EnzymesIB MYP Biology: Revision notes
Section 1
What are enzymes?
Enzymes are biological catalysts: they speed up chemical reactions in living things without being used up. They are made of protein.
Enzymes are specific. Each one has a region called the active site with a particular shape. The molecule it works on, the substrate, has a complementary shape and fits into the active site, forming an enzyme-substrate complex. Products are released and the enzyme can be used again. This is the lock-and-key model.
Enzymes are not used up in a reaction, and they are not living. They are protein molecules.
Section 2
Digestive enzymes
Digestive enzymes break large insoluble molecules into small soluble ones:
- Amylase: starch to maltose
- Protease: protein to amino acids
- Lipase: lipids (fats) to fatty acids and glycerol
Because enzymes are specific, amylase cannot digest protein and protease cannot digest fat.
Section 3
Effect of temperature
As temperature rises from low values, enzyme and substrate particles move faster and collide more often, so the rate of reaction increases. Every enzyme has an optimum temperature; for human enzymes this is about 37 °C.
Above the optimum the rate drops quickly. High temperatures change the shape of the active site: the enzyme is denatured. The substrate no longer fits and the reaction stops. Denaturation is permanent, so cooling the enzyme does not bring it back.
Section 4
Effect of pH
Each enzyme also has an optimum pH. Pepsin in the stomach works best at about pH 2 (acid), while most enzymes in the small intestine work best at about pH 8 (slightly alkaline). Amylase in saliva works best close to pH 7.
Values of pH far from the optimum change the shape of the active site and denature the enzyme.
In a question about denaturation always mention the active site changing shape and the substrate no longer fitting.
Section 5
Investigating the rate of enzyme activity
A typical investigation uses amylase and starch. Samples of the mixture are tested with iodine at timed intervals; the iodine turns blue-black while starch is present and stays orange-brown when all the starch is digested.
- Independent variable: the one you change, e.g. temperature or pH (using buffers).
- Dependent variable: the one you measure, e.g. the time for the starch to disappear.
- Control variables: keep constant, e.g. volume and concentration of starch and enzyme.
The rate can be compared using : a shorter time means a faster rate. Use a water bath to control temperature, wear eye protection and repeat to calculate a mean.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Enzymes
- A patient in Dubai whose pancreas is damaged cannot make enough of its digestive enzymes. Her doctor prescribes capsules containing the enzyme lipase to be taken with every meal.Explain why lipase can digest fat but cannot digest protein.2 marks
- A student in Seoul investigates how temperature affects the activity of the enzyme amylase. She mixes 5 cm³ of starch solution with 2 cm³ of amylase solution in a test tube kept in a water bath. Every 30 seconds she puts a drop of the mixture into iodine solution on a spotting tile and records the time at which the iodine stops turning blue-black. She repeats this at 20 °C, 30 °C, 40 °C, 50 °C and 60 °C. The times taken were 240 s, 120 s, 60 s and 90 s for the first four temperatures. At 60 °C the iodine was still turning blue-black after 600 s.Describe the trend in the results between 20 °C and 40 °C.2 marks
- The enzyme pepsin, a protease made in the human stomach, works best at about pH 2. Another protease made by the pancreas works in the small intestine, where the pH is about 8.Explain why pepsin would not digest protein well if it were carried into the small intestine.3 marks
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).