Topic 5 MCQs IGCSE Review

• A

• B

• C

• D

Answer explanation

The correct answer is D. (A protein that functions as a biological catalyst) because:

Enzymes are proteins (formed from chains of amino acids) which increase the rate of chemical reactions without being changed by it. This makes them catalysts, and because they are found in living organisms they are ‘biological catalysts’ (many catalysts in chemistry are metals which are obviously not biological!)

A and B are incorrect because enzymes, as catalysts, are not used up in the process of speeding up reactions.

C is incorrect because enzymes are made from protein, not carbohydrates.

• A

• B

• C

• D

Answer explanation

The correct answer is B. (They are not changed by the reaction) because:

A feature of all catalysts is that they remain unchanged at the end of a reaction, and can go on to catalyse subsequent reactions.

A is incorrect because although enzymes, as biological catalysts, are made from proteins, other catalysts not found in living organisms are not made of proteins (many metals can act as catalysts in certain chemical reactions for example).

C and D are incorrect because catalysts are not changed by the chemical reactions they speed up.

• A

• B

• C

• D

Answer explanation

The correct answer is B because:

Different enzymes work best at different pH levels, each enzyme will have an optimum pH. Enzymes are made from proteins, which themselves are made from amino acids chemically bonded together. Enzymes also move about randomly in a fluid; they don’t ‘choose’ to collide with substrates. 

The other answers are incorrect as they do not describe features of enzymes that are true for all enzymes. 

• A

• B

• C

• D

Answer explanation

The correct answer is D. (Each enzyme works best at a different pH) because:

For enzyme 1, the rate of reaction is quickest at pH 1.5, making this its optimum pH. For enzyme 2, the rate of reaction is quickest at pH 7, making this its optimum pH. And for enzyme 3, the rate of reaction is quickest at pH 14, making this its optimum pH.

A is incorrect because the graph gives no information about temperature. 

B is incorrect because only enzyme 2 works best in neutral conditions.

C is incorrect because only enzyme 1 works best in acidic conditions.

• A

• B

• C

• D

Answer explanation

The correct answer is B. (enzymes) because:

Enzymes are biological catalysts which help ensure metabolic reactions take place in living organisms.

A is incorrect because although enzymes are proteins, this is not the correct term for the answer. 

C is incorrect because although enzymes are catalysts, the question stem refers to living organisms which means that the catalyst has to be a biological catalyst.

D is incorrect because hormones do not ensure that metabolic reactions take place. 

• A

• B

• C

• D

Answer explanation

The correct answer is B because:

• For most enzymes found in living organisms, the optimum (highest rate of enzyme activity) temperature is usually between 30°C to 50°C

• Increasing the temperature from 0°C  to 50°C would increase the activity of the enzyme as the more energy the molecules have the faster they move, and the number of successful collisions between substrate molecules and enzymes increases, leading to the reaction taking less time to occur (increasing the rate)

• Heating to high temperatures (beyond the optimum) will break/alter the bonds that hold the enzyme together and it will lose its shape. This is known as denaturation. Substrates cannot fit into the active site of denatured enzymes as they have lost their complementary structures, so above optimum temperatures there is a rapid decrease in enzyme activity

• Denaturation is irreversible - once enzymes are denatured they cannot regain their proper shape and activity will stop

A and D are incorrect because both graphs show enzyme activity as being extremely high at very low temperatures and very high temperatures, which is not possible - at low temperatures, the frequency of collisions is low (due to molecules having less kinetic energy) and at higher temperatures enzymes denature (due to destabilisation of enzyme structure permanently distorting the shape of the active site).

C is incorrect because this graph shows a gradual decrease in enzyme activity above the optimum temperature, which is incorrect as enzyme activity past the optimum decreases rapidly leading to denaturation.

• A

• B

• C

• D

Answer explanation

The correct answer is D. (It may increase or decrease the rate) because:

• Whether the rate of enzyme activity increases or decreases depends on what the optimum pH for the enzyme is. In mammals, it would depend if the enzyme acts inside cells, or if it were a digestive enzyme

• If the increase in pH takes the pH away from the optimum in either direction, then the rate will slow down

• If the increase in pH moves the pH towards the optimum, then the rate will increase

The other answers are incorrect because they do not correctly state how changing the pH will affect the rate of reaction. 

• A

• B

• C

• D

Answer explanation

The correct answer is B because:

Increasing the temperature (X) initially increases the rate of reaction; but above the optimum temperature (Y) the enzyme denatures.

The other answers are incorrect as these do not correctly describe the shape of the graph. 

• A

• B

• C

• D

Answer explanation

The correct answer is D. (pH) because:

If the condition being investigated is pH, then this is the independent variable and should be different in each test tube to get a good range of results.

A is incorrect because the temperature is one factor that can affect enzyme activity and should not be changed if pH is being altered.

B is incorrect because the concentration of the enzyme is another factor that needs to be kept the same (not changed) to make the experiment fair as it can also affect enzyme activity.

C is incorrect because the volume of the lipid solution is one of the things that needs to be kept the same to make the experiment valid; the more lipid (substrate) there is, the longer it will take for the reaction to complete.

• A

• B

• C

• D

Answer explanation

The correct answer is C because:

Increasing the temperature from 15°C to 30°C increases the kinetic energy of both enzymes and their substrates, which consequently increases the frequency of effective collisions. Increasing the temperature from 15°C to 30°C does not alter the shape of the active site. 

The other answers are incorrect as they do not correctly describe the effect of increasing temperature.