explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere

Transport in Plants – Cambridge AS & A‑Level Biology (Topic 7)

7.1 Structure of Transport Tissues

Dicot stem (transverse section)

• Xylem – located towards the centre, consists of vessel elements (large diameter, perforation plates) and tracheids (narrow, with pits). Provides a continuous water‑conducting column.
• Phloem – external to the xylem, composed of sieve‑tube elements (living cells with sieve plates) and companion cells (metabolically active, control loading/unloading).
• Vascular cambium – lateral meristem that adds secondary xylem (inward) and secondary phloem (outward) in woody stems.

Root (cross‑section)

• Root epidermis – outermost layer; in mature roots often replaced by root hairs (increase surface area for water uptake).
• Cortex – parenchyma cells through which water moves radially.
• Endodermis – single cell layer with the Casparian strip (suberin lamella) that blocks apoplastic flow, forcing water to cross the plasma membrane (symplastic route).
• Pericycle – gives rise to lateral roots; lies just inside the endodermis.
• Vascular cylinder – central xylem and phloem (similar arrangement to the stem).

Leaf (cross‑section)

• Epidermis – outer layer, contains stomata (pores) surrounded by guard cells.
• Mesophyll – palisade (columnar cells) and spongy (irregular cells with intercellular air spaces) where photosynthesis and transpiration occur.
• Veins – bundles of xylem (water upward) and phloem (sugar transport outward).

7.2 Transport Mechanisms

7.2.1 Root Water Uptake – Apoplast and Symplast Pathways

• Apoplastic route – water moves through cell walls and intercellular spaces without crossing membranes.
• Symplastic route – water enters the cytoplasm via plasma‑membrane aquaporins, then passes from cell to cell through plasmodesmata.
• The Casparian strip in the endodermis blocks the apoplastic pathway; water must enter the symplast, ensuring that solutes are screened before entering the xylem.
• Root‑pressure (generated by active uptake of ions into the xylem) can push water upward, especially at night or in well‑watered plants.

7.2.2 Transpiration – Evaporation and Diffusion

Transpiration is the loss of water from aerial parts of the plant, principally the leaves, to the atmosphere. It occurs in two consecutive stages.

Step 1 – Evaporation

• Water from mesophyll cells and intercellular air spaces moves to the inner surface of the epidermis.
• At the epidermal surface the liquid water changes to vapour – this is evaporation.
• The vapour first enters the thin boundary layer of still air that clings to the leaf surface (see sketch in Figure 1).

Step 2 – Diffusion

• Water vapour diffuses from the leaf surface, through the boundary layer, into the surrounding air.
• Diffusion is driven by a vapour‑pressure (or water‑potential) gradient: Ψ_leaf < Ψ_air.
• When the gradient disappears, net water loss stops.

The overall rate of transpiration therefore depends on the water‑potential gradient between the leaf interior and the atmosphere.

7.2.3 Cohesion‑Tension Theory (Water Movement in the Xylem)

• Evaporation from the leaf creates a very negative water potential in the mesophyll.
• This negative potential generates a tensile (pulling) force that is transmitted down the continuous water column in the xylem.
• Water molecules stick together (cohesion) and to the walls of the vessels (adhesion), allowing the column to transmit the tension without breaking.
• If the tension becomes excessive, air bubbles (cavitation) can form, interrupting the column.

7.2.4 Phloem Transport – Pressure‑Flow (Mass‑Flow) Hypothesis

• Source (e.g., mature leaf) loads sucrose into sieve‑tube elements via active transport (H⁺‑ATPase in companion cells creates a proton gradient).
• Loading raises the solute potential (Ψ_s) → more negative, causing water to enter osmotically, raising the pressure potential (Ψ_p) in the source sieve tubes.
• The resulting hydrostatic pressure gradient drives bulk flow of sap toward the sink (e.g., growing root, fruit) where sucrose is unloaded, lowering Ψ_s and allowing water to exit.
• Flow is rapid and bidirectional only in the sense that different sieve tubes can act as source or sink simultaneously.

Water Potential (Ψ)

For the Cambridge syllabus the simplified equation is:

\$\Psi = \Psis + \Psip\$

• Ψ_s (solute potential) – always negative; the more solutes, the more negative the value.
• Ψ_p (pressure potential) – positive in turgid cells, zero in dead cells, negative when the water column is under tension (xylem).

Note: Matric (Ψ_m) and gravitational (Ψ_g) potentials are advanced topics and are not required for the exam.

Worked example – calculating leaf water potential

Given:

• Solute potential, Ψ_s = –0.15 MPa
• Pressure potential, Ψ_p = +0.05 MPa (turgor in a well‑watered leaf)

Calculation:

\$\Psi_{\text{leaf}} = (-0.15) + (+0.05) = -0.10\ \text{MPa}\$

If the surrounding air has an effective water potential of ≈ 0 MPa, water will move from the leaf (more negative) to the air → transpiration occurs.

Common pitfalls (exam tip box)

• Always keep track of signs: Ψ_s is negative, Ψ_p can be positive (turgor) or negative (tension).
• Do not forget to include both components; many students incorrectly set Ψ_p = 0 for a leaf under tension.
• When comparing water potentials, the direction of flow is from the higher (less negative) to the lower (more negative) value.

Factors Influencing the Rate of Transpiration

Stomatal Regulation and Guard‑Cell Physiology

• Guard cells flank each stomatal pore and control its aperture.
• In light, K⁺ and Cl⁻ are actively taken up; water follows osmotically, the guard cells swell, and the stomata open → increased transpiration and CO₂ uptake.
• During drought, low leaf water potential triggers loss of K⁺/Cl⁻, water exits, guard cells shrink and stomata close → water loss is reduced.
• Other signals that promote closure: high internal CO₂, the hormone abscisic acid (ABA), and low blue light.

Suggested Diagrams (for revision)

1. Leaf cross‑section showing (a) mesophyll cells, (b) intercellular air spaces, (c) stomata with guard cells, (d) thin boundary layer, and arrows indicating the two‑stage water movement (evaporation → diffusion).
2. Root cross‑section highlighting the apoplastic and symplastic pathways, Casparian strip, and direction of root‑pressure flow.
3. Phloem pressure‑flow model from a source leaf to a sink organ (e.g., fruit), with loading/unloading steps.