Lesson 10 of 12 / Nutrition and transport in flowering 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 and explain four environmental effects on transpiration and wilting.Syllabus
Syllabus K224 / K225, Topic 8(k). Investigate and explain four environmental effects on transpiration and 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: Scale, Labels, Accuracy of plotting and the Plotted line (SLAP). Each is usually worth a mark, so a careless graph can lose marks even if the trend is right.
Labels: put the factor you changed on the x-axis and the factor you measured on the y-axis. Label each axis with a quantity and a unit, such as "Time / min". Scale: use even steps, such as 1, 2, 5 or 10 per large square, and make the points cover at least half of the grid each way.
Accuracy: mark each point with a small cross, within half a small square. Plotted line: draw one smooth curve or a ruled straight line that passes close to the points. If the question says join the points, use ruled lines from point to point. Do not draw the line past the first or last point.
For two sets of results on one graph, use different symbols, draw a line for each and add a key. To read a value, draw a dashed line from the axis to your line and across to the other axis, then give the value with its unit. For a straight line, the rate is the gradient: the change on the y-axis divided by the change on the x-axis.
Each source of error needs its effect. If air gets into the xylem because the shoot was cut in air, less water is taken up, so the rate is too low; cut it under water. A leaking joint makes the bubble move at the wrong rate; seal the joints. If the room warms up, later readings are too high; keep the temperature steady.
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?