Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
256
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Rennin
concentration
/ %
0.0
Time to reach end-point / s
1st
reading
did not
clot
2nd
reading
did not
clot
3rd
reading
did not
clot
Mean
did not
clot
0.2 67.2 68.9 67.8 68.0
0.4 48.1 46.9 47.3 47.4
0.6 30.1 31.9 30.1 30.7
0.8 20.3 19.2 19.9 19.8
1.0 13.1 18.9 12.7 12.9
Table P1.1 Results for an experiment to investigate the effect
of enzyme concentration on the rate of activity of rennin. The
reading in bold italics is an anomalous result and has been
excluded from the calculation of the mean.
The table is drawn with ruled columns, rows and a
border. The purpose of a results table is to record your
results clearly, so that you and others can easily see
what they are, and so that you can use them easily to
draw a graph or to make calculations. Drawing neat,
clear lines makes it much easier to see the results at
a glance.
The columns are clearly headed with the quantity and
its unit. (Use SI units.) Sometimes, you might want
to arrange the table the other way round, so that it is
the rows that are headed. Sometimes, both rows and
columns might need to include units. The important
thing to remember is that the units go in the heading,
not with the numerical entries in the table.
The results are organised in a sensible sequence. The
values for the rennin concentration go up from the
lowest to the highest.
The independent variable (rennin concentration) comes
first, followed by the readings of the dependent variable
(time taken to reach end-point).
Each measurement of the dependent variable is taken
to the same number of decimal places. You would
have used a stopwatch to take these readings, and it
probably gave a reading to one hundredth, or even one
thousandth, of a second. So the first reading on the
watch could have been 67.207. However, as we have
seen, it is very difficult to judge this end-point, so to
suggest that you can time it to the nearest thousandth
of a second is not sensible. You can perhaps justify,
however, recording the values to the nearest one
tenth of a second, rounding up or down the reading on
the watch.
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The values calculated for the mean are given to the
same number of decimal places as the individual
readings. This is very important to remember. If you
have only recorded the individual readings to the
nearest one tenth of a second, then it is wrong to
suggest you can calculate the mean to one hundredth
or one thousandth of a second.
In the last row, the readings for the rennin at a
concentration of 1% contain an anomalous result. The
second reading (shown in bold italics) is clearly out of
line with the other two, and looks much too close to the
readings for the 0.8% rennin solution. You can’t know
what went wrong here, but something clearly did. If
you are in a position to do so, the best thing to do about
an anomalous result is to measure it again. However,
if you can’t do that, then you should ignore it. Do not
include it in your calculation of the mean.
The mean for this row is therefore calculated as
(13.1 + 12.7) ÷ 2 = 12.9.
The first row of the table records that the milk ‘did
not clot’. An alternative way of recording this would
be to record the time as infinite (symbol: ∞). This can
then be converted to a rate like all the other results by
calculating
1
time . Note that ∞ 1 = 0 (zero rate).
QUESTION
Constructing a line graph
You will generally want to display the results in a table as a
graph. Figure P1.12 shows a line graph constructed using
the results in Table P1.1.
Once again, there are several important points to note
about this graph, which you should always bear in mind
whenever you construct and complete a graph.
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P1.4 Look back at the experiment described in Question
P1.1, and your answers to Questions P1.2 and P1.3.
Construct a results table, with full headings, in which
you could record your results.
The independent variable goes on the x-axis
(horizontal axis), and the dependent variable on the
y-axis (vertical axis).
Each axis is fully labelled, including the units. Usually,
you can simply copy the headings that you have used in
the results table.
The scale on each axis goes up in equal intervals, such
as 1 s, 2 s, 5 s or 10 s intervals. You would not therefore,
have an axis that read 20 °C, 30 °C, 50 °C, 60 °C, 80 °C.