📚 Transport in Plants Exam Practice | 植物运输 真题精练
Mastering transport in plants is essential for A-level Biology, and focused exam practice can turn challenging concepts into easy marks. This revision article walks you through high-yield topics, typical exam questions, and examiner tips to boost your confidence.
掌握植物运输对A-level生物至关重要，有针对性的真题精练能将难点转化为得分点。本文梳理高频考点、典型真题和评分技巧，助你冲刺高分。
1. The Xylem and Phloem – Structure Meets Function | 木质部与韧皮部：结构与功能
Xylem vessels are dead, hollow tubes formed from cells arranged end-to-end, with no end walls and lignin-thickened walls. They transport water and mineral ions upwards from roots to leaves and provide mechanical support.
木质部导管是由端对端排列的细胞形成的死细胞空心管，无端壁，细胞壁木质化加厚。它们将水和无机离子从根部向上运输到叶片，并提供机械支撑。
Phloem sieve tubes are living cells with perforated sieve plates, allowing mass flow of sucrose and amino acids. Companion cells carry out metabolic activities to load solutes into sieve tubes actively.
韧皮部筛管是活细胞，具有穿孔的筛板，使蔗糖和氨基酸能进行集流运输。伴胞执行代谢活动，主动将溶质装载到筛管中。
Typical exam question: ‘Explain how the structure of xylem vessels is adapted for the transport of water.’ A top answer mentions: lignified walls prevent collapse under tension, no end walls form continuous columns, bordered pits allow lateral movement, and narrow diameter helps maintain the water column via cohesion.
典型真题：“解释木质部导管结构如何适应水分运输。”高分答案要点：木质化壁防止在张力下塌陷，无端壁形成连续水柱，具缘纹孔允许横向移动，较窄的管径有助于通过内聚力维持水柱。
| Feature / 特征 | Xylem / 木质部 | Phloem / 韧皮部 |
|---|---|---|
| Living or dead / 死活 | Dead at maturity | Living |
| Cell wall / 细胞壁 | Lignified | Cellulose, no lignin |
| Direction of flow / 流向 | Upwards | Up and down |
| Main transported substances / 主要运输物 | Water and mineral ions | Sucrose and amino acids |
2. Water Uptake and the Apoplast/Symplast Pathways | 水分吸收及质外体/共质体途径
Water enters root hair cells by osmosis because the soil water has a higher water potential than the root hair cell sap. The large surface area of root hairs and thin cellulose walls speed up uptake.
水分通过渗透进入根毛细胞，因为土壤水的水势高于根毛细胞液。根毛巨大的表面积和薄的纤维素壁加速了吸收。
From the root hair, water travels to the xylem via three pathways: the apoplast (through cell walls and intercellular spaces), the symplast (through cytoplasm and plasmodesmata), and the vacuolar pathway. The Casparian strip in the endodermis blocks the apoplast route, forcing water into the symplast, which allows selective mineral uptake.
从根毛开始，水分通过三条途径向木质部运输：质外体途径（沿细胞壁和细胞间隙）、共质体途径（经细胞质和胞间连丝）以及液泡途径。内皮层中的凯氏带阻断质外体途径，迫使水分进入共质体，从而实现选择性吸收矿物质。
Exam practice: ‘Explain the role of the Casparian strip.’ A full-mark response should state that it is a band of suberin impermeable to water, forcing water and dissolved ions to pass through the selectively permeable plasma membrane of endodermal cells, which controls ion uptake and prevents backflow.
真题精练：“解释凯氏带的作用。”满分回答应指出它是由木栓质形成的防水带，迫使水和溶解的离子通过内皮层细胞的选择性透性膜，从而控制离子吸收并防止回流。
3. Transpiration and the Cohesion-Tension Mechanism | 蒸腾作用与内聚力-张力机制
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. It creates a water potential gradient that pulls water up through the xylem.
蒸腾作用是水分从植物地上部分（主要通过气孔）以水蒸气形式散失的过程。它形成水势梯度，将水牵引上木质部。
The cohesion-tension theory explains this process. Water molecules cohere due to hydrogen bonding, forming a continuous column in the xylem. Tension (negative pressure) is generated at the leaf surface as water evaporates, pulling the whole water column upwards.
