SPEC 3.3.4.2 - MASS TRANSPORT IN PLANTS
Mass Transport in Plants
Xylem and cohesion-tension theory. Phloem and the mass flow hypothesis. Tracer and ringing evidence.
Why Plants Need Transport Systems
Plants are large, multicellular organisms. Diffusion alone is too slow to supply every cell, so plants have two separate vascular tissues:
Xylem
Transports water and dissolved mineral ions from roots to the rest of the plant. One-way flow: upwards.
Phloem
Transports organic substances — mainly sucrose, plus amino acids. Flow direction depends on need (source → sink), so can be up OR down the plant.
Big Picture
Xylem and phloem run side-by-side in vascular bundles — from roots, through stem, into leaves. In this lesson we explain how substances move through each tissue and what evidence supports the proposed mechanisms.
Xylem Structure
XYLEM
Plant vascular tissue that transports water and mineral ions from roots to leaves. Made of dead, hollow, lignified cells joined end-to-end.
Features Adapted for Function
Dead cells → no cell contents → hollow tubes allow uninterrupted water flow
No end walls between cells → form continuous tubes from roots to leaves
Lignified walls → lignin makes walls strong and waterproof → prevents vessels collapsing under tension (negative pressure) and stops water leaking out sideways
Pits in the walls → allow water to move laterally between vessels (useful if one vessel becomes blocked)
Narrow diameter → helps maintain the water column by cohesion
Linking Structure to Function
Every feature of xylem is aimed at supporting a continuous, unbroken column of water being pulled upwards under tension. Lose the column (air bubble / "embolism") and transport in that vessel stops — hence the pits provide a lateral bypass.
Transpiration
TRANSPIRATION
The loss of water vapour from the aerial parts of a plant, mainly through the stomata on the underside of leaves.
1
Water evaporates from the surface of mesophyll cells into the air spaces of the leaf.
2
Water vapour accumulates in these air spaces → higher water vapour concentration inside the leaf than outside.
3
Water vapour diffuses out through the open stomata down the water vapour concentration gradient (high inside leaf → low outside).
Why It Matters
Transpiration is a side-effect of having stomata open for gas exchange (CO₂ in for photosynthesis). But it also does important jobs: drives water transport up the xylem, cools the leaf, and delivers mineral ions in the transpiration stream.
The Pathway: Root → Leaf
1
Uptake at the root: Water moves from the soil into root hair cells by osmosis (soil water has higher water potential than root cell cytoplasm).
2
Across the root cortex: Water moves from cell to cell (by osmosis) or via the cell walls, down a water potential gradient → into the xylem.
3
Up the xylem: Water moves up the stem in a continuous column — the mechanism is the cohesion-tension theory (next slide).
4
Into the leaf: Water leaves the xylem in the leaf → moves across mesophyll cells by osmosis → evaporates from cell surfaces → diffuses out through stomata.
Key Idea
Throughout the pathway, water moves down a water potential gradient — from high (soil) to low (air outside the leaf). No energy is required from the plant to move water upward; the evaporation from leaves provides the driving force.
Cohesion-Tension Theory
COHESION-TENSION THEORY
The mechanism of water transport in the xylem: transpiration from leaves creates tension (negative pressure) that pulls a continuous column of water up through the xylem. Water molecules are held together by cohesion (hydrogen bonds) between them.
1
Water evaporates from mesophyll cell surfaces and diffuses out through stomata (transpiration).
2
This lowers the water potential of the mesophyll cells → they draw water from neighbouring cells → ultimately from the xylem in the leaf.
3
Removal of water from the top of the xylem creates tension (negative pressure) throughout the water column.
4
Cohesion: water molecules are attracted to each other by hydrogen bonds → they stick together, so the tension is transmitted down the whole column → water is pulled up from the roots.
5
Adhesion: water molecules are also attracted to the xylem walls → helps maintain the column and prevent break-up.
2024 AS Examiner Alert
In June 2024, only a minority of students could clearly state that tension (negative pressure) is CREATED BY the evaporation of water from leaves. Don't just describe what happens in a plant — make the causal link: evaporation → water leaves xylem at the top → tension created → column pulled up by cohesion.
Evidence for Cohesion-Tension
The cohesion-tension theory is supported by multiple lines of observation:
Evidence 1 — Tree Trunk Diameter
Tree trunks show a measurable decrease in diameter during the day (when transpiration is highest) and recover at night. This matches the prediction that xylem is under tension during the day — the walls are pulled inwards.
Evidence 2 — Broken Xylem Stops Water Flow
If a xylem vessel is broken / air is introduced (an "air lock"), water stops moving up that vessel. This shows the water column is under tension and requires continuity — consistent with the theory.
Evidence 3 — Dye Uptake
Cut stems placed in dye show the dye moving up the xylem, the same direction as water, and only through the xylem.
Evidence 4 — Pressure Measurements
Direct measurements inside xylem give negative pressure values (tension) during the day, consistent with water being pulled up rather than pushed up.
Factors Affecting Transpiration Rate
Anything that changes either the rate of evaporation from mesophyll surfaces or the diffusion gradient for water vapour out of the leaf will alter transpiration rate.
