Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
It would be dangerous to maintain a body temperature
of 40 °C, as even a slight rise above this would begin to
denature enzymes.
Enzymes from other organisms may have different
optimum temperatures. Some enzymes, such as those
found in bacteria which live in hot springs (Figure 3.10),
have much higher optimum temperatures. Some plant
enzymes have lower optimum temperatures, depending on
their habitat.
Rate of reaction
optimum pH
60
pH and enzyme activity
Figure 3.11 shows how the activity of an enzyme is affected
by pH. Most enzymes work fastest at a pH of somewhere
around 7 – that is, in fairly neutral conditions. Some,
however, such as the protease pepsin, which is found in
Figure 3.10 Not all enzymes have an optimum temperature of
40 °C. Bacteria and algae living in hot springs such as this one
in Yellowstone National Park, USA, are able to tolerate very
high temperatures. Enzymes from such organisms are proving
useful in various industrial applications.
1 3 5 7 9 11 13
pH
Figure 3.11 The effect of pH on the rate of an enzymecontrolled
reaction.
the acidic conditions of the stomach, have a different
optimum pH.
pH is a measure of the concentration of hydrogen ions
in a solution. The lower the pH, the higher the hydrogen
ion concentration. Hydrogen ions can interact with the
R groups of amino acids – for example, by affecting
ionisation (the negative or positive charges) of the groups.
This affects the ionic bonding between the groups (page 42),
which in turn affects the three-dimensional arrangement
of the enzyme molecule. The shape of the active site may
change and therefore reduce the chances of the substrate
molecule fitting into it. A pH which is very different from
the optimum pH can cause denaturation of an enzyme.
When investigating pH, you can use buffer solutions
(Chapter P1). Buffer solutions each have a particular pH
and maintain it even if the reaction taking place would
otherwise cause pH to change. You add a measured
volume of the buffer to your reacting mixture.
QUESTIONS
3.4 How could you carry out an experiment to determine
the effect of temperature on the rate of breakdown of
hydrogen peroxide by catalase?
3.5 Proteases are used in biological washing powders.
a How would a protease remove a blood stain on
clothes?
b Most biological washing powders are
recommended for use at low washing
temperatures. Why is this?
c Washing powder manufacturers have produced
proteases which can work at temperatures higher
than 40 °C. Why is this useful?
QUESTION
3.6 Trypsin is a protease secreted in pancreatic juice,
which acts in the duodenum. If you add trypsin to
a suspension of milk powder in water, the enzyme
digests the protein in the milk, so that the suspension
becomes clear.
How could you carry out an investigation into
the effect of pH on the rate of activity of trypsin?
(A suspension of 4 g of milk powder in 100 cm 3 of water
will become clear in a few minutes if an equal volume
of a 0.5% trypsin solution is added to it.)