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Enzymes and chemical digestionIB MYP Sciences: Revision notes

Section 1

What are enzymes?

Enzymes are proteins that act as biological catalysts: they speed up chemical reactions in living things without being used up. Without enzymes, digestion would be far too slow to keep us alive.

Digestive enzymes break down large insoluble food molecules into small soluble ones that can be absorbed.

Key termsenzymecatalyst

Section 2

The lock and key model

Each enzyme has a special-shaped region called the active site. The molecule it works on, the substrate, fits the active site like a key fits a lock. The enzyme and substrate join, the reaction happens and the products leave. The enzyme is unchanged and can be used again.

Because the shape of the active site only fits one kind of substrate, enzymes are specific: amylase cannot digest protein, and protease cannot digest fat.

Key termsactive sitesubstratespecific

Section 3

The digestive enzymes

  • Amylase breaks down starch into maltose (and then glucose). It is made in the salivary glands (acts in the mouth) and the pancreas (acts in the small intestine).
  • Protease breaks down protein into amino acids. It is made in the stomach (pepsin), the pancreas and the small intestine.
  • Lipase breaks down fats into fatty acids and glycerol. It is made in the pancreas and acts in the small intestine.
Key termsamylaseproteaselipase
Exam tip

Link the enzyme name to the substrate and the products: amylase (starch), protease (protein), lipase (lipids).

Section 4

Temperature and pH

Enzymes work best at an optimum temperature and pH.

  • As temperature rises, enzyme and substrate move faster and collide more often, so the rate increases up to the optimum (about 37 °C for human enzymes).
  • Above the optimum the enzyme's shape changes and the active site no longer fits the substrate. The enzyme is denatured, and this cannot be reversed.
  • Each enzyme has an optimum pH. Pepsin works best in acid (about pH 2) in the stomach, but amylase works best at about pH 7. Too far from the optimum pH also denatures an enzyme.
Key termsoptimumdenatured
Common mistake

Enzymes are not 'killed' by high temperatures, because they were never alive. They are denatured.

Section 5

Investigating enzyme activity (criteria B and C)

A typical investigation uses amylase and starch, tested with iodine.

  1. Change one independent variable (temperature or pH).
  2. Measure the time taken for the starch to disappear, by testing drops with iodine at regular intervals until it stops turning blue-black.
  3. Keep other variables the same (volumes, concentrations, temperature or pH).
  4. Repeat and calculate a mean.

A shorter time means a faster reaction. When processing the results, describe the trend, explain it using ideas about collisions and denaturing, and evaluate how reliable the method was.

Key termsindependent variabledependent variablecontrol variable

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Enzymes and chemical digestion

  1. A manufacturer in Gothenburg sells a biological washing powder that contains the enzymes protease and lipase. The powder removes food stains from clothes in water at 40 °C, but it works poorly in boiling water.
    Explain why the protease in the powder does not break down grease stains.2 marks
  2. Pepsin is a protease made in the stomach, where hydrochloric acid gives the gastric juice a pH of about 2. Amylase in saliva works best at about pH 7. Swallowed food, mixed with saliva, passes from the mouth into the stomach and then into the small intestine, where bile and pancreatic juice raise the pH to about 8.
    Explain why pepsin stops working when it passes into the small intestine.2 marks
  3. A student in Johannesburg mixes amylase solution with starch solution at five different temperatures. At each temperature she tests a drop of the mixture with iodine solution every 15 seconds, until the drop no longer turns blue-black. At 10 °C this took 300 seconds, at 20 °C it took 150 seconds, at 30 °C 80 seconds and at 40 °C 45 seconds. At 60 °C the drop still turned blue-black after 600 seconds.
    Describe the results of the investigation.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).