Cambridge International A Level Biology Revision Guide

Chapter P1: Practical skills for AS
Measuring the dependent variable
You may be told exactly how to measure the dependent
variable. Sometimes, however, you may have to decide the
best way to do this.
In an enzyme experiment such as the rennin one,
there are three possible methods for taking measurements.
(You can remind yourself about ways of measuring
reaction rate by looking back at pages 57–58 in Chapter 3.)
■■
You could determine the initial rate of reaction –
taking measurements very quickly to find how much
product has been formed, or how much substrate has
disappeared, in the first minute or so of the reaction.
■■
■■
You could leave the reaction to take place until it has
completely finished – that is, all the substrate has been
converted to product – and record the time taken to do
this.
You could time how long it takes to reach a clearly
identifiable stage of the reaction, called an end-point.
Let us say that, for the rennin reaction, you will use the
last of these three methods. You will measure the time
taken for the rennin to produce clots of milk that stick
to the sides of the test tube in which the reaction is taking
place. You add the rennin to the milk in the tube, and
twist the tube gently. The end-point is the moment when
you first see little clots remaining on the sides of the tube
(Figure P1.3).
The following boxes describe apparatus and techniques
which you might not use but should be aware of. Box P1.1
describes using a colorimeter to measure colour changes as
a dependent variable, while Box P1.2 describes how to use
a haemocytometer to count cells. You won’t be expected to
use these in an examination but you may be shown these
apparatus in class or in a video or animation.
Figure P1.3 Determining the end-point of the clotting of milk
by rennin.
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QUESTION
P1.1 A student is investigating the effect of temperature on
the activity of amylase on starch.
a Identify the independent variable and the
dependent variable in this investigation.
b Suggest a suitable range for the independent
variable. Explain why you have chosen this range.
c Suggest a suitable interval for the independent
variable. Explain why you have chosen this
interval.
QUESTION
P1.2 Look back at the experiment described in Question
P1.1. Suggest two ways in which you could
measure the dependent variable.