📚 IGCSE AQA Biology: Cell Membrane Revision | IGCSE AQA 生物：细胞膜考点精讲
The cell membrane is a fundamental structure in all living organisms. It controls the entry and exit of substances, protects the cell, and allows communication with the external environment. Mastering this topic is essential for your IGCSE AQA Biology exam. This article covers every key point you need to know, from structure to transport mechanisms, all explained in clear bilingual sections.
细胞膜是所有生物体中一个基础结构。它控制物质的进出，保护细胞，并实现与外界环境的交流。掌握这一主题对你的 IGCSE AQA 生物考试至关重要。本文将涵盖从结构到运输机制的所有关键考点，并以清晰的中英双语逐点讲解。
1. Structure of the Cell Membrane | 细胞膜的结构
The cell membrane, or plasma membrane, forms a thin barrier around the cell. It is mainly composed of phospholipids arranged into a bilayer, with proteins and carbohydrates embedded or attached to it. Under the electron microscope, it appears as two dark lines separated by a light band.
细胞膜（质膜）在细胞周围形成一道很薄的屏障。它主要由排列成双层的磷脂分子构成，其中镶嵌或附着有蛋白质和碳水化合物。在电子显微镜下，它呈现为被一个浅色亮带隔开的两个暗线。
The membrane is about 7–10 nm thick, making it extremely thin but remarkably flexible. Its components are not rigidly fixed; they can move laterally, which is crucial for its functions.
该膜厚约7–10纳米，极其薄但非常柔韧。其组分并非固定不动，它们可以侧向移动，这对它的功能至关重要。
2. The Fluid Mosaic Model | 流体镶嵌模型
We describe the cell membrane using the fluid mosaic model. The ‘fluid’ part refers to the phospholipid bilayer and proteins moving within the layer, like icebergs floating on a sea. The ‘mosaic’ part refers to the pattern of different proteins scattered throughout the lipid bilayer.
我们用流体镶嵌模型来描述细胞膜。“流体”指的是磷脂双层和蛋白质在层内移动，如同冰山漂浮在海洋上。“镶嵌”指的是不同的蛋白质分散于脂双层中形成的图案。
This model was proposed by Singer and Nicolson in 1972 and remains the accepted explanation. It helps us understand how transport proteins, receptors, and other molecules can change position and function dynamically.
该模型由辛格和尼科尔森于1972年提出，至今仍是被接受的解释。它帮助我们理解转运蛋白、受体和其他分子如何动态地改变位置和功能。
3. Phospholipid Bilayer | 磷脂双分子层
Each phospholipid molecule has a hydrophilic (water‑loving) phosphate head and two hydrophobic (water‑fearing) fatty acid tails. In water, phospholipids spontaneously arrange into a bilayer: the heads face outward toward the watery environments inside and outside the cell, while the tails face inward, shielded from water.
每个磷脂分子有一个亲水（喜水）的磷酸头端和两条疏水（厌水）的脂肪酸尾端。在水中，磷脂自发地排列成双层：头端朝向细胞内部和外部的含水环境，尾端朝内，避免接触水。
The tails are made of saturated or unsaturated fatty acids. Unsaturated tails with kinks increase membrane fluidity. The bilayer acts as a selective barrier – only small, non‑polar molecules can pass through directly.
尾部由饱和或不饱和脂肪酸组成。带有扭结的不饱和尾部能增加膜的流动性。该双层作为一个选择性屏障——只有小的非极性分子能直接穿过。
Head: hydrophilic – PO₄²⁻ | 头端：亲水 – PO₄²⁻
Tail: hydrophobic – CH₂ chains | 尾端：疏水 – CH₂链
4. Membrane Proteins | 膜蛋白
Proteins are embedded in the phospholipid bilayer either partially (peripheral proteins) or spanning the entire membrane (integral proteins). They perform a wide range of functions, and many are glycoproteins with carbohydrate chains attached.
