Hydrogen peroxide breaks down so slowly on its own that you can hardly see it happen. Add a scrap of raw potato and the cylinder foams over. Enzymes are the proteins that make every reaction in a cell fast enough to keep the organism alive, and each one works because of the shape of its active site.
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.
Fig. 1: the substrate fits the complementary active site, an enzyme-substrate complex forms, then the products leave and the unchanged enzyme can work again
Section 01
What a catalyst does.
Pour hydrogen peroxide with a little washing-up liquid into a measuring cylinder. It breaks down into water and oxygen, but so slowly that almost nothing happens. Now add manganese(IV) oxide, raw potato or boiled potato and watch the oxygen foam rise.
Added
Foam
Afterwards
nothing
almost none
reaction far too slow to see
manganese(IV) oxide
fast, up to about 100 cm3
unchanged: a non-biological catalyst
raw potato
fast, up to about 95 cm3
unchanged: contains the enzyme catalase
boiled potato
none
catalase denatured by boiling
Catalyst: a substance that increases the rate of a chemical reaction and is not changed by the reaction. Pour off the foam, add fresh peroxide and the same catalyst works again.
Enzymes: proteins that act as biological catalysts. They are involved in all metabolic reactions, building molecules up (synthesis) and breaking them down (digestion, respiration).
Why life needs them: at body temperature most reactions in cells would be far too slow to sustain life. Enzymes raise the reaction rate to the level the organism needs.
Examiner trap
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.
Section 02
Shape is everything: the active site.
Bring the right substrate up to the enzyme, then try a molecule of a different shape. Watch the stage readout change as the substrate binds, reacts and leaves as products.
Active site: a small region of the enzyme with a particular shape. Its shape is complementary to the shape of the substrate, so the substrate fits into it.
Enzyme action: the substrate fits into the active site, the reaction happens and the products are formed and released. The enzyme is unchanged.
Extended: enzyme-substrate complex. When the substrate is held in the active site the two together form an enzyme-substrate complex. The reaction takes place in the complex, the products leave and the active site is free for another substrate molecule.
Extended: specificity. Only a substrate with a complementary shape fits and binds to the active site. A molecule of a different shape does not fit, so no complex forms and nothing happens. Each enzyme catalyses one reaction (or a few very similar ones): amylase digests starch but not protein.
Examiner trap
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.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the catalysts tab, which substance gives no foam at all when added to hydrogen peroxide?
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After the foam has risen, the sim says the raw potato is "unchanged, still works". What does this show?
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In the active site tab, what does the stage readout say while the substrate is held in the active site?
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When you bring in "a different molecule", why does no reaction happen?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
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Which statement describes a catalyst?
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Enzymes are best described as:
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Extended: an enzyme acts on one substrate only. The best explanation is that:
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Why do living organisms need enzymes?
Skill unlocked
Enzymes as catalysts, mastered.
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