IGCSE Biology 0610: Enzymes, Temperature and pH - Study Guide PDF with Answers

How an enzyme works

Enzymes are proteins that speed metabolic reactions sufficiently for living cells. A substrate interacts with an enzyme's active site; products leave, and the enzyme can catalyse another reaction. The active site's shape and chemical properties make an enzyme specific to a substrate or related substrates. A useful model is a complementary fit, but the enzyme is not a rigid key in every molecular detail. An enzyme changes reaction rate; it does not change the overall products specified by the reaction or get permanently used up in the normal catalytic cycle.

Temperature has two competing effects

At low temperatures, particles have less kinetic energy, so successful encounters are less frequent. Warming usually increases rate towards an optimum. Above a suitable optimum, heat can disrupt the enzyme's three-dimensional structure. If its active site no longer binds the substrate effectively, the rate falls: the enzyme is denatured. Cooling a typical denatured enzyme does not simply restore its original activity. Do not say that low temperature alone denatures it; a cold enzyme can often work again when warmed.

pH changes the active site

Each enzyme has a pH range and an optimum for its conditions. A pH far from that range can alter bonds affecting its shape and reduce substrate fit; extreme pH may denature it. Not every enzyme has an optimum at pH 7: stomach enzymes and enzymes in other environments can differ. Always describe the specific data supplied rather than assuming a universal optimum or that a slower reaction means no enzyme molecules are present.

Interpret an enzyme-rate investigation

A fair comparison changes one independent variable, such as temperature, while controlling enzyme concentration, substrate amount, pH and observation time. Measure a dependent variable tied to reaction progress, for example product made per minute. A bar or line graph should label axes and units. If a data series rises then falls, locate its highest measured rate; the true optimum could lie between tested temperatures. Repeats and mean results help reveal unusual measurements without turning an estimate into a certainty.

1. Read a temperature dataset

An enzyme gives 2, 6, 10 and 3 units of product per minute at 15, 25, 35 and 45 degrees C respectively. What can be concluded?

1. The highest measured rate is 10 units/min at 35 degrees C.
2. The measured optimum among these four temperatures is 35 degrees C; an untested temperature nearby could be slightly better.
3. The fall by 45 degrees C is consistent with loss of effective active-site shape, but the data alone do not prove the molecular mechanism.

2. Design a fair pH comparison

A class compares activity at pH 4, 7 and 10. Identify the changed variable, measured variable and two controls.

1. Independent variable: pH of the reaction mixture.
2. Dependent variable: a consistent measure of rate, such as product formed per minute.
3. Keep temperature, enzyme quantity, substrate quantity and observation time consistent; two of these controls would suffice.

3. Explain a cold versus hot result

A reaction is slow at 5 degrees C and also slow after heating its enzyme strongly. Why can these observations have different causes?

1. At 5 degrees C, particles have lower kinetic energy and fewer effective encounters; the enzyme may remain structurally intact.
2. Strong heating may change the active site's shape through denaturation, so substrate binding is impaired.
3. Warming the cold mixture can raise its rate, whereas cooling a denatured sample may not restore it.