Lesson 10 of 12 / Nutrition and transport in plants
Plot and read a transpiration graph
What turns a sketch into a graph that earns every mark?
I can plot potometer results to the marked rules, read values and rates from the graph, and state sources of error with their effects.
I can investigate environmental effects on transpiration and explain wilting.Syllabus
Syllabus K325, Topic 10(m). Investigate environmental effects on transpiration and explain wilting.
About 7 min
Make a guess. You are not marked.
You plot six readings that rise in a rough curve. Should your line join every point dot to dot?
Show the answer
Draw one smooth best-fit line or curve.
Unless told to join points, draw a single smooth line through the trend. It may miss some points.
The key idea
Put the independent variable on the x-axis, label both axes "quantity / unit", choose an even scale that fills at least half the grid, plot each point exactly, and draw one clear line without going past your data. Read values with construction lines and quote them with units.
The independent variable is the one you change, such as time or air movement. The dependent variable is the one you measure, such as the distance a bubble moves. A gradient is the steepness of a line: the change in y divided by the change in x.
Labels. Time is the independent variable, so it goes on the x-axis. Each label gives the quantity and its unit: Time / min and Distance moved by bubble / mm.
Invented readings, not measured data. Each step adds what an examiner checks; skipping one usually costs its mark.
Explanation
Graph questions are marked on four things, often remembered as SLAP: Scale, Labels, Accuracy of plotting and the Plotted line. Each one is usually worth a mark, so a correct trend drawn carelessly can still lose most of the marks.
Labels. The independent variable goes on the x-axis and the dependent variable on the y-axis. Each axis label has a quantity and a unit, written "Time / min" and "Distance moved by bubble / mm". Scale. Choose even steps, such as 1, 2, 5 or 10 units per large square, never awkward steps such as 3 or 7. The plotted points should cover at least half of the grid in each direction. The axes do not have to start at zero, but each must be labelled with numbers at regular intervals.
Accuracy. Plot each point with a small cross or a dot in a circle, placed to within half a small square. Plotting is checked point by point, so take each value straight from the table. Plotted line. Draw a single smooth best-fit curve or a ruled straight line that passes through or close to the points, with about as many points on each side. If the question asks you to join the points, use ruled straight lines from point to point. Do not continue the line beyond the first and last points: that is extrapolation, and the data do not support it.
For two data sets on the same axes, use different symbols, such as crosses and circles, draw a separate line for each, and add a key. To read a value off your graph, draw a dashed construction line from the axis to your line and across to the other axis, then quote the reading with its unit. For a straight line, the rate is the gradient: distance moved divided by time. If the line curves, the rate at one time is the gradient of a tangent, a ruled line that just touches the curve at that time. A tangent at 4 min passing through (0 min, 4 mm) and (8 min, 20 mm) gives (20 - 4) / (8 - 0) = 2.0 mm/min.
Potometer readings carry their own sources of error, and each needs its effect. If air enters the xylem because the shoot was cut in air, the water column breaks and uptake is underestimated; cut the shoot under water. A leaking joint lets water in or out, so the bubble moves at the wrong rate; seal the joints and check that the bubble does not move with the shoot removed. If the room warms during a run, later readings are too high for the stated condition; keep the temperature constant and let the shoot settle for 5 minutes before timing.
Step by step
- 1
Labels
Independent variable on x, dependent on y; each label is quantity / unit.
- 2
Scale
Even steps; points fill at least half the grid in both directions.
- 3
Accuracy
Small crosses, each within half a small square of its true position.
- 4
Plotted line
One best-fit line or ruled point-to-point lines as instructed, ending at the first and last points.
Worked example
Still air and a fan
Invented potometer readings, distance moved by the bubble every 2 min from 0 to 10 min: still air 0, 5, 11, 15, 20, 25 mm; with a fan 0, 9, 18, 26, 36, 45 mm. Describe how you would plot these, then compare the mean rates.
One way to explain it
Time / min goes on the x-axis from 0 to 10, and Distance moved by bubble / mm on the y-axis from 0 to 50 in steps of 5 or 10 per large square. Plot still air with crosses and the fan with circles, draw a ruled best-fit line for each and add a key. Over 10 min the still-air rate is 25 / 10 = 2.5 mm/min and the fan rate is 45 / 10 = 4.5 mm/min, an 80% increase.
Why this answer works
- Time is changed in fixed steps, so it is the independent variable on x.
- A y scale to 50 mm uses most of the grid, because the largest reading is 45 mm.
- Percentage increase = (4.5 - 2.5) / 2.5 x 100 = 80%.
Watch out for this
A student says: "A line of best fit must pass through every point and start at the origin." What is wrong with this?
Show the answer
A best-fit line follows the overall trend and may miss some points. It goes through the origin only if the data include that point, and it stops at the first and last points.
Check your understanding
The largest bubble distance in a table is 45 mm and the grid has 10 large squares up the y-axis. Which y-axis scale is best?
Show the answer
5 mm per large square, so the axis reaches 50 mm.
It is an even step, it reaches 45 mm and the points fill most of the grid.
Quick recall
What turns a sketch into a graph that earns every mark?