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Module 13 - Food & digestion

Module 13 - Food & Digestion - Revision

Here are the key ideas from Module 13. Make sure that you also look at your written notes and the sites on the links page.

Food and Food groups

Learning Resource

An ICT research exercise is available for this module. Students are expected to research questions on the food groups and their functions.

Nutrition Links Page (opens in a new window)

Question sheet (150k)

You require Adobe Acrobat Reader (get it here) to read this file.

Why do we need food?

We need to eat food for lots of different reasons:

  • For energy to live, move and survive
  • To grow, and to repair damaged cells and tissues in our body.
  • To keep warm
  • To prevent diseases

Different foods contain different nutrients to help us to achieve these things. Here are the main nutrients you need to know about:

  • CARBOHYDRATES (sugars and starch) for ENERGY
  • PROTEINS for GROWTH and repairing our cells and tissues
  • FATS for energy and WARMTH
  • VITAMINS and MINERALS to keep us healthy and to PREVENT DISEASES.


Type of food

Why we need it


As starch in cereals (potatoes, bread, pasta, rice). As sugars in sweet foods and drinks (chocolate, soft drinks etc.)

For energy. Most of our carbohydrate should be in the form of starch not as sugars.


Meat, fish, nuts, lentils

For growing and repairing our tissues. Children who receive too little protein may grow too slowly or suffer from a condition called kwashiorkor.


Meat, cakes, fried foods, fast food

For warmth and for energy. Too much fat in our diet leads to severe health problems including heart attacks.

Vitamins and minerals

Lots of foods but especially fruit and vegetables

A lack of some vitamins may lead to particular illnesses; e.g. a lack of Vitamin C (from oranges, limes etc.) leads to a condition called scurvy. Not having enough iron (a mineral) in the diet leads to anaemia.

You also need a lot of water - about 75% of your body is made of water, remember. It is recommended that you drink about 4 pints of water (or fruit juice / low sugar squash) every day!

Some foods contain lots of fibre. This cannot be digested but it helps to keep our digestive system healthy.

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What is a 'balanced diet'?

A balanced diet contains the right amounts of different food groups to keep us healthy. To illustrate a balanced diet, scientists sometimes use a food pyramid:

The food pyramid shows how much of different types of foods you should be eating:

  • Breads and cereals should make up the major part of your diet
  • You should eat lots of vegetables and fruit
  • You should eat some (but not too much) meat (or veggie alternative) and dairy products every day
  • You should eat as few fats, oils and sweets as possible


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Testing for food groups

  • For information on how to test for the different food groups in food see the guide.

Digestion and the Digestive System

What does the digestive system do?

  • The digestive system breaks down larger pieces of food into smaller pieces of food. This can be done physically (e.g. the teeth breaking down food in the mouth) or chemically (e.g. chemicals called enzymes breaking down large molecules of food into smaller molecules of food - e.g. breaking down starch (a large carbohydrate) into sugars, which are smaller carbohydrates)
  • Once the food has been broken down, the molecules of food are absorbed from the digestive system into the blood in the small intestine. The nutrients can then be carried round the body to where they are needed.

Are all types of food digested?


  • Fibre cannot be digested at all. It gives your food bulk so that your digestive system can push on it. Most of faeces (poo) is made up of fibre.
  • Vitamins and minerals are small molecules anyway, so they don't have to be digested at all - they can pass into the blood from the small intestine.

For information on the what the different parts of the digestive system do, see the links on our links page.

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What are enzymes and how do they work?

  • Enzymes are special chemicals in the digestive system that break down large food molecules into smaller food molecules. These small molecules can then move from the small intestine into the blood.
  • Starch is broken down into sugars by an enzyme
  • Proteins are broken down into amino acids by enzymes
  • Fats are broken down into fatty acids by enzymes

See the article and animation at to see more on how food is broken down.

What conditions do enzymes work best in?

  • Enzymes in the body work best at body temperature, which is about 37 degrees Celsius. At very hot temperatures, enzymes are destroyed (denatured) and stop working.
  • Enzymes can also be damaged by changes to the pH (how acid or alkali it is). Enzymes in the stomach, though, work at very low pH (strong acid) because the stomach contains acid.

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Fascinating Digestive System Facts

(you don't need to know these, but they are interesting all the same)

  • We each eat about 500kg (½ ton) of food per year.
  • We each produce 1.7 litres (3 pints) of saliva every day.
  • Muscles contract in waves to move the food down the oesophagus. This means that food would get to a person's stomach, even if they were standing on their head.
  • An adult stomach can hold approximately 1.5 litres (2 ½ pints) of material.
  • The digestive system of an adult human is about 8m long.
  • Every day 11.5 litres (20 pints) of digested food, liquids and digestive juices flow through the digestive system, but only 100mls (one sixth of a pint) is lost in faeces (poo)
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