📚 Cell Membranes | A-Level WJEC Biology Revision | 细胞膜 A-Level WJEC 生物考点精讲
Cell membranes are fundamental to life, acting as dynamic barriers that control the movement of substances in and out of cells. In WJEC A-Level Biology, you must understand the fluid mosaic model, the roles of phospholipids, proteins, cholesterol, and glycocalyx, and how these components contribute to selective permeability and cell communication. This article provides a detailed, bilingual breakdown of every key concept to help you master the topic.
细胞膜是生命活动的基础，它是控制物质进出细胞的动态屏障。在 WJEC A-Level 生物学考试中，你需要掌握流动镶嵌模型，理解磷脂、蛋白质、胆固醇和糖被膜的作用，以及这些组分如何共同实现选择透过性和细胞通讯。本文将通过中英双语详细解析每一个重要考点，帮助你扎实掌握该主题。
1. Phospholipid Bilayer | 磷脂双分子层
The core structure of all cell membranes is the phospholipid bilayer. Each phospholipid molecule has a hydrophilic (water‑attracting) phosphate head and two hydrophobic (water‑repelling) fatty acid tails. In an aqueous environment, phospholipids spontaneously arrange themselves so that the heads face the external and internal watery solutions, while the tails point inwards, away from water. This arrangement forms a stable barrier that is impermeable to most water‑soluble molecules and ions.
所有细胞膜的核心结构是磷脂双分子层。每个磷脂分子都有一个亲水的磷酸头部和两条疏水的脂肪酸尾部。在水环境中，磷脂自发排列成头部朝向外侧和内侧水溶液、尾部朝内远离水的结构。这种排列形成了一个稳定的屏障，使大多数水溶性分子和离子无法自由通过。
2. Membrane Proteins | 膜蛋白
Proteins are scattered throughout the phospholipid bilayer, either embedded partially or spanning the entire membrane. Integral proteins, such as channel proteins and carrier proteins, traverse the membrane and facilitate the transport of specific substances. Peripheral proteins are bound to the surface and often act as enzymes or anchors for the cytoskeleton. Membrane proteins also serve as receptors for hormones, neurotransmitters, and other signalling molecules, enabling the cell to respond to its environment.
蛋白质散在分布于磷脂双分子层中，有的部分嵌入，有的贯穿整个膜。内在蛋白（如通道蛋白和载体蛋白）横跨膜，辅助特定物质的运输。外在蛋白结合在膜表面，常作为酶或细胞骨架的锚定点。膜蛋白还可以作为激素、神经递质等信号分子的受体，使细胞能对周围环境作出反应。
3. Cholesterol | 胆固醇
Cholesterol molecules are interspersed among phospholipids in animal cell membranes. They have a small hydrophilic hydroxyl group and a large hydrophobic steroid ring. Cholesterol regulates membrane fluidity: at high temperatures, it restrains phospholipid movement, reducing fluidity; at low temperatures, it prevents fatty acid tails from packing too tightly, thereby maintaining flexibility. Cholesterol also contributes to the mechanical stability of the membrane, reducing its permeability to small water‑soluble molecules.
胆固醇分子穿插在动物细胞膜的磷脂之间。它们有一个亲水的小羟基和疏水的大甾环结构。胆固醇调节膜的流动性：在高温时，它限制磷脂运动，降低流动性；在低温时，它阻止脂肪酸尾部过度紧密排列，从而保持膜的柔韧性。胆固醇还有助于提高膜的机械稳定性，降低对小分子水溶性物质的通透性。
4. Glycolipids and Glycoproteins | 糖脂与糖蛋白
On the extracellular surface of the membrane, carbohydrate chains are covalently attached to lipids (glycolipids) or to proteins (glycoproteins). Together they form the glycocalyx, a fuzzy coat that protects the cell from mechanical and chemical damage. These carbohydrate markers are essential in cell‑cell recognition, cell adhesion, and immune responses. For example, the ABO blood group antigens are glycolipids on red blood cell membranes.
