Wednesday, 8 April 2009

iGCSE Biology - Food and Digestion

  • Five main food groups: carbohydrates, lipids, proteins, minerals, and vitamins.
  • We also need fibre and water.
  • Carbohydrates:

              Make up 5% of mass of human body.

                   Main “fuel” for supplying cells with energy.

                   Cells release this energy by oxidising glucose, which is a                        carbohydrate.

                   Glucose is found in many sweet-tasting foods.

                   Fructose is found in fruit.

                   Lactose is found in milk.

                   Sucrose is ordinary table sugar.

                   Sugars can cause tooth decay.

                   Most of the carbohydrates we use come from starch, which is                        insoluble.

                   Starch is a polymer of glucose.

                   Starch is found only in plants, and the equivalent of it in animals           is glycogen.

                   Another carbohydrate is cellulose, found in plant cell walls.                             Humans cannot digest this, and therefore forms roughage in                             the gut.

  • Lipids:

                   Make up 10% of human body.

                   If solid at room temperature called fats, if liquid at room                     temperature called oils.

                   Fats come from animals, oils come from plants.

                   Fat is used to insulate the human body as well as to protect                    organs by surrounding them.

                   Lipids are made up of glycerol and fatty acids.

                   Excess lipids, especially saturated fats such as cholesterol, are           unhealthy and have been linked to heart disease.

  • Proteins:

              Make up 18% of human body.

                   Needed for growth and repair.

                   Meat, fish, cheese, and eggs are rich in protein.

                   Protein deficiency disease is kwashiorkor.

                   Proteins are polymers, made up of amino acids.

                   Shape is crucial to function of protein.

  • Minerals:

                   Calcium deficiency is rickets.

                   Iron deficiency is anaemia.


Amount in an adult body (g)

Location or role in body

Examples of foods rich in minerals



Making teeth and bones

Dairy products, fish, bread, vegetables



Making teeth and bones; part of many chemicals, e.g. DNA

Most foods



In body fluids, e.g. blood

Common salt, most foods



In body fluids, e.g. blood

Common salt, most foods



Making bones, found inside cells

Green vegetables



Part of haemoglobin in red blood cells, helps carry oxygen

Red meat, liver, eggs, some vegetables e.g. spinach

  • Vitamins:



Use in body


Source in food



Making chemical in retina, protecting surface of the eye.

Night blindness, damaged cornea

Fish liver oils, liver butter, carrots



Helps with cell respiration


Yeast extract, cereals



Helps with cell respiration

Poor growth, dry skin

Green vegetables, eggs, fish



Helps with cell respiration


Liver, meat, fish



Sticks together cell linings


Fresh fruit and vegetables



Helps bones absorb calcium and phosphate

Rickets, poor teeth

Fish liver oils


  • Food tests:

                   Starch test:

1.       Add dilute iodine solution.

2.     If it goes blue-black, starch is present.

              Glucose test:

1.       Place in test-tube, and add 2cm of water.

2.     Add several drops of Benedict’s solution.

3.     Boil the test-tube in a water bath.

4.     If it goes brick-red, glucose is present.

Protein test:

1.       Add to 2cm of water in test-tube and mix.

2.     Add potassium hydroxide solution.

3.     Add copper sulphate solution.

4.     If it goes mauve, protein is present.

Lipid test:

1.       Add ethanol and shake.

2.     Add to cold water.

3.     If an emulsion forms, lipids are present.

·        Energy content of food is measured in kilojoules (kJ).

·        To find the energy content of food do the following:

1.       Find mass of food.

2.     Place 20cm3 of water into a boiling tube.

3.     Measure the temperature of the water.

4.     Light the food in a Bunsen.

5.     When it lights, hold under the boiling tube until it no longer burns.

6.     Measure final temperature of water.

7.    (final temperature – temperature at start) x 20 x 4.2

                           Mass of food (g)

·        Food is broken down through digestion so that it can be absorbed.

·        Digestion is speeded up by enzymes.

·        Digestion can be chemical or mechanical.

·        Muscles move food along the gut.

              There are two muscle layers in wall of the intestine.

                   Circular muscles and longitudinal muscles contract and relax in                        succession, causing narrowing and widening of the gut.

                   Waves of muscle contraction push food along the gut.

                   This is called peristalsis.

  • The mouth, stomach and first part of intestine (duodenum) break down food using enzymes.
  • Digested food is absorbed in the last part of the small intestine (ileum).
  • The large intestine absorbs water from the remains and stores faeces.
  • Digestive Enzymes:

Class of Enzyme


Digestive Action

Source of Enzyme

Where it acts in the gut



Starch à maltose

Salivary glands


Starch à maltose


Small intestine

Maltose à glucose

Small intestine wall

Small intestine



Proteins à peptides

Stomach wall



Proteins à peptides


Small intestine


Peptides à amino acids

Small intestine wall

Small intestine



Lipids à glycerol and fatty acids


Small intestine

  • Process of digestion:
    1. Saliva, containing amylase, moistens the food in the mouth.
    2. The chewed food passes through oesophagus to the stomach.
    3. Stomach secretes hydrochloric acid, killing bacteria on food.
    4. Pepsin breaks down protein in the stomach.
    5. Food is held in stomach by a sphincter muscle, and when this relaxes food is released into the duodenum.
    6. Amylase, trypsin, and lipase (all made by pancreas) are added to the food.
    7. Bile, produced in the liver and stored in the gall bladder, passes down the bile duct onto the food, emulsifying it and increasing its surface area. This means the lipase can act faster on it.
    8. Once the food has been broken down, it is absorbed in the ileum.
    9. Ileum has very large surface area: very long, folded upon itself, and contains villi, each with their own microvilli. Villi are small extrusions from wall of ileum.
    10. Most food products enter capillaries in villi.
    11. Products of fat digestion enter a lacteal, part of the lymphatic system, and form lymph. This is later drained into blood
    12. Blood vessels in ileum form hepatic portal vein which leads to liver, which “processes” food molecules.
    13. The food molecules are assimilated: absorbed into cells from the blood.

·        First part of large intestine, the colon, absorbs water to form faeces.

·        Faeces stored in rectum until expelled by the anus.


  1. i love the notes in this, mr/ms. author keep up the extraodinary work :))))) thank you very much.

  2. Thank you for this!! Its hard to find good revision notes for IGCSE Science! Keep up the good work :D

  3. i would love to thank you so much nd its actually very hard to find notes for igsce..but amzing its totally for mine's... :D

  4. you can find more on

  5. Very good and useful.
    please send notes for other chapters

  6. tht was so gud nd saved ma tym

  7. you can find more @ it has even got other subjects such as chemistry, physics, ICT, Economics, Accounting, Business studies and English!