📚 GCSE WJEC Biology: Cell Membrane Exam Focus | GCSE WJEC 生物：细胞膜 考点精讲
The cell membrane is a fundamental topic in GCSE WJEC Biology. Understanding its structure and function is essential for explaining how substances move into and out of cells, and for grasping key processes such as osmosis, diffusion, and active transport. This revision guide covers all the examinable points you need, presented in clear bilingual sections.
细胞膜是 GCSE WJEC 生物学的基础课题。理解其结构与功能对于解释物质如何进出细胞以及掌握渗透、扩散和主动运输等关键过程至关重要。本复习指南涵盖了所有你需要掌握的考点，以清晰的中英双语形式呈现。
1. Cell Membrane Overview | 细胞膜概述
The cell membrane, also known as the plasma membrane, is a thin layer that surrounds every cell. It acts as a barrier between the cell’s internal environment and the outside world. In WJEC exams, you must be able to describe its main roles: controlling the movement of substances in and out of the cell, maintaining cell shape, and allowing cell recognition and communication.
细胞膜，也称为质膜，是包裹每个细胞的一层薄膜。它充当细胞内部与外部环境之间的屏障。在 WJEC 考试中，你必须能够描述其主要功能：控制物质进出细胞、维持细胞形状，以及实现细胞识别和通讯。
All cell membranes have a similar basic structure, described by the fluid mosaic model. You should be able to label a diagram of the membrane, identifying phospholipids, proteins, glycoproteins, glycolipids, and cholesterol. Remember that only plant and bacterial cells have a cell wall outside the membrane, which provides additional support.
所有细胞膜都具有类似的基本结构，可用流动镶嵌模型来描述。你应该能够标注细胞膜示意图，识别磷脂、蛋白质、糖蛋白、糖脂和胆固醇。请记住，只有植物和细菌细胞在膜外还有细胞壁，这提供额外的支持。
2. Fluid Mosaic Model | 流动镶嵌模型
The fluid mosaic model is the accepted explanation of cell membrane structure. ‘Fluid’ refers to the fact that the phospholipids and proteins can move laterally within the layer, like a two-dimensional liquid. ‘Mosaic’ describes the pattern of different proteins embedded in the phospholipid bilayer, like tiles in a mosaic.
流动镶嵌模型是对细胞膜结构被广泛接受的解释。“流动”指的是磷脂和蛋白质可以在层内横向移动，就像二维液体一样。“镶嵌”描述了不同蛋白质嵌在磷脂双分子层中的图案，就像马赛克中的瓷砖。
This model helps explain how the membrane is flexible and self-sealing, and how proteins can change position to allow certain molecules to cross. In WJEC exam questions, you might be asked to explain why the membrane is described as ‘fluid’ and as ‘mosaic’, so memorise the meaning of these terms.
该模型有助于解释细胞膜为何柔韧且能自修复，以及蛋白质如何改变位置以允许特定分子通过。在 WJEC 考题中，你可能会被要求解释为什么细胞膜被描述为“流动的”和“镶嵌的”，因此请记住这些术语的含义。
3. Phospholipid Bilayer | 磷脂双分子层
The foundation of the membrane is a double layer of phospholipid molecules. Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic (water-hating) fatty acid tails. The heads point outwards towards the watery environments inside and outside the cell, while the tails point inwards, away from water.
细胞膜的基础是双层磷脂分子。每个磷脂分子有一个亲水（喜水）头部和两个疏水（厌水）脂肪酸尾部。头部朝外，指向细胞内外两侧的水环境，而尾部朝内，远离水环境。
This arrangement forms a stable barrier that prevents water-soluble substances like ions and glucose from passing through freely. Only very small, non-polar molecules such as oxygen and carbon dioxide can diffuse directly through the phospholipid bilayer. This selective barrier is key to homeostasis.
这种排列形成了一道稳定的屏障，阻止水溶性物质（如离子和葡萄糖）自由通过。只有非常小的非极性分子（如氧气和二氧化碳）可以直接通过磷脂双分子层扩散。这种选择屏障对于稳态至关重要。
| Phospholipid Part | 磷脂部分 | Property | 特性 | Location | 位置 |
|---|---|---|
| Head | 头部 | Hydrophilic | 亲水 | Faces outward and inward (aqueous) | 朝外和朝内（水环境） |
| Tails | 尾部 | Hydrophobic | 疏水 | Inside the bilayer, away from water | 双分子层内部，远离水 |
4. Membrane Proteins | 膜蛋白
Proteins are scattered throughout the membrane, some spanning the whole bilayer (integral proteins) and others attached to one surface (peripheral proteins). Channel proteins and carrier proteins are two types you must know for WJEC: channel proteins form pores that allow specific ions to diffuse through, while carrier proteins change shape to move molecules across the membrane.
蛋白质散布在整个细胞膜中，一些贯穿整个双分子层（整合蛋白），另一些附着在某一表面（外周蛋白）。通道蛋白和载体蛋白是 WJEC 要求掌握的两种类型：通道蛋白形成孔道，允许特定离子扩散通过；载体蛋白则改变形状将分子运过膜。
Membrane proteins also act as receptors for hormones and other signalling molecules, and as enzymes. In exam diagrams, you might be asked to label a channel protein and a carrier protein, and explain the difference between them in terms of mechanism and energy requirement.
