📚 Year 12 CIE Biology Mock Test Analysis: Cell Membranes & Transport | CIE 12年级生物模拟卷解析：细胞膜与物质运输
This mock test analysis focuses on the Cell Membranes and Transport unit for Year 12 CIE Biology. It provides a selection of exam-style questions and detailed explanations of the key concepts, common pitfalls, and marking points. Working through these examples will sharpen your understanding of the fluid mosaic model, diffusion, osmosis, active transport, and related experimental techniques.
本模拟卷解析围绕CIE 12年级生物的“细胞膜与物质运输”单元。我们精选了考试风格的问题，并对核心概念、常见错误和评分要点进行了详细讲解。通过这些例题的训练，你将更深入地理解流体镶嵌模型、扩散、渗透、主动运输以及相关的实验技术。
1. Fluid Mosaic Model Structure | 流体镶嵌模型结构
Question: Describe the arrangement of phospholipids and proteins in the fluid mosaic model of cell membranes. (4 marks)
题目：描述细胞膜流体镶嵌模型中磷脂和蛋白质的排列方式。（4分）
Marks are given for stating that the membrane is made of a phospholipid bilayer, where the hydrophilic phosphate heads face outwards towards the aqueous environment and the hydrophobic fatty acid tails face inwards, away from water.
得分点：膜由磷脂双分子层构成，亲水的磷酸头部朝外，朝向水环境，疏水的脂肪酸尾部朝内，背离水。
Proteins are scattered throughout the bilayer; integral proteins extend into or completely span the membrane, while peripheral proteins are attached to the surface of the membrane or to integral proteins.
蛋白质散布在双分子层中；整合蛋白嵌入或贯穿整个膜，外周蛋白则附着在膜表面或整合蛋白上。
The model is described as ‘fluid’ because phospholipids and proteins can move laterally within the layer, and ‘mosaic’ refers to the pattern of proteins floating in the phospholipid sea.
该模型被称为“流动”是因为磷脂和蛋白质可以在层内横向移动，“镶嵌”则指蛋白质在磷脂海洋中漂浮所形成的图案。
2. Diffusion & Concentration Gradients | 扩散与浓度梯度
Question: Explain how the rate of simple diffusion across a membrane is affected by the concentration gradient and the thickness of the exchange surface. (3 marks)
题目：解释浓度梯度和交换表面的厚度如何影响跨膜的简单扩散速率。（3分）
A steeper concentration gradient increases the rate of diffusion because more particles move from the region of higher concentration to the region of lower concentration per unit time, following Fick’s law.
浓度梯度越大，扩散速率越快，因为单位时间内有更多的粒子从高浓度区域向低浓度区域运动，符合菲克定律。
Reducing the thickness of the exchange surface decreases the distance that molecules must travel, so particles cross the membrane more quickly, increasing the rate of diffusion.
减小交换表面的厚度会缩短分子必须穿行的距离，因此粒子能更快地穿过膜，从而提高了扩散速率。
3. Osmosis and Water Potential | 渗透与水势
Question: A plant cell with a water potential (ψ) of –600 kPa and a pressure potential (ψp) of +300 kPa is placed in a sucrose solution of ψ = –400 kPa. Predict the net movement of water and explain the result. (3 marks)
题目：一个水势（ψ）为–600 kPa、压力势（ψp）为+300 kPa的植物细胞被放入ψ = –400 kPa的蔗糖溶液中。预测水的净移动方向并解释结果。（3分）
Water always moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential; –400 kPa is higher than –600 kPa, so water will move into the cell by osmosis.
水总是从水势较高（负值较小）的区域向水势较低（负值较大）的区域移动；–400 kPa 高于 –600 kPa，因此水将通过渗透作用进入细胞。
As water enters, the cell’s volume increases, pushing the membrane against the cell wall. The pressure potential rises, and the cell becomes turgid, eventually reaching equilibrium when the cell’s water potential equals –400 kPa.
随着水进入，细胞体积增大，细胞膜压向细胞壁。压力势升高，细胞变得硬挺，最终当细胞水势等于–400 kPa时达到平衡。
4. Active Transport & ATP | 主动运输与ATP
Question: Outline the role of the sodium–potassium (Na⁺/K⁺) pump in a neurone and explain why it is classified as active transport. (4 marks)
题目：概述神经元中钠钾泵的作用，并解释为什么它被归为主动运输。（4分）
The Na⁺/K⁺ pump moves three Na⁺ ions out of the axon and two K⁺ ions into the axon against their concentration gradients, using the hydrolysis of ATP to provide the required energy.
钠钾泵将三个 Na⁺ 离子泵出轴突，将两个 K⁺ 离子泵入轴突，两者都是逆浓度梯度进行的，需要水解ATP提供能量。
It is classified as active transport because it moves ions from a lower concentration to a higher concentration, a process that requires metabolic energy and is directly coupled to ATP hydrolysis.
它被归为主动运输，因为它将离子从浓度较低处移至浓度较高处，这个过程需要代谢能量，并且直接与ATP水解相耦联。
The pump helps maintain the resting potential by creating a steep electrochemical gradient, essential for the generation of action potentials.
该泵通过建立巨大的电化学梯度来维持静息电位，这对动作电位的产生至关重要。
5. Endocytosis & Exocytosis | 胞吞与胞吐
Question: Distinguish between endocytosis and exocytosis, providing one example of each. (4 marks)
题目：区分胞吞和胞吐，并各举一例。（4分）
Endocytosis is the process by which a cell engulfs large molecules or particles by folding the cell membrane inward to form a vesicle that pinches off into the cytoplasm. Exocytosis is the reverse process, where vesicles inside the cell fuse with the plasma membrane, releasing their contents to the outside.
胞吞是细胞通过将细胞膜向内折叠，形成小囊并掐断进入细胞质，从而包裹大分子或颗粒的过程。胞吐则相反，是细胞内的囊泡与质膜融合，将其内容物释放到细胞外的过程。
An example of endocytosis is the uptake of cholesterol carried by low‑density lipoproteins (LDLs) into liver cells. An example of exocytosis is the secretion of digestive enzymes from pancreatic cells into the pancreatic duct.
胞吞的一个例子是肝细胞通过低密度脂蛋白（LDL）摄取胆固醇。胞吐的一个例子是胰腺细胞分泌消化酶进入胰管。
6. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
Question: Describe and explain how increasing temperature affects the rate of diffusion across a cell membrane. (3 marks)
题目：描述并解释升高温度如何影响跨细胞膜的扩散速率。（3分）
Increasing temperature raises the kinetic energy of both the diffusing molecules and the phospholipids in the membrane; molecules move faster and collide more frequently, increasing the rate of diffusion.
升高温度增加了扩散分子和膜中磷脂的动能；分子移动更快，碰撞更频繁，从而提高了扩散速率。
At higher temperatures, the phospholipid bilayer becomes more fluid as the hydrocarbon tails vibrate more vigorously, which makes the membrane more permeable to small, non‑polar molecules, further increasing the diffusion rate.
在较高温度下，由于烃链振动加剧，磷脂双分子层变得更加流动，使膜对小分子非极性物质的通透性增加，进一步提高了扩散速率。
7. Interpretation of Experimental Data: Beetroot Membrane Permeability | 实验数据解析：甜菜根膜通透性
Question: The table shows absorbance readings of the pigment betalain leaking from beetroot discs at different temperatures. Describe and explain the trend. (4 marks)
| Temperature / °C | Absorbance at 550 nm |
|---|---|
| 10 | 0.15 |
| 30 | 0.35 |
| 50 | 0.90 |
| 70 | 1.80 |
题目：表格显示了不同温度下甜菜根圆片泄漏的甜菜红素吸光度。描述并解释其变化趋势。（4分）
As temperature increases, the absorbance of betalain in the surrounding solution rises, indicating a greater leakage of pigment and therefore increased membrane permeability.
随着温度升高，周围溶液中甜菜红素的吸光度上升，这表明色素泄漏增多，因此膜的通透性增大。
Below 50 °C, the rise is gradual and often attributed to increased fluidity of the phospholipid bilayer, which allows more pigment to escape through the lipid matrix. Above 50 °C, the sharp increase in absorbance is due to denaturation of membrane proteins, which form permanent gaps through the bilayer, causing a massive leakage of pigment.
