Cambridge International A Level Biology Revision Guide

Chapter P1: Practical skills for AS
Making conclusions
A conclusion is a simple, well-focused and clear statement
describing what you can deduce from the results of your
experiment. The conclusion should relate to the initial
question you were investigating or the hypothesis you
were testing.
For example, the results shown in Table P1.1 could lead
you to write a conclusion like this:
The greater the concentration of rennin, the
shorter the time taken to reach the end-point. An
increase in rennin concentration increases the
rate of reaction.
Describing data
You may be asked to describe your results in detail. You
can do this from a table of results, but it is often best
done using the graph that you have drawn. The graph is
likely to show more clearly any trends and patterns in the
relationship between your independent and dependent
variables.
For example, you could describe the results shown in
Figure P1.12 (page 257) like this:
When no rennin was present, no end-point was
reached (time taken = ∞), indicating that no
reaction was taking place. At a concentration
of 0.2% rennin, the end-point was reached in a
mean time of 68.0 seconds. As the concentration
of rennin increased, the mean time to reach
the end-point decreased, with the shortest
mean time (12.9 s) occurring at a concentration
of 1% rennin. This indicates that the rate of
reaction increases as the concentration of rennin
increases.
The line on the graph is a curve with decreasing
gradient, not a straight line, so the relationship
between concentration of rennin and the rate
of reaction is not proportional (linear). The
curve is steepest for the lower concentrations of
rennin, gradually flattening out for the higher
concentrations. This shows that a 0.2% increase
in rennin concentration has a greater effect on
reaction rate at low rennin concentrations than at
high rennin concentrations.
There are several points to bear in mind when you
are describing results shown on a graph. (These same
points are important when you are dealing with graphs
on structured question papers, as well as on practical
examination papers.)
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Begin by describing the overall trend – the overall
relationship between what is shown on the x-axis and
on the y-axis.
Look for any changes in gradient on the graph, and
describe these. In this case, the change in gradient is
a steady one (the gradient gets gradually less and less
as the rennin concentration increases). Sometimes,
there are sharp changes in gradient at particular key
points, and you should focus on those and describe the
gradient changes and precisely where they occur.
Quote figures from the graph. You will need to pick
on points of particular interest (for example, where
gradient changes occur), and quote the coordinates of
those points – that is, you should state both the x-axis
value and the y-axis value.
Take great care not to use phrases that suggest
something is happening over time, if time is not shown
on the x-axis. For example, it would be totally wrong to
say that the gradient of the graph in Figure P1.12
(page 257) ‘is steep at first and gradually gets
less’. ‘At first’ suggests time – but the x-axis shows
concentration, not time. Words such as ‘more quickly’,
‘slower’ and ‘rapidly’ should all be avoided unless the
x-axis shows time.
Making calculations from data
You may be asked to carry out a calculation from a set of
results – either the results that you have collected, or a set
of results that is presented to you.
It is very important to show every single step in any
calculation that you make. For example, you might be
given a set of five measurements and asked to find the
mean value. You should set out your calculation clearly,
like this:
measurements: 12.5 μm, 18.6 μm, 13.2 μm,
10.8 μm, 11.3 μm
(12.5 + 18.6 + 13.2 + 10.8 + 11.3)
mean =
5
= 66.4
5
= 13.3 μm
Remember that, even though your calculator will show an
answer of 13.28, you must give your answer to only one
decimal place, the same as for the original measurements.
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