The digestive system: GCSE Biology

The digestive system is an organ system in which several organs work together to digest and absorb food. Digestive enzymes break large food molecules into small molecules that dissolve and can pass into the blood: amylase breaks down starch, proteases break down proteins and lipases break down fats. For GCSE you need the job of each organ, where each enzyme is made, and what bile does.

The organs of the digestive system

Food passes along one long tube, the gut, from mouth to anus. The liver, gall bladder and pancreas are not part of the tube, but they add juices to it.

Follow a meal through each organ in the Human Physiology course, which shows what happens at every stop.

Digestive enzymes: what they do and where they are made

Each enzyme catalyses one specific reaction, because only one kind of molecule fits its active site (the lock and key model). See enzymes as biological catalysts for how that works.

The small intestine also makes enzymes in its lining that finish the job. The products are absorbed and used to build new carbohydrates, lipids and proteins, and some glucose is used in respiration.

Match each enzyme to its food, products and where it is made in this question from the Human Physiology course.

What does bile do?

The liver makes bile, and the gall bladder keeps it until food arrives in the small intestine. Bile is not an enzyme. It has two jobs:

1. It is alkaline, so it neutralises the hydrochloric acid from the stomach.
2. It emulsifies fat, breaking it into small droplets, which increases the surface area.

Both make lipase work faster: the alkaline conditions suit it, and the larger surface area gives it more fat to act on at once.

Absorption in the small intestine

The small intestine is about 3 metres long in a living person. Its lining is covered in millions of villi, tiny finger-like projections 0.5 to 1 mm long, and the cells on them have even smaller microvilli. Together with folds in the lining, they give an absorbing surface of about 200 m², which OpenStax compares to more than 100 times the surface area of your skin. Villi have a good blood supply, so absorbed molecules are carried away quickly, keeping a steep concentration gradient.

Worked examples

Example 1: a rate of digestion

In the amylase practical (required practical 5), drops of the mixture are tested with iodine every 30 seconds. Iodine turns blue-black while starch is present and stays brown once it has all been digested. At pH 7 the starch is all gone after 150 seconds. Calculate the rate as 1/time.

• Rate = 1 ÷ 150 = 0.0067 s⁻¹ (to 2 significant figures). A shorter time means a faster rate.

Example 2: comparing two pH values

At pH 5 the same digestion takes 240 seconds. How many times faster is the reaction at pH 7?

1. Rate at pH 5 = 1 ÷ 240 = 0.0042 s⁻¹.
2. Rate at pH 7 ÷ rate at pH 5 = 240 ÷ 150 = 1.6 times faster at pH 7.
3. pH changes the shape of the active site, so amylase works faster at some pH values than others.

Common mistakes

• Calling bile an enzyme. It emulsifies fat and neutralises acid, but breaks no bonds.
• Saying bile is made in the gall bladder. The liver makes it; the gall bladder only holds it.
• Saying lipase makes glycerol and amino acids. Lipase gives glycerol and fatty acids; proteases give amino acids.
• Forgetting the pancreas. It makes all three types of enzyme.
• Saying the large intestine absorbs food. It mainly absorbs water; food molecules are absorbed in the small intestine.

Practise with a short timed paper

The digestive system is in the Organisation topic of AQA GCSE Biology and Combined Science. The daily GCSE Biology Higher and GCSE Combined Science Higher papers are short timed papers of 8 questions, with a new one every day.

Read next

Biology · Biological moleculesBiological catalysts are enzymes: how enzymes workBiological catalysts are called enzymes. How enzymes work, the active site and lock and key model, temperature, pH and denaturation, and digestive enzymes.