Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Answers to end-of-chapter questions
11 a see Figure 3.14b. Award 1 mark for each
correct label;;;[3]
b inhibitor A had no effect on V max;
and increased K m; [2]
c inhibitor B decreased V max;
and had no effect on K m; [2]
d inhibitor A is competitive, B is noncompetitive;
A is competitive because:
it increased K m / did not affect V max;
decreased the affinity of the enzyme for its
substrate;
the substrate is competing with the inhibitor
for the active site;
the inhibition is overcome by increasing
substrate concentration; [max. 4]
Or
Alternative ways of explaining the same
marking points;
B is non-competitive because:
it did not affect K m /decreased V max;
did not affect the affinity of the enzyme for its
substrate;
the substrate is not competing with the
inhibitor for the active site;
the inhibition cannot be overcome by
increasing substrate concentration;
e i Z;[1]
ii Y;[1]
iii X;[1]
Reasons:
Accept any valid points up to a maximum of
2 marks for each inhibitor, for example:
ii the lines Y and Z cross the y-axis at the
same point, which is 1/V max;
therefore V max is the same for both;
line Y meets the x-axis at a less negative
value than line Z;
therefore K m is increased; [max. 2]
iii the lines X and Z cross the y-axis at the
same point, (which is –1/K m );
therefore both have the same K m;
line X crosses the y-axis higher than line Z,
so 1/V max has a higher value;
therefore V max has a lower value; [max. 2]
[Total: 18]
Cambridge International AS and A Level Biology © Cambridge University Press 2014