膜蛋白还可作为激素和其他信号分子的受体，以及酶。在考试示意图中，你可能需要标注通道蛋白和载体蛋白，并从机制和能量需求方面解释它们的区别。
5. Carbohydrates in the Membrane | 细胞膜上的糖类
Carbohydrates are found on the outer surface of the cell membrane, bound to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate chains act as identification tags, allowing cells to recognise each other. This is crucial for the immune system to distinguish self from non-self, and for cell adhesion in tissues.
糖类位于细胞膜的外表面，与蛋白质结合形成糖蛋白，或与脂类结合形成糖脂。这些糖链充当识别标签，使细胞能够相互识别。这对于免疫系统区分自身与非自身以及组织中的细胞粘附至关重要。
In WJEC, you need to know that glycoproteins and glycolipids are involved in cell signalling and recognition. They can also bind to specific molecules, triggering cellular responses. Remember that only the outer surface has carbohydrates; the inner surface does not, which is a common comparison point.
在 WJEC 中，你需要知道糖蛋白和糖脂参与细胞信号传递和识别。它们还能与特定分子结合，触发细胞反应。请记住，只有外表面有糖类，内表面没有，这是一个常见的比较点。
6. Cholesterol in the Membrane | 细胞膜中的胆固醇
Cholesterol is a lipid molecule found within the hydrophobic region of the phospholipid bilayer. Its main function is to regulate membrane fluidity: at high temperatures, cholesterol restrains the movement of phospholipids, making the membrane less fluid; at low temperatures, it prevents phospholipids from packing too tightly together, maintaining fluidity and preventing the membrane from becoming rigid.
胆固醇是一种脂类分子，位于磷脂双分子层的疏水区域。其主要功能是调节细胞膜的流动性：在高温下，胆固醇限制磷脂的移动，使膜流动性降低；在低温下，它防止磷脂过于紧密堆积，维持流动性，避免细胞膜变得僵硬。
Cholesterol also contributes to the mechanical stability of the membrane. In animal cells, cholesterol is essential; plant cell membranes contain related compounds called phytosterols. For WJEC, you simply need to recall the role of cholesterol in maintaining membrane fluidity and stability.
胆固醇还有助于细胞膜的机械稳定性。在动物细胞中，胆固醇至关重要；植物细胞膜则含有称为植物固醇的相关化合物。对于 WJEC，你只需记住胆固醇在维持膜流动性和稳定性方面的作用。
7. Selective Permeability | 选择透过性
The cell membrane is selectively permeable (or semi-permeable), meaning it allows some substances to cross but not others. This property is due to the hydrophobic core of the bilayer and the presence of specific transport proteins. Small, non-polar molecules (O₂, CO₂) diffuse freely, but ions and large polar molecules require protein channels or carriers.
细胞膜具有选择透过性（或半透性），这意味着它允许某些物质通过，但不允许其他物质通过。这一特性源于双分子层的疏水核心以及特定转运蛋白的存在。小的非极性分子（O₂、CO₂）可以自由扩散，但离子和大极性分子需要蛋白通道或载体。
WJEC questions often ask you to explain why the membrane is described as selectively permeable. Use examples: oxygen passes through easily, but glucose requires a carrier protein. Water, though polar, can cross slowly due to its small size, but osmosis mainly occurs through aquaporins.
WJEC 考题经常要求你解释为什么细胞膜被描述为选择透过性。举例说明：氧气容易通过，但葡萄糖需要载体蛋白。水虽然极性，但因分子小可以缓慢通过，但渗透主要通过水通道蛋白进行。
8. 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, meaning it does not require metabolic energy (ATP). In living organisms, diffusion is vital for gas exchange, nutrient uptake, and waste removal.
扩散是粒子从高浓度区域向低浓度区域的净移动，顺浓度梯度进行。这是一个被动过程，不需要代谢能量（ATP）。在生物体内，扩散对于气体交换、营养摄取和废物排出至关重要。
Factors affecting the rate of diffusion include temperature, concentration gradient, surface area, and diffusion distance. You must be able to apply Fick’s law, which states: rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of membrane. WJEC may ask you to calculate rates or explain how a factor alters rate.
影响扩散速率的因素包括温度、浓度梯度、表面积和扩散距离。你必须能够应用菲克定律：扩散速率 ∝（表面积 × 浓度差）÷ 膜厚度。WJEC 可能会要求你计算速率或解释某个因素如何改变速率。
Rate of diffusion ∝ (Surface area × Concentration difference) ÷ Distance
9. Osmosis | 渗透
Osmosis is a special case of diffusion, referring specifically to the movement of water molecules across a selectively permeable membrane from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution). It is also a passive process and does not require energy.
渗透是扩散的一种特殊情况，特指水分子通过选择透过性膜，从较高水势（稀溶液）区域移动到较低水势（浓溶液）区域。这也是一个被动过程，不需要能量。
Water potential is measured in kilopascals (kPa); pure water has a water potential of 0 kPa, and solutions have negative values. Water always moves towards a more negative water potential. WJEC exam questions often involve describing the effects of different solutions on animal and plant cells (e.g., turgid, flaccid, plasmolyzed).
水势以千帕斯卡（kPa）为单位；纯水的水势为 0 kPa，溶液的水势为负值。水总是向更负的水势移动。WJEC 考题常涉及描述不同溶液对动植物细胞的影响（例如，胀大、松弛、质壁分离）。
| Solution Type | 溶液类型 | Animal Cell Effect | 动物细胞效应 | Plant Cell Effect | 植物细胞效应 |
|---|---|---|
| Hypotonic | 低渗 | Burst (lysis) | 胀破（溶解） | Turgid (normal, healthy) | 胀大（正常、健康） |
| Isotonic | 等渗 | Normal shape | 正常形状 | Flaccid | 松弛 |
| Hypertonic | 高渗 | Shriveled (crenation) | 皱缩（皱缩） | Plasmolyzed | 质壁分离 |
10. Active Transport | 主动运输
Active transport is the movement of molecules or ions across the membrane against their concentration gradient, from a region of lower concentration to a region of higher concentration. This process requires energy in the form of ATP, and uses specific carrier proteins. Carrier proteins use energy from ATP to change shape and transport the solute.
主动运输是分子或离子逆浓度梯度跨膜移动的过程，即从低浓度区域向高浓度区域移动。这一过程需要以 ATP 形式的能量，并使用特定的载体蛋白。载体蛋白利用 ATP 的能量改变形状并运输溶质。
A typical WJEC example is the uptake of mineral ions by root hair cells from the soil, where the ion concentration inside the cell is already higher than in the soil. Other examples include glucose absorption in the small intestine and the sodium-potassium pump in nerve cells. Active transport allows cells to accumulate essential substances even when the external supply is scarce.
典型的 WJEC 示例是根毛细胞从土壤中吸收矿质离子，此时细胞内的离子浓度已经高于土壤中的浓度。其他例子包括小肠中的葡萄糖吸收以及神经细胞中的钠-钾泵。主动运输使细胞即使在外界供应稀少时也能积累必需物质。
11. Endocytosis and Exocytosis | 胞吞与胞吐
For very large molecules or particles, the cell uses bulk transport mechanisms: endocytosis (taking in) and exocytosis (secreting out). In endocytosis, the membrane folds inward to form a vesicle around the substance. In exocytosis, vesicles containing the substance fuse with the membrane and release their contents outside. Both require energy (ATP).
对于非常大的分子或颗粒，细胞采用批量运输机制：胞吞（摄入）和胞吐（分泌）。在胞吞过程中，细胞膜向内折叠，形成一个包裹物质的囊泡。在胞吐过程中，含有该物质的囊泡与细胞膜融合，并将内容物释放到外部。两者都需要能量（ATP）。
WJEC may ask you to distinguish between these and the other transport methods. Remember that endocytosis and exocytosis do not involve movement through the phospholipid bilayer; instead, the membrane itself changes shape. These processes are used for secreting enzymes, hormones, and for white blood cells engulfing pathogens (phagocytosis).
WJEC 可能会要求你区分这些运输方式与其他运输方式。请记住，胞吞和胞吐不涉及通过磷脂双分子层的运动；相反，细胞膜本身会改变形状。这些过程用于分泌酶、激素以及白细胞吞噬病原体（吞噬作用）。
12. Factors Affecting Transport and Exam Tips | 影响运输的因素与考试技巧
All transport processes are affected by temperature. For diffusion and osmosis, higher temperature increases kinetic energy, thus increasing rate, until proteins denature and membrane integrity is lost. For active transport, temperature affects enzyme activity in ATP production, so rate also increases to an optimum then falls. pH and metabolic poisons (like cyanide) can inhibit active transport by blocking ATP synthesis.
所有运输过程都受温度影响。对于扩散和渗透，高温增加动能，从而提高速率，直到蛋白质变性、膜完整性丧失。对于主动运输，温度影响 ATP 产生中的酶活性，因此速率也会升高到最佳值然后下降。pH 和代谢毒物（如氰化物）可通过阻断 ATP 合成来抑制主动运输。
In WJEC Biology exams, always link the structure of the membrane to its function. When describing substance movement, specify the type of transport, whether energy is needed, and the direction of the concentration gradient. Use precise terminology: ‘water potential’ not just ‘concentration of water’; ‘carrier protein’ for active transport; ‘channel protein’ for facilitated diffusion. Practice labelling diagrams and interpreting data from osmosis experiments, such as measuring changes in mass or length of potato cylinders.
在 WJEC 生物学考试中，始终将细胞膜的结构与其功能联系起来。在描述物质移动时，要明确运输类型、是否需要能量以及浓度梯度方向。使用精确术语：“水势”而不仅仅是“水浓度”；主动运输用“载体蛋白”；协助扩散用“通道蛋白”。练习标注示意图，并解读渗透实验数据，例如测量薯条质量或长度的变化。
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