AQA Biology 4.2.3.2
Transpiration
Water evaporates from leaves and more is drawn up from the roots. Open and close a stoma, change the weather around a potometer and watch the bubble, then work out the rate of water uptake.
Triple Science · Foundation tier
Start here
Washing on the line.
Wet washing hung out on a line dries in an hour one day, and is still damp by evening on another.
If you had to guess, will a plant lose water faster on a hot, windy day, or a cool, still one?
Stomata and guard cells
Water leaves through the stomata.
Water evaporates from the leaves and leaves as water vapour, mostly through the stomata. More water is drawn up the xylem from the roots to replace it: this is the transpiration stream. Each stoma is a pore between two guard cells, which control gas exchange and water loss. Open and close it.
In light, the guard cells open the stoma. Carbon dioxide gets in for photosynthesis, and water vapour gets out.
Most stomata are on the lower surface of the leaf: in Plant tissues. Water enters root hair cells by osmosis: in Cell specialisation.
Four factors · the potometer
Change the conditions, watch the bubble.
A potometer measures how fast a leafy shoot takes up water. As the shoot takes water in, an air bubble moves along the capillary tube. The further it moves in one minute, the faster the water uptake. Change one condition at a time.
Temperature
Light
Humidity
Air movement
Change one condition. The bubble moved 22 mm in 1 minute.
A potometer measures water uptake. That is close to, but not exactly, the water lost: a little is used by the plant.
Equation · rate of water uptake
The rate is how far the bubble moves in each minute. Time must be in minutes for a rate in mm/min: seconds ÷ 60 gives minutes, so 90 s = 1.5 min.
Equation
Rate of water uptake
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
rate = distance ÷ time
- x
- distance moved by the bubble
- mm
- R
- rate
- mm/min
- t
- time
- min
Repeat each reading, then find the mean: add the readings, then divide by how many there are.
mean = (8 + 10 + 9) ÷ 3 = 9 mm/min
Two steps · volume of water taken up
The water the shoot takes up fills a cylinder of the capillary tube. Step 1 finds its volume from the tube’s radius and the distance the bubble moved. You are usually given the diameter, so halve it first, as its own line:
r = diameter ÷ 2
Keep every length in mm, so the volume comes out in mm3. π is a fixed number, about 3.14.
Equation
Volume of water taken up
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
volume = π r² × distance
- V
- volume
- mm³
- π
- pi, about 3.14
- r
- radius of the tube
- mm
- x
- distance moved by the bubble
- mm
Step 2 divides that volume by the time, for a rate in mm3/min.
Equation
Rate from volume
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
rate = volume ÷ time
- V
- volume
- mm³
- R
- rate
- mm³/min
- t
- time
- min
Think again · humid air
“Humid air makes plants lose more water.”
Your call
ChooseThe air around a plant becomes very humid. What happens to the rate of transpiration?
Sort
Faster or slower?
Place each change.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Explain
- The factor, then why water vapour leaves faster or slower.
- Describe
- How to use a potometer, one factor at a time.
- Predict
- What the bubble will do when a condition changes.
- Calculate
- Distance ÷ time. Or halve, π r² × distance, then ÷ time.
Key fact
Transpiration is the loss of water vapour from the leaves, mostly through the stomata. It is faster when it is warmer, brighter, drier and windier.
Key note · AQA 4.2.3.2
Transpiration
- Transpiration is the loss of water vapour from the leaves, mostly through the stomata. The transpiration stream carries water up the xylem from the roots.
- Each stoma is a pore between two guard cells, which control gas exchange and water loss: open in light, closed in the dark.
- Faster when warmer, brighter, drier (low humidity) and windier.
- A potometer measures water uptake by how far an air bubble moves in a set time. Change one factor; keep the others the same.
- Rate of water uptake = distance moved by the bubble ÷ time. Repeat, then find the mean.
- Volume taken up = π r² × distance (halve the diameter first); then rate = volume ÷ time.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Not sure why humid air slows transpiration?
Bubble distances and tube sizes are made-up round figures for practice.