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AQA BiologyOrganisation

Digestive system and enzymes

Explain digestion and interpret enzyme experiments.

Start here

The key idea

Enzymes are biological catalysts.Digestive enzymes break large insoluble molecules into smaller soluble molecules that can be absorbed.

Enzyme action in digestionThe active site fits one substrate, then releases smaller products.
Enzyme action in digestionThe active site fits one substrate, then releases smaller products.enzymesubstratesoluble products
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

Enzymes as biological catalysts

Enzymes are large protein molecules that speed up the rate of chemical reactions in the body without being used up.Each enzyme has a specific active site whose shape is complementary to one type of substrate, described by the lock-and-key model.Digestive enzymes break large insoluble food molecules into small soluble molecules that can be absorbed into the blood.

2

Effect of temperature and pH

As temperature increases the rate of an enzyme reaction increases because molecules collide more often, until the optimum temperature is reached.Above the optimum the enzyme denatures: the active site changes shape so the substrate no longer fits.Each enzyme also has an optimum pH; extremes of pH also denature the enzyme by changing the shape of the active site.

3

Digestive enzymes

Carbohydrases such as amylase break down carbohydrates (starch) into simple sugars; amylase is made in the salivary glands, pancreas and small intestine.Proteases break down proteins into amino acids and are made in the stomach, pancreas and small intestine.Lipases break down lipids (fats) into fatty acids and glycerol and are made in the pancreas and small intestine.

4

Bile and the role of the liver

Bile is produced in the liver, stored in the gall bladder and released into the small intestine.Bile is alkaline so it neutralises the acid that leaves the stomach, creating the alkaline conditions enzymes in the small intestine need.Bile also emulsifies fats, breaking large fat droplets into smaller ones to increase the surface area for lipase to act on.

Key terms

Definitions to learn

Enzyme

A protein that acts as a biological catalyst, speeding up reactions without being used up.

Active site

The region of an enzyme where the substrate binds; its shape is specific to the substrate.

Denatured

When an enzyme's active site changes shape so the substrate no longer fits, stopping the reaction.

Substrate

The molecule that an enzyme acts on.

Bile

An alkaline liquid made in the liver that neutralises stomach acid and emulsifies fats.

Emulsify

To break large fat droplets into smaller droplets, increasing surface area for digestion.

Worked example

Explain why amylase activity falls sharply above its optimum temperature.

1

Higher temperature initially increases collisions.

2

Above the optimum, bonds holding the enzyme shape break.

3

The active site changes shape, so the substrate no longer fits.

Final answer

The enzyme denatures and fewer enzyme-substrate complexes form.

Exam habit

Name the specific enzyme and its substrate. 'Amylase breaks down starch into maltose' scores full marks.'It breaks down food' scores nothing. Say 'active site changes shape' not 'enzyme dies'.

Watch out

Do not say the enzyme dies. State that its active site changes shape.

