Biology • Year 11 • Module 2 • Lesson 18
Comparing Transport Systems: Plants and Animals
Apply the xylem/artery and phloem/vein comparisons to real data, graph interpretation, and scenario questions.
1. Interpret data, changes in transport medium composition
The table below shows the approximate concentrations of key substances in xylem sap and blood plasma at different points in the organism. Use the data to answer the questions. 8 marks
| Substance | Xylem sap, root end (entering) | Xylem sap, leaf end (after transit) | Blood plasma, aorta (leaving heart) | Blood plasma, venule (leaving capillary bed) |
|---|---|---|---|---|
| O2 (relative units) | trace | trace | high (100) | low (20) |
| CO2 (relative units) | trace | trace | low (40) | high (90) |
| Glucose (relative units) | absent | absent | high (100) | low (30) |
| Dissolved minerals (Ca2+, NO3−, etc.) | moderate (50) | low (15) | present (regulated) | present (regulated) |
| Sucrose | absent | absent | absent | absent |
1.1 Describe what happens to O2 and CO2 concentration in the blood as it passes through an active tissue capillary bed. 2 marks
1.2 Explain why dissolved minerals in xylem sap are at a lower concentration at the leaf end than at the root end. 2 marks
1.3 The table shows sucrose is absent from both xylem sap and blood plasma. Using lesson content, explain where sucrose is found in a plant and why it does not appear in xylem sap. 2 marks
1.4 Identify one similarity and one difference in how the composition of xylem sap and blood plasma changes in transit. 2 marks
2. Interpret graph, pressure profiles in plant and animal transport
The stylised graph below shows fluid pressure at different points in a plant's vascular system (left panel) and in the human circulatory system (right panel). Study both panels and answer the questions. 6 marks
Stylised pressure profiles, relative values only. Based on Lesson 18 Card 1 (xylem/artery pressure comparison) and Card 5 (five-vessel table).
2.1 Describe the direction of the pressure gradient in xylem versus blood in an artery. How does this reflect the different driving mechanisms? 2 marks
2.2 In the animal panel, blood pressure drops sharply between the arteriole and capillary. Explain why this drop occurs and why it is essential for exchange to take place. 2 marks
2.3 The phloem curve shows positive pressure that decreases from source to sink. Explain the mechanism that creates this pressure at the source end. 2 marks
3. Diagram critique, three errors to find
A student has drawn the following notes comparing xylem and arteries. There are three biological errors. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
Student notes: "Xylem and arteries are both made of living cells. Both operate under positive pressure, the heart pushes blood into arteries while root pressure pushes water into xylem. Xylem needs thick walls because the positive pressure could burst them, just like arteries. Because both systems have pressure, neither needs valves. Phloem is passive, like veins, neither uses any energy."
3.1 Error 1: What is wrong?
Correction:
3.2 Error 2: What is wrong?
Correction:
3.3 Error 3: What is wrong?
Correction:
4. Apply to a new scenario, designing a plant for faster water transport
A plant biologist proposes genetically engineering a plant species to replace dead xylem vessel elements with living cells that actively pump water using ATP, claiming this would increase the speed of water transport to leaves on hot days. 6 marks
4.1 Explain one structural reason why xylem vessel elements are normally dead at maturity, using lesson content. 2 marks
4.2 Identify one cost and one benefit of the biologist's proposal. 2 marks
4.3 The lesson says "plants can afford to wait for passive forces." Using this idea, evaluate whether the biologist's proposal is consistent with how plant biology works. 2 marks
Q1.1, O2 and CO2 in capillary bed (2 marks)
O2 concentration falls as it diffuses from blood into respiring tissue cells (which consume O2 in cellular respiration) [1]. CO2 concentration rises as it diffuses from tissue cells (which produce CO2 as a waste product) into the blood [1].
Q1.2, Mineral decrease in xylem from root to leaf (2 marks)
As xylem sap moves upward, leaf cells and other living cells along the pathway selectively absorb mineral ions (e.g. NO3−, Ca2+) from the xylem for use in cell processes such as protein synthesis and enzyme function [1]. This removal of ions reduces their concentration in the xylem sap before it reaches the leaf end [1].
Q1.3, Sucrose in phloem not xylem (2 marks)
Sucrose is transported in phloem, not xylem [1]. Xylem carries only water and dissolved inorganic minerals, sucrose and other organic photosynthates are actively loaded into phloem sieve tubes by companion cells. The two vascular tissues transport entirely different substances [1].
