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3. Why do we need food?
We need to consume a balanced diet which contains a variety of food in the proportions shown in the diagram.
The body needs:
Carbohydrates which provide energy.
Proteins for repair and growth.
Vitamins and minerals to keep the body healthy.

4. Food Groups Pop Quiz

5. What is a balanced diet?

6. What do carbohydrates provide?
Starchy foods contain carbohydrates, which are
made of long chains of identical small sugar molecules.
Carbohydrate
1 Sugar molecule

7. What do carbohydrates provide?
The body has to break these large food molecules up into smaller pieces.
The small sugar molecules are very useful. The body can use them to make…
This energy is used to make the body work.
ENERGY

8. What do proteins provide?
As with carbohydrates, proteins are made of chains of chemicals. However, instead of the chain containing identical molecules, in protein these molecules are different.
Protein is made up of chains of amino acids. There are over 20 different kinds of amino acid.
Protein is used to allow the body to grow and to repair the body.

9. What happens to food in the digestive system?

10. Mechanical digestion
Food is firstly broken down (mechanical breakdown) into smaller parts by the teeth in the mouth. It is then mixed with saliva and swallowed.

11. The stomach
Food enters the stomach, which is basically a muscular bag, filled with hydrochloric acid (HCl).
Two things happen here:
Chemical breakdown of the food.
Microbes are destroyed.
food enters from the gullet
cross section of stomach
digested food leaves
muscle tissue

12. The small intestine
Food enters the small intestine from the stomach. It passes along the small intestine where the soluble food is taken into the blood through the walls.

13. The large intestine
The remains of the food are then passed on to large intestine (colon).
All that is left is waste material and water. The body will want to leave the waste material within the digestive system but the water is valuable and so it is re-absorbed here.
The waste material is passed to the rectum where it is stored until it leaves the body through the anus.

14. Why breakdown large molecules into smaller ones?
The body must go through mechanical and chemical digestion to convert large insoluble molecules into smaller soluble ones.
Carbohydrate molecule
Protein molecule
(folded up)
Fat molecule
This is because smaller molecules can pass through the walls of the small intestine and then dissolve into the blood stream. Larger molecules cannot do this.
Sugar

15. Digestion quiz

16. Enzymes
Enzymes are chemicals which break the larger molecules down into smaller molecules. There are three types of enzymes that you need to know about.

17. Enzymes The three types of enzymes are:
Carbohydrase chops carbohydrates into smaller sugar molecules.
Protease cuts proteins into the soluble amino acids.
Lipase breaks fats down into the smaller fatty acids and glycerol.

18. Proteins digestion
Proteins are digested in the stomach by an enzyme called Protease. This enzyme needs to work in an acidic environment. Protease breaks proteins (chains of different molecules) down into up to 20 different amino acids (four of which are shown).
Protease
Folded up protein chain
Amino acids

19. Starch/ carbohydrates
Carbohydrates are chains of identical sugar molecules. The enzyme called Carbohydrase breaks the chemical bonds between the individual sugar molecules (called glucose) as part of digestion.
Carbohydrase
Long carbohydrate
Small sugar

20. Fats Fats are digested in two stages:
Firstly bile (released by the gall bladder) allows the fat to “mix” with water by breaking the fat up into smaller droplets. This is called emulsification.
Bile
Secondly, an enzyme called Lipase breaks the fats down into the smaller fatty acid molecules and glycerol.
Lipase +
Fat
Fatty
acid
Glycerol