FactorChangeEffect on RateWhy
Light intensity↑ Light↑ RateStomata open wider in light (for photosynthesis) → larger surface for water vapour to diffuse out
Temperature↑ Temp↑ RateWater molecules gain kinetic energy → faster evaporation; air holds more water vapour → steeper gradient
Humidity↑ Humidity↓ RateMore water vapour outside the leaf → reduced concentration gradient between leaf and air
Wind speed↑ Wind↑ RateRemoves water vapour from around stomata → maintains steep concentration gradient
Link to Fick's Law
All these factors map directly onto Fick's Law: rate of diffusion ∝ surface area × concentration difference ÷ distance. Stomata opening changes surface area; humidity/wind change the concentration difference.
Measuring Transpiration: The Potometer
A potometer measures water uptake by a cut shoot — which is a good estimate of transpiration rate (most water taken up is lost by transpiration).
1
Set up underwater — cut the shoot and insert it into the apparatus underwater so no air bubbles enter. Air bubbles break the water column and stop measurements.
2
Seal joints with petroleum jelly to prevent air leaks or water evaporation from anywhere except the leaves.
3
Introduce a single air bubble into the capillary tube → measure how far the bubble moves in a fixed time. Bubble movement = water uptake.
4
Change one variable (e.g. light intensity, temperature, humidity, wind speed) while controlling the others. Repeat, and take the mean.
Key Limitation
A potometer measures water uptake, not directly transpiration. A small amount of water is used in photosynthesis and kept in cells as they grow. But because >99% of water taken up is lost by transpiration, uptake is a very good approximation.
Phloem Structure
PHLOEM
Plant vascular tissue that transports organic substances — mainly sucrose (and some amino acids) — from sources to sinks.
Sieve Tube Elements
Living cells but with very few organelles
Joined end-to-end to form long tubes
End walls are perforated sieve plates — let sap flow through
Little cytoplasm → sap flows easily
Companion Cells
Adjacent to each sieve tube element
Full set of organelles including many mitochondria
Provide ATP for active loading of sucrose into the phloem
Control the metabolism of the sieve tubes
Translocation: Source to Sink
TRANSLOCATION
The transport of organic substances (mainly sucrose) in the phloem from sources to sinks.
Source
A region that produces or releases sucrose into the phloem. Typically: photosynthesising leaves. Also: tubers and bulbs at the start of a growing season (when stored starch is mobilised).
Sink
A region that uses or stores sucrose. Examples: roots, growing tips (meristems), developing fruits, storage organs (tubers, bulbs) building up stores.
Same Organ, Different Roles
A potato tuber is a sink in summer (being loaded with starch from leaves) but becomes a source in spring (as starch is hydrolysed to sucrose and exported to new growing shoots). So source and sink are functional, not fixed — it depends on the plant's current needs.
The Mass Flow Hypothesis
This is AQA's required explanation for how sucrose moves through phloem. The driver is a hydrostatic pressure gradient from source to sink.
1
At the source (e.g. a leaf): sucrose is actively loaded into companion cells and then into sieve tube elements (uses ATP from the many mitochondria in companion cells).
2
This lowers the water potential of the phloem sap at the source → water enters the phloem from adjacent xylem by osmosis.
3
Water entering increases the volume → hydrostatic pressure rises in the phloem at the source.
4
At the sink: sucrose is unloaded (used for respiration or converted to starch for storage). Water potential of phloem sap rises → water leaves by osmosis → hydrostatic pressure in the phloem falls at the sink.
5
Result: a pressure gradient from source (high P) to sink (low P) → the sucrose solution flows by bulk (mass) flow from source to sink through the sieve tubes.
Evidence FOR Mass Flow
1. Aphid Stylet Experiments
Aphids feed on phloem by inserting their mouthparts (stylets) into sieve tubes. If the aphid body is cut off leaving the stylet behind, phloem sap oozes out — showing the phloem is under positive pressure, as the mass flow hypothesis predicts.
2. Sap Flows Faster Than Diffusion
The rate of sap movement in phloem (~1 m hr⁻¹) is far faster than diffusion could achieve — consistent with a bulk-flow mechanism driven by a pressure gradient.
3. Sucrose Concentration Gradient
Sucrose concentration in the phloem is higher at sources than at sinks — as expected if sucrose is loaded at sources and unloaded at sinks.
4. Metabolic Inhibitors Stop Translocation
Adding metabolic poisons (e.g. cyanide, which blocks respiration) stops translocation — consistent with the hypothesis that loading at the source requires ATP from companion-cell mitochondria.
Evidence AGAINST Mass Flow
AQA explicitly requires you to evaluate evidence for and against the hypothesis. It's still called a hypothesis because some observations don't fit neatly:
1. Different Solutes Move at Different Rates
In a simple bulk-flow model, all solutes should move at the same rate (they're all being carried by the same stream of water). But in reality, sucrose and amino acids move at different rates — suggesting selective transport mechanisms, not just passive bulk flow.
