How do plants move water up the xylem and sugars through the phloem without a pump?
Mass transport in plants: transport of water in the xylem by the cohesion-tension theory and transpiration; transport of organic substances in the phloem by mass flow (the source-to-sink translocation model) and supporting evidence.
An AQA A-Level Biology answer on mass transport in plants. Explains transpiration and the cohesion-tension theory of water movement in the xylem, the factors affecting transpiration rate, and translocation of sugars in the phloem by the mass flow hypothesis with its supporting evidence.
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What this dot point is asking
AQA wants you to explain how water moves up the xylem by the cohesion-tension theory driven by transpiration, the factors that affect transpiration rate, and how organic substances are translocated in the phloem by the mass flow hypothesis - including the evidence for and against it.
Two transport tissues
Plants have two mass transport tissues:
- Xylem carries water and dissolved mineral ions upwards, from roots to leaves. Xylem vessels are dead, hollow tubes with no end walls (a continuous column) and walls strengthened with lignin.
- Phloem carries organic substances (mainly sucrose) from sources to sinks, in either direction. Phloem sieve tubes are living cells with perforated sieve plates, supported by companion cells.
Transpiration
Transpiration is the evaporation of water from the leaves, mainly through the stomata. It is the engine that pulls water up the plant.
Water enters the roots by osmosis (often aided by active transport of mineral ions into the root, lowering root water potential), travels up the xylem, and evaporates from the leaf - a continuous transpiration stream.
Factors affecting transpiration rate:
| Factor | Effect on rate | Why |
|---|---|---|
| Light | Increases | Stomata open in the light for photosynthesis, so more water escapes |
| Temperature | Increases | Faster evaporation and a steeper water-vapour gradient |
| Humidity | Decreases | A humid atmosphere reduces the water-potential gradient out of the leaf |
| Air movement (wind) | Increases | Removes humid air at the leaf surface, maintaining a steep gradient |
A potometer measures the rate of water uptake as a proxy for transpiration rate under different conditions.
The cohesion-tension theory
This explains how water reaches the top of a tall plant without a pump.
- Transpiration pull. Water evaporates from mesophyll cell surfaces and leaves through the stomata, lowering the leaf's water potential.
- Tension. Water is drawn out of the top of the xylem to replace it, putting the water column under tension (negative pressure).
- Cohesion. Water molecules are polar and hydrogen-bond to each other, so they stick together as a continuous, unbroken column.
- Column pulled up. As water leaves at the top, the whole column is pulled up the xylem.
- Adhesion. Water molecules also adhere to the lignified xylem walls, helping the column resist gravity.
Translocation in the phloem: the mass flow hypothesis
Translocation is the movement of organic substances (mainly sucrose) through the phloem from a source (where they are made or stored, e.g. leaves) to a sink (where they are used or stored, e.g. roots, growing buds, fruit).
The mass flow hypothesis explains it:
Evidence for and against mass flow
Supporting evidence:
- Sap is released when phloem is cut, showing it is under pressure.
- The concentration of sucrose is higher in the source (leaves) than the sink.
- Removing a ring of bark (and phloem) causes sugar to accumulate above the ring (a "ringing" experiment).
- Aphid stylets show sap flows faster nearer the source, consistent with a pressure-driven flow.
Objections / counter-evidence:
- Sieve plates would seem to obstruct mass flow (though they may have a supporting role).
- Not all solutes move at the same rate, and some move in different directions at the same time, which simple mass flow struggles to explain fully.
Try this
Q1. Explain the roles of cohesion and adhesion in moving water up the xylem. [2 marks]
- Cue. Cohesion (hydrogen bonding between water molecules) holds the water as a continuous column so it is pulled up as one; adhesion (attraction of water to the xylem walls) helps the column resist gravity and supports the upward movement.
Q2. Describe how a potometer could be used to investigate the effect of wind speed on transpiration rate. [3 marks]
- Cue. Set up the potometer with a leafy shoot, measure the distance an air bubble moves in a set time to find the rate of water uptake; repeat at different wind speeds using a fan while keeping temperature, light and humidity constant; compare the rates.
Q3. Explain how sucrose is loaded into the phloem at a source and how this drives mass flow. [4 marks]
- Cue. Sucrose is actively loaded into sieve tubes by companion cells using ATP, lowering water potential; water enters from the xylem by osmosis, raising hydrostatic pressure at the source; the higher pressure at the source than the sink drives sap to flow by mass flow towards the sink, where sucrose is unloaded.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 AQA Paper 25 marksDescribe the cohesion-tension theory of water movement up the xylem of a plant.Show worked answer →
A 5-mark answer needs evaporation, the tension it creates, cohesion, the continuous column, and adhesion.
- Point 1 (transpiration pull)
- Water evaporates from the surface of mesophyll cells and diffuses out of the leaf through the stomata (transpiration). This lowers the water potential in the leaf.
- Point 2 (tension)
- Water is drawn out of the xylem to replace it, putting the water column under tension (negative pressure).
- Point 3 (cohesion)
- Water molecules are polar and form hydrogen bonds with one another, so they stick together (cohesion). This makes the water move as one continuous column.
- Point 4 (continuous column pulled up)
- As water leaves the top, the whole column is pulled up the xylem from the roots - a continuous, unbroken column.
- Point 5 (adhesion)
- Water molecules also adhere to the walls of the xylem vessels, helping the column resist gravity and supporting the upward pull.
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