在膜的细胞外侧表面，碳水化合物链以共价键结合在脂质上形成糖脂，或结合在蛋白质上形成糖蛋白，共同构成糖被膜（糖萼）。这层绒毛状外衣能保护细胞免受机械和化学损伤。这些糖标记在细胞识别、细胞粘附和免疫反应中起关键作用。例如，ABO 血型抗原就是红细胞膜上的糖脂。
5. The Fluid Mosaic Model | 流动镶嵌模型
The fluid mosaic model describes the cell membrane as a dynamic, quasi‑fluid structure where phospholipids and proteins can move laterally within the layer. The ‘fluid’ aspect refers to the ability of lipids and proteins to drift in the plane of the bilayer, while the ‘mosaic’ aspect reflects the patchwork of proteins embedded in the phospholipid matrix. Experimental evidence, such as cell fusion studies and freeze‑fracture electron microscopy, supports this model.
流动镶嵌模型将细胞膜描述为一个动态的、准流体结构，磷脂和蛋白质能在膜的平面内进行侧向移动。“流动”指脂质和蛋白质可在双分子层平面内漂移，“镶嵌”则反映蛋白质像马赛克一样嵌在磷脂基质中。细胞融合实验和冷冻断裂电镜等实验证据均支持这一模型。
6. Selective Permeability and Factors Affecting It | 选择透过性及其影响因素
The cell membrane is selectively permeable, meaning it allows some substances to cross easily while restricting others. Lipid‑soluble molecules (e.g. oxygen, carbon dioxide, steroid hormones) diffuse through the phospholipid bilayer directly. Small uncharged polar molecules (e.g. water, urea) pass slowly, whereas ions and larger polar molecules require transport proteins. Factors that affect permeability include temperature, pH, solvent concentration, and the presence of detergents, which can disrupt the phospholipid bilayer.
细胞膜具有选择透过性，即允许某些物质轻易通过，而限制其他物质。脂溶性分子（如氧气、二氧化碳、类固醇激素）可直接穿过磷脂双分子层。小而不带电荷的极性分子（如水、尿素）通过较慢，而离子和较大的极性分子则需要转运蛋白。影响通透性的因素包括温度、pH、溶剂浓度以及清洁剂的存在，这些都可能破坏磷脂双分子层的完整性。
7. Passive Transport: Simple Diffusion and Osmosis | 被动运输：简单扩散与渗透
Simple diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration down a concentration gradient; it does not require metabolic energy (ATP). Small non‑polar molecules and lipid‑soluble substances cross the membrane by simple diffusion. Osmosis is a special case of diffusion involving the movement of free water molecules from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration) through a partially permeable membrane.
简单扩散是分子顺浓度梯度从高浓度区域向低浓度区域的净移动，不需要代谢能（ATP）。小分子非极性物质和脂溶性物质通过简单扩散穿过膜。渗透是扩散的一种特殊形式，指自由水分子通过部分透性膜，从水势较高（溶质浓度较低）的区域向水势较低（溶质浓度较高）的区域移动。
8. Facilitated Diffusion | 易化扩散
Facilitated diffusion allows large or polar molecules (e.g. glucose, amino acids) and ions to cross the membrane down their concentration gradient without using ATP. This process relies on two types of integral membrane proteins: channel proteins, which form hydrophilic pores for ions (often gated), and carrier proteins, which undergo a conformational change to shuttle specific molecules across. Like simple diffusion, facilitated diffusion is passive and saturable.
易化扩散使较大的极性分子（如葡萄糖、氨基酸）和离子能顺浓度梯度穿过膜，无需消耗 ATP。该过程依赖两种内在膜蛋白：通道蛋白，为离子形成亲水孔道（常为门控）；载体蛋白，通过自身构象改变将特定分子转运过去。与简单扩散一样，易化扩散也是被动运输，并具有饱和性。
9. Active Transport | 主动运输
Active transport moves substances against their concentration gradient, from a region of lower concentration to a region of higher concentration. This process requires carrier proteins and a direct input of metabolic energy, usually from the hydrolysis of ATP. The sodium‑potassium pump (Na⁺/K⁺‑ATPase) is a classic example: it actively transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell per ATP molecule hydrolysed, maintaining the resting potential in neurons and osmotic balance in cells.