蛋白质以两种方式嵌入磷脂双层：部分嵌入（外周蛋白）或贯穿整个膜（整合蛋白）。它们执行多种功能，许多蛋白质因附有碳水化合物链而成为糖蛋白。
The main types of membrane proteins you need to know for the exam include:
考试中你需要了解的膜蛋白主要类型包括：
Channel proteins – form a hydrophilic pore that allows specific ions (e.g., Na⁺, K⁺) to diffuse through. | 通道蛋白 – 形成一个亲水孔道，允许特定离子（如Na⁺、K⁺）通过扩散。
Carrier proteins – bind to specific molecules, change shape, and transport them across, used in facilitated diffusion and active transport. | 载体蛋白 – 与特定分子结合，改变构象并将其转运过膜，用于易化扩散和主动运输。
Receptor proteins – receive chemical signals (e.g., hormones) and trigger a response inside the cell. | 受体蛋白 – 接受化学信号（如激素）并触发细胞内的响应。
Enzymes – catalyse reactions at the membrane surface (e.g., ATP synthase in respiration). | 酶 – 在膜表面催化反应（例如呼吸作用中的ATP合酶）。
Glycoproteins – contribute to cell recognition and adhesion; they often act as antigens. | 糖蛋白 – 参与细胞识别和黏附；它们常作为抗原发挥作用。
| Protein Type | Function | Example |
| Channel | Allows passive movement of ions | K⁺ channel in nerve cells |
| Carrier | Facilitated diffusion and active transport | Glucose transporter GLUT4 |
| Receptor | Binds to insulin to activate glucose uptake | Insulin receptor |
5. Carbohydrates on the Membrane | 膜上的碳水化合物
Short carbohydrate chains attach to proteins (forming glycoproteins) or to lipids (forming glycolipids) on the outer surface of the membrane. This sugary coating is called the glycocalyx. It is involved in cell‑to‑cell recognition, adhesion, and protection.
短碳水化合物链附在膜外表面的蛋白质（形成糖蛋白）或脂类（形成糖脂）上。这一糖衣层称为糖萼。它参与细胞间识别、黏附和保护。
Some glycoproteins act as antigens, allowing the immune system to distinguish self from non‑self. In blood transfusions, ABO antigens are glycoproteins on red blood cell membranes – a classic exam context.
一些糖蛋白作为抗原，使免疫系统能够区分自身和非自身。在输血中，ABO抗原就是红细胞膜上的糖蛋白——一个经典的考试情境。
6. Permeability of the Membrane | 细胞膜的通透性
The cell membrane is selectively permeable (partially permeable). This means it allows some substances to cross but restricts others. Small, non‑polar molecules like O₂ and CO₂ diffuse through the bilayer easily. Water, despite being polar, is small enough to pass slowly. Ions and large polar molecules (glucose, amino acids) require transport proteins.
细胞膜是选择渗透性（部分渗透性）的。这意味着它允许一些物质穿过，而限制另一些。小而非极性的分子（如O₂和CO₂）容易通过双层扩散。水虽然极性，但因体积足够小也能缓慢穿过。离子和大极性分子（葡萄糖、氨基酸）需要转运蛋白。
Permeability can be affected by temperature (higher temperature increases fluidity and permeability), pH, and the presence of organic solvents (ethanol can dissolve lipids). This is often tested in practical contexts.
通透性可能受温度（升高温度会增加流动性和通透性）、pH和有机溶剂的存在（乙醇可溶解脂类）影响。这一点常在实验情境中考查。
7. Diffusion | 扩散
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process – it does not require metabolic energy. The rate of diffusion depends on several factors.
扩散是粒子沿浓度梯度从高浓度区域向低浓度区域的净移动。这是一种被动过程——不需要代谢能量。扩散速率取决于多个因素。
Factors affecting diffusion: concentration gradient (steeper gradient → faster rate), temperature (higher temperature → particles have more kinetic energy), surface area to volume ratio (larger surface area → faster exchange), and diffusion distance (shorter distance → faster).
