The shape of the lesson
By the end of the lesson, learners can
- Temperature and the optimum: Mix amylase with starch in a tube in a water bath.
- pH: every enzyme has its own optimum: Choose pepsin, amylase or trypsin and drag the pH slider (or the dot on the graph) from pH 1 to pH 12.
- Below the optimum (Extended): more kinetic energy, more frequent effective collisions, more enzyme-substrate complexes.
- Above the optimum: active site changes shape, substrate no longer fits, enzyme denatured; permanent.
- pH: each enzyme has its own optimum; on either side, shape and fit get worse; far away, denatured.
- Rate = 1 ÷ time taken. Low temperature slows an enzyme but does not denature it.
One picture every idea returns to
Enzyme activity rises with temperature up to an optimum temperature, then falls quickly as the enzyme is denatured: the shape of its active site changes, so the substrate no longer fits. Each enzyme also has an optimum pH; on either side of it activity falls, and far from it the enzyme is denatured. Denaturation is permanent.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Warm helps, hot destroys.. Warm an enzyme and it works faster, up to a point. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Temperature and the optimum | Mix amylase with starch in a tube in a water bath. Project tab 3 of the lab (Amylase and starch at different temperatures) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: pH: every enzyme has its own optimum | Choose pepsin, amylase or trypsin and drag the pH slider (or the dot on the graph) from pH 1 to pH 12. Project tab 4 of the lab (Effect of pH on pepsin, amylase and trypsin) and let a student drive it while the class predicts. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 22 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in pH: every enzyme has its own optimum, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Low temperature does not denature an enzyme. | At 10 °C the molecules simply move slowly and collide less often; warm it up and it works again. Only high temperatures (and extreme pH) denature. Also avoid saying the enzyme is "killed": enzymes are molecules, not living things. |
| The optimum pH is not always 7. | Pepsin works best in acid and is denatured at pH 7. Read the optimum off the graph for the enzyme in the question. |
Temperature, pH and enzyme activity: worksheet
6 questions, 22 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by the optimum temperature of an enzyme and what is meant by denaturation.
Plan an investigation to find the effect of temperature on the activity of amylase, using starch solution, iodine solution and a spotting tile.
A student found that the starch disappeared after 5 minutes at 25 °C, 2 minutes at 40 °C and 10 minutes at 55 °C. Calculate the rate at each temperature in min−1, and state which temperature was nearest the optimum.
Extended. Explain the shape of a graph of the rate of an enzyme-controlled reaction against temperature, from 10 °C to 70 °C, with an optimum at 40 °C.
The table shows the activity of two digestive enzymes, X and Y, at different pH values (as a percentage of each enzyme's maximum).
| pH | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|
| Enzyme X / % | 100 | 15 | 0 | 0 | 0 |
| Enzyme Y / % | 0 | 2 | 45 | 100 | 40 |
(a) State the optimum pH of each enzyme. (b) Suggest where in the alimentary canal each enzyme works, with a reason. (c) Explain why enzyme X has no activity at pH 8.
Extended. A tube of amylase and starch at 10 °C digests the starch slowly. A second tube is heated to 70 °C and digests almost no starch. Both are then moved to 37 °C. Predict and explain what happens in each tube.
Answer key and marking notes
- Optimum temperature: the temperature at which the enzyme works fastest (the rate of reaction is highest).
- Denaturation: a permanent change in the shape of the enzyme and its active site, so the substrate no longer fits and the enzyme stops working.
- Put drops of iodine solution into the wells of a spotting tile.
- Place separate tubes of starch solution and amylase in a water bath at the first temperature (for example 10 °C) until they reach that temperature, then mix them and start a stop-clock.
- At regular intervals (for example every 30 s) put one drop of the mixture into a fresh well of iodine; record the time when the iodine stays orange-brown (no starch left).
- Repeat at other temperatures, for example 20, 30, 40, 50 and 60 °C, and calculate rate = 1 ÷ time.
- Control: volume and concentration of starch and amylase, pH (use a buffer); repeat each temperature and calculate a mean.
- 25 °C: 1 ÷ 5 = 0.20 min−1; 40 °C: 1 ÷ 2 = 0.50 min−1.
- 55 °C: 1 ÷ 10 = 0.10 min−1.
- 40 °C gave the highest rate, so it was nearest the optimum.
- From 10 °C to 40 °C the rate increases.
- Enzyme and substrate molecules gain kinetic energy and move faster, so there are more frequent effective collisions and more enzyme-substrate complexes form.
- The rate is highest at the optimum temperature, 40 °C.
- Above 40 °C bonds holding the enzyme's shape break, so the active site changes shape and is no longer complementary to the substrate.
- The substrate no longer fits, fewer effective collisions occur, and the rate falls steeply until the enzyme is fully denatured and the rate is zero.
- (a) X: pH 2. Y: pH 8.
