Diet, the digestive system and teeth
🎯What you need to be able to do
- Describe a balanced diet; state sources and the importance of each nutrient; state the causes of scurvy and rickets.
- Identify the organs of the digestive system and describe their roles in ingestion, digestion, absorption, assimilation and egestion.
- Describe physical digestion, the types and structure of human teeth, and the role of the stomach.
- Outline the role of bile in emulsifying fats EXTENDED.
📚The biology
A balanced diet
A balanced diet contains all the nutrients in the correct proportions, with enough energy for the person’s age, sex and activity.
carbohydrates (rice, bread, pasta): the main source of energy
fats and oils (butter, oils, nuts): an energy store; insulation
proteins (meat, fish, eggs, beans): growth and repair; making enzymes and antibodies
vitamin C (citrus fruit, green vegetables): healthy skin and gums; lack causes scurvy
vitamin D (oily fish, eggs; made in skin in sunlight): absorbing calcium; lack causes rickets
calcium (milk, cheese): strong bones and teeth; lack also causes rickets
iron (red meat, spinach): making haemoglobin in red blood cells
fibre (vegetables, wholegrain cereals): bulk for the intestine to push food along; prevents constipation
water: the solvent for reactions and transport; replaces water lost in sweat and urine
The digestive system
ingestion: taking substances such as food and drink into the body (mouth)
digestion: the breakdown of food (mouth, stomach, small intestine)
absorption: movement of nutrients from the intestines into the blood (small intestine; water also in the colon)
assimilation: uptake and use of nutrients by cells
egestion: removal of undigested food from the body as faeces (rectum, anus)
- Salivary glands secrete saliva containing amylase. The oesophagus carries food to the stomach.
- The stomach churns food and secretes gastric juice (protease and hydrochloric acid).
- The pancreas secretes pancreatic juice with amylase, protease and lipase into the duodenum.
- The liver makes bile, which is stored in the gall bladder and released into the duodenum.
- The small intestine (duodenum and ileum) completes digestion and absorbs the products; the colon absorbs some water; the rectum stores faeces until they leave through the anus.
Physical digestion
Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. It increases the surface area of food for enzymes to act on in chemical digestion. Teeth chew food; the muscular wall of the stomach churns it.
Bile EXTENDED
EXTENDED Bile emulsifies fats and oils: it breaks large droplets into many small ones, increasing the surface area for lipase to act on. Bile contains no enzymes. (It also neutralises stomach acid — see 7b.)
✏️Worked example
A student eats a sandwich.
(a) Name the process and the organ for each: (i) the food enters the mouth; (ii) glucose passes into the blood; (iii) undigested fibre leaves the body. [3]
(b) Explain how chewing helps chemical digestion. [2]
(c) EXTENDED The student’s gall bladder has been removed. Suggest why they are advised to eat less fat. [2]
(a) (i) Ingestion, mouth. (ii) Absorption, small intestine. (iii) Egestion, anus.
(b) Chewing breaks food into smaller pieces, increasing the surface area for enzymes to act on, so chemical digestion is faster.
(c) Bile can no longer be stored and released in large amounts after a meal, so less fat is emulsified; with a smaller surface area, lipase digests fat more slowly and undigested fat may cause discomfort.
Check it. The five process words each describe a different step, in order: ingestion → digestion → absorption →
assimilation → egestion.
Egestion is not excretion. Faeces are undigested food that never entered the body’s cells; excretion removes waste products of
metabolism (topic 13).
📝Practise
In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.
1. (Multiple choice.) What is absorption? A: taking food into the mouth. B: the breakdown of food. C: movement of nutrients from the intestines into the blood. D: use of nutrients by cells.
C. D is assimilation.
2. (Theory.) Explain why a person whose diet lacks iron may feel tired and short of breath. [2]
Iron is needed to make haemoglobin; with less haemoglobin, red blood cells carry less oxygen to the cells for respiration.
3. (Theory.) State why fibre is important in the diet, even though it is not digested. [1]
It adds bulk, so the muscles of the intestine can push food along (prevents constipation).
4. (Theory.) Explain why a growing teenager needs plenty of calcium and protein. [2]
Calcium for growing strong bones and teeth; protein for growth and repair of tissues (making new cells).
5. (Theory.) Name the organ that (a) makes bile, (b) stores bile, (c) secretes lipase into the duodenum. [3]
(a) Liver. (b) Gall bladder. (c) Pancreas.
6. (Theory.) Describe the role of the stomach in physical digestion. [1]
Its muscular walls contract to churn the food, breaking it into smaller pieces and mixing it with gastric juice.
7. (Theory.) Relate the shape of molars to their function. [2]
They have broad, flat surfaces with cusps, which crush and grind food into small pieces.
8. (Theory.) EXTENDED Explain why emulsification increases the rate of fat digestion, even though bile contains no enzymes. [2]
It breaks large fat droplets into many small droplets, giving a much larger surface area on which lipase can act.
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- NHS — the Eatwell Guide to a balanced diet
- BBC Bitesize — the digestive system, animated