📚 2.5 Biological Membranes: Exam Practice | 2.5 生物膜真题精练
This article provides targeted exam practice for A-Level Biology topic 2.5 Biological Membranes. We walk through the fluid mosaic model, transport mechanisms, experimental methods, and typical questions that appear time and again in past papers. Every section pairs essential knowledge with exam-style reasoning so you can sharpen your answers and avoid the traps that lose marks.
本文为A-Level生物2.5生物膜专题提供针对性的真题精练。我们围绕流动镶嵌模型、物质运输机制、实验方法及历年高频考题展开解析。每一节都将核心知识与真题思维配对，帮助你打磨答案并避开那些容易丢分的陷阱。
1. The Fluid Mosaic Model | 流动镶嵌模型
The fluid mosaic model describes the cell membrane as a dynamic bilayer of phospholipids with proteins, glycoproteins, glycolipids and cholesterol scattered throughout. “Fluid” refers to the lateral movement of phospholipids and many membrane proteins; “mosaic” refers to the patchwork of different components embedded in the bilayer. In an exam, you are expected to link the model’s features to function — for example, how the hydrophobic core forms a barrier to water-soluble substances, while channel and carrier proteins provide selective permeability.
流动镶嵌模型将细胞膜描述为由磷脂双分子层构成的动态结构，其中散布着蛋白质、糖蛋白、糖脂和胆固醇。”流动”是指磷脂和许多膜蛋白可以侧向移动；”镶嵌”则指嵌在双分子层中的不同组分形成的拼图。考试中你需要将模型特征与功能联系起来——例如，疏水核心如何形成对水溶性物质的屏障，而通道蛋白和载体蛋白如何实现选择性通透。
Typical question: “Explain how the fluid mosaic model accounts for the properties of cell membranes.” Always mention the phospholipid bilayer as a barrier, intrinsic and extrinsic proteins, the role of cholesterol, and how the model explains selective permeability and membrane flexibility.
典型考题：“解释流动镶嵌模型如何解释细胞膜的特性。” 回答时一定要提及磷脂双分子层作为屏障、内在蛋白与外在蛋白、胆固醇的作用，以及该模型如何解释选择透过性和膜的柔韧性。
2. Phospholipids and Membrane Structure | 磷脂与膜结构
A phospholipid has a hydrophilic (polar) phosphate-containing head and two hydrophobic (non-polar) fatty acid tails. In water, they spontaneously arrange into a bilayer with hydrophobic tails facing inward, shielded from water. Exam markers look for the idea that this arrangement creates a selectively permeable barrier: small, non-polar molecules like O₂ and CO₂ diffuse through readily, while ions and large polar molecules such as glucose require specific transport proteins.
磷脂分子含有一个亲水（极性）的磷酸头端和两条疏水（非极性）的脂肪酸尾端。在水中，它们自发排列成双分子层，疏水尾端朝内，避开水。阅卷人看重的是这样一种认识：这种排列形成了一道选择性通透屏障——小而非极性的分子（如O₂和CO₂）可以自由扩散，而离子和大极性分子（如葡萄糖）则需要特定的转运蛋白。
Remember to use the words ‘hydrophilic’ and ‘hydrophobic’ correctly. If asked about the arrangement, refer to the phospholipid bilayer as the fundamental structural unit. Avoid confusing polarity with charge—phosphate groups are polar but carry a negative charge at physiological pH, which contributes to the hydrophilic nature.
记得正确使用”亲水”和”疏水”这两个词。如果被问到排列方式，要以磷脂双分子层为基础结构单元进行阐述。避免混淆极性与电荷——磷酸基团是极性的，在生理pH下带负电，这有助于其亲水性。
3. Membrane Proteins and Their Functions | 膜蛋白及其功能
Proteins embedded in the membrane perform diverse roles: channel proteins form pores for passive ion movement; carrier proteins undergo conformational change to transport specific molecules (facilitated diffusion or active transport); glycoproteins act as receptors for hormones and neurotransmitters, and are also involved in cell recognition; enzymes such as ATP synthase catalyse reactions at the membrane surface. Intrinsic proteins span the bilayer; extrinsic proteins are on the surface. Exam answers gain credit by giving a named example along with the function.
