A balanced diet and the food groups
A balanced diet means eating the right amounts of every food group your body needs. Miss one out and something starts to go wrong.
| Food group | Why you need it | Good sources |
|---|---|---|
| Carbohydrates | Main energy supply | Bread, rice, pasta |
| Proteins | Growth and repair of cells | Meat, fish, eggs, beans |
| Lipids (fats) | Energy store and insulation | Oils, butter, nuts |
| Vitamins | Keep processes working (e.g. vitamin C prevents scurvy) | Fruit, vegetables |
| Minerals | Small amounts for health (e.g. iron for blood) | Red meat, leafy greens |
| Fibre | Keeps food moving through the gut | Wholegrains, vegetables |
| Water | Needed for every reaction in the body | Drinks and most foods |
Mechanical vs chemical digestion
Digestion happens in two ways at once, and it helps to keep them apart in your head.
- Mechanical digestion
- Physically breaking food into smaller pieces, for example by chewing with teeth or churning in the stomach. The molecules stay the same, they are just smaller lumps.
- Chemical digestion
- Using enzymes to break large molecules into small soluble ones, for example starch into glucose. The molecules themselves are changed.
The two work together. Chewing gives a bigger surface area, which lets the enzymes of chemical digestion work faster. This is why swallowing food whole would slow everything down.
One quick way to tell them apart
If the molecule is only made smaller in size, that is mechanical. If a big molecule is turned into a different, smaller molecule, that is chemical. Chewing is mechanical; an enzyme is chemical.
The alimentary canal in order
The alimentary canal is the long tube food travels through, from mouth to anus. Learn the order, because questions love to test it.
| Order | Part | What happens there |
|---|---|---|
| 1 | Mouth | Teeth chew (mechanical); saliva adds amylase to start digesting starch |
| 2 | Oesophagus | A muscular tube that pushes food down to the stomach |
| 3 | Stomach | Churns food and adds acid and protease to digest protein |
| 4 | Small intestine | Most digestion is completed and small molecules are absorbed into the blood |
| 5 | Large intestine | Water is absorbed from the leftover material |
| 6 | Rectum and anus | Undigested waste (faeces) is stored and removed |
The liver and pancreas are not part of the tube itself, but they help by releasing bile and enzymes into the small intestine.
The key enzymes and bile
Three groups of digestive enzymes handle the three food molecules. Each one breaks a large molecule into small soluble units that can be absorbed.
| Enzyme | Breaks down | Into |
|---|---|---|
| Amylase (a carbohydrase) | Starch | Simple sugars (glucose) |
| Protease | Proteins | Amino acids |
| Lipase | Lipids (fats) | Fatty acids and glycerol |
Bile is made in the liver, stored in the gall bladder and released into the small intestine. It is not an enzyme, so it does no chemical digestion itself, but it does two useful jobs:
- It is alkaline, which neutralises the acid arriving from the stomach so the enzymes of the small intestine can work at their optimum pH.
- It emulsifies fats, breaking large fat droplets into many tiny ones. This gives a bigger surface area for lipase to work on, speeding up fat digestion.
Where this is assessed
Recalling the canal, enzymes and bile is Criterion A (knowing and understanding). Investigating how bile or temperature affects fat digestion is Criterion B (inquiring and designing) and Criterion C (processing and evaluating), while discussing diet and health links to Criterion D (reflecting on the impacts of science).
Check yourself
1. List the parts of the alimentary canal in order from mouth to anus. +
Mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus. The liver and pancreas help but are not part of the tube.
2. Bile is not an enzyme, so how does it help digest fat? +
It emulsifies fat, breaking large droplets into many small ones. This increases the surface area so lipase can digest the fat faster. Bile is also alkaline, giving lipase its correct pH.
3. Which enzyme digests protein, and what is the product? +
Protease breaks protein down into amino acids.
Part of the Biology library. Spotted an error or want a topic added? That feedback makes the notes better.