Mass transport in plants
AQA
A-Level
Xylem vessels are dead, hollow cells stacked end to end with their end walls broken down, and with walls thickened by lignin, so they form a continuous open tube from root to leaf. Cohesion-tension theory explains how water rises through them without any cell doing work. Water evaporates from the surfaces of mesophyll cells and diffuses out through the stomata, which is transpiration. This lowers the water potential of those cells, so water is drawn out of the xylem in the leaf, and that creates tension, a negative pressure, at the top of the column. Water molecules are polar and form hydrogen bonds with one another, so they stick together; this cohesion means the entire column is pulled up as a single unit. Water also adheres to the lignified walls. The column must be unbroken, which is why lignin holding the vessel open matters and why an air bubble stops the flow entirely.
12
Objectives
12
Exam Tips
18
Pitfalls
18
Key Terms
20
Mark Points
Subtopics in this area
Xylem as the tissue that transports water in the stem and leaves of plants. The cohesion-tension theory of water transport in the xylem.
Phloem as the tissue that transports organic substances in plants.
The mass flow hypothesis for the mechanism of translocation in plants. The use of tracers and ringing experiments to investigate transport in plants.
Students should be able to: recognise correlations and causal relationships interpret evidence from tracer and ringing experiments and to evaluate the evidence for and against the mass flow hypothesis.
Mass transport in plants Revision Guide
Learning Objectives
What you need to know and understand
- Explain how evaporation from a leaf creates the tension that pulls water up a xylem vessel.
- Explain the importance of hydrogen bonding between water molecules to cohesion-tension.
- Predict the effect on water transport of an air bubble in a xylem vessel or of a vessel that has lost its lignin.
- Describe how sucrose is loaded into a sieve tube element, naming the role of the companion cell and of ATP.
- Compare the structure of phloem with that of xylem and relate each difference to what the tissue transports.
- Identify sources and sinks in a given plant and state the direction of transport between them.
- Explain how loading sucrose at a source generates the hydrostatic pressure that drives mass flow.
- Predict and explain the appearance of a ringed stem above and below the ring.
- Describe how radioactive tracers and autoradiography are used to investigate transport in plants.
- Evaluate a set of results as evidence for and against the mass flow hypothesis, quoting data on both sides.
- Explain why a correlation between two variables in a transport experiment does not establish cause.
- Identify a limitation of a ringing or tracer experiment and state what further evidence would strengthen the conclusion.
Marking Points
Key points examiners look for in your answers
- one mark for an unbroken continuous column of water, with no barrier to its movement
- one mark for cohesion between water molecules caused by hydrogen bonding between polar molecules
- one mark for evaporation or transpiration from the leaf creating tension in the column
- one mark for water being pulled up the xylem by that tension rather than pushed up from below
- one mark for a correct reference to cohesion-tension causing the movement of water, credited as a principle mark where the detailed points are not gained
- one mark for sucrose being actively transported, or co-transported with hydrogen ions, into the phloem or sieve tube
- one mark for naming the companion or transfer cell as the site at which loading occurs
- one mark for movement from source to sink, with a named sink such as respiring cells, a storage organ, a root tip or a developing fruit
- one mark for sucrose being unloaded at the sink by active transport or by facilitated diffusion
- one mark for naming sieve tube elements and sieve plates as the structures through which transport occurs
- Describe active loading of sucrose into the phloem at the source, which lowers the water potential.
- Explain that water enters the phloem from the xylem by osmosis, raising the hydrostatic pressure at the source.
- State that unloading of sucrose at the sink raises water potential, causing water to leave and pressure to fall.
- Explain that mass flow of solution occurs down the hydrostatic pressure gradient from source to sink.
- Describe evidence from tracer experiments, such as tracking radioactive 14C-labelled sugars in the phloem using autoradiography.
- Identify a point as evidence for the mass flow hypothesis and quote the data that support it.
- Identify a point as evidence against the mass flow hypothesis and quote the data that support it.
- Identify an alternative explanation for a result, such as another variable that changed at the same time.
- Describe a relationship as a correlation rather than a cause where no mechanism has been demonstrated.
- State a limitation of the investigation, such as only one species or one tissue having been tested.
Examiner Tips
Expert advice for maximising your marks
- 💡Build the answer on three keywords: continuous column, cohesion by hydrogen bonding, tension from transpiration.
- 💡Start the explanation at the leaf, because that is where the driving force is generated.
- 💡If the detail deserts you, stating clearly that cohesion-tension moves the water can still earn a mark.
- 💡Name sucrose every time, and name a specific sink rather than saying the rest of the plant.
- 💡Contrast phloem with xylem in one line: living against dead, two-way against one-way, sucrose against water and mineral ions.
- 💡Sieve tube and sieve element are both accepted for phloem cell, so use whichever term you are confident with.
- 💡Always use the term 'hydrostatic pressure' rather than just 'pressure', and clearly state which end is high and which is low.
- 💡In ringing experiment questions, remember that the xylem remains intact, so the upward transport of water is unaffected.
- 💡When evaluating evidence for mass flow, clearly distinguish between what the data shows and what it fails to rule out.
- 💡Split the answer physically into for and against; the mark scheme is usually split the same way.
- 💡Every point should carry a figure or an observation, not just a claim.
- 💡Testing only one species, or only one site, is nearly always worth a limitation mark.
Common Mistakes
Pitfalls to avoid in your exam answers
- writing chain of water molecules, which mark schemes ignore, instead of column of water
- describing water being pushed up from the roots when the driving force is a pull from the leaves
- confusing cohesion between water molecules with adhesion to the vessel wall
- saying hydrogen bonds between water and lignin are what hold the column up
- forgetting that xylem vessels are dead, and describing them using energy to move water
- saying phloem transports glucose when the sugar carried is sucrose
- describing phloem as dead tissue, which confuses it with xylem
- assuming transport is always downwards from the leaves
- omitting the companion cell, so the answer contains no source of ATP for loading
- writing transports food where the mark requires a named organic substance
- Describing the driving force as a concentration gradient of sucrose; Correction: specify that it is a hydrostatic pressure gradient that drives the bulk flow of sap.
- Stating that water enters the phloem by active transport; Correction: water always moves by osmosis down a water potential gradient.
- Predicting swelling below the ring in a ringing experiment; Correction: swelling occurs above the ring because the downward flow of sugars is interrupted.
- Claiming a tracer experiment proves mass flow; Correction: tracers only prove that the phloem carries the labelled organic compounds, not the mechanism of flow itself.
- Writing every point as evidence for and never addressing what the data fail to show. Correction: explicitly consider evidence against and what cannot be concluded.
- Concluding that a correlation between sucrose concentration and flow rate proves mass flow. Correction: state that the data show a correlation and suggest another variable might be responsible unless a causal mechanism is proven.
- Quoting sieve plates as evidence for mass flow when they are normally raised as evidence against it. Correction: sieve plates are an obstacle to bulk flow and are used as evidence against.
- Treating sucrose and amino acids moving in opposite directions in the same tissue as evidence against mass flow. Correction: this is only evidence against if they are in the same sieve tube; opposite directions in different sieve tubes are compatible with mass flow.