Q1.4, Similarity and difference (2 marks)
Similarity: In both xylem sap and blood plasma, the composition of the transport medium changes as it passes through metabolically active tissue, substances are removed from the fluid by active cells [1].
Difference: Xylem sap carries only inorganic ions (minerals), which decrease in transit; blood plasma carries both organic (glucose, O2) and inorganic substances, with O2 and glucose decreasing while CO2 increases, a much more complex composition change [1].
Q2.1, Pressure gradients (2 marks)
Xylem operates under negative pressure (tension) that becomes more negative from root to leaf, the column is pulled from the leaf end by transpiration. Arteries operate under positive pressure that decreases from aorta to capillary, blood is pushed from behind by ventricular contraction. Both systems show a pressure gradient driving fluid flow, but in xylem the driving force is at the top (evaporation at the leaf) while in arteries it is at the bottom (cardiac contraction) [1 each].
Q2.2, Pressure drop at arteriole/capillary (2 marks)
Blood pressure drops sharply across the arterioles because they have narrow lumens and a muscular wall that creates high resistance, most arterial pressure is dissipated here [1]. This reduction to very low pressure in capillaries is essential because high pressure in capillaries would force fluid out of vessels too rapidly and prevent the controlled diffusion exchange of O2, CO2, glucose and waste that must occur across the thin capillary walls [1].
Q2.3, Phloem positive pressure at source (2 marks)
Companion cells actively load sucrose into sieve tube elements at the source leaf using ATP-driven membrane transport proteins [1]. This raises the solute concentration inside the sieve tube, lowering its water potential and causing water to enter by osmosis from surrounding xylem, this osmotic water entry increases hydrostatic (turgor) pressure inside the phloem, driving bulk flow toward the low-pressure sink [1].
Q3, Diagram critique (6 marks)
3.1 Error 1 ("xylem made of living cells"): Xylem vessel elements are dead at maturity, their cytoplasm, nucleus and organelles have been removed, leaving hollow tubes. Living cytoplasm would obstruct the water column and impose osmotic resistance. Correction: xylem vessels are dead; arteries are made of living cells. [1 + 1]
3.2 Error 2 ("xylem operates under positive pressure / thick walls resist bursting"): Xylem operates under negative pressure (tension), not positive. The thick lignified walls resist collapse inward, not bursting outward. It is arteries that operate under positive pressure and have walls that resist bursting. Correction: xylem is under negative pressure, lignin prevents the vessel from collapsing inward under tension. [1 + 1]
3.3 Error 3 ("phloem is passive, uses no energy"): Phloem transport requires ATP. Companion cells actively load sucrose into sieve tubes at the source, which is an active process consuming metabolic energy. It is xylem (not phloem) that is passive at the vessel level. Correction: phloem requires ATP for active sucrose loading; xylem is passive at the vessel (no ATP consumed in the xylem itself). [1 + 1]
Q4.1, Why xylem cells are dead (2 marks)
Xylem vessel elements die at maturity so that their cytoplasm, nucleus and organelles are removed, creating an empty hollow lumen [1]. Living cytoplasm would obstruct the water column (imposing resistance to flow) and would impose osmotic resistance, water molecules would have to cross living membranes rather than flowing freely through the continuous hollow tube. Death is therefore essential for unobstructed low-resistance bulk flow [1].
Q4.2, Cost and benefit (2 marks)
Benefit: Living pump cells could potentially deliver water faster by active transport, maintaining water supply to leaves during rapid transpiration on hot days [1].
Cost: Living cells in the vessel would obstruct the lumen, impose osmotic barriers, and require continuous ATP expenditure, increasing the plant's metabolic energy demands significantly. The current passive system is essentially free (solar-powered) [1].
Q4.3, Evaluate against "plants can wait for passive forces" (2 marks)
The lesson notes that plants are sessile (rooted) and can tolerate slower delivery, their cells do not have the same instant high-volume O2 demands as animal neurons or heart muscle [1]. Replacing passive xylem with an active ATP-driven pump would undermine the plant's fundamental energy strategy: currently, solar energy (transpiration) powers water transport at zero metabolic cost. Converting to active pumping would impose continuous metabolic energy costs that the plant's physiology is not adapted to bear, making the proposal inconsistent with plant biology [1].