Revision - GCSE biology - the digestive system

The digestive system

{diagram of digestive system needed}

Organ What it does
Mouth The teeth chew up food using mechanical digesting into easy to swallow balls (a bolus). The chewing increases surface area.
Salivary glands Produces the enzyme amylase which brakes down starch into simple sugars (if you chew bread for ages it begins to taste sweet). It also produces mucus which covers the food and lets its slide down the gullet
Oesophagus (gullet) Takes food down from the mouth to the stomach
Stomach It pummels food with its muscular walls.
It creates protease (pepsin) which dissolves proteins into amino acids.
It produces hydrochloric acids for two reasons; To kill bacteria, and to give the pepsin the correct pH to operate in (pH 2 – very acidic). The stomach produces mucus which stops the acid damaging it.
Liver Where bile is produces. Bile emulsifies (breaks down) fat, and neutralises the stomach acid so the conditions are right for the enzymes on the small intestine
Gall bladder Where bile is stored before being injected into the small intestine
Pancreas Creates the enzymes protease, amylase and lipase. These go down the pancreatic duct and enter in the duodenum
Small intestine Produces protease, amylase and lipase. Nutrients from the food are absorbed into the blood. Is very long for increased surface area, and is covered with villi for the same reason (more on villi later)
Large intestine Excess water is absorbed from the food
Anus Where the faeces leave your body

When we swallow the epiglottis (which is a small flap of skin) covers the glottis (which is the opening of the wind pipe) and stops food falling down.


  • There is muscular tissue all along the inside of the gut. It pushes the bolus along, contracting (peristaltic squeeze) and opening the circular (inner) and longitudinal (outer) muscle
  • Along the gut there is glandular tissue which produces enzymes and mucus
  • There are also lots of villi (sing. villus)
  • They are good for digesting food because;
  • They have a large surface area, they are covered in micro-villi, they have very thin walls, they have an excellent blood supply and there are bloody loads of them


  • Break big molecules into smaller ones
  • Big molecules include fats, proteins and starch


  • Pepsin – Proteins into amino acids
  • Amylase – Starch into simple sugars
  • Trypase – Same as pepsin
  • Lipase – Fat into fatty acids and glycerol
  • Maltase – Breaks maltose into glucose

Food tests

  • Iodine test - Brown iodine goes black if there is starch
  • Biuret test – First add sodium hydroxide NaOH to the solution and shake well. Then add pale blue copper sulphate solution. If it turns purple then there is protein
  • Benedict’s test – Add blue benedict’s solution into the test tube, bring to the boil, if an orange precipitate forms then simple sugars are present
Enzyme Amylase Protease Lipase
Made Salivary glands, pancreas and small intestine Stomach (Where it is called pepsin) pancreas and the small intestine Pancreas and small intestine
Does Starch into glucose and maltose Proteins into amino acids Fat into fatty acids and glycerol
Diagram Image:Amylase.jpg Image:Protease.jpg Image:Lipase.jpg


  • Starch
    • In bread, potatoes and muesli
  • Protein
    • In eggs, fish and meat
  • Fat
    • In butter, cooking oil and sausages


  • Bile is strongly alkaline, which means that it neutralizes the stomach acid. It also emulsifies (brakes into lots of little drops) which gives the lipase a bigger surface area (The bile salts break up the bile)
  • These smaller molecules can then be absorbed into the blood in the small intestine (using active uptake). *They then travel to where they are needed

Also See

Here are the other comprehensive GCSE Biology notes by Prometheus: