📚 Cell Membranes for IGCSE CIE Biology | IGCSE CIE 生物：细胞膜考点精讲
The cell membrane (also called the plasma membrane) is a fundamental structure found around every living cell. It separates the cell contents from the external environment, controls what enters and leaves the cell, and plays a vital role in communication and recognition. For IGCSE CIE Biology, you must understand its structure, how substances move across it, and the effects of osmosis on plant and animal cells.
细胞膜（又称质膜）是每个活细胞周围的基本结构。它将细胞内容物与外部环境分开，控制物质的进出，并在细胞通讯与识别中起关键作用。IGCSE CIE 生物考试要求你掌握细胞膜的结构、物质跨膜运输的方式，以及渗透作用对动植物细胞的影响。
1. Introduction to Cell Membranes | 细胞膜简介
All cells, whether prokaryotic or eukaryotic, are surrounded by a cell surface membrane. In plant cells, a cell wall lies outside the membrane, but the membrane is still the living boundary that directly controls transport. Internal membranes also enclose organelles like mitochondria and chloroplasts, forming compartments for specific reactions.
所有细胞，不论是原核还是真核，都被细胞表面膜包裹着。植物细胞在膜外还有细胞壁，但细胞膜仍然是直接控制运输的活体界限。细胞内部的膜系统还包裹着线粒体、叶绿体等细胞器，形成不同反应区室。
Under the light microscope, the membrane is too thin to be seen clearly, but its properties are inferred from experiments. Using electron microscopy and biochemical analysis, scientists revealed the detailed structure, leading to the fluid mosaic model, which is the core of this topic.
在光学显微镜下，膜太薄无法清晰观察，但可以通过实验推断其性质。科学家借助电子显微镜和生化分析揭示了膜的详细结构，提出了流体镶嵌模型，这是本专题的核心。
2. The Fluid Mosaic Model | 流体镶嵌模型
The fluid mosaic model describes the cell membrane as a double layer of phospholipid molecules with various proteins embedded or attached. The term 'fluid' indicates that the phospholipids and proteins can move laterally within the layer, giving the membrane flexibility. 'Mosaic' refers to the pattern formed by the scattered protein molecules among the phospholipids.
流体镶嵌模型将细胞膜描述为磷脂双分子层，其中嵌入或附着着多种蛋白质。“流体”意味着磷脂和蛋白质可以在膜内横向移动，使膜具有流动性；“镶嵌”则指蛋白质分子散在磷脂之间形成的图案。
This model was proposed by Singer and Nicolson in 1972 and replaced earlier models. It has been supported by freeze-fracture electron microscopy, which splits the membrane along the middle of the bilayer and reveals proteins bulging out.
该模型由 Singer 和 Nicolson 于1972年提出，取代了更早的模型。冷冻断裂电子显微技术沿磷脂双分子层中部劈开细胞膜，显示出凸起的蛋白质，强有力地支持了这一模型。
3. Components of the Membrane | 膜的组成成分
The main components are phospholipids, proteins, and small amounts of carbohydrates and cholesterol (in animal cells).
膜的主要成分是磷脂、蛋白质，以及少量糖类和胆固醇（动物细胞中含有）。
Phospholipid molecules: each has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty acid tails. In the bilayer, the heads face outwards towards the watery environments (cytoplasm and extracellular fluid), while the tails face inwards, creating a hydrophobic core. This arrangement acts as a barrier to most water-soluble substances.
磷脂分子：每个分子有一个亲水（喜水）的磷酸头部和两条疏水（厌水）的脂肪酸尾部。在双分子层中，头部朝外，分别面向细胞质和细胞外液；尾部朝内，形成疏水核心。这种排列对大多数水溶性物质起屏障作用。
Membrane proteins: these are scattered throughout the bilayer. Some are intrinsic (integral) and span the entire membrane, while others are extrinsic (peripheral) and are attached to one surface. Functions include transport (channel proteins, carrier proteins), receptors, enzymes, and structural support.
膜蛋白：它们散在于双分子层中。有的为内在（整合）蛋白，贯穿整个膜层；有的为外在（周边）蛋白，只附着在某一侧表面。功能包括运输（通道蛋白、载体蛋白）、受体、酶和结构支持等。
Cholesterol: in animal cell membranes, cholesterol molecules are inserted between phospholipids. They regulate membrane fluidity – preventing it from becoming too fluid at high temperatures and too stiff at low temperatures. Plant cell membranes usually lack cholesterol but have other sterols.
胆固醇：动物细胞膜中，胆固醇分子插在磷脂之间。它们调节膜的流动性——防止高温时膜过于流动，低温时过于僵硬。植物细胞膜通常不含胆固醇，但有其他固醇。
Glycoproteins and glycolipids: short carbohydrate chains are attached to some proteins and lipids on the outer surface, forming a 'glycocalyx'. These act as recognition sites, allowing cells to identify each other, and are involved in cell adhesion and immune responses.
