Biology • Year 11 • Module 2 • Lesson 16
Plant Transport Systems: Xylem and Phloem
Apply cohesion-tension theory and the pressure-flow hypothesis to experimental data, novel scenarios and diagram analysis.
1. Interpret a ringing experiment
A botanist removes a complete ring of bark (the outer layers including all phloem) from a eucalyptus stem, leaving the xylem intact. The ring is 5 cm wide. She then observes the plant over four weeks. Her measurements are recorded in the table below. 10 marks
| Week | Observation above ring | Observation below ring | Water status of whole plant |
|---|---|---|---|
| 0 (baseline) | Normal growth, green leaves | Normal root growth | Normal |
| 1 | Slight swelling just above ring; leaves normal | Root growth slows | Normal |
| 2 | Visible bulge of tissue above ring; leaves still photosynthesising | Root tips turning yellow, growth halted | Normal |
| 4 | Large callus swelling above ring; leaves begin to yellow | Roots dying; plant destabilising | Normal until Week 4 then declining |
1.1 What is the function of a ringing experiment in studying vascular transport? 1 mark
1.2 Explain why swelling occurs above the ring and not below it. Link your answer to phloem function and the pressure-flow hypothesis. 3 marks
1.3 Why did root growth slow and eventually stop, even though xylem (and therefore water supply) was intact? 2 marks
1.4 Why did the water status of the whole plant remain normal for three weeks despite the ring removing all phloem? 2 marks
1.5 Predict what would have happened if the scientist had removed the xylem (leaving phloem intact) instead of the phloem. Give one observation that would differ. 2 marks
2. Interpret graph, xylem tension and soil water potential
A sunflower plant is grown in a controlled environment. The graph below shows the xylem water potential (measured at the base of the stem, in MPa) and soil water potential over 12 hours, from 6 am to 6 pm on a warm day. 8 marks
Figure 2.1. Xylem water potential (measured at stem base) and soil water potential over one day in a controlled sunflower experiment. Negative values indicate tension below atmospheric pressure.
2.1 At 6 am, both the xylem and soil water potential are close to 0 MPa. What does this tell you about transpiration at this time? 1 mark
2.2 Describe the trend in xylem water potential between 8 am and 1 pm. Explain this trend in terms of the cohesion-tension theory. 3 marks
2.3 At 1 pm, the xylem water potential (−1.4 MPa) is much lower than the soil water potential (−0.05 MPa). Explain why this difference drives water uptake from the soil. 2 marks
2.4 After 3 pm, xylem water potential begins to recover (becomes less negative). Suggest a reason for this recovery. 2 marks
3. Apply source-sink to a potato plant
In spring, a potato plant produces new leaves (photosynthesising) at the shoot tips and new tubers (starch-storing organs) underground. At the same time, last year's stored starch in existing tubers is being broken down to sucrose to fuel new shoot growth. 7 marks
3.1 Identify which structures are acting as phloem sources and which are acting as phloem sinks in this plant in spring. 2 marks
3.2 Explain, using the pressure-flow hypothesis, how sucrose moves from a photosynthesising leaf (source) to a developing tuber (sink). Your answer must include the terms active loading, osmosis, turgor pressure, and bulk flow. 4 marks
3.3 When the potato is dormant in winter, the tubers release stored sucrose to fuel any metabolic activity. How does this change the source/sink status of the tubers? 1 mark
4. Identify errors in a student diagram
A student drew the diagram below to show how water moves from soil to leaf in a plant. The diagram contains three biological errors. Identify each error and write the correct version. 6 marks, 2 per error: 1 identify, 1 correct
Student diagram description:
- Label on root hair cells: "Water enters by active transport using ATP"
- Arrow at endodermis: "Water and all minerals pass freely through cell walls past this point"
- Label on xylem vessels: "Xylem vessels are living cells that pump water upward using energy from companion cells"
4.1 Error 1: What is wrong?
Correction:
4.2 Error 2: What is wrong?
Correction:
4.3 Error 3: What is wrong?
Correction:
Q1.1, Function of a ringing experiment (1 mark)
Removing a ring of bark (phloem) while leaving xylem intact allows the two transport systems to be separated and studied independently, demonstrating which tissues are responsible for which functions (e.g. sugar transport vs water transport). [1 mark for identifying separation of function / isolating phloem]
Q1.2, Why swelling occurs above the ring (3 marks)
Phloem transports sucrose downward (from source leaves above to root sinks below) via the pressure-flow mechanism. Removing the phloem ring interrupts this downward flow [1]. Sucrose produced by photosynthesis in leaves cannot be transported past the ring [1]. Sucrose accumulates above the ring in phloem tissue, raising the osmotic concentration there; water follows by osmosis, causing the tissue to swell (form a callus) [1].
Marking notes. 1 mark for identifying that phloem carries sucrose downward; 1 mark for identifying that the ring blocks sucrose movement; 1 mark for explaining accumulation/osmosis causing swelling.
Q1.3, Root growth stops (2 marks)
Roots depend on phloem-delivered sucrose (from photosynthesis in leaves) as their primary energy source for cellular respiration and growth [1]. With phloem removed, no sucrose can reach roots; roots starve of organic nutrient supply and eventually die even though water delivery via xylem remains intact [1].
