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3.6.1Exchange and Transport

Transpiration — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 41 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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3.6.1 Transpiration — AQA A Level Biology 7402 notes: Explain how water moves from the soil to the atmosphere through a plant, using the cohesion-tension theory, and evaluate the factors that change the rate of transpiration.Transpiration is the loss of water vapour from the aeri…

  • Apoplast pathway
  • Symplast pathway
  • Casparian strip
  • root pressure
  • xerophytes
  • Exam-style question 1

Section 1 of 8What transpiration is and why it matters

Transpiration is the loss of water vapour from the aerial parts of a plant, mostly through the stomata of the leaves.

What transpiration is and why it matters
It is an unavoidable consequence of gas exchange: stomata must open to let CO2 diffuse in for photosynthesis,
  • and water vapour diffuses out down its water potential gradient at the same time.
Transpiration is not simply wasteful,
  • because the flow of water it drives brings mineral ions to the leaves, supplies water for photosynthesis and cell expansion, keeps cells turgid for support and carries heat away as water evaporates.

Water leaves the plant in a continuous stream. The pathway can be summarised as: soil water, root hair cell, root cortex, endodermis, xylem, leaf mesophyll, air spaces, stomata, atmosphere.

Section 2 of 8Water uptake by the roots

Root hair cells have a large surface area and a lower water potential than the soil solution because their cytoplasm contains dissolved solutes and ions taken up by active transport. Water therefore enters by osmosis. It then crosses the cortex by two routes.

1Apoplast pathway
Apoplast pathway through the cell walls and the spaces between the cellulose microfibrils.
  • There is little resistance, and water is pulled along by the transpiration stream.
2Symplast pathway
Symplast pathway through the cytoplasm of cells, linked from cell to cell by plasmodesmata.

At the endodermis the apoplast route is blocked by the waterproof Casparian strip (a band of suberin). Water must enter the symplast, which allows the plant to control which ions enter the xylem. Active transport of ions into the xylem lowers its water potential, so water follows by osmosis and a small root pressure is generated.

Section 3 of 8The cohesion-tension theory

The main driving force for water movement up the xylem is evaporation from the leaves, not root pressure. The sequence is:

Water evaporates from the moist walls of mesophyll cells into the air spaces and diffuses out through open stomata down a water potential gradient.
Loss of water lowers the water potential of the mesophyll cells, so water is drawn in from neighbouring cells and from the xylem by osmosis.
Tension
Removal of water from the top of the xylem creates a tension (negative pressure) in the water column.
Cohesion
Water molecules are polar and form hydrogen bonds with each other. This cohesion means the water column is pulled up as a continuous unbroken stream.
Adheres
Water also adheres to the hydrophilic lignin walls of the xylem, which helps support the column against gravity.

Because of the tension, xylem vessels are pulled inwards, so the diameter of a tree trunk measurably shrinks during the day when transpiration is high. The thick lignified walls stop the vessels collapsing.

Section 4 of 8Factors affecting the rate of transpiration

FactorEffect on rateReason
Light intensityIncreasesStomata open in the light so more diffusion occurs
TemperatureIncreasesMore kinetic energy, faster evaporation and diffusion, and the air holds more water vapour so the gradient is steeper
HumidityDecreases as humidity risesWater potential of the air rises, so the gradient from leaf to air is smaller
Wind speedIncreasesMoves saturated air away from the stomata, maintaining a steep gradient
Soil water availabilityDecreases when lowGuard cells lose turgor and stomata close

When a plant wilts, cells lose turgor and the stomata close, so transpiration falls and the plant conserves water.

Section 5 of 8Adaptations of xerophytes and links

Plants in dry habitats are called xerophytes. Their adaptations reduce water loss by reducing the surface area for evaporation or by lengthening the diffusion pathway or lowering the gradient.

1
Thick waxy cuticle: reduces evaporation through the epidermis.
2
Sunken stomata, often in pits: trap a layer of humid air, reducing the gradient.
3
Rolled leaves, as in marram grass: trap humid air around the stomata.
4
Hairs on the epidermis: trap humid air and reduce air movement.
5
Reduced leaves or spines: smaller surface area.
6
Extensive or deep root systems: reach more water.

This topic links to osmosis and water potential (unit 2), to gas exchange in plants, and to the practical in the next lesson, where a potometer is used to measure water uptake as an estimate of transpiration.

Section 6 of 8Exam-style questions

  1. Explain how water moves up the xylem from the roots to the leaves. [4]4 marks

  2. A student measured the mass of a leafy shoot before and after 3 hours in a windy, warm room. The mass fell from 12.5 g to 11.0 g. Calculate the percentage loss in mass and suggest why the real loss by transpiration might be slightly different from the mass change. [3]3 marks

  3. Explain why a plant in a hot, dry, windy environment may wilt even if the soil is moist. [3]3 marks

Section 7 of 8Key terms

TermMeaning
TranspirationLoss of water vapour from the aerial parts of a plant, mainly through the stomata.
CohesionAttraction between water molecules due to hydrogen bonding.
AdhesionAttraction between water molecules and the walls of the xylem.
Casparian stripWaterproof band of suberin in the endodermis that blocks the apoplast pathway.
Apoplast pathwayWater movement through cell walls and spaces between cells.
Symplast pathwayWater movement through the cytoplasm via plasmodesmata.
TensionNegative pressure in the xylem caused by water loss from the leaves.
XerophyteA plant adapted to survive in conditions with little available water.

Section 8 of 8Exam technique

1
Saying water is 'sucked up' by the roots. Uptake is passive and is driven mainly by evaporation from the leaves (tension), not by the roots pulling.
2
Confusing cohesion with adhesion. Cohesion is attraction between water molecules; adhesion is attraction between water and the xylem walls.
3
Saying humidity increases the rate of transpiration. Higher humidity reduces the gradient and slows transpiration.
4
Stating that transpiration is a process of active transport. It requires no ATP; it is evaporation followed by diffusion and osmosis.
5
Writing that water evaporates 'from the stomata'. It evaporates from the mesophyll cell walls and diffuses out through the stomata.

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