糖蛋白与糖脂：一些膜蛋白和膜脂的外侧连着短糖链，形成“糖萼”。它们充当识别位点，让细胞相互识别，并参与细胞黏附和免疫应答。
4. Functions of the Cell Membrane | 细胞膜的功能
The cell membrane has three major roles: selective permeability (control of movement), cell communication, and compartmentalisation.
细胞膜有三大主要功能：选择性通透性（控制物质进出）、细胞通讯和区室化。
It is selectively permeable, meaning it allows some substances to cross easily while restricting others. Small, non-polar molecules (e.g., oxygen, carbon dioxide) diffuse rapidly through the bilayer. Water, although polar, can pass slowly through because it is small; aquaporins speed up this process. Large polar molecules (e.g., glucose) and ions require specific transport proteins. This property is essential for maintaining the internal environment of the cell.
它具有选择性通透性，即允许某些物质轻易通过而限制其他物质。小的非极性分子（如氧气、二氧化碳）可快速透过双分子层。水虽为极性分子，但因体积小可缓慢通过；水孔蛋白加速这一过程。大的极性分子（如葡萄糖）和离子则需要特定的运输蛋白。这一特性对于维持细胞内环境至关重要。
Receptor proteins on the membrane bind to signal molecules (e.g., hormones), triggering a response inside the cell. This is cell signalling. Membrane-bound enzymes catalyse reactions. Adhesion proteins help cells stick together to form tissues.
膜上的受体蛋白与信号分子（如激素）结合，引发胞内反应，即细胞信号转导。膜结合酶催化反应。黏附蛋白帮助细胞相互连接形成组织。
5. Passive Transport – Diffusion | 被动运输：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, requiring no energy from the cell. The particles move because of their random kinetic energy.
扩散是粒子从高浓度区域向低浓度区域的净移动，顺着浓度梯度进行。这是一种被动过程，不需要细胞提供能量。粒子因自身随机动能而移动。
Diffusion continues until the particles are evenly distributed, reaching equilibrium. Molecules such as oxygen, carbon dioxide, and urea enter and leave cells by simple diffusion directly through the phospholipid bilayer. The rate depends on concentration gradient, particle size, temperature, and the surface area of the membrane.
扩散将持续进行，直至粒子均匀分布，达到平衡。氧气、二氧化碳、尿素等分子通过简单扩散直接穿过磷脂双分子层进出细胞。其速率取决于浓度梯度、粒子大小、温度和膜表面积。
6. Passive Transport – Osmosis | 被动运输：渗透
Osmosis is a special case of diffusion. It is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane. Water potential describes the tendency of water to move and is affected by solute concentration and pressure.
渗透是一种特殊的扩散。它是水分子从高水势（稀溶液）区域通过部分通透膜向低水势（浓溶液）区域的净移动。水势描述水分子的运动趋势，受溶质浓度和压力的影响。
In the context of IGCSE, you can describe it simply: water moves from a more dilute solution to a more concentrated one across a membrane. The membrane allows water through but restricts many solutes. Pure water has the highest water potential; adding solute lowers water potential (more negative).
在 IGCSE 语境中可简单描述为：水通过半透膜从较稀溶液向较浓溶液移动。膜允许水通过但限制多数溶质。纯水水势最高；加入溶质会降低水势（更负）。
Water potential (ψ) is lowered by solute (ψₛ) and raised by pressure (ψₚ): ψ = ψₛ + ψₚ
水势 (ψ) 因溶质 (ψₛ) 降低，因压力 (ψₚ) 升高：ψ = ψₛ + ψₚ
7. Osmosis in Plant and Animal Cells | 动植物细胞中的渗透作用
The effects of osmosis differ in plant and animal cells because plant cells have a strong cell wall that resists swelling.
渗透对动植物细胞的影响不同，因为植物细胞有坚硬的细胞壁，能够抵抗膨胀。
Animal cells in a hypotonic solution (higher water potential outside): water enters by osmosis. The cell swells and may burst (lysis). In a hypertonic solution (lower water potential outside): water leaves, the cell shrinks and becomes crenated. In an isotonic solution, there is no net water movement, and the cell remains normal.
动物细胞在低渗溶液（外界水势较高）中：水进入细胞，细胞膨胀甚至破裂（溶破）。在高渗溶液（外界水势较低）中：水流出，细胞皱缩。在等渗溶液中，无水净移动，细胞保持正常。
Plant cells in a hypotonic solution: water enters the vacuole and cytoplasm, causing the cell to swell and push against the cell wall. The cell becomes turgid (firm). This turgor pressure supports the plant. In a hypertonic solution: water leaves the vacuole, the cytoplasm shrinks and pulls away from the wall – this is plasmolysis. The plant wilts. In an isotonic solution, the cell becomes flaccid (limp), and plasmolysis is just beginning.
植物细胞在低渗溶液中：水进入液泡和细胞质，细胞膨胀并推挤细胞壁，成为硬挺状态。这种膨压支持植物体。在高渗溶液中：水从液泡流出，细胞质皱缩并从细胞壁脱离——这称为质壁分离，植株萎蔫。在等渗溶液中，细胞变得松弛，质壁分离刚开始发生。
| Condition | Animal cell | Plant cell |
|---|---|---|
| Hypotonic solution | Swelling, lysis | Turgid |
| Isotonic solution | Normal | Flaccid |
| Hypertonic solution | Shrivelled (crenated) | Plasmolysed |
英文表格解读：三种渗透条件下动物细胞与植物细胞的不同状态比较。
8. Active Transport | 主动运输
Active transport is the movement of molecules or ions across a membrane against their concentration gradient – from a lower to a higher concentration – using energy from respiration (ATP). Carrier proteins in the membrane bind the specific solute and change shape to move it across.
主动运输是分子或离子逆着浓度梯度跨越膜的运动——从低浓度到高浓度——需要利用呼吸作用释放的能量（ATP）。膜上的载体蛋白与特定溶质结合并改变形状，将其转运过膜。
This process is vital for absorbing nutrients when external concentrations are low. Examples include uptake of mineral ions by root hair cells from the soil, absorption of glucose by kidney tubules back into the blood, and moving ions across gill membranes in fish. In active transport, cells can maintain internal concentrations very different from the surroundings.
当外界浓度较低时，主动运输对吸收营养至关重要。例子包括根毛细胞从土壤吸收矿质离子、肾小管将葡萄糖重吸收入血，以及鱼鳃膜上的离子转运。借助主动运输，细胞能维持与外界环境差异极大的内部浓度。
9. Factors Affecting Movement Across Membranes | 影响跨膜运输的因素
The rate of diffusion is increased by a steeper concentration gradient, higher temperature (increases kinetic energy), larger surface area of membrane, and shorter diffusion distance. For osmosis, the water potential gradient is the driving force.
浓度梯度越陡、温度越高（增加动能）、膜表面积越大、扩散距离越短，扩散速率越快。对于渗透，水势梯度是驱动力。
Active transport rate depends on the number of carrier proteins, the availability of ATP, and temperature. Temperature affects enzyme-controlled respiration that produces ATP, as well as membrane fluidity. At very high temperatures, proteins denature and active transport stops. Inhibitors like cyanide, which block respiration, also stop active transport.
主动运输速率取决于载体蛋白数量、ATP 供给和温度。温度影响酶控呼吸作用产生 ATP 的速率以及膜的流动性。在极高温度下，蛋白质变性，主动运输停止。氰化物等呼吸抑制剂也会抑制主动运输。
10. Investigating Diffusion and Osmosis | 扩散和渗透的实验探究
A common investigation of diffusion uses a porous pot (or agar jelly) containing an indicator, placed in a liquid. Visking (dialysis) tubing is the classic model for osmosis. This tubing is partially permeable: it allows small molecules like water to pass, but not larger molecules like starch or sucrose.
常见的扩散研究使用含指示剂的多孔瓷壶（或琼脂凝胶）浸入液体中。透析管是经典的渗透模型。这种管具部分通透性：允许水等小分子通过，但不允许淀粉或蔗糖等较大分子通过。
In a typical experiment, a Visking tubing bag is filled with a concentrated sucrose solution and tied at both ends. The bag is weighed and placed in a beaker of distilled water. After a set time, the bag has gained mass because water has moved in by osmosis from a higher water potential (distilled water) to a lower water potential (sucrose solution). This demonstrates the directional movement of water and can be used to measure water potential of solutions.
典型实验中，透析管袋内装浓蔗糖溶液，两端扎紧。称重后放入蒸馏水烧杯中。一定时间后，袋的质量增加，说明水从水势较高的蒸馏水通过渗透进入水势较低的蔗糖溶液。这演示了水的定向移动，可用于测定溶液的水势。
Living tissue can also be used: potato cylinders placed in different sucrose concentrations change in length and mass. If immersed in a solution with the same water potential as the potato cells, there is no net change. Plotting change in mass against concentration allows the estimation of the potato tissue's water potential where the graph crosses zero change.
活体组织也可以用于实验：马铃薯圆柱在不同蔗糖浓度中，长度和质量发生变化。如果浸泡在与马铃薯细胞水势相同的溶液中，则无净变化。绘制质量变化对浓度的关系图，零变化点可估测组织的水势。
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