嵌入膜中的蛋白质行使多种功能：通道蛋白形成孔道供离子被动移动；载体蛋白通过构象变化运输特定分子（易化扩散或主动运输）；糖蛋白作为激素和神经递质的受体，同时也参与细胞识别；酶（如ATP合酶）在膜表面催化反应。内在蛋白贯穿双层；外在蛋白位于膜表面。答题时若给出具体的例子连同功能，更易得分。
Common pitfall: Saying “carrier proteins use energy” without distinguishing between facilitated diffusion (no ATP) and active transport (uses ATP). Always specify whether the process is passive or active.
常见误区：只说”载体蛋白消耗能量”，却不区分易化扩散（不需要ATP）和主动运输（需要ATP）。务必指明过程是被动还是主动。
4. Cholesterol and Membrane Fluidity | 胆固醇与膜流动性
Cholesterol is a lipid found in animal cell membranes, positioned between phospholipid fatty acid tails. It acts as a fluidity buffer: at high temperatures it reduces fluidity by restraining phospholipid movement; at low temperatures it prevents tight packing of tails, thus maintaining fluidity and preventing the membrane from becoming too rigid. Exams frequently ask about the effect of cholesterol on membrane permeability. Link this to the concept of membrane stability and the maintenance of concentration gradients.
胆固醇是存在于动物细胞膜中的一种脂质，位于磷脂脂肪酸尾之间。它充当流动性缓冲剂：高温时通过限制磷脂移动来降低流动性；低温时阻止尾部的紧密堆积，从而维持流动性，防止膜变得过于僵硬。考试中常问胆固醇对膜通透性的影响。要将其与膜的稳定性及维持浓度梯度联系起来。
Also note that plant cell membranes lack cholesterol but contain other sterols, and that the presence of cholesterol makes animal cell membranes less permeable to small water-soluble molecules and ions.
还需注意，植物细胞膜不含胆固醇而含有其他固醇，且胆固醇的存在使动物细胞膜对小的水溶性分子和离子的通透性降低。
5. Factors Affecting Membrane Permeability | 影响膜通透性的因素
Temperature, pH, and solvents (such as ethanol) can alter membrane permeability. As temperature rises, phospholipids gain kinetic energy, increasing fluidity and permeability; beyond a certain point, proteins denature, creating large gaps, and the membrane loses integrity. High concentrations of ethanol dissolve lipids, disrupting the bilayer. Graph questions often show a sharp increase in permeability after a critical temperature or solvent concentration. You need to explain the two-phase effect: an initial gradual change due to increased fluidity, then a steep rise due to protein denaturation.
温度、pH值和溶剂（如乙醇）可改变膜的通透性。随着温度上升，磷脂获得动能，流动性增加，通透性升高；超过某一点后，蛋白质变性，形成较大的间隙，膜失去完整性。高浓度乙醇会溶解脂质，破坏双分子层。图表题常显示在某个临界温度或溶剂浓度后通透性急剧上升。你需要解释两阶段效应：起初因流动性增加导致的缓慢变化，随后因蛋白质变性造成的急剧上升。
Practise questions that ask you to interpret data on the leakage of beetroot pigment (betalain) under different conditions. Always relate the depth of colour in the surrounding solution to membrane damage.
练习那些要求你解释甜菜根色素（甜菜苷）在不同条件下泄漏数据的问题。始终将周围溶液颜色的深浅与膜损伤联系起来。
6. Passive Transport: Diffusion and Facilitated Diffusion | 被动运输：扩散与易化扩散
Simple diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, without the use of membrane proteins or metabolic energy. Facilitated diffusion also occurs down a concentration gradient but requires channel or carrier proteins. Channel proteins provide a hydrophilic passage; carrier proteins bind the solute and change shape. Both forms are passive and cease when equilibrium is reached. Use the terms ‘concentration gradient’, ‘kinetic energy’ and ‘selectivity’ precisely.
简单扩散是指粒子在无膜蛋白和代谢能量参与的情况下，沿浓度梯度从较高浓度区域净移动到较低浓度区域。易化扩散同样沿浓度梯度进行，但需要通道蛋白或载体蛋白。通道蛋白提供亲水通道；载体蛋白结合溶质并改变构象。二者均属被动运输，达到平衡时停止。准确使用”浓度梯度”、”动能”和”选择性”这些术语。
Diagram questions may ask you to draw or label the curve for facilitated diffusion versus simple diffusion: both plateau, but the facilitated diffusion curve is often steeper at low concentrations and shows saturation as carriers become occupied.
图表题可能要求你绘制或标注易化扩散与简单扩散的曲线：两者都会趋于平缓，但易化扩散曲线在低浓度时往往更陡，并随着载体被饱和而呈现饱和现象。
7. Osmosis and Water Potential Calculations | 渗透与水势计算
Osmosis is the net diffusion of water molecules across a partially permeable membrane from a region of higher water potential (ψ, psi) to a region of lower water potential. Water potential Ψ = Ψₛ + Ψₚ, where Ψₛ is solute potential (always negative or zero) and Ψₚ is pressure potential (usually positive inside plant cells). Pure water has a water potential of 0 kPa. In exam calculations, adding a solute lowers Ψₛ, making water potential more negative, and water moves towards the more negative ψ.
渗透是水分子通过部分透性膜从较高水势（ψ）区域向较低水势区域的净扩散。水势 Ψ = Ψₛ + Ψₚ，其中 Ψₛ 为溶质势（总是负值或零），Ψₚ 为压力势（在植物细胞内通常为正）。纯水的水势为0 kPa。在考试计算中，加入溶质会降低 Ψₛ，使水势变得更负，水分子便向更负的ψ移动。
Ψ = Ψₛ + Ψₚ
Example: If a plant cell has Ψₛ = -700 kPa and Ψₚ = 400 kPa, its water potential is -300 kPa. If the surrounding solution has Ψ = -500 kPa, water moves out of the cell (from -300 to -500 kPa).
示例：若一个植物细胞的 Ψₛ = -700 kPa，Ψₚ = 400 kPa，则其水势为 -300 kPa。若周围溶液的 Ψ = -500 kPa，则水从细胞流出（从 -300 到 -500 kPa）。
8. Active Transport and Co-transport | 主动运输与协同运输
Active transport moves molecules or ions against their concentration gradient using energy from ATP hydrolysis. Carrier proteins act as pumps. A classic A-level example is the sodium-potassium pump (Na⁺/K⁺ ATPase), which extrudes 3Na⁺ and imports 2K⁺ per ATP. Co-transport, such as the sodium-glucose symporter in the small intestine, couples the movement of Na⁺ down its electrochemical gradient to the uphill transport of glucose. Exams test your ability to distinguish between primary active transport (direct ATP use) and secondary active transport (uses an ion gradient set up by primary transport).
主动运输利用ATP水解提供的能量，将分子或离子逆浓度梯度运送。载体蛋白充当泵。A-level经典例子是钠钾泵（Na⁺/K⁺ ATP酶），每水解一个ATP泵出3个Na⁺、泵入2个K⁺。协同运输，如小肠中的钠-葡萄糖同向转运体，将Na⁺沿电化学梯度下移与葡萄糖上坡运输相偶联。考试考察你能否区分初级主动运输（直接使用ATP）和次级主动运输（利用初级运输建立的离子梯度）。
State clearly whether a protein is a carrier, a pump, or a cotransporter. Use phrases like “the energy from the Na⁺ gradient is used to pull glucose into the cell” for secondary active transport.
要清楚说明某蛋白质是载体、泵还是协同转运体。描述次级主动运输时，使用”利用Na⁺梯度的能量将葡萄糖拉入细胞”这类表述。
9. Endocytosis and Exocytosis | 胞吞与胞吐
Large molecules (e.g. proteins, polysaccharides) or particles enter and leave the cell via bulk transport. Endocytosis involves the plasma membrane folding inward to form a vesicle, while exocytosis involves vesicles fusing with the plasma membrane to release contents. Both processes require ATP and involve cytoskeleton rearrangement. Phagocytosis is ‘cell eating’ (solid particles); pinocytosis is ‘cell drinking’ (fluids). Examiners often ask for a comparison between these processes and carrier-mediated transport, highlighting that bulk transport moves large quantities but is not selective at the molecular level.
