Home / iGCSE Biology (0610)-4.1 Biological molecules – iGCSE Style Questions Paper 3

iGCSE Biology (0610)-4.1 Biological molecules – iGCSE Style Questions Paper 3

IGCSE Biology (0610) Practice Questions Paper 3 - All Chapters

Question

(a) Complete the sentences to describe enzymes.
Use words or phrases from the list.
Each word or phrase may be used once, more than once or not at all.

Enzymes are proteins. All proteins are made up from a chain of smaller ………….. called …………… .
All proteins contain the chemical elements ……………., …………… and …………….. .

(b) Enzymes function as biological catalysts. Describe what is meant by the term catalyst.
(c) Fig. 1.1 shows a reaction catalysed by an enzyme.
Use Fig. 1.1 to describe enzyme action.
(d) Some bacteria are found living in very high temperatures.
Fig. 1.2 shows the effect of temperature on the activity of an enzyme in these bacteria.

Using the information in Fig. 1.2:
(i) Identify the optimum temperature for this enzyme.
(ii) Identify the temperature when the enzyme is completely denatured. Explain how you identified your choice.
(iii) Calculate the difference in enzyme activity between \(20~^\circ\text{C}\) and \(40~^\circ\text{C}\).
Space for working.
(iv) The mean body temperature for humans is \(37~^\circ\text{C}\). On Fig. 1.2, sketch the curve for the activity of an enzyme found in humans. [2]
▶️ Answer/Explanation
Detailed solution

(a)
Enzymes are proteins. All proteins are made up from a chain of smaller molecules called amino acids.
All proteins contain the chemical elements carbon, hydrogen and nitrogen (or oxygen).
(Note: Proteins consist of Carbon, Hydrogen, Oxygen, and Nitrogen. Since there are three blanks, listing any three of these from the list is acceptable, but Nitrogen is the key element distinguishing proteins from carbohydrates and fats).

(b)
A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction (it can be used again).

(c)
Using the “lock and key” hypothesis shown in Fig 1.1:

  • The substrate has a specific shape that is complementary to the enzyme’s active site.
  • The substrate binds to the active site to form an enzyme-substrate complex.
  • The reaction occurs (breakdown or synthesis), and products are formed.
  • The products leave the active site, and the enzyme remains unchanged.

(d)(i)
Optimum temperature: \(50 \pm 1\) \(^\circ\text{C}\).
(This is the temperature corresponding to the peak of the curve on the graph).

(d)(ii)
Temperature: \(66\) \(^\circ\text{C}\).
Explanation: There is zero activity / no enzyme activity at this point.
(At this temperature, the enzyme has denatured, meaning the active site has lost its shape and can no longer bind to the substrate).

(d)(iii)
Calculation:
At \(40~^\circ\text{C}\), activity = \(10\) arbitrary units.
At \(20~^\circ\text{C}\), activity = \(2.8\) arbitrary units (each small square on the y-axis represents \(0.4\) units; the point is 2 squares above 2).
Difference: \(10 – 2.8 = \mathbf{7.2}\) arbitrary units.

(d)(iv)
Sketch:
The curve should have the same shape as the bacterial enzyme (gradual rise to a peak, then sharp decline). The optimum (peak) should be shifted to the left, centered around \(37~^\circ\text{C}\) \(\pm 2\).

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