iGCSE Biology (0610)-5.1 Enzymes - iGCSE Style Questions Paper 1

Question

Which graph shows the effect of $pH$ on enzyme activity?

▶️ Answer/Explanation

Enzymes have an optimum $pH$ at which they function most efficiently. As the $pH$ moves away from this optimum—either becoming too acidic or too alkaline—the chemical bonds holding the enzyme’s active site together begin to break. This leads to denaturation, where the active site changes shape and the substrate can no longer fit.

Consequently, enzyme activity is highest at the peak and drops off to zero on either side, creating a characteristic bell-shaped curve. Graph C correctly illustrates this relationship, showing a single clear peak of maximum activity.

✅ Answer: (C)

Question

The graph shows the effect of temperature on an enzyme-controlled reaction.

At which point on the graph is the enzyme completely denatured?

▶️ Answer/Explanation

Enzymes are biological catalysts that have an optimum temperature (Point B) where the reaction rate is highest. As temperature increases beyond this point, the kinetic energy causes the weak bonds maintaining the enzyme’s specific 3D shape to break. This process is called denaturation. At Point C, the enzyme’s active site is beginning to deform, causing a rapid drop in activity. By Point D, the rate of reaction has fallen to zero because the enzyme molecules are completely denatured, meaning the active site no longer fits the substrate, and the reaction can no longer occur.
✅ Answer: (D)

Question

Enzyme R digests protein in the stomach.

Four test-tubes are set up. Each contains the same amounts of protein and enzyme R. The test-tubes are kept at different levels of pH and temperature, as shown in the table.

In which test-tube will protein digestion be quickest?

▶️ Answer/Explanation

The correct answer is B.

Stomach enzymes like Enzyme R (pepsin) function optimally in highly acidic conditions ($pH \text{ } 2$) created by gastric acid. Furthermore, enzymatic reactions accelerate as temperature increases toward the human biological optimum of approximately $37^\circ\text{C}$ due to increased molecular collisions. While test-tube A has the correct pH, its lower temperature ($20^\circ\text{C}$) results in a slower reaction. Test-tube B provides the ideal combination of an acidic environment and a near-body temperature ($35^\circ\text{C}$), ensuring the highest rate of protein digestion.

iGCSE Biology (0610)-5.1 Enzymes – iGCSE Style Questions Paper 1

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