Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
58
different rates in each of the five reactions. It is only at
the very beginning of the reaction that we can be sure that
differences in reaction rate are caused only by differences
in enzyme concentration.
To work out the initial rate for each enzyme
concentration, we can calculate the slope of the curve
30 seconds after the beginning of the reaction, as explained
earlier. Ideally, we should do this for an even earlier stage of
the reaction, but in practice this is impossible. We can then
plot a second graph, Figure 3.7b, showing the initial rate of
reaction against enzyme concentration.
a
b
Total volume O 2 collected / cm 3
Initial rate of reaction / cm 3 O 2 min −1
10
9
8
7
6
5
4
3
2
1
initial volume of extract
4.0 cm 3
0
0 30 60 90 120 150 180 210 240
Time / s
8
7
6
5
4
3
2
1
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Enzyme concentration / cm 3 of celery extract
3.0 cm 3
2.0 cm 3
1.0 cm 3
0.5 cm 3
Figure 3.7 The effect of enzyme concentration on the rate of
an enzyme-catalysed reaction. a Different volumes of celery
extract, which contains catalase, were added to the same
volume of hydrogen peroxide. Water was added to make the
total volume of the mixture the same in each case. b The rate
of reaction in the first 30 s was calculated for each enzyme
concentration.
This graph shows that the initial rate of reaction
increases linearly. In these conditions, reaction rate is
directly proportional to the enzyme concentration. This
is just what common sense says should happen. The more
enzyme present, the more active sites will be available
for the substrate to slot into. As long as there is plenty of
substrate available, the initial rate of a reaction increases
linearly with enzyme concentration.
BOX 3.1: Measuring reaction rate
It is easy to measure the rate of the catalase–hydrogen
peroxide reaction, because one of the products is a gas,
which is released and can be collected. Unfortunately, it
is not always so easy to measure the rate of a reaction. If,
for example, you wanted to investigate the rate at which
amylase breaks down starch, it would be very difficult to
observe the course of the reaction because the substrate
(starch) and the product (maltose) remain as colourless
substances in the reaction mixture.
The easiest way to measure the rate of this reaction is
to measure the rate at which starch disappears from the
reaction mixture. This can be done by taking samples from
the mixture at known times, and adding each sample to
some iodine in potassium iodide solution. Starch forms a
blue-black colour with this solution. Using a colorimeter,
you can measure the intensity of the blue-black colour
obtained, and use this as a measure of the amount of
starch still remaining. If you do this over a period of time,
you can plot a curve of ‘amount of starch remaining’
against ‘time’. You can then calculate the initial reaction
rate in the same way as for the catalase–hydrogen
peroxide reaction.
It is even easier to observe the course of this reaction
if you mix starch, iodine in potassium iodide solution and
amylase in a tube, and take regular readings of the colour
of the mixture in this one tube in a colorimeter. However,
this is not ideal, because the iodine interferes with the rate
of the reaction and slows it down.
QUESTION
3.1 a In the breakdown of starch by amylase, if you
were to plot the amount of starch remaining
against time, sketch the curve you would
expect to obtain.
b How could you use this curve to calculate the
initial reaction rate?