Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
a
b
Rate of respiration
Rate of respiration
Temperature
Temperature
Figure P2.1 Using sketch graphs to show predicted results: a rate of respiration of yeast will increase with temperature;
b as temperature increases, the rate of respiration will increase up to a maximum temperature, above which it will fall.
492
Identifying variables
Both the hypotheses on the previous page include reference
to the independent and dependent variables. Make sure
that you know which is which. It is usually a good idea to
make a clear statement about these as part of your plan.
You also need to think about the controlled variables –
the ones that you will keep constant as you change the
independent variable. It is very important to include only
variables that really might have an effect on your results
– the key variables – and not just give a long list including
unimportant ones.
Figure P2.2 Apparatus for measuring the rate of respiration
of yeast.
You may have done an experiment to investigate the
relationship between temperature and rate of respiration
of yeast, using a respirometer, like the experiment on
page 280, and you may think that this would be a much
better way of doing this investigation. You would be
right – but if the question asks you to use the syringe
apparatus, then that is what you must do.
Changing the independent variable
You should be prepared to describe how you will change
the independent variable. In this experiment, this means
changing the temperature, and you should already know
how to do this using a water bath. You should describe
how you would do this, and how you would measure the
different values of temperature.
There are many other possible independent variables
that a question could involve. For example, the
independent variable could be pH, in which case you
would use buffer solutions. It could be concentration. By
now, you should be familiar with using a stock solution to
make up solutions with lower concentrations, using the
serial dilution technique – this is described on page 248 in
Chapter P1. At A level, however, you could also be asked
QUESTION
P2.1 Describe how you could use the apparatus in
Figure P2.2 to measure the rate of respiration
of yeast.