Describe how water vapour loss is related to leaf structure.

Transport in Plants – Water‑Vapour Loss and Leaf Structure

Learning Objective (AO1)

Describe how water‑vapour loss (transpiration) is related to leaf structure and to the main environmental factors that influence its rate.

Core Requirements (8.1 – 8.4)

1. Xylem and Phloem

• Xylem – conducts water and dissolved minerals from roots to shoots. Consists of long, dead cells with thick lignified walls that form continuous tubes.
• Phloem – conducts organic nutrients (mainly sucrose) from source tissues (e.g., mature leaves) to sink tissues (e.g., growing roots, fruits). Composed of living sieve‑tube elements and companion cells.

2. Water‑Movement Pathway (Core Requirement 8.3)

All steps required by the syllabus are listed below and numbered exactly as expected.

1. Water is absorbed by root hairs from the soil.
2. It passes radially through the root cortex into the root xylem vessels.
3. Water moves upward through the stem xylem to the leaf petiole.
4. In the leaf, water is delivered via the leaf veins (xylem) to the mesophyll cells.
5. Water evaporates from the moist cell walls into the intercellular air spaces.
6. It diffuses out of the leaf through open stomata into the atmosphere.

3. Translocation (Core Requirement 8.4)

• Sucrose is produced in photosynthetic (source) cells of mature leaves.
• It is actively loaded into the phloem sieve‑tube elements, lowering the water potential.
• Water follows by osmosis, creating a pressure gradient that drives the bulk flow of sap toward sink tissues (e.g., roots, developing fruits).
• At the sink, sucrose is unloaded and used for growth or storage.

How Leaf Structure Controls Transpiration

Each anatomical feature influences either the ease with which water vapour can leave the leaf (stomatal conductance, g_s) or the resistance offered by the boundary layer.

Simplified Transpiration Relationship (AO1)

Transpiration rate increases when:

• more stomata are open (greater stomatal conductance, g_s), and
• the difference between the humidity inside the leaf and the surrounding air is larger (greater vapour‑pressure deficit, VPD).

For students who wish to see the formal expression, see the box below.

Environmental Drivers (Supplementary 8.3)

• Temperature – raises leaf temperature, increasing VPD and the kinetic energy of water molecules.
• Wind speed – removes the saturated air layer, thinning the boundary layer and raising VPD.
• Relative humidity – low external humidity creates a larger VPD; high humidity reduces it.
• Light intensity – blue light activates guard‑cell proton pumps, opening stomata and therefore increasing g_s.

Wilting – Link to Turgor Pressure

• Transpiration removes water faster than it can be replaced → leaf cells lose water.
• Loss of water reduces cell turgor pressure (the pressure exerted by the vacuole against the cell wall).
• When turgor falls below a critical level the leaf wilts, signalling that the plant must close stomata or increase water uptake.

Leaf Orientation, Phototropism & Transpiration (Link to 14.5)

• Leaves normally grow towards the light (positive phototropism) to maximise photosynthesis.
• The same orientation also increases exposure to solar heat and wind, which raises leaf temperature and reduces the boundary layer, thereby increasing transpiration.
• Thus, leaf orientation is a trade‑off between carbon gain and water loss.

Practical Investigation Ideas (AO3)

Diagram‑Labeling Practice (AO1 – Sense Organs 14.2)

Provide students with a labelled cross‑section of a leaf showing:

• Cuticle
• Upper epidermis
• Palisade mesophyll
• Spongy mesophyll
• Lower epidermis
• Stomata with guard cells
• Trichomes (leaf hairs)
• Veins (xylem and phloem)

Ask them to:

1. Label each part.
2. Draw arrows to show the direction of water movement from the xylem to the atmosphere.
3. Indicate where photosynthetic CO₂ enters the leaf.

Summary

Transpiration links leaf anatomy, environmental conditions and the physical properties of water. Structural traits such as stomatal density, guard‑cell control of aperture, cuticle thickness, leaf area, orientation and trichomes directly modify stomatal conductance and the boundary‑layer resistance. Temperature, wind, humidity and light alter the vapour‑pressure deficit, the driving force for water loss. Together these factors explain how plants balance the need for CO₂ uptake with the risk of excessive water loss, and they provide a solid basis for practical investigations that meet AO3 requirements.