Human nutrition and digestion
A balanced diet, the parts of the digestive system, and how food is digested and absorbed.
📌 Key points
- A balanced diet includes carbohydrates and fats (energy), proteins (growth and repair), vitamins C and D, mineral ions calcium and iron, fibre and water.
- Vitamin C deficiency causes scurvy; vitamin D or calcium deficiency causes rickets. Iron is needed to make haemoglobin; fibre prevents constipation.
- The alimentary canal: mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon and rectum) and anus. The salivary glands, pancreas, liver and gall bladder are associated organs.
- Ingestion, mechanical and chemical digestion, absorption, assimilation and egestion are the stages of nutrition.
- Mechanical digestion by teeth and the churning stomach breaks food into smaller pieces, increasing the surface area for enzymes. A tooth crown is covered by hard enamel, with dentine and a pulp cavity inside.
- Amylase (from salivary glands and pancreas) digests starch to maltose; maltase on the membranes of the small intestine digests maltose to glucose.
- Proteases digest proteins to amino acids: pepsin in the stomach and trypsin in the small intestine. Lipase (from the pancreas) digests fats to fatty acids and glycerol.
- Hydrochloric acid in the stomach kills harmful microorganisms in food and gives an acid pH for pepsin. Bile (made in the liver, stored in the gall bladder) neutralises acid in the duodenum and emulsifies fats, increasing their surface area for lipase.
- Most digested food and water is absorbed in the small intestine. Villi and microvilli give a large surface area; villi have thin walls, capillaries for glucose and amino acids, and lacteals for fats.
- Cholera bacteria release a toxin that makes the small intestine secrete chloride ions; water follows by osmosis, causing diarrhoea and dehydration. It is treated with oral rehydration therapy.
📖 Key terms
- Ingestion
- Taking substances such as food and drink into the body through the mouth.
- Digestion
- Breaking down large, insoluble food molecules into small, soluble molecules.
- Absorption
- Movement of nutrients from the intestines into the blood.
- Assimilation
- Uptake and use of nutrients by cells.
- Egestion
- Passing out of undigested food, as faeces, through the anus.
- Emulsification
- Breaking large fat drops into small droplets, increasing their surface area.
✏️ Worked example
Explain how the structure of a villus makes absorption efficient.
- Villi and microvilli give the small intestine a very large surface area for absorption.
- The wall of each villus is one cell thick, so the diffusion distance is short.
- Each villus has a dense network of capillaries; blood flow carries nutrients away and keeps a steep concentration gradient.
- A lacteal in each villus absorbs fatty acids and glycerol.
- Answer: large surface area, short diffusion distance and a good blood supply all increase the rate of absorption.
Do not mix up egestion (removing undigested food) with excretion (removing waste made by metabolism). Faeces are egested, not excreted.
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