- (b) X works in the stomach, which is acidic (hydrochloric acid); Y works in the small intestine (duodenum), which is slightly alkaline.
- (c) At pH 8 the active site of X has changed shape: the enzyme is denatured.
- So its substrate no longer fits and no enzyme-substrate complexes form.
- The tube from 10 °C now digests starch quickly: at 10 °C the enzyme was not denatured, its molecules just had little kinetic energy and few effective collisions.
- At 37 °C the molecules have more kinetic energy, so effective collisions are more frequent.
- The tube from 70 °C still digests no starch: the amylase was denatured, and the change in the shape of the active site is permanent, so starch cannot fit.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
In the starch and amylase investigation, a drop of the mixture is added to iodine solution. Which result shows that starch is still present?
- AThe iodine turns blue-black
- BThe iodine stays orange-brown
- CThe iodine turns brick-red
- DThe iodine turns purple
At 35 °C all the starch is digested by amylase after 2.5 minutes. What is the rate of reaction?
- A0.40 min−1
- B2.5 min−1
- C0.25 min−1
- D4.0 min−1
The graph shows how the rate of two digestive enzymes, A and B, changes with pH. Which statement is correct?
- AA could work in the stomach and B in the small intestine
- BA and B both have their optimum at pH 7
- CB works fastest in acid and A in alkali
- DA is denatured at pH 2 and B at pH 8
A student investigates the effect of temperature on the rate at which amylase digests starch. Which variable must be kept the same at every temperature?
- AThe pH of the mixture
- BThe temperature of the water bath
- CThe time taken for the starch to disappear
- DThe colour of the iodine after each test
An enzyme is boiled and then cooled to its optimum temperature. It no longer works. Which word describes what has happened to it?
- Adenatured
- Binhibited by the cold
- Cused up
- Ddigested
(Extended) An enzyme reaction is faster at 30 °C than at 15 °C. Which explanation is correct?
- AMolecules have more kinetic energy, so effective collisions are more frequent
- BThe active site is denatured at 15 °C and recovers at 30 °C
- CMore enzyme molecules are produced when the temperature rises
- DThe substrate becomes complementary to the active site only when warm
(Extended) Above the optimum temperature the rate of an enzyme reaction falls steeply even though molecules are moving faster. Why?
- AThe active site changes shape so the substrate no longer fits
- BThe molecules move too fast for the substrate to stay bound
- CThe substrate is broken down by the heat before it binds
- DThe enzyme runs out of energy at high temperature
(Extended) Trypsin works in the small intestine with an optimum pH of about 8. Some trypsin is placed in a solution at pH 2 and then returned to pH 8, but it digests very little protein. What is the best explanation?
- AAt pH 2 its active site changed shape permanently, so protein no longer fits
- BAt pH 2 its molecules lost kinetic energy, so they collide less at pH 8
- CAt pH 2 the protein substrate was denatured, so it no longer needs digesting
- DAt pH 2 trypsin was used up, so less of it is left to work at pH 8
Quiz key
Iodine solution turns from orange-brown to blue-black when starch is present. If it stays orange-brown, all the starch has been digested. Brick-red is a positive Benedict's test for reducing sugar, and purple is a positive biuret test for protein.
Rate = 1 ÷ 2.5 = 0.40 min−1. Writing 2.5 just repeats the time; 0.25 is 1 ÷ 4; and 4.0 confuses 1 ÷ 0.25 with 1 ÷ 2.5.
A peaks at pH 2, which suits the acidic stomach, and B peaks at pH 8, which suits the slightly alkaline small intestine. Neither has its optimum at pH 7. B works best in alkali, not acid. pH 2 and pH 8 are where each enzyme works fastest, so they cannot be where it is denatured.
pH affects enzyme activity, so it must be controlled (for example with a buffer). The water bath temperature is the independent variable, the time taken is the dependent variable, and the iodine colour is how the result is observed.
Boiling denatures the enzyme: its shape, including the active site, changes permanently, so the substrate no longer fits even after cooling. Cooling itself does not stop an enzyme permanently, enzymes are not used up in reactions, and boiling does not digest the enzyme.
Raising the temperature below the optimum gives enzyme and substrate molecules more kinetic energy, so they move faster and collide more often, forming more enzyme-substrate complexes per second. Low temperature does not denature enzymes, a warmer test tube does not make new enzyme, and the substrate's shape fits the active site at both temperatures.
Above the optimum, bonds holding the enzyme's shape break. The active site is no longer complementary to the substrate, so fewer effective collisions form enzyme-substrate complexes and the enzyme is denatured. Speed of movement on its own would increase the rate; the substrate is not destroyed by heat; and enzymes do not use up an energy supply.
Far from its optimum, the extra H+ ions alter the bonds that hold trypsin's shape, so the active site changes and the enzyme is denatured; the change is permanent, so protein still does not fit at pH 8. pH does not change kinetic energy, any change to the substrate is not why the enzyme fails, and enzymes are not used up.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.