Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
You could also be asked to calculate a gradient at one
or more points on your graph. Figure P1.15 explains how
to do this when there is a straight line, and also when the
line is curved.
QUESTION
P1.5 a Choose two different points on the graph in
Figure P1.12 (page 257), and calculate the gradient
at each point. Remember to show all the steps in
your calculation fully and clearly.
b Use your calculated values to add to the
description of the results given on page 259.
A third type of calculation you could be asked to do is
to find the percentage change. To do this:
1 find the difference between the first reading and
the second reading, by subtracting one from the other
2 divide this value by the first reading, and multiply by 100;
this figure gives you the percentage change – remember
to state whether the change is an increase or
a decrease.
For example, imagine that the mass of a plant on day
1 was 250 g. On day 5, after it had lost a lot of water by
transpiration, its mass was 221 g.
change in mass = 250 – 221 = 29 g
change in mass
percentage change in mass =
original mass × 100
= 29
250 × 100
= 11.6% decrease
260
y
x 2
point on line
y
x 1
point on line
y 1
y 2
tangent
point on curve
x
x
To determine the gradient of a straight line
graph:
1 Select two points which are at least half as far
apart as the length of the line of the graph.
2 Draw a right-angle triangle between these
points.
3 Calculate the gradient using the lengths of
the triangle sides x 1 and y 1 :
y 1
gradient =
x 1
y 2
Figure P1.15 Calculating the gradients of a straight line and a curve at a point.
To determine the gradient at a point on a
curved graph:
1 Draw a tangent to the curve at that point,
making sure it is at least half as long as the
line of the graph.
2 Draw a right-angle triangle on the tangent.
3 Calculate the gradient using the lengths of
the triangle sides x 2 and y 2 :
gradient =
x 2