Lesson 5: Transport in Plants

Plants cannot walk to water or pump blood — yet a coast redwood lifts water 100 metres without a single moving part, and every leaf exports sugar through living pipelines. Plant transport is a triumph of physics: osmosis, cohesion, tension and pressure gradients doing the work of hearts. This lesson covers water uptake from the soil (water potential, osmosis, the apoplast/symplast/vacuolar pathways), xylem and the cohesion–tension theory of the transpiration stream, factors affecting transpiration and how the potometer measures it, xerophytic adaptations, and phloem translocation via the mass-flow hypothesis with its evidence.

Water potential and uptake from the soil

Water potential (Ψ, psi) measures the tendency of water to move: pure water has Ψ = 0 kPa, and all solutions have negative Ψ (solute lowers it). Water moves from higher (less negative) to lower (more negative) water potential by osmosis. Root-hair cells have lower Ψ than soil water (dissolved ions, sugars), so water enters by osmosis; mineral ions enter by diffusion and active transport (against the gradient, using ATP from respiration) — the active uptake of ions lowers Ψ further, drawing in more water.

Three pathways cross the root cortex: the apoplast (through cell walls, fast, but blocked at the endodermis by the waterproof Casparian strip), the symplast (through cytoplasm via plasmodesmata, selective) and the vacuolar pathway (through vacuoles). The Casparian strip forces all water and ions through the endodermal cell membranes (the symplast route), giving the plant control over what enters the xylem — toxic ions can be excluded, needed ions actively pumped in.

Root hairs massively increase surface area for absorption (Fick's law again); mycorrhizal fungi extend the absorptive network further (Lesson 14). Waterlogging suffocates roots (no oxygen for respiration → no ATP for active ion uptake → wilting despite wet soil) — a favourite "explain the observation" question linking respiration, active transport and water uptake.

Xylem and the cohesion–tension theory

Xylem vessels are dead, hollow tubes: no cytoplasm, no end walls, walls strengthened with lignin (waterproof, and resisting the tension that would collapse them). This structure is pure function — an inert pipeline. Water moves up in the transpiration stream driven by transpiration pull from the leaves, explained by the cohesion–tension theory.

The theory in four steps: (1) water evaporates from mesophyll cell walls into leaf air spaces and diffuses out through stomata (transpiration), lowering the water potential of mesophyll cells; (2) these cells draw water from the xylem by osmosis; (3) this creates tension (negative pressure) in the xylem water column; (4) the column is pulled up as a continuous thread because water molecules cohere (hydrogen bonding) to each other and adhere to the xylem walls. Capillarity plays only a minor role — the column is pulled, not pushed.

Evidence for cohesion–tension: water continues to rise in stems even when the roots are removed (a cut stem in water still transpires — ruling out root pressure as the main driver); tensions measured in xylem by pressure probes match predictions; and vulnerability to cavitation (air bubbles breaking the column under extreme tension — drought embolism) confirms the column really is under tension. Root pressure (osmotic push from the roots) contributes only in small plants at night. Exam answers must give the four steps plus at least one piece of evidence.

Transpiration — factors and measurement

Transpiration rate increases with: higher temperature (faster evaporation, steeper gradient), lower humidity (steeper water-potential gradient between leaf and air), air movement/wind (sweeps away the humid boundary layer, maintaining the gradient) and light (stomata open for photosynthesis). These factors are tested with a potometer: a cut shoot in a capillary tube, measuring water uptake (assumed approximately equal to transpiration over short periods) as an air bubble moves.

Potometer technique: cut the shoot underwater (to prevent air locks), seal joints with petroleum jelly, ensure no air bubbles except the marker bubble, and allow equilibration. Change one variable at a time (fan on/off, humid bag vs dry air, dark vs light). Limitations: it measures water uptake, not transpiration directly (some water is used in photosynthesis or retained as cells grow); cutting the shoot wounds it; conditions are artificial. Suggest improvements: longer equilibration, repeats, controlling temperature with a water bath.

Data interpretation: calculate rates (distance moved ÷ time, or volume per unit time using the capillary's cross-section), plot bar charts for discrete variables, and explain each result via the gradient/evaporation mechanism. "Explain why the bubble moves faster in windy conditions" → wind removes the saturated boundary layer → steeper water-potential gradient → faster diffusion of water vapour → faster uptake. Always complete the causal chain.

Xerophytic adaptations and water conservation

Xerophytes reduce transpiration while keeping stomata functional: thick waxy cuticles (waterproof barrier), sunken stomata in pits lined with hairs (trapping moist air, reducing the gradient), rolled leaves (marram grass — stomata on the inner surface), reduced surface area (cactus spines are modified leaves; photosynthesis moves to the fleshy stem) and water-storage parenchyma. Each adaptation maps onto the transpiration factors: cuticle and pits reduce the effective gradient; small area reduces the surface for loss.

Stomatal control is active: guard cells open and close via K⁺ fluxes and osmosis (Lesson 3), and the hormone abscisic acid (ABA, Lesson 17) triggers closure during drought. Wilting itself reduces transpiration (drooping leaves expose less area) — a last-resort response. Hydrophytes show the reverse adaptations: large thin leaves, stomata on upper surfaces, reduced cuticles — maximising gas exchange where water is unlimited.