Examiner tips

How to score full marks

  • 1Say the enzyme is 'denatured', not 'killed' — enzymes are not alive.
  • 2When explaining bile and fat, use 'increases surface area for lipase / for the enzyme to work on' — emulsification does not chemically digest fat itself.
  • 3For the lock-and-key model, always link the substrate shape being COMPLEMENTARY to the active site.
Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Where is bile made and what does it do?
Mark scheme
  1. 1.Name its source.
  2. 2.Give both digestive roles.
Bile is made in the liver. It neutralises stomach acid and emulsifies fat.
2Why does emulsifying fat speed up digestion?
Mark scheme
  1. 1.Link droplet size to surface area.
It forms smaller droplets, increasing surface area for lipase action.
3A student tests amylase at different pH values. Name one control variable.
Mark scheme
  1. 1.Keep one factor other than pH constant.
Any suitable control, such as temperature, enzyme concentration or starch concentration.
4Name the products formed when lipase breaks down a lipid.[2 marks]
Mark scheme
  1. 1.Recall the two products of fat digestion.
Fatty acids (1) and glycerol (1).
5State where bile is made and where it is stored.[2 marks]
Mark scheme
  1. 1.Recall the two organs involved.
Made in the liver (1); stored in the gall bladder (1).
6Explain why a high temperature stops an enzyme working.[3 marks]
Mark scheme
  1. 1.Link the temperature to the shape of the active site.
At high temperature the enzyme denatures (1); the active site changes shape (1) so the substrate no longer fits / can no longer bind (1).
7Describe how bile helps the digestion of fats in the small intestine.[4 marks]
Mark scheme
  1. 1.Consider both the pH change and the effect on fat droplets.
Bile is alkaline so it neutralises the acid from the stomach (1), providing the optimum/alkaline pH for enzymes in the small intestine (1); it emulsifies fats, breaking large droplets into smaller ones (1), which increases the surface area for lipase to act on, speeding up digestion (1).
8A student measured how quickly amylase digested starch at pH values from 2 to 10, finding the fastest reaction at pH 7. Explain these results.[5 marks]
Mark scheme
  1. 1.Relate the optimum pH to the active site, then explain the slower rates at extremes.
Amylase has an optimum pH of 7 (1), at which the active site is the correct shape so the substrate binds and reaction rate is fastest (1). At pH values far from 7 (e.g. 2 or 10) the enzyme is denatured (1) because the active site changes shape so starch no longer fits (1), so little or no reaction occurs (1).
9State the name of the substrate for amylase and the product it produces.[2 marks]
Mark scheme
  1. 1.Recall the carbohydrase substrate and product.
Substrate: starch (1); product: (maltose /) simple sugars (1).
10Explain why digestive enzymes must break large food molecules into smaller ones before they can be absorbed into the blood.[3 marks]
Mark scheme
  1. 1.Link molecule size to the ability to cross the intestinal wall.
Large molecules such as starch and proteins are insoluble and too large to pass through the wall of the small intestine (1); enzymes break them down into small soluble molecules such as glucose and amino acids (1); which can then diffuse or be absorbed through the intestinal lining into the bloodstream (1).
11Describe the lock-and-key model of enzyme action.[4 marks]
Mark scheme
  1. 1.Describe the active site and how the substrate fits.
  2. 2.Explain what this means for specificity.
Each enzyme has an active site with a specific shape (1); the substrate has a complementary shape to the active site and fits into it like a key in a lock (1); forming an enzyme-substrate complex (1); this specificity means each enzyme only catalyses one type of reaction (1).
12A student designs an experiment to investigate the effect of pH on protease activity, using egg white protein as the substrate. Describe how the student should make the experiment a fair test.[4 marks]
Mark scheme
  1. 1.Identify the independent variable.
  2. 2.List the variables that must be controlled.
The independent variable is pH (1); the student should keep temperature constant (1); keep the concentration and volume of protease and substrate constant (1); measure the same dependent variable each time, e.g. time for protein to break down or change in turbidity (1).
13Proteases in the stomach work at pH 2, but proteases in the small intestine work best at pH 8. Explain the significance of this difference.[4 marks]
Mark scheme
  1. 1.Link pH to active site shape for each enzyme.
  2. 2.Connect to the conditions in each organ.
Each protease has an active site shaped for a different optimum pH (1); the stomach protease (pepsin) works at pH 2 because the stomach produces hydrochloric acid (1); the small intestine protease works at pH 8 because bile has neutralised the acid (1); using different enzymes for different pH environments ensures digestion can continue along the digestive system (1).
14Evaluate the use of biological washing powders that contain protease and lipase enzymes, compared to non-biological powders.[5 marks]
Mark scheme
  1. 1.Give advantages linked to enzyme action.
  2. 2.Give disadvantages, including temperature and allergic reactions.
  3. 3.Draw a conclusion.
Biological powders contain proteases and lipases that break down protein and fat stains specifically (1); they work effectively at lower temperatures, saving energy (1); however they may cause skin irritation or allergic reactions in some people (1); enzymes are denatured at high temperatures so they do not work in hot washes needed for some fabrics (1); overall biological powders are more effective for most everyday stains but may not be suitable for everyone (1).
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