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2.2.3Transport Over Larger Distances

Transpiration

5 min read 9 parts Lesson 13 of 20 1 quiz

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Section 1 of 9What is transpiration?

Transpiration is the loss of water vapour from the leaves of a plant, mainly through the stomata, and the movement of water through the plant that results. It happens as follows:

1
Water in the cell walls of the spongy mesophyll evaporates into the air spaces inside the leaf.
2
Water vapour diffuses out of the leaf through the open stomata, down a concentration gradient into the drier air outside.
3
The loss of water from the leaf pulls more water up through the xylem, because the water molecules stick together (cohesion) in a continuous column.
4
More water enters the root hair cells by osmosis to replace it.

The flow is called the transpiration stream. It supplies water for photosynthesis, keeps cells turgid so that the plant is supported, carries mineral ions to the leaves and cools the leaf as water evaporates.

Section 2 of 9Control by the stomata

The stomata open in the light so that carbon dioxide can enter for photosynthesis, and this inevitably allows water vapour out.

Control by the stomata
If the plant loses water faster than it can take it up, it begins to wilt as cells lose turgor.
To prevent this, the guard cells close the stomata, for example when it is dark or when the plant is short of water.
Plants in dry places often have adaptations that reduce water loss, such as a thick waxy cuticle, few stomata or leaves reduced to spines.

Section 3 of 9Factors affecting the rate of transpiration

FactorEffect on rateExplanation
Light intensityHigher light increases rateStomata open wider in the light
TemperatureHigher temperature increases rateWater evaporates faster and molecules diffuse faster
Air movement (wind)More wind increases rateWind removes water vapour from near the leaf, keeping the concentration gradient steep
HumidityHigher humidity decreases rateThe air already contains a lot of water vapour, so the concentration gradient is less steep

On a hot, dry, windy and sunny day, transpiration is fastest. On a still, humid, cool, dull day it is slowest.

Section 4 of 9Measuring transpiration with a potometer

Potometer apparatus for measuring water uptake
Potometer apparatus for measuring water uptake

A potometer measures the rate at which a cut shoot takes up water, which is taken as an estimate of the rate of transpiration. The method is:

1
Cut a leafy shoot under water, so that no air enters the xylem.
2
Fit it into the potometer under water and make the joints airtight, for example with petroleum jelly.
3
Introduce an air bubble into the capillary tube and record its starting position on the scale.
4
Record how far the bubble moves in a measured time, such as 5 minutes.
5
Calculate the rate as distance moved divided by time (mm per minute).

You can then change one factor, for example turning on a fan, and keep the others constant. Water uptake is not exactly the same as water lost, because some water is used in photosynthesis and some stays in the plant, so it is only an estimate.

Section 5 of 9Calculations and graphs

If the bubble moves 36 mm in 6 minutes, the rate is 36 / 6 = 6 mm per minute.

Calculations and graphs
If you know the internal radius of the capillary tube, the volume of water taken up is the cross-sectional area (pi x r squared) multiplied by the distance moved.
Results are often plotted as rate against the factor being tested.
For light intensity, the curve rises and then levels out, because another factor becomes limiting, such as the number of stomata or the humidity.

Section 6 of 9Wilting and turgor

Turgid and wilted plants compared
Turgid and wilted plants compared

The cells of a well-watered plant are turgid, pushing against their cell walls and supporting the leaves and stems.

Wilting and turgor
If the water lost by transpiration is greater than the water taken up, cells become flaccid and the plant wilts.
The plant recovers when water is supplied again.
Water loss and support are therefore directly linked,
  • and a water shortage reduces photosynthesis because guard cells close the stomata and limit carbon dioxide.

Transpiration and the plant's other needs

Transpiration can seem wasteful, since a plant may lose hundreds of times more water than it uses in photosynthesis, but it cannot be avoided.

1
The same stomata that let carbon dioxide diffuse in let water vapour diffuse out.
2
The plant manages the trade-off: stomata are open when light is available, and closed in darkness or drought.
3
On a hot day the rate of photosynthesis may be limited if the stomata close to save water,
  • so transpiration, photosynthesis and temperature are all connected.
4
In examination questions, always explain a change in rate by referring to the concentration gradient of water vapour between the air spaces and the outside air.

Section 7 of 9Exam-style questions

  1. Explain why a plant loses water faster on a windy day than on a still day. [3]3 marks

  2. In a potometer experiment the bubble moved 45 mm in 9 minutes in still air and 90 mm in 9 minutes with a fan. Calculate the rate in each condition and the percentage increase caused by the fan. [3]3 marks

  3. Suggest two precautions needed to make the potometer results reliable. [2]2 marks

Section 8 of 9Key terms

TermMeaning
TranspirationLoss of water vapour from the leaves by evaporation and diffusion.
Transpiration streamThe flow of water up the xylem from roots to leaves.
CohesionThe attraction between water molecules that keeps them together in a column.
PotometerApparatus used to measure the rate of water uptake by a shoot.
TurgidDescribes a cell full of water and pressing against its wall.
FlaccidDescribes a cell that has lost water and is soft.
WiltingDrooping of a plant when its cells lose turgor.
HumidityThe amount of water vapour in the air.

Section 9 of 9Exam technique

1
Saying that water is pumped up by the roots: it is pulled up by evaporation and cohesion.
2
Saying that transpiration happens only through the roots or that stomata open at night to cool the leaf.
3
Describing higher humidity as increasing the rate: it decreases the gradient and slows loss.
4
Forgetting that a potometer measures uptake, which only approximates transpiration.
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