内聚力-张力理论解释了这一过程。水分子因氢键内聚，在木质部中形成连续水柱。随着水分蒸发，叶面产生张力（负压），将整个水柱向上拉。
Water transport = cohesion + adhesion + tension
水分运输 = 内聚力 + 附着力 + 张力
Classic exam question: ‘Describe the cohesion-tension theory of water movement.’ Key points for the answer: (1) Transpiration from leaves lowers water potential in leaf cells. (2) Water moves from xylem into leaf cells down a water potential gradient. (3) Creating tension in the xylem. (4) Water molecules cohere via H-bonds, so the whole column is pulled up. (5) Adhesion of water to xylem walls helps combat gravity. (6) Lignified walls prevent xylem collapse.
经典真题：“描述水分运动的内聚力-张力理论。”答案要点：(1) 叶蒸腾降低叶细胞水势。(2) 水沿水势梯度从木质部进入叶细胞。(3) 在木质部产生张力。(4) 水分子通过氢键内聚，因此整个水柱被向上拉。(5) 水与木质部壁附着力有助于对抗重力。(6) 木质化壁防止导管塌陷。
4. Measuring Transpiration: Potometers and Pitfalls | 测量蒸腾：蒸腾计与常见误区
A potometer is a device used to measure water uptake by a leafy shoot, which gives an indirect estimate of transpiration rate. The distance moved by an air bubble in a capillary tube over time is recorded.
蒸腾计是用于测量带叶枝条吸水量的装置，间接估算蒸腾速率。记录毛细管中气泡一段时间内移动的距离。
It is essential to ensure an airtight seal, cut the stem under water to prevent air locks, and allow the plant to acclimatise. Remember, a potometer measures water absorption, not transpiration directly; not all absorbed water is transpired immediately.
必须确保装置密封，在水中切割茎部以防气泡栓塞，并让植物适应环境。注意，蒸腾计测量的是吸水率而非直接蒸腾，因为吸收的水分并非全部立即蒸腾。
Exam question: ‘Describe how you would use a potometer to investigate the effect of wind on transpiration rate.’ You would set up the potometer, position a fan at different distances or speeds, measure bubble movement over a fixed time, calculate rate, and control variables such as temperature and humidity. Compare with no wind as a control.
真题：“描述如何利用蒸腾计研究风对蒸腾速率的影响。” 设置蒸腾计，将风扇置于不同距离或速度，测量固定时间内气泡移动距离，计算速率，并控制温度、湿度等变量。以无风作为对照。
5. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
Four main environmental factors influence transpiration: light intensity, temperature, humidity, and wind speed. Light stimulates stomatal opening; higher temperatures increase kinetic energy and water vapour diffusion; high humidity reduces the water potential gradient; wind removes humid air around the leaf, increasing the gradient.
四个主要环境因子影响蒸腾：光照强度、温度、湿度和风速。光照促使气孔张开；较高温度增加分子动能和水汽扩散；高湿度减小水势梯度；风带走叶面周围潮湿空气，增大梯度。
Exam staple: ‘Explain the effect of increasing temperature on transpiration.’ A thorough answer should mention: increases kinetic energy of water molecules, faster evaporation from mesophyll cell walls, increases water vapour concentration in sub-stomatal cavity, steeper diffusion gradient, and also warms the leaf, raising transpiration. Remember to link to stomatal aperture if relevant.
常考题：“解释温度升高对蒸腾的影响。” 完整回答应提到：增加水分子动能，加速从叶肉细胞壁蒸发，提高气孔下腔水汽浓度，增大扩散梯度，同时使叶片升温，蒸腾增强。如有关联，也可提到气孔开度。
- Light / 光照: increases stomatal opening and therefore transpiration / 促进气孔开放，蒸腾增加
- Temperature / 温度: increases evaporation and diffusion / 加速蒸发与扩散
- Humidity / 湿度: reduces water potential gradient, decreases transpiration / 降低水势梯度，蒸腾减弱
- Wind / 风: removes boundary layer, steepens gradient / 移走边界层，增大梯度
6. Phloem Loading and the Mass Flow Hypothesis | 韧皮部装载与集流假说
Sucrose is actively loaded into sieve tubes at the source (e.g. leaves) by companion cells using proton pumps and co-transport proteins. This lowers the water potential inside the sieve tube, causing water to enter from the xylem by osmosis.