影响扩散的因素：浓度梯度（梯度越陡，速率越快）、温度（温度越高，粒子动能越大）、表面积体积比（表面积越大，交换越快）以及扩散距离（距离越短，越快）。
In living organisms, O₂ diffuses from alveoli into blood capillaries, and CO₂ diffuses from respiring cells into the blood. These examples frequently appear in exam questions.
在生物体内，O₂从肺泡扩散进入毛细血管，CO₂从呼吸细胞扩散入血。这些例子经常出现在考题中。
Rate of diffusion ∝ (surface area × concentration difference) / diffusion distance
扩散速率 ∝ (表面积 × 浓度差) / 扩散距离
8. Osmosis | 渗透作用
Osmosis is a special case of diffusion involving water molecules. It is the net movement of water from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a selectively permeable membrane.
渗透作用是涉及水分子的扩散特例。它是水通过选择渗透性膜从水势较高区域（稀溶液）向水势较低区域（浓溶液）的净移动。
Water potential (ψ) is measured in kilopascals (kPa). Pure water has a water potential of zero. Adding solutes lowers the water potential (makes it more negative). The formula is:
水势（ψ）以千帕（kPa）为单位。纯水的水势为零。加入溶质会降低水势（使其更负）。公式为：
ψ = ψₛ + ψₚ
water potential = solute potential + pressure potential | 水势 = 溶质势 + 压力势
In animal cells, if placed in a hypotonic solution, water enters by osmosis and the cell may burst (lysis). In a hypertonic solution, water leaves and the cell shrinks (crenation). Plant cells become turgid when water enters, as the cell wall exerts pressure preventing bursting. In a hypertonic environment, plant cells undergo plasmolysis – the cell membrane pulls away from the cell wall.
在动物细胞中，若置于低渗溶液中，水通过渗透作用进入，细胞可能胀破（溶血）；在高渗溶液中，水流出，细胞皱缩（质壁分离）。植物细胞吸水时变得硬挺（膨胀），因为细胞壁施加压力防止破裂。在高渗环境中，植物细胞发生质壁分离——细胞膜与细胞壁脱离。
9. Active Transport | 主动运输
Active transport is the movement of molecules or ions against their concentration gradient (from low to high concentration). This process requires energy in the form of ATP and uses specific carrier proteins. It is essential for absorbing nutrients from dilute solutions.
主动运输是分子或离子逆浓度梯度（从低浓度到高浓度）的移动。该过程需要ATP形式的能量并使用特定载体蛋白。对于从稀溶液中吸收营养物质来说，这是必不可少的。
During active transport, the carrier protein binds to the solute, changes shape using energy from ATP hydrolysis, and releases the solute on the other side of the membrane. The energy conversion is:
在主动运输中，载体蛋白与溶质结合，利用ATP水解的能量改变形状，在膜的另一侧释放溶质。能量转化如下：
ATP + H₂O → ADP + Pⁱ (+ energy)
ATP + H₂O → ADP + Pⁱ (+ 能量)
Key examples: uptake of nitrate ions (NO₃⁻) by root hair cells in plants, and absorption of glucose into the blood from the small intestine (in the villi). In both cases, concentration gradients are unfavourable for diffusion.
关键例子：植物根毛细胞对硝酸根离子（NO₃⁻）的吸收，以及葡萄糖从小肠绒毛吸收入血。在这两种情况下，浓度梯度都不利于扩散。
10. Factors Affecting Transport Across Membranes | 影响跨膜运输的因素
Several variables influence the rate of diffusion, osmosis, and active transport. Understanding these helps in designing experiments and interpreting data.
