Catalysts in living things — questions

1. state

Which statement describes a catalyst?

Circle one answer.

• A substance that is used up as it speeds up a reaction
• A substance that slows a reaction down and is not changed
• A substance that increases the rate of a reaction and is not changed by it
• A product that forms when a reaction is fast

Nudge (only if you are stuck): Two things are true of every catalyst: what it does to the reaction, and what happens to the catalyst itself.

2. complete

Complete the sentences about enzymes.

Enzymes are …

that work as biological …

They are involved in all … reactions in living things.

Nudge (only if you are stuck): Think about what kind of molecule an enzyme is, and what job it does in a cell.

3. identify

Match each term to its meaning.

Write the number of the matching option.

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

• Active site: ______
• Substrate: ______
• Enzyme–substrate complex: ______
• Product: ______

Nudge (only if you are stuck): Look at the three-stage diagram of enzyme action in the reading.

4. sequence

Put the stages of enzyme action in order.

Write the number of each step in order.

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

• First: ______
• Second: ______
• Third: ______
• Fourth: ______
• Last: ______

Nudge (only if you are stuck): Start with the two molecules apart, and end with the enzyme ready to work again.

5. explain

A few drops of saliva can break down a whole test tube of starch. Explain how such a small amount of amylase can break down so much starch.

Nudge (only if you are stuck): What happens to an amylase molecule after its products leave the active site?

6. explain

Amylase breaks down starch but has no effect on protein. Which statement explains why?

Circle one answer.

• Amylase only works in the mouth, where there is no protein
• Only starch has a shape that fits amylase’s active site
• Protein is too large to be broken down by any enzyme
• Protein molecules move too slowly to meet amylase

Nudge (only if you are stuck): Compare the shape of each substrate with the shape of amylase’s active site.

7. state

Why do all living organisms need enzymes?

Circle one answer.

• Enzymes are food that cells store
• Enzymes give organisms energy
• Enzymes make the reactions of life fast enough to keep the organism alive
• Enzymes make reactions happen that could never happen at all

Nudge (only if you are stuck): Think about how fast reactions would be at the temperature an organism lives at.

8. identify

Which of these are metabolic reactions in your body that need enzymes? Choose all that apply.

Tick every correct statement.

• Respiration in muscle cells
• Joining amino acids to build a protein
• Breaking down starch in the gut
• Sweat evaporating from the skin

Nudge (only if you are stuck): A metabolic reaction is a chemical reaction in a living thing. Which change makes no new substance?

9. explain

A broad bean seed stores starch. It takes in water and grows a root within a day, using sugar from its starch store. Explain why the seed could not grow without amylase, even though starch can break down slowly without any enzyme.

Nudge (only if you are stuck): Compare how fast the growing root needs sugar with how fast starch breaks down on its own.

10. identify

Amylase acts on starch but not on protein. What makes an enzyme specific like this?

Circle one answer.

• The shape of its active site fits only its own substrate
• It is used up after one reaction
• It only works at one temperature
• It is made of a different protein for every reaction in the body

Nudge (only if you are stuck): Think about the lock and key: what has to fit?

11. predict / explain

Milk sugar, lactose, is broken down into glucose and galactose by the enzyme lactase. Lactase is added to milk to make lactose-free milk. (a) Predict whether lactase would break down sucrose (ordinary sugar). (b) Explain your prediction.

(a) Prediction

(b) Explanation

Nudge (only if you are stuck): Lactose and sucrose are both sugars made of two units, but they have different shapes.

12. explain

When you cut your finger, the blood forms a clot within a few minutes. Making the clot is a chain of chemical reactions in the blood, each catalysed by an enzyme. (a) Explain why the clotting reactions must be catalysed by enzymes. (b) A student says: "Enzymes are only needed to digest food." Use your knowledge of enzymes to correct the student.

(a) Why enzymes are needed for clotting

(b) Correct the student

Nudge (only if you are stuck): (a) How fast must the clot form, and how fast would the reactions be at body temperature without a catalyst? (b) Which reactions in cells involve enzymes?