Catalysts in living things — answer key

1. CQ1 · Practice

Model answer. c. A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction.

A full answer includes

• I chose c.

Also accept

• c

Watch for

• “A catalyst is used up as the reaction goes on.” — A catalyst is not changed by the reaction, so it can be used again.
• “A catalyst makes a reaction happen that could not happen at all.” — A catalyst speeds up a reaction that could already happen, only very slowly.

2. CQ2 · Practice

Model answer. Enzymes are proteins. They work as biological catalysts, and they are involved in all metabolic reactions: the chemical reactions that build up and break down substances in living things.

A full answer includes

• g1: proteins.
• g2: catalysts.
• g3: metabolic (all the chemical reactions of the organism).

Also accept

• proteins; catalysts; metabolic

Watch for

• “Enzymes are living things.” — Enzymes are protein molecules made by cells; they are not alive.

3. CQ3 · Practice

Options as numbered on the pupil sheet: 1 — The part of the enzyme with a shape the substrate fits into; 2 — The molecule the enzyme acts on; 3 — The enzyme with the substrate held in its active site; 4 — A new molecule made by the reaction.

Model answer. Active site: the part of the enzyme whose shape is complementary to the substrate. Substrate: the molecule the enzyme acts on. Enzyme–substrate complex: the enzyme and substrate fitted together. Product: a molecule made by the reaction, which then leaves the active site.

A full answer includes

• Active site: the part of the enzyme the substrate fits into.
• Substrate: the molecule the enzyme acts on.
• Enzyme–substrate complex: the enzyme with the substrate held in its active site.
• Product: a new molecule made by the reaction.

Also accept

• active site: part of the enzyme; substrate: molecule acted on; complex: fitted together; product: new molecule made

Watch for

• “The active site is part of the substrate.” — The active site is a part of the enzyme; the substrate fits into it.

4. CQ4 · Practice

Options as numbered on the pupil sheet: 1 — The substrate collides with the enzyme at its active site; 2 — The substrate fits into the active site; 3 — An enzyme–substrate complex forms; 4 — The substrate is changed into products; 5 — The products leave and the enzyme is unchanged.

Model answer. The substrate collides with the enzyme at its active site; it fits because the shapes are complementary; an enzyme–substrate complex forms; the substrate is changed into products; the products leave the active site and the enzyme, unchanged, can be used again.

A full answer includes

• Substrate collides with the active site → substrate fits the active site → enzyme–substrate complex forms → substrate is changed into products → products leave; enzyme unchanged.

Also accept

• collide, fit, complex, products formed, products leave

Watch for

• “The enzyme changes into the products.” — The substrate changes into the products; the enzyme comes out unchanged.

5. CQ5 · Practice

Model answer. Amylase is a catalyst: it is not changed by the reaction. When maltose leaves the active site, the same amylase molecule is free to bind another part of a starch chain. Each molecule is used over and over again, so a small amount of amylase breaks down a large amount of starch.

A full answer includes

• Amylase is a catalyst, so it is not changed by the reaction.
• After the products (maltose) leave the active site, the same amylase molecule can take another starch molecule.
• So each amylase molecule is used again and again, many times a second.

Also accept

• catalyst, unchanged, reused for more substrate

Watch for

• “Saliva contains as many amylase molecules as there are starch molecules.” — Only a little amylase is needed, because each molecule is used again and again.

6. CQ6 · Practice

Model answer. b. Only starch has a shape complementary to amylase’s active site, so only starch fits and forms an enzyme–substrate complex. Protein has a different shape and does not fit.

A full answer includes

• I chose b.

Also accept

• b

Watch for

• “Protein is too large for any enzyme.” — Proteases break down proteins; the point is that protein does not fit amylase’s active site.

7. CQ7 · Practice

Model answer. c. Without enzymes the reactions of life would be far too slow at the temperatures organisms live at; enzymes give them the rate needed to stay alive.

A full answer includes

• I chose c.

Also accept

• c

Watch for

• “Enzymes give organisms energy.” — Enzymes speed up reactions such as respiration, which releases energy; they are not the energy source.

8. CQ8 · Practice

Model answer. a, b and c: respiration, building proteins and breaking down starch are all chemical reactions in the body, and enzymes are involved in all of them. Sweat evaporating (d) is a change of state, not a chemical reaction.

A full answer includes

• I chose a, b and c.

Also accept

• a, b, c

Watch for

• “Only digestion needs enzymes.” — Enzymes are involved in all metabolic reactions, inside every cell as well as in the gut.

9. CQ9 · Practice

Model answer. The growing root needs sugar quickly, for respiration and to build new cells. Without an enzyme starch breaks down far too slowly at the temperature of the soil. Amylase speeds up the breakdown, so sugar is made fast enough to keep the seedling alive and growing.

A full answer includes

• The root needs sugar quickly, within hours, for respiration and growth.
• Without amylase starch breaks down far too slowly.
• Amylase catalyses the breakdown, giving the rate the seedling needs to stay alive and grow.

Also accept

• sugar needed quickly; uncatalysed breakdown too slow; amylase gives the rate needed

Watch for

• “The seed could wait until the starch breaks down by itself.” — That would take far too long: the seedling would die before it had enough sugar.

10. CQ10 · Practice

Model answer. a. The active site has a shape complementary to one substrate (or a few very similar ones), so only that substrate fits and forms an enzyme–substrate complex.

A full answer includes

• I chose a.

Also accept

• a

Watch for

• “Enzymes choose which molecules to work on.” — Enzymes do not choose; only a substrate whose shape fits the active site can form a complex.

11. CT1 · Apply

Model answer. (a) Lactase would not break down sucrose. (b) Enzymes are specific. The active site of lactase has a shape complementary to lactose. Sucrose has a different shape, so it does not fit into the active site, no enzyme–substrate complex forms and it is not broken down.

A full answer includes

• (a) No, lactase would not break down sucrose.
• (b) Enzymes are specific: lactase’s active site is complementary to lactose; sucrose has a different shape, does not fit, so no enzyme–substrate complex forms.

Also accept

• (a) no; (b) specific, active site complementary to lactose only, sucrose does not fit

Watch for

• “Any enzyme that breaks down a sugar breaks down all sugars.” — Each enzyme is specific to the shape of its own substrate.

12. CT2 · Apply

Model answer. (a) A clot has to form within minutes, or the organism loses too much blood. At body temperature the clotting reactions would be far too slow without a catalyst. Enzymes catalyse them, so they happen fast enough to keep the organism alive, and the enzymes are not used up. (b) Enzymes are proteins that act as biological catalysts in all metabolic reactions: for example respiration in every cell, building proteins from amino acids and clotting, as well as digestion.

A full answer includes

• (a) The clot must form within minutes, or too much blood is lost; at body temperature the reactions would be far too slow without enzymes; enzymes speed them up to the rate needed.
• (b) Enzymes are proteins involved in all metabolic reactions, for example respiration in every cell and building proteins, not only digestion.

Also accept

• (a) clot needed quickly; uncatalysed reactions too slow; enzymes speed them up; (b) enzymes are proteins in all metabolic reactions, e.g. respiration

Watch for

• “The body could just heat the blood to speed up clotting.” — Living things cannot heat their blood enough; enzymes make reactions fast at body temperature.
• “Only digestive enzymes are real enzymes.” — Every cell uses enzymes, for respiration, building molecules and much more.