Questions on plant exchange usually give you a table of conditions and a measurement, then ask you to describe the pattern, calculate a rate and explain it. The three tasks use different skills, so treat them as three separate steps.
This lesson sits in biological mechanisms in mixed tasks and builds on the chain from the membrane lesson. All numbers here are invented for teaching.
What is happening inside the leaf?
Water evaporates from the surfaces of cells inside the leaf. The water vapour then diffuses out through the stomata, which are small pores, down a concentration gradient. This loss is called transpiration.
Four conditions change the steepness of that gradient: light (opens stomata), temperature (faster evaporation), humidity (humid air reduces the gradient) and wind (removes humid air near the leaf).
Worked example (invented data)
A potometer measures how much water a leafy shoot takes up in 15 minutes under four conditions.
| Condition | Water taken up in 15 min (cm³) | Rate (cm³/min) |
|---|---|---|
| Shade, still air | 1.2 | 0.08 |
| Bright light, still air | 3.0 | 0.20 |
| Bright light, windy | 4.5 | 0.30 |
| Humid, still air | 0.9 | 0.06 |
Step 1, calculate each rate: rate = volume ÷ time. 1.2 ÷ 15 = 0.08. 3.0 ÷ 15 = 0.20. 4.5 ÷ 15 = 0.30. 0.9 ÷ 15 = 0.06.
Step 2, describe the pattern: the highest rate is bright light with wind (0.30 cm³/min). The lowest is humid, still air (0.06 cm³/min).
Step 3, compare with numbers: bright light against shade is 0.20 ÷ 0.08 = 2.5 times faster. Wind adds 0.30 − 0.20 = 0.10 cm³/min, which is a 50% increase on still air.
Step 4, explain: in bright light the stomata are more open, so more water vapour diffuses out. Wind removes the humid air around the leaf, which keeps the gradient steep. In humid air the gradient is shallow, so less vapour leaves.
The mistake to watch for
Mistaken answer: “Wind makes the plant take up more water because wind pushes the water up the stem.”
The data shows the rate of uptake rose, but wind does not push water. The mechanism is the steeper concentration gradient at the leaf surface, and the uptake rises because the water lost is replaced from the roots. Start from the leaf and work back down the plant.
Another slip is leaving out the unit. A rate of “0.20” scores less than “0.20 cm³/min”.
Check yourself
1. A shoot takes up 2.4 cm³ in 20 minutes. What is the rate?
Show answer
2.4 ÷ 20 = 0.12 cm³/min. Check: 0.12 × 20 = 2.4.
2. A shoot has a rate of 0.18 cm³/min. How much water does it take up in 25 minutes?
Show answer
0.18 × 25 = 4.5 cm³. Check: 0.18 × 100 = 18, and 25 is a quarter of 100, so 18 ÷ 4 = 4.5.
3. Which of the four conditions in the table would you expect to give the lowest rate, and why?
Show answer
Humid, still air (0.06 cm³/min). The air outside already holds a lot of water vapour, so the concentration gradient is shallow and diffusion out of the stomata is slow.
Where this leads next
Continue with tracing energy between a biological process and a physical measurement. The scientific investigation critic helps you judge whether a potometer method controls the right variables, and the mixed practice set tests the full chain.
If table questions are where your marks slip, our teachers can build that habit in online one-to-one Combined Science tuition.