Revise Graphs: Choosing, Drawing & Reading Them for Biology 3 (IAL) WBI13 (AS Level) — revision notes and instant AI marking. Free to start.
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Edexcel IAL Biology • Unit 3: Practical Skills in Biology I
Graphs: Choosing, Drawing & Reading Them
📈 Big idea: Choosing the wrong type of graph loses marks before you draw a single point. Continuous independent variable means a line graph; categories mean a bar chart. After that, most of the marks are mechanical — axes, labels, units, scale, plotting, line — and every one of them is yours for the taking.
Summary — What This Topic Covers
Choosing between a line graph, a bar chart and a scatter graph
Which variable goes on which axis, and why
Choosing a scale that uses the paper
Drawing a line of best fit — and when not to join the points
Reading a gradient, and taking values from a curve
1. Choosing the Right Graph
IV IS… USE EXAMPLE
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continuous (any value in a range) line graph temperature, pH,
concentration, time
categoric (named groups) bar chart species, tissue type,
light/dark
two continuous, neither controlled scatter graph height vs mass
(+ line of best fit) in a population
frequency of continuous data histogram number of leaves in
(bars touch) each length band
The distinction
A bar chart has gaps between the bars because the categories are separate. A histogram has no gaps because the data is continuous and the bars represent adjoining ranges.
Common mistake
Drawing a bar chart for temperature. Temperature is continuous — 25 °C exists between 20 and 30, so the bars would be hiding real values. Continuous data always gets a line.
2. Axes
The rule
Independent variable on the x-axis (horizontal), dependent variable on the y-axis (vertical). What you changed goes across; what you measured goes up.
Label both axes with quantity / unit, exactly as in the table
Choose a scale that makes the plotted points fill at least half of each axis
Use sensible intervals — 1, 2, 5, 10 and their multiples. Never 3, 7 or 15
The axes do not have to start at zero, but say so if they do not
✗ points crammed into the bottom-left corner → scale mark lost
✗ x-axis in steps of 3 → hard to read between points
✓ points spread across most of the grid, steps of 10
3. Plotting and the Line
Plot each point as a small neat cross or an encircled dot — a blob hides where the value actually is
Draw a line of best fit where the relationship is expected to be smooth: a single smooth line with roughly equal numbers of points either side
Do not join the points dot-to-dot when a trend is expected — that implies the wiggles between points are real
Do join point to point only when each reading is a separate measurement with no expected trend between them
Anomalies on a graph
Plot the anomalous point, circle it, and ignore it when drawing the line of best fit. Do not leave it out of the graph — that is hiding data.
What mark schemes look for
A line of best fit that passes through the scatter, not through every point. A line bending to hit each dot is the commonest way to lose the line mark.
4. Reading a Gradient
The method
Choose two points on the line — never two of your plotted data points — as far apart as possible. gradient = Δy ÷ Δx, and the unit is the y-unit divided by the x-unit.
points chosen on the line: (10 s, 4.0 cm³) and (50 s, 18.0 cm³)
Δy = 18.0 − 4.0 = 14.0 cm³
Δx = 50 − 10 = 40 s
gradient = 14.0 ÷ 40 = 0.35 cm³ s⁻¹
draw the triangle on the graph — it is worth a mark on its own
For a curve, the gradient changes. To find the rate at a particular moment, draw a tangent at that point and take its gradient. The initial rate is the gradient of the tangent at t = 0, which for many enzyme reactions is the only fair measure, because substrate is being used up as the reaction proceeds.
Easy marks
Draw the gradient triangle on the graph and label its sides. It costs seconds and examiners award it separately from the arithmetic.
5. Describing What a Graph Shows
"Describe the graph" means: state the trend, quote figures, and name any change of pattern.
✗ "The rate goes up then goes down."
✓ "The rate increases from 0.1 cm³ s⁻¹ at 10 °C to a maximum of
0.8 cm³ s⁻¹ at 40 °C, then falls sharply to 0.1 cm³ s⁻¹ at 60 °C.
The optimum temperature is therefore 40 °C."
Worth remembering
Always quote figures with units from the graph. A description with no numbers rarely scores above one mark, however accurate it is.
Practice Questions
Practice Question 1
A student investigates the effect of temperature on enzyme activity and draws a bar chart. Explain why this is inappropriate and state what they should have drawn.
Practice Question 2
Describe how to determine the rate of reaction at 20 s from a curved graph of volume of gas against time.
Practice Question 3
A student's graph has all its points in the lower left quarter of the grid. State the problem and how to fix it.
Practice Question 4
Explain the difference between a bar chart and a histogram.
What to Memorise
Continuous IV → line graphCategoric IV → bar chart with gapsContinuous frequency → histogram, bars touchIV on x, DV on yPoints fill at least half the axesIntervals of 1, 2, 5, 10 — never 3Line of best fit, not dot-to-dotCircle anomalies, ignore them for the lineGradient = Δy ÷ Δx from the LINECurve → tangent
Concepts Checklist
Exam Tips
What mark schemes look for
Axes labelled with units, a sensible scale, accurate plotting and a line of best fit. These four are often four separate marks, all available regardless of your data.
The trap
Joining the points dot-to-dot when the relationship should be smooth. It says the zigzags between readings are real measurements, and it loses the line mark.
Easy marks
Draw the gradient triangle. It is a mark on its own and takes five seconds.
Worth remembering
"Describe the graph" always wants figures with units, and the point where the pattern changes — the optimum, the plateau, the intercept.