蔗糖在源端（如叶片）通过伴胞的质子泵和协同转运蛋白被主动装载到筛管中。这降低了筛管内水势，使水分从木质部经渗透进入。
The increased hydrostatic pressure at the source forces phloem sap to flow towards sinks (growing roots, fruits) where sucrose is unloaded, lowering pressure and maintaining a pressure gradient. This mass flow hypothesis is the widely accepted model for translocation.
源端升高的静水压推动韧皮部汁液流向库（生长中的根、果实），蔗糖在此卸出，压力降低，维持压力梯度。这一集流假说是目前广为接受的韧皮部运输模型。
Phloem translocation: source (high pressure) → sink (low pressure)
韧皮部运输：源（高压）→ 库（低压）
Exam question: ‘Describe the mass flow hypothesis for translocation in the phloem.’ Marking points include: active loading of sucrose into sieve tubes, lowering of water potential, entry of water by osmosis, increase in hydrostatic pressure, mass flow down pressure gradient, unloading at sink (diffusion/facilitated diffusion), and water leaving the sieve tube and returning to xylem.
真题：“描述韧皮部运输的集流假说。”得分点：蔗糖主动装载入筛管，水势下降，水分通过渗透进入，静水压升高，沿压力梯度集流，在库端卸出（扩散/易化扩散），水分离开筛管返回木质部。
7. Evidence for Translocation | 韧皮部运输的证据
Evidence supporting mass flow comes from several classic experiments. Ringing (removing a ring of bark) causes swelling above the ring due to accumulation of sugars, showing that phloem is the route. Radioactive tracers like 14C incorporated into sucrose are detected moving downwards in the phloem.
支持集流假说的证据来自几个经典实验。环割（去除一圈树皮）导致环上膨大，因为糖类积累，证明韧皮部是运输途径。放射性示踪剂如14C标记蔗糖，在韧皮部向下移动被检测到。
Aphid stylets inserted into sieve tubes allow collection of phloem sap, demonstrating high sucrose concentration and positive pressure. When aphids are removed, sap continues to ooze, confirming pressure-driven flow.
将蚜虫口针插入筛管可收集韧皮部汁液，显示高蔗糖浓度和正压。移除蚜虫后汁液继续渗出，证实压力驱动的流动。
Common exam command: ‘Evaluate the evidence for the mass flow hypothesis.’ You must mention that while ringing, tracers, and aphid studies support the hypothesis, arguments against include: the process requires energy and there is bidirectional flow in some conditions, which the model does not fully explain. Also, sieve plates could theoretically create resistance.
常见指令词：“评价集流假说的证据。” 需指出环割、示踪、蚜虫研究支持假说，但也可提出反驳：该过程需要能量，某些情况下存在双向运输，模型不能完全解释；筛板理论上可能产生阻力。
8. Water Potential and Solute Potential Calculations | 水势与溶质势计算
Water potential (Ψw) is the sum of solute potential (Ψs) and pressure potential (Ψp): Ψw = Ψs + Ψp. Water always moves from regions of higher (less negative) water potential to regions of lower (more negative) water potential.
水势(Ψw)是溶质势(Ψs)和压力势(Ψp)之和：Ψw = Ψs + Ψp。水分总是从水势较高（负值较小）的区域流向水势较低（负值更大）的区域。
Ψw = Ψs + Ψp
Worked example: A plant cell has a solute potential of -800 kPa and a pressure potential of 500 kPa. Its water potential is Ψw = -800 + 500 = -300 kPa. If this cell is placed in pure water (Ψw = 0), water enters the cell by osmosis, increasing Ψp until water potential equilibrium is approached.
