Higher Biology 2025, Question 8: reading a dual-axis graph and nailing percentage increase

01Identify the variables in the passage

It is tempting to skip straight to the graph. Do not. The passage tells you the dependent variables are heart rate and blood lactate concentration, and the independent variable is the athlete''s running speed. It also says an athlete, singular, which is very often the evaluation point later on.

02Break the graph into four parts

Take each part in turn before you answer anything.

Title: here the introductory passage does that job. Axes: running speed is on the x-axis, heart rate on the left y-axis, blood lactate concentration on the right y-axis, two separate y-axes, which is the whole trap. Legend: note which line is which now, not later. Trend: both variables increase as running speed increases.

03Describing changes using values from the graph

The phrase "values from the graph" means quote data points. The question is about blood lactate, so use the dotted line and the right-hand axis, and the range is the full length of the x-axis. It is worth two marks, which tells you the trend changes at some point.

As running speed increases from 2.0 to 2.4 m/s, blood lactate concentration remains constant at 1.4 mmol/L. From 2.4 to 4.8 m/s, blood lactate concentration increases from 1.4 to 6.3 mmol/L.

"Remains constant to 2.4 m/s then increases" is one mark, not two. Units and values are what buy the second mark.

04Easy mistakes

Reading the wrong y-axis or the wrong trend line. Sense-check your answer, a single-figure human heart rate should stop you dead. Misreading a point as 6.2 or 6.4 instead of 6.3: count up from one whole number to the next to check the scale. Missing units: state the units for each variable at least once.

05Percentage increase is not percentage change

For the heart rate calculation, use the solid line and the left-hand y-axis, across the whole x-axis: 122 bpm at 2.0 m/s and 180 bpm at 4.8 m/s.

Percentage increase is the difference divided by the original value, multiplied by 100. Distinguish it from percentage change: a change usually needs a sign, an increase or decrease does not, and adding one can cost you the mark.

06Reading between the lines

When the value you need sits between two gridlines, follow it across with a ruler. On this paper the answer was 1.7 mmol/L, a mark lost purely to eyeballing is the most annoying mark to lose.