大分子（如蛋白质、多糖）或颗粒通过胞吞吐大量进出细胞。胞吞是质膜向内凹陷形成囊泡，胞吐是囊泡与质膜融合释放内容物。两个过程均需ATP并涉及细胞骨架重排。吞噬作用为”细胞进食”（固体颗粒）；胞饮作用为”细胞饮用”（液体）。考官常要求比较胞吞吐与载体介导运输，强调胞吞吐可移动大量物质，但在分子层面不具有选择性。
When answering, link the fluid mosaic model to the ability of membranes to fuse and form vesicles — the fluidity of phospholipids is essential.
答题时，要将流动镶嵌模型与膜能够融合形成囊泡的能力联系起来——磷脂的流动性至关重要。
10. Investigating Membrane Permeability: Beetroot Practical | 探究膜通透性：甜菜根实验
The beetroot core practical is a staple. Beetroot cells contain a red pigment, betalain, which leaks out when membranes are damaged. Typical variables include temperature (e.g. water baths at 0, 20, 40, 60, 80°C) or ethanol concentration. The absorbance or percentage transmission of the surrounding solution is measured using a colorimeter. A rise in absorbance indicates greater pigment leakage. Common exam questions: plot a graph, describe the trend, explain the rise at high temperatures (protein denaturation, phospholipid fluidity increase, bilayer breakdown), and evaluate limitations (size of beetroot discs, equilibration time, colorimeter calibration).
甜菜根核心实验是必考内容。甜菜根细胞含有红色素甜菜苷，当膜受损时会泄漏。典型变量包括温度（如0, 20, 40, 60, 80°C水浴）或乙醇浓度。用比色计测量周围溶液的吸光度或透光率百分比。吸光度升高表明色素泄漏更多。常见考题：绘制图表，描述变化趋势，解释高温下升高现象（蛋白质变性、磷脂流动性增加、双层破坏），并评价实验局限性（甜菜根圆片大小、平衡时间、比色计校准）。
Always frame your explanation: “As temperature increases, phospholipids gain kinetic energy, making the membrane more fluid and permeable. Above a certain temperature, membrane proteins denature, opening larger holes, so betalain leaks out more rapidly.”
始终按这样的思路解释：”随着温度升高，磷脂获得动能，使膜更具流动性和通透性。超过某一温度后，膜蛋白变性，形成更大的孔洞，因此甜菜苷更快泄漏。”
11. Exam Technique and Common Pitfalls | 答题技巧与常见误区
Misreading command words is a major source of lost marks. ‘Describe’ asks for a factual account; ‘Explain’ requires reasons and mechanisms; ‘Suggest’ means apply biological principles to an unfamiliar context. When comparing simple and facilitated diffusion, explicitly mention the use of proteins and saturation. For osmosis questions, always state that water moves from a high water potential to a low water potential, and support your answer with calculated ψ values. In membrane structure questions, draw and label clearly: you might be asked to include phospholipids, proteins, glycoproteins, and cholesterol, with the hydrophobic and hydrophilic regions marked.
误读指令词是丢分的主要原因。”描述”要求事实陈述；”解释”需要给出原因和机制；”建议”则是将生物学原理应用于陌生情境。在比较简单扩散和易化扩散时，要明确提到蛋白质的使用及饱和现象。回答渗透问题时，务必说明水从高水势移向低水势，并用计算出的ψ值支持答案。在膜结构题中，绘图并清晰标注：可能会要求画出磷脂、蛋白质、糖蛋白和胆固醇，并标出疏水区和亲水区。
Another frequent error is stating that “ATP is used in facilitated diffusion” — it is not. Facilitated diffusion is passive. Also, avoid vague phrases like “the membrane breaks down”; instead, say “the phospholipid bilayer becomes disrupted” or “transmembrane proteins denature”. Precision in vocabulary is the hallmark of a top-grade answer.
另一个常见错误是说”易化扩散使用ATP”——实际上并非如此，易化扩散是被动的。此外，避免使用”膜的破裂”这类模糊表述；应该说”磷脂双分子层受到破坏”或”跨膜蛋白变性”。用词精准是高分答案的标志。
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