IGCSE · Practice questions · Enzymes as biological catalysts

A lock, a key and no change.

Six original Cambridge-style questions on catalysts, enzymes as biological catalysts, the shape of the active site, the enzyme-substrate complex and why each enzyme acts on only one substrate.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Fit, react, release.

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.

Recall

[3 marks]

Define a catalyst and describe what an enzyme is.

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. ✓

Explain

[2 marks]

Explain why enzymes are important to all living organisms.

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. ✓

Explain

[4 marks]

Extended. Explain how an enzyme molecule catalyses the breakdown of its substrate. Use the terms active site, enzyme-substrate complex and product.

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. ✓

Data

[4 marks]

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.

(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. ✓

Explain

[3 marks]

Extended. Amylase digests starch but has no effect on protein. Explain why.

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. ✓

Analysis

[3 marks]

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.

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. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.