Q1.4, Water status normal for three weeks (2 marks)
Water transport occurs through xylem, which was not removed in the ringing experiment [1]. Xylem continues to deliver water and minerals from roots to leaves, so the plant's water status is unaffected for as long as root function is adequate, water transport is independent of phloem [1].
Q1.5, Removing xylem instead (2 marks)
Removing the xylem while leaving phloem intact would disrupt water supply. Water could not travel upward past the ring, so leaves above the ring would wilt and dry out rapidly [1]. A key difference from the phloem ringing experiment: wilting would occur quickly (within hours or days) because transpiration-driven water deficit could not be replenished, whereas in the phloem-ring experiment the shoot above remained healthy for weeks [1]. Accept other valid observations, e.g. minerals would not reach leaves / leaves would yellow; sugar transport to roots via phloem would initially continue normally.
Q2.1, Transpiration at 6 am (1 mark)
At 6 am, stomata have just opened (or are still partly closed) and light intensity is low, so transpiration is minimal or negligible. There is little or no water deficit at the leaf, so no tension develops in the xylem and its water potential is close to that of the soil. [1 mark for minimal transpiration / stomata closed or just opening]
Q2.2, Trend 8 am to 1 pm and explanation (3 marks)
Between 8 am and 1 pm, xylem water potential decreases (becomes more negative), dropping from approximately −0.2 MPa to −1.4 MPa [1]. This occurs because as light intensity and temperature rise during the morning, transpiration rate increases, water evaporates from leaf mesophyll cells faster than it can be replaced [1]. This creates a greater water deficit at the leaf end of the xylem, increasing tension (negative pressure) in the water column; the higher tension is transmitted via cohesion to root xylem, lowering the water potential of the whole column [1].
Q2.3, Driving water uptake from soil (2 marks)
Water moves from regions of higher water potential to regions of lower water potential [1]. At 1 pm, the xylem water potential (−1.4 MPa) is substantially lower than soil water potential (−0.05 MPa), so water moves from soil into root hair cells by osmosis and into root xylem, maintaining the supply of water to replace that lost by transpiration [1].
Q2.4, Recovery after 3 pm (2 marks)
After 3 pm, light intensity falls as the sun moves lower in the sky [1]. Stomata begin to close in response to lower light or due to daily circadian rhythms, reducing transpiration rate. Less water is lost from the leaf, so the tension in the xylem decreases and water potential recovers toward equilibrium with the soil [1]. Accept also: temperature drops in late afternoon, reducing evaporation rate; stomata close to reduce water loss.
Q3.1, Sources and sinks in spring (2 marks)
Sources: New photosynthesising leaves at the shoot tips (producing sucrose); old tubers releasing stored sucrose (acting as sources while mobilising starch). Sinks: Growing root tips; new tubers developing underground; actively growing shoot tips and meristems. [1 mark for correct sources, 1 mark for correct sinks]
Q3.2, Pressure-flow mechanism, leaf to tuber (4 marks)
At the source leaf, companion cells use ATP to actively load sucrose from mesophyll cells into sieve tube elements [1]. This raises the solute concentration in source sieve tubes, lowering their water potential; water enters by osmosis from adjacent xylem and mesophyll cells, raising turgor pressure at the source end [1]. The turgor pressure difference between the high-pressure source (leaf) and the low-pressure sink (tuber) drives bulk flow of phloem sap through the sieve tubes toward the tuber [1]. At the tuber (sink), sucrose is removed from the sieve tubes (used for respiration or converted to starch), lowering solute concentration, water potential rises and water exits to xylem, maintaining the pressure gradient that drives continued flow [1].
Q3.3, Source/sink status change in winter (1 mark)
When tubers release stored sucrose to fuel metabolism in winter, they switch from being phloem sinks (which store sucrose) to phloem sources (which release sucrose into the phloem for delivery to other parts of the plant, such as growing meristems in early spring). [1 mark for correctly identifying the shift from sink to source]
Q4, Student diagram errors (6 marks)
4.1 Error 1, "Water enters by active transport using ATP." Water entry into root hair cells is passive. Correction: water enters root hair cells by osmosispassive movement down a water potential gradient from the soil (higher water potential) to the root hair cell cytoplasm (lower water potential due to high solute concentration). No ATP is required. [1 identify + 1 correct]
4.2 Error 2, "Water and all minerals pass freely through cell walls past this point (endodermis)." The Casparian strip at the endodermis blocks the apoplast pathway. Correction: the Casparian strip (a waxy suberin band) prevents water and minerals from passing freely through cell walls at the endodermis; they must cross the plasma membrane of endodermal cells, enabling selective mineral uptake. [1 identify + 1 correct]
4.3 Error 3, "Xylem vessels are living cells that pump water upward using energy from companion cells." Xylem vessels are dead, and water movement through xylem is passive. Correction: xylem vessel elements are dead at maturity (cell contents are removed) and no energy is required at the xylem itself, water is pulled upward by transpiration-generated tension (cohesion-tension theory). Companion cells support phloem, not xylem. [1 identify + 1 correct]