主动运输逆浓度梯度进行，将物质从低浓度区域运往高浓度区域。该过程需要载体蛋白，并直接消耗代谢能量——通常来自 ATP 的水解。钠钾泵（Na⁺/K⁺‑ATP 酶）是经典例子：每水解一分子 ATP，它主动将 3 个 Na⁺ 运出细胞，同时将 2 个 K⁺ 运入细胞，从而维持神经元的静息电位和细胞的渗透平衡。
10. Endocytosis and Exocytosis | 胞吞与胞吐
Large molecules and particles are transported across the membrane via vesicles in processes that require energy. Endocytosis involves the membrane engulfing material to form a vesicle that brings substances into the cell. Phagocytosis (‘cell eating’) takes in solid particles, while pinocytosis (‘cell drinking’) takes in dissolved substances. Exocytosis is the reverse process: secretory vesicles fuse with the plasma membrane to release their contents to the outside, important for exporting proteins, neurotransmitters, and waste.
大分子和颗粒通过耗能的囊泡过程进出细胞膜。胞吞是细胞膜包裹物质形成囊泡，将物质摄入细胞内。吞噬（“细胞吞噬”）摄入固体颗粒，而胞饮（“细胞饮用”）摄入溶解物质。胞吐则是反向过程：分泌囊泡与质膜融合，将其内容物释放到细胞外，这对于输出蛋白质、神经递质和废物至关重要。
11. Practical: Effect of Temperature on Membrane Permeability | 实验：温度对膜通透性的影响
A common WJEC‑required practical uses beetroot cylinders to investigate how temperature affects membrane permeability. Beetroot cells contain betalain, a red pigment that leaks out when the tonoplast and plasma membrane are damaged. After placing beetroot discs in water baths at different temperatures (e.g. 0–70 °C) for a fixed time, the absorbance or colour intensity of the surrounding water is measured using a colorimeter. Higher absorbance indicates greater membrane damage and increased permeability. Results typically show a sharp rise in pigment leakage above about 45 °C, due to protein denaturation and increased fluidity of the phospholipid bilayer.
WJEC 常考的一个实验是用甜菜根圆片探究温度对膜通透性的影响。甜菜根细胞含有甜菜红素，当液泡膜和质膜受损时，红色色素会泄漏出来。将甜菜根圆片在不同温度（如 0–70 °C）的水浴中放置固定时间后，用比色计测量周围水的吸光度或颜色强度。吸光度越高，表明膜损伤越大，通透性越强。结果通常显示大约 45 °C 以上色素泄漏急剧上升，原因是蛋白质变性和磷脂双分子层流动性增加。
12. Water Potential and Osmotic Regulation | 水势与渗透调节
Water potential (Ψ) measures the tendency of water to move from one area to another. Pure water at standard pressure has a water potential of 0 kPa. The addition of solutes lowers water potential (makes it more negative). Osmosis occurs from regions of higher water potential to regions of lower water potential. Plant cells use their cell walls and vacuoles to regulate osmotic changes: a turgid cell has a high pressure potential pushing the membrane against the wall, while plasmolysis occurs when the cell membrane pulls away from the wall in a hypertonic solution. Animal cells risk lysis in hypotonic solutions and crenation in hypertonic solutions, highlighting the importance of osmoregulation.
水势（Ψ）衡量水分从一处向另一处移动的趋势。标准压力下的纯水水势为 0 kPa。加入溶质会降低水势（使其变得更负）。渗透作用总是从水势高的区域向水势低的区域进行。植物细胞利用细胞壁和液泡来调节渗透变化：充盈的细胞具有较高的压力势，将细胞膜压向细胞壁；而在高渗溶液中，细胞膜脱离细胞壁，发生质壁分离。动物细胞在低渗溶液中可能破裂（溶血），在高渗溶液中会皱缩，这凸显了渗透调节的重要性。
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