The shape of the lesson
By the end of the lesson, learners can
- What a catalyst does: Pour hydrogen peroxide with a little washing-up liquid into a measuring cylinder.
- Shape is everything: the active site: Bring the right substrate up to the enzyme, then try a molecule of a different shape.
- Catalyst: increases the rate of a reaction and is not changed by the reaction.
- Enzyme: a protein that acts as a biological catalyst in all metabolic reactions.
- Active site: shape complementary to the substrate; say complementary, not the same.
- Extended: substrate + enzyme → enzyme-substrate complex → products + unchanged enzyme.
One picture every idea returns to
A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. Enzymes are proteins that are involved in all metabolic reactions, where they act as biological catalysts. Each enzyme has an active site whose shape is complementary to its substrate; the substrate fits, reacts and leaves as products, and the enzyme is free to be used again.
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, Speeding up life, unchanged.. Hydrogen peroxide breaks down so slowly on its own that you can hardly see it happen. 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: What a catalyst does | Pour hydrogen peroxide with a little washing-up liquid into a measuring cylinder. Project tab 1 of the lab (Hydrogen peroxide with different catalysts) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Shape is everything: the active site | Bring the right substrate up to the enzyme, then try a molecule of a different shape. Project tab 2 of the lab (Substrate binding to the active site) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 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, 19 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 Shape is everything: the active site, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| An enzyme is not used up and is not a reactant. | Do not write that the enzyme "is broken down" or "becomes the product": it leaves the reaction unchanged and catalyses the next one. |
| The active site is complementary to the substrate, not "the same shape" as it. | Say complementary: the two shapes fit together like a key in a lock. |
Enzymes as biological catalysts: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
Define a catalyst and describe what an enzyme is.
Explain why enzymes are important to all living organisms.
Extended. Explain how an enzyme molecule catalyses the breakdown of its substrate. Use the terms active site, enzyme-substrate complex and product.
A student added equal masses of four materials to 20 cm3 of hydrogen peroxide with washing-up liquid and measured the height of the oxygen foam after 2 minutes.
| Material added | Foam after 2 min / cm3 |
|---|---|
| none | 1 |
| manganese(IV) oxide | 96 |
| raw liver | 88 |
| boiled liver | 1 |
(a) Which result is the control, and why is it needed? (b) Explain the results for raw and boiled liver.
Extended. Amylase digests starch but has no effect on protein. Explain why.
After the foam stopped rising, the student poured away the liquid and added fresh hydrogen peroxide to the same piece of manganese(IV) oxide and the same piece of raw liver. Predict what happened and explain how this supports the idea that both are catalysts.
Answer key and marking notes
- A catalyst is a substance that increases the rate of a chemical reaction.
- It is not changed by the reaction (so it can be used again).
- An enzyme is a protein that functions as a biological catalyst and is involved in all metabolic reactions.
- Metabolic reactions such as respiration and digestion would happen far too slowly at body temperature without a catalyst.
- Enzymes speed these reactions up to a rate that is fast enough to sustain life.
- The substrate has a shape complementary to the active site of the enzyme, so it fits into the active site.
- The enzyme and substrate together form an enzyme-substrate complex.
- The reaction takes place and the substrate is broken down into products.
- The products leave the active site; the enzyme is unchanged and can bind another substrate molecule.
- (a) The tube with nothing added is the control; it shows that hydrogen peroxide breaks down only very slowly on its own, so any extra foam is caused by the material added.
- (b) Raw liver contains the enzyme catalase, which catalyses the breakdown of hydrogen peroxide into water and oxygen, so a lot of foam forms.
- Boiling denatures the catalase: its active site changes shape.
- So the substrate no longer fits and the boiled liver gives no more foam than the control.
- Enzymes are specific.
- The active site of amylase has a shape that is complementary to starch, so starch can fit and form an enzyme-substrate complex.
- Protein molecules have a different shape that does not fit the active site, so no complex forms and no reaction is catalysed.
- Both produced a lot of foam again.
- The manganese(IV) oxide and the catalase in the liver were not used up or changed by the first reaction.
- A catalyst increases the rate of a reaction without being changed, so it can be used again and again.
