Biology • Year 11 • Module 2 • Lesson 18
Comparing Transport Systems: Plants and Animals
Lock in the core vocabulary and the structural features of all five vessel types before tackling comparison questions.
1. Complete the five-vessel comparison table
Fill in every empty cell using your lesson notes. Each row tests a different feature across all five vessel types. 20 marks (1 per cell)
| Feature | Xylem | Phloem | Artery | Vein | Capillary |
|---|---|---|---|---|---|
| Organism | |||||
| Contents | Oxygenated blood | ||||
| Direction of flow | Toward heart | ||||
| Driving force | Transpiration pull (cohesion-tension) | ||||
| Pressure type | Positive (turgor) | ||||
| Energy required? | No, passive diffusion | ||||
| Wall structure | Thin, less muscle | ||||
| Living cells in wall? | Yes (sieve tubes + companion cells) | ||||
| Valves present? | No | ||||
| Primary function |
2. Term–definition match
Match each term with its definition by writing the correct letter in the right-hand column. Each letter is used once. 10 marks
| # | Term | Definition (shuffled) | Letter |
|---|---|---|---|
| 2.1 | Cohesion-tension mechanism | A. The driving force for phloem transport, created when sucrose is actively loaded at the source leaf, raising osmotic pressure and pushing sap toward sinks. | |
| 2.2 | Transpiration pull | B. Reinforcing polymer deposited in xylem cell walls that resists collapse under negative pressure (tension). | |
| 2.3 | Pressure-flow hypothesis | C. The physical explanation for xylem transport: water molecules stick together (cohesion) and to vessel walls (adhesion), allowing tension created by evaporation to pull the column upward. | |
| 2.4 | Turgor pressure gradient | D. The type of pressure operating in xylem vessels, a below-atmospheric pulling force that draws water upward. | |
| 2.5 | Lignin | E. Water evaporation from leaf mesophyll cells that generates the tension driving xylem flow; powered by solar energy, not ATP. | |
| 2.6 | Sieve tube element | F. A pocket-shaped valve in veins that prevents backflow of blood when venous pressure is low. | |
| 2.7 | Companion cell | G. The living phloem cell that actively loads sucrose into adjacent sieve tubes, providing the energy for phloem transport. | |
| 2.8 | Negative pressure (tension) | H. The thin-walled living cell in phloem that conducts photosynthate from source to sink; lacks a nucleus at maturity. | |
| 2.9 | Pocket valve | I. The mechanism explaining phloem transport: bulk flow from high-pressure source (sucrose-loaded) to low-pressure sink (sucrose-unloaded). | |
| 2.10 | Convergent evolution | J. The independent evolution of similar structures or functions in unrelated lineages in response to the same environmental pressures. |
3. True or false, with correction
Circle T or F. If the statement is false, write the correct version on the line provided. 10 marks (1 for T/F, 1 for the correction where needed)
3.1 Xylem vessels are composed of living cells that actively pump water through the plant. T / F
3.2 Both xylem and arteries carry fluid under pressure away from the driving source. T / F
3.3 Phloem transports sugars in one direction only, always from leaf downward to roots. T / F
3.4 Arteries operate under negative (tension) pressure, while xylem operates under positive pressure. T / F
3.5 Veins carry blood toward the heart at low positive pressure, assisted by pocket valves and skeletal muscle compression. T / F
4. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 What is the function of lignin in xylem vessel walls?
4.2 What is the function of elastic fibres in artery walls?
4.3 What is the function of companion cells in phloem transport?
4.4 What is the function of pocket valves in veins?
4.5 What is the function of the single-cell-thick wall of capillaries?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they relate. Each arrow must carry a linking phrase (e.g. "is driven by", "carries", "resists", "requires"). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: transpiration pull · xylem · water + minerals · lignin · left ventricular contraction · artery
Q1, Five-vessel comparison table
Xylem: Organism, Plant. Contents, Water + dissolved minerals. Direction, Upward (roots to leaves). Pressure, Negative (tension). Energy, No (passive). Wall, Thick, lignified. Living cells?, No (dead at maturity). Valves?, No. Function, Deliver water and minerals upward.