Comparing xerophytes, mesophytes and hydrophytes is a classic 6-marker: describe each habitat's water challenge, then explain three adaptations per plant type with mechanisms. The strongest answers quantify the trade-off: every anti-transpiration adaptation potentially limits CO₂ uptake and photosynthesis — desert plants grow slowly because water conservation constrains carbon gain.

Phloem — translocation and the mass-flow hypothesis

Phloem transports assimilates (mainly sucrose, plus amino acids and hormones) from sources (photosynthesising leaves, or storage organs in spring) to sinks (roots, fruits, growing buds). Sieve-tube elements are living but have no nucleus and few organelles, with perforated sieve plates; companion cells (with nuclei, dense cytoplasm, many mitochondria) service them via plasmodesmata, loading sucrose and providing ATP. This division of labour is a key structure–function point.

The mass-flow hypothesis: (1) sucrose is actively loaded into sieve tubes at the source (lowering their water potential); (2) water follows by osmosis from the xylem, raising hydrostatic pressure; (3) at the sink, sucrose is unloaded (used or stored), water potential rises, water leaves by osmosis, lowering pressure; (4) the pressure gradient drives bulk flow of sap from source to sink. It is a pressure-driven bulk flow, not diffusion — explaining the observed speeds (~1 m h⁻¹, far faster than diffusion could manage).

Evidence: aphid stylets (aphids pierce sieve tubes; sap exudes under pressure — and the exudate is sucrose-rich, confirming both content and pressure); ringing experiments (removing a ring of bark/phloem stops sugar reaching roots below it, which starve — while xylem transport continues); radioactive tracers (¹⁴CO₂ fed to a leaf appears in the roots via the phloem); and metabolic poisons (cyanide stops translocation by halting ATP-dependent loading). Limitations: sieve plates should impede flow more than observed; not all solutes move at the same speed; the mechanisms of loading/unloading are still debated. Evaluation questions ask you to weigh evidence against limitations — conclude that mass flow is the best-supported model despite the gaps.

Worked example — potometer rate calculation

A potometer's capillary has diameter 1.0 mm (radius 0.5 mm). In still air the bubble moves 24 mm in 10 minutes. Volume taken up = π r² × length = π × 0.25 × 24 ≈ 18.8 mm³, so the uptake rate = 18.8 ÷ 10 ≈ 1.9 mm³ min⁻¹. With a fan running, the bubble moves 57 mm in 10 minutes (≈ 4.5 mm³ min⁻¹ — the same π r² conversion). Explain the increase: the fan removes the humid boundary layer clinging to the leaf, steepening the water-potential gradient between leaf air spaces and the atmosphere, so water vapour diffuses out faster and uptake rises to replace it. Note also what the instrument cannot tell you: how much of the uptake was lost by transpiration versus retained — a precise answer calls this an estimate of transpiration, not a direct measure.

Comparing xylem and phloem answers most "compare and contrast" questions directly — xylem: dead, hollow, lignified vessels; water + mineral ions; one direction (up); driven by transpiration pull (tension); no ATP required. Phloem: living sieve tubes + companion cells; sucrose/amino acids; source-to-sink (direction varies seasonally); driven by pressure gradients built by active loading; requires ATP. Learn the comparison as a table, not prose.

Common mistakes

• "Water is pushed up the xylem by root pressure" as the main mechanism. Root pressure is small and absent in tall trees; the stream is pulled by transpiration under tension. Direction of force is the mark.
• Confusing the pathways. Apoplast = cell walls only (blocked by the Casparian strip); symplast = cytoplasm via plasmodesmata; vacuolar = across vacuoles. The strip forces water through membranes, giving selectivity.
• "The potometer measures transpiration." It measures uptake, and only approximates loss. Writing "rate of transpiration = …" without the caveat loses the evaluation mark.
• Mass flow without the water. Sucrose loading alone moves nothing: the marks are for loading → lower Ψ → water enters by osmosis → hydrostatic pressure rises. Miss the osmotic step and the mechanism collapses.
• Ψ sign errors. Water always moves from less negative Ψ to more negative Ψ. "Water moves from high to low water potential" is only correct if you treat "low" as "more negative" — say so explicitly.

Exam Experience

Q1 (data). A potometer with a capillary of radius 0.4 mm records a bubble movement of 40 mm in 8 minutes. Calculate the rate of water uptake in mm³ min⁻¹ and state one reason this only estimates the transpiration rate. [4 marks] Q2. Describe the mechanism of mass flow in the phloem, and give one piece of evidence supporting it. [5 marks]

Mark scheme — Q1: Volume = π × 0.4² × 40 = π × 0.16 × 40 ≈ 20.1 mm³ [1]; rate = 20.1 ÷ 8 ≈ 2.5 mm³ min⁻¹ [1]. Estimate because some water is used in photosynthesis or retained by growing cells [1]; or the shoot is cut/wounded and conditions are artificial [1].

Mark scheme — Q2: Sucrose is actively loaded into sieve tubes at the source [1]; water potential falls, water enters by osmosis from the xylem [1]; hydrostatic pressure rises at the source; at the sink sucrose is unloaded, water leaves, pressure falls [1]; sap flows in bulk down the pressure gradient [1]. Evidence (any one): aphid stylet exudate is sucrose-rich and under pressure / ringing a stem stops sugar movement below the ring / ¹⁴C tracer moves from leaf to sink via phloem [1].