计算实例：一个植物细胞溶质势为-800 kPa，压力势为500 kPa。其水势 Ψw = -800 + 500 = -300 kPa。若将此细胞置于纯水中 (Ψw = 0)，水通过渗透进入细胞，Ψp 增大，直至水势接近平衡。
Exam tip: Always state units (kPa or MPa) and indicate whether water moves in or out. Use the sign convention correctly; pure water is 0, solutes make potentials negative.
考试技巧：务必标明单位 (kPa 或 MPa)，并说明水分进出方向。正确使用符号约定：纯水为0，溶质使势值变负。
9. Common Exam Command Words and Model Answers | 常见指令词与标准答案
‘Describe’ questions require stating facts or processes without explanation, e.g., ‘Describe the pathway of water from root cortex to xylem.’ Answer by listing sequential structures: root hair, epidermis, cortex via apoplast/symplast, endodermis, pericycle, xylem.
“Describe”（描述）类问题要求陈述事实或过程而不需解释，例如“描述水分从根皮层到木质部的途径”。回答依次列出结构：根毛、表皮、经质外体/共质体通过皮层、内皮层、中柱鞘、木质部。
‘Explain’ questions demand reasons and mechanisms. For instance, ‘Explain why transpiration rate increases in windy conditions.’ Link wind to the removal of humid air, steepening the water potential gradient between leaf and atmosphere, leading to faster diffusion of water vapour out of the stomata.
“Explain”（解释）类问题要求给出原因和机制。例如“解释为何有风时蒸腾速率升高”。将风与移走潮湿空气联系起来，增大叶片与大气之间的水势梯度，导致水汽更快扩散出气孔。
‘Suggest’ questions often involve applying knowledge to novel scenarios, e.g., ‘Suggest why a plant wilts at high temperature.’ Link to high transpiration, reduced water uptake, loss of turgor pressure. No detailed knowledge of the specific plant needed.
“Suggest”（建议）类问题常要求将知识应用于新情境，例如“试解释为何植物在高温下萎蔫”。关联高蒸腾、吸水减少、膨压丧失。无需该植物的特殊知识。
Model answer structure: Start with a clear topic sentence, then add supporting details using ‘because’, ‘leading to’, ‘so that’. Quantify where possible (use data from graphs or tables).
标准答案结构：以清晰的主题句开始，然后使用“because”、“leading to”、“so that”提供支持细节。尽可能量化（使用图表或表格中的数据）。
10. Top Tips for the Transport in Plants Exam | 植物运输考试高分秘诀
1) Always link structure to function explicitly. Don’t just say ‘xylem has lignin’ – say ‘lignin strengthens walls to prevent collapse under the tension generated by transpiration’. 2) Use precise terminology: cohesion, adhesion, hydrostatic pressure, water potential, mass flow, Casparian strip, symplast, apoplast. 3) When drawing a potometer or translocation diagram, label fully and indicate the direction of flow with arrows.
1) 始终将结构与功能明确关联。不要只说“木质部有木质素”，而要说“木质素加固细胞壁，防止在蒸腾产生的张力下塌陷”。2) 使用精确术语：内聚力、附着力、静水压、水势、集流、凯氏带、共质体、质外体。3) 绘制蒸腾计或运输图时，完整标注并用箭头标明流向。
4) Practice calculations with negative water potentials – many candidates make sign errors. 5) Prepare to compare and contrast xylem and phloem, or transpiration and translocation, in table form. 6) Anticipate data-response questions: interpret graphs of distance moved by bubble versus time, and suggest reasons for anomalies.
4) 练习含负值的水势计算——很多学生在符号上犯错。5) 准备用表格形式比较木质部与韧皮部，或蒸腾与运输。6) 预料数据响应题：解释气泡移动距离与时间的关系图，并给出异常情况的可能原因。
Finally, keep answers concise but complete. A 2-mark question needs two distinct points; a 5-mark question often expects five teaching points or a structured description. Time management is crucial – don’t overwrite on short-answer items.
最后，答案要简洁但完整。2 分题需要两个不同的点；5 分题往往需要五个得分点或有序的描述。时间管理至关重要——不要在简答题上过度书写。
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