for starch (blue-black), Biuret for protein (blue to purple) and emulsion test for fat (milky white).