2. Role of Sieve Plates
Sieve plates would impede pure bulk flow — they are a physical barrier. If phloem were a straightforward mass-flow pipe, we might not expect these perforated end-walls. Their function in the mechanism remains debated.
3. Active Loading Required
Pure mass flow is a passive process driven by pressure alone. But translocation requires ATP (shown by metabolic inhibitor experiments) — this ATP is used for active loading at the source, meaning the system is not purely a simple passive bulk-flow pipe.
Exam Skill
AQA questions on mass flow often ask you to evaluate — you must present BOTH sides. If you only write the "for" evidence, you cap your marks at the lower end of the scheme. Always include at least one "against" point.
Ringing Experiments
A ringing experiment removes a complete ring of bark (including the phloem, which lies in the bark) from around a tree trunk, leaving the xylem intact underneath.
1
Tissue above the ring swells (with sucrose accumulating) because sucrose made in the leaves cannot travel past the cut down to the roots.
2
Tissue below the ring eventually dies — the roots are deprived of sucrose.
3
Water transport (xylem) is unaffected — the leaves still receive water.
What This Shows
Phloem is responsible for transporting organic substances (sucrose), not the xylem — because cutting it stops sugar movement but not water movement.
Phloem transport is normally downward from leaves to roots (at least in summer, when leaves are the main source).
Tracer Experiments
Radioactive ¹⁴C (a radioactive isotope of carbon) is used to track where newly made organic substances go in the plant.
1
A plant is placed in an atmosphere containing ¹⁴CO₂. In the leaves, photosynthesis incorporates the ¹⁴C into glucose, which is converted to ¹⁴C-labelled sucrose for transport.
2
After a period of time, the plant is placed against photographic film (autoradiography). Areas containing ¹⁴C expose the film.
3
Results: the radioactive label appears only in the phloem, moving from the leaves (source) to sinks (roots, fruits, growing tips).
What This Shows
The location of the label confirms: (i) phloem is the tissue transporting sucrose, (ii) movement is from source to sink, (iii) leaves (where photosynthesis occurred) are the source. All consistent with the mass flow hypothesis.
Xylem vs Phloem: Side by Side
FeatureXylemPhloem
What it transportsWater + mineral ionsSucrose + amino acids
CellsDead, hollow, lignified vesselsLiving sieve tubes + companion cells
End wallsAbsent — continuous tubePerforated sieve plates
Direction of flowONE way: roots → leaves onlyBOTH ways: source to sink (up or down)
Driving forceTranspiration pull (tension, negative pressure)Hydrostatic pressure gradient (positive pressure)
Energy requirementPassive — no ATP from plant needed for transport itselfActive loading at source requires ATP
MechanismCohesion-tension theoryMass flow hypothesis
Practice Question
Using the cohesion-tension theory, explain how water moves from the roots of a tree to the leaves at the top. (5 marks)
MP1
Water evaporates from mesophyll cells in the leaves and diffuses out through open stomata (transpiration).
MP2
This evaporation creates tension (a negative pressure) at the top of the xylem — the critical causal link.
MP3
Water molecules are attracted to each other by hydrogen bonds (cohesion), so they stick together in a continuous column.
MP4
The tension is transmitted down the whole column → water is pulled up from the roots.
MP5
Adhesion of water to the xylem walls also helps maintain the column / water moves into the roots by osmosis down a water potential gradient.
Examiner tip
The crucial mark often missed is MP2 — many students describe the process without explicitly linking evaporation → tension. Always make that causal connection.
Summary
The Essentials
Xylem = water + mineral ions, one-way up. Dead, hollow, lignified cells. Driven by cohesion-tension .
Cohesion-tension theory: evaporation from leaves (transpiration) creates tension at the top of the xylem → hydrogen bonds between water molecules (cohesion) transmit the pull down → water pulled up from roots. Adhesion to walls helps.
Transpiration rate ↑ with light, temperature, wind; ↓ with humidity. Measured using a potometer .
Phloem = sucrose + amino acids, source to sink (can be up or down). Living sieve tubes with perforated sieve plates; adjacent companion cells with many mitochondria provide ATP for loading.
Mass flow hypothesis: active loading at source → ↓ water potential → water enters by osmosis → ↑ hydrostatic pressure. Unloading at sink has opposite effect. Pressure gradient drives bulk flow.
Evidence FOR mass flow: aphid stylets show positive pressure; rate faster than diffusion; sucrose gradient source→sink; metabolic poisons block transport.
Evidence AGAINST: different solutes move at different rates; sieve plates seem to impede flow.
Ringing experiments: removing the phloem ring stops sugar transport but not water → proves phloem transports organic solutes.
Tracer experiments (¹⁴C): label moves from photosynthesising leaves to sinks via phloem.
Top Exam Tips
Cohesion-tension: always make the causal link — evaporation creates tension — don't just describe.
Mass flow: state "active loading" (using ATP) at the source, not just "loading."
Evaluate means present BOTH for and against. Don't just list supporting evidence.
Watch terminology: water moves by osmosis (never "active transport"). Ions move by active transport or diffusion (never "osmosis").
Next: Why Plants Need Transport Systems