在50 °C以下，上升较为平缓，通常归因于磷脂双分子层流动性增加，使更多色素通过脂质基质逸出。在50 °C以上，吸光度急剧上升是由于膜蛋白变性，在双分子层中形成永久空隙，导致色素大量泄漏。
8. Multiple Choice Practice: Identifying Transport Mechanisms | 选择题练习：识别运输机制
Question: Which of the following transport processes requires a specific carrier protein but does not directly use ATP? A. Osmosis B. Facilitated diffusion C. Active transport D. Exocytosis
题目：下列哪种运输过程需要特定的载体蛋白但不直接使用ATP？ A. 渗透 B. 易化扩散 C. 主动运输 D. 胞吐
The correct answer is B, facilitated diffusion. Carrier proteins undergo a conformational change to transport molecules down their concentration gradient, so no direct input of metabolic energy is needed.
正确答案是B，易化扩散。载体蛋白通过构象变化将分子顺浓度梯度运输，因此不需要直接输入代谢能。
Osmosis involves water moving through partially permeable membranes, often via aquaporins, but it is not mediated by a carrier protein that changes shape. Active transport directly uses ATP, and exocytosis involves vesicle fusion, not a carrier protein.
渗透涉及水通过部分透性膜，常经过水通道蛋白，但不涉及构象变化的载体蛋白。主动运输直接使用ATP，胞吐涉及囊泡融合而非载体蛋白。
9. Water Potential Calculation
Question: A flaccid plant cell has a solute potential (ψs) of –850 kPa and a pressure potential (ψp olute potential is unchanged. Calculate the final pressure potential of the cell. (2 marks)
题目：一个萎蔫植物细胞的溶质势（ψs）为 –850 kPa，压力势（ψp）为 0 kPa。将其放入纯水（ψ = 0 kPa）中。平衡时细胞的溶质势不变。计算细胞最终的压力势。（2分）
At equilibrium, the water potential of the cell must equal the water potential of the external solution: ψcell = 0 kPa. Using ψ = ψs + ψp, we have 0 = –850 + ψp, so ψp = +850 kPa.
平衡时，细胞的水势必须等于外界溶液的水势：ψcell = 0 kPa。根据 ψ = ψs + ψp，代入得 0 = –850 + ψp，因此 ψp = +850 kPa。
This positive pressure potential represents the turgor pressure that builds up as water enters, pushing the membrane against the rigid cell wall and preventing further net influx of water.
这个正的压力势代表当水进入时建立的膨压，使细胞膜紧贴刚性细胞壁，阻止水的进一步净流入。
10. Common Pitfalls and Exam Tips | 常见错误与考试技巧
A frequent mistake is confusing water potential with solute concentration. Remember that water potential is a measure of the tendency of water to move; pure water has the highest water potential (0 kPa), and adding solutes lowers it to negative values. Always state that water moves from a high (less negative) to a low (more negative) water potential.
一个常见错误是混淆水势与溶质浓度。记住水势是水移动趋势的量度；纯水的水势最高（0 kPa），加入溶质会使其降低为负值。始终要说明水从较高（负值较小）的水势向较低（负值较大）的水势移动。
When describing active transport, many students forget to mention that it requires a carrier protein and moves substances against their concentration gradient. The phrase ‘using energy from ATP’ must be explicitly linked to the conformational change of the pump protein.
在描述主动运输时，许多学生忘记提及它需要载体蛋白并逆浓度梯度运输物质。“利用ATP提供的能量”必须明确与泵蛋白的构象变化相关联。
In data questions, always refer to the values provided. For example, ‘absorbance increased from 0.15 at 10 °C to 1.80 at 70 °C’ is better than ‘the absorbance went up’. Link the numbers to biological explanations such as protein denaturation at high temperatures.
在数据题中，一定要引用所提供的数值。例如，“吸光度从10 °C时的0.15上升到70 °C时的1.80”比“吸光度上升了”更好。将数值与生物学解释联系起来，如高温下蛋白质变性。
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