若干变量影响扩散、渗透和主动运输的速率。理解这些有助于实验设计和数据解读。
Temperature: Higher temperature increases kinetic energy, so particles move faster. Diffusion rate increases, but if too hot, membrane proteins can denature and active transport stops. | 温度：升高温度增加动能，粒子运动更快。扩散速率增加，但若过热，膜蛋白可能变性，主动运输停止。
Concentration gradient: The steeper the difference, the faster the rate of passive transport. No such effect on active transport once carriers are saturated. | 浓度梯度：浓度差越陡，被动运输速率越快。一旦载体饱和，对主动运输则无此影响。
Surface area to volume ratio: A larger surface area relative to volume increases exchange efficiency. This explains the flattened shape of red blood cells, the folded inner membrane of mitochondria, and root hair cells. | 表面积体积比：相对于体积更大的表面积可提高交换效率。这解释了红细胞扁平形状、线粒体内膜折叠以及根毛细胞的结构。
Membrane thickness: Thinner membranes reduce diffusion distance; alveoli and capillaries have thin walls (one cell thick) for rapid gas exchange. | 膜厚度：较薄的膜减少扩散距离；肺泡和毛细血管壁很薄（仅单层细胞），以实现快速气体交换。
Solvent and inhibitors: Ethanol dissolves lipids, making membranes leaky. Metabolic poisons (cyanide) stop ATP production, halting active transport but not diffusion. | 溶剂和抑制剂：乙醇溶解脂类，使膜渗漏。代谢毒物（氰化物）阻止ATP生成，从而停止主动运输但不影响扩散。
11. Experiment: Investigating Diffusion Using Visking Tubing | 实验：用透析管研究扩散
A common practical uses visking tubing (a selectively permeable membrane) to model diffusion. You can fill the tubing with starch solution and place it in a beaker of water containing iodine. Or fill with glucose and test the external water with Benedict’s solution.
一个常见实验用透析管（一种选择渗透性膜）模拟扩散。你可以将透析管装满淀粉溶液，放入含碘水的烧杯中。或者装满葡萄糖，然后用本氏液检测外部水。
The procedure in brief: Tie one end of the tubing, add the test solution, tie the other end, and immerse in a beaker of water or iodine solution. After 20–30 minutes, observe any colour change. Starch is too large to pass through, but iodine (small) can enter and cause a blue‑black colour inside. Glucose molecules are small enough to diffuse out and give a positive Benedict’s test.
简略步骤：系紧管子一端，加入测试溶液，系紧另一端，浸入水或碘溶液的烧杯中。20–30分钟后，观察任何颜色变化。淀粉太大无法通过，但碘（小分子）可以进入并在内部产生蓝黑色。葡萄糖分子足够小，能扩散出去，使得本氏液检测呈阳性。
This experiment demonstrates that the membrane allows small molecules to pass but restricts larger ones – supporting the concept of selective permeability.
该实验表明膜允许小分子通过，限制大分子——支撑了选择渗透性的概念。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
When answering exam questions, always use precise terminology. Do not say ‘cell wall’ when you mean ‘cell membrane’ – plant cells have both! Specify that the membrane is selectively permeable. Use ‘water potential’ rather than ‘water concentration’ when discussing osmosis.
回答考题时，始终使用精确术语。当你想表达“细胞膜”时，不要说成“细胞壁”——植物细胞两者都有！明确细胞膜具有选择渗透性。讨论渗透作用时，使用“水势”而非“水浓度”。
Common mistakes include: confusing diffusion with osmosis, forgetting that active transport requires both carrier proteins and ATP, and misapplying the water potential equation. Also, always state that facilitated diffusion uses channel or carrier proteins but is still passive.
常见错误包括：混淆扩散与渗透，忘记主动运输需要载体蛋白和ATP两者，以及误用水势方程。此外，始终要说明易化扩散使用通道或载体蛋白但仍是被动过程。
Drawings of the fluid mosaic model should label phospholipid bilayer (with head and tail), intrinsic and extrinsic proteins, and glycoproteins. Use a scale bar if asked. Remember that the membrane is fluid; proteins are embedded, not on the surface like a skin.
绘制流体镶嵌模型时应标注磷脂双层（含头尾端）、内在和外周蛋白、糖蛋白。如果要求，使用比例尺。记住膜是流动的；蛋白质是嵌入的，而非像皮肤一样在表面。
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