Phloem: Organism, Plant. Contents, Sucrose, amino acids, hormones. Direction, Any (source to sink). Driving force, Turgor pressure gradient (active loading). Energy, Yes (ATP for sucrose loading). Wall, Thin, unlignified. Valves?, No. Function, Distribute photosynthate to sinks.
Artery: Organism, Animal. Direction, Away from heart. Driving force, Left ventricular contraction. Pressure, High positive (~120 mmHg). Energy, Yes (cardiac). Wall, Thick, muscular + elastic. Living cells?, Yes. Function, Deliver O2 and nutrients to tissues.
Vein: Organism, Animal. Contents, Deoxygenated blood. Driving force, Residual pressure + muscle compression. Pressure, Low positive (~5–10 mmHg). Energy, Yes (indirect). Wall, Thin, less muscle. Living cells?, Yes. Valves?, Yes (pocket valves). Function, Return deoxygenated blood to heart.
Capillary: Organism, Animal. Contents, Blood (exchange in both directions). Direction, Delivers/collects in tissues. Driving force, Arterial pressure gradient. Pressure, Very low. Wall, One cell thick (endothelium). Living cells?, Yes. Valves?, No. Function, Exchange O2, CO2, nutrients, waste.
Q2, Term–definition matches
2.1 C • 2.2 E • 2.3 I • 2.4 A • 2.5 B • 2.6 H • 2.7 G • 2.8 D • 2.9 F • 2.10 J
Q3, True / false with correction
3.1 False. Xylem vessels are dead at maturity, their cytoplasm and nucleus have been removed, leaving hollow tubes. Water moves through xylem passively by transpiration pull, not by active pumping by living cells.
3.2 True. Both xylem (pulled from above by transpiration) and arteries (pushed from behind by the heart) carry fluid under pressure away from the source.
3.3 False. Phloem transport is bidirectional, it moves sugars from any source (e.g. mature leaf) to any sink (e.g. growing root tip, developing fruit) in either direction, depending on where sources and sinks are located.
3.4 False. It is the reverse: arteries operate under high positive pressure (pushed by the heart), while xylem operates under negative pressure (tension, water is pulled, not pushed).
3.5 True.
Q4.1, Function of lignin
Lignin is a rigid structural polymer deposited in xylem vessel walls that prevents the vessel from collapsing inward under the negative pressure (tension) generated by transpiration pull. Without lignin, xylem vessels would crumple like a straw sucked too hard.
Q4.2, Function of elastic fibres in arteries
Elastic fibres in artery walls stretch during systole (when the heart contracts and blood pressure peaks) and recoil during diastole, smoothing the pulsatile blood flow between heartbeats. They also prevent the artery from bursting under the high positive pressure of arterial blood (~120 mmHg).
Q4.3, Function of companion cells
Companion cells actively load sucrose into adjacent sieve tube elements using ATP-driven membrane transport proteins. This raises the solute concentration in the sieve tube, creating the osmotic turgor pressure that drives phloem transport by the pressure-flow mechanism.
Q4.4, Function of pocket valves
Pocket valves in veins are one-way mechanical gates that prevent backflow of blood. Because venous blood travels at very low pressure, sometimes against gravity, without valves blood would flow backward under its own weight. Valves snap open when muscle contractions push blood forward and snap shut if pressure would reverse.
Q4.5, Function of the single-cell-thick capillary wall
The single-cell-thick wall minimises the diffusion distance between blood plasma and surrounding tissue cells. According to Fick's law, rate of diffusion is inversely proportional to membrane thickness, so the thinnest possible wall maximises the rate at which O2, CO2, glucose, and waste products can exchange between blood and tissues.
Q5, Sample concept map
Valid arrows include: transpiration pull drives flow through → xylem • xylem carries → water + minerals • lignin reinforces walls of → xylem • xylem requires lignin to resist collapse under negative pressure • left ventricular contraction drives flow through → artery • artery unlike xylem, operates under positive pressure. Award 1 mark per correctly labelled, directionally accurate arrow (max 6).