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.
Which statement about a catalyst is correct?
- AIt speeds up a reaction and is not changed at the end
- BIt speeds up a reaction and is used up as it works
- CIt is changed into the products of the reaction
- DIt only works on reactions inside living cells
What type of molecule is an enzyme?
- Aprotein
- Bcarbohydrate
- Cfat (lipid)
- Dmineral ion
Raw potato is added to 10 cm3 of hydrogen peroxide and foam rises for 4 minutes, then stops. When fresh hydrogen peroxide is poured onto the same potato, foam forms again. Why did the foam stop the first time?
- AAll the hydrogen peroxide had been broken down
- BAll the catalase had been used up in the reaction
- CThe catalase had been denatured by the oxygen
- DThe potato had been changed into water and oxygen
Which statement describes the role of enzymes in living organisms?
- AThey are involved in all metabolic reactions as biological catalysts
- BThey are only involved in digesting food in the gut
- CThey provide the energy needed for every reaction in cells
- DThey are only found in animal cells, not plant cells
Without enzymes, the reactions of respiration would still happen at body temperature, but extremely slowly. What would be the result for the organism?
- AEnergy would not be released fast enough to sustain life
- BThe organism would overheat from too much energy
- CThe products would build up and become toxic quickly
- DThe organism would respire faster to make up for it
(Extended) The diagram shows an enzyme and its substrate before and after binding. Which row correctly names P, Q and R?
- AP substrate; Q active site; R enzyme-substrate complex
- BP active site; Q substrate; R enzyme-substrate complex
- CP product; Q active site; R enzyme-product complex
- DP substrate; Q product; R denatured enzyme
(Extended) Catalase breaks down hydrogen peroxide but has no effect on starch. Which explanation is correct?
- AStarch is not complementary in shape to catalase's active site
- BStarch is too large a molecule to collide with catalase
- CCatalase is denatured as soon as it touches starch
- DCatalase has no active site until peroxide is present
(Extended) One molecule of catalase can break down millions of hydrogen peroxide molecules a minute. Which feature of enzyme action makes this possible?
- AProducts leave the active site, so it is free to bind another substrate
- BEach enzyme molecule splits into new enzyme molecules after a reaction
- CThe active site changes shape to fit a new substrate each time
- DThe enzyme releases energy that breaks substrates without binding them
Quiz key
A catalyst increases the rate of a chemical reaction and is not changed by it, so it can be used again. It is not used up and does not become a product. Many catalysts, such as manganese(IV) oxide, are not biological, so they work outside cells too.
Enzymes are proteins, which is why heat and extreme pH can change their shape. Carbohydrates such as starch and fats are often the substrates of enzymes, not enzymes themselves, and mineral ions are not large enough to have an active site.
The substrate (hydrogen peroxide) ran out. The enzyme catalase was not used up, which is why the same potato works again with fresh peroxide. Oxygen does not denature catalase, and the potato is a catalyst, so it is not turned into products.
Enzymes catalyse all metabolic reactions, including respiration, photosynthesis and protein synthesis, in every kind of organism. Digestion is only one example. Enzymes do not supply energy; they speed up reactions. Plant cells, such as potato cells with catalase, contain enzymes too.
Enzymes raise reaction rates to the level needed to sustain life. Without them, respiration would release energy far too slowly for the cell's needs. Slow reactions do not cause overheating or a rapid build-up of products, and the organism cannot speed up respiration without its enzymes.
P is the substrate that fits into Q, the active site (the region of the enzyme with a complementary shape). Together they form R, the enzyme-substrate complex. Products only form after the reaction in the complex, and nothing in the diagram is denatured: the substrate fits the active site exactly.
Enzyme specificity depends on the shape and fit of the active site: only a substrate with a complementary shape can bind and form an enzyme-substrate complex. Starch can collide with catalase but does not fit. Starch does not denature catalase, and the active site is always present.
After the reaction in the enzyme-substrate complex, the products are released and the enzyme is unchanged, so the active site can immediately bind another substrate. Enzymes do not divide, do not change their active site to fit different substrates, and must bind the substrate to catalyse the reaction.
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.