📚 A-Level Edexcel Biology: Cell Membrane – Key Concepts and Exam Focus | A-Level Edexcel 生物：细胞膜 考点精讲
The cell membrane is a fundamental component of all cells, acting as a selectively permeable barrier. Understanding its structure and function is essential for A-Level Edexcel Biology, appearing in topics such as transport across cell membranes and cell recognition. This article provides an in-depth revision guide covering the fluid mosaic model, transport mechanisms, factors affecting permeability, and core practicals.
细胞膜是所有细胞的基本组成部分，充当选择性通透屏障。理解其结构与功能对于A-Level Edexcel 生物学至关重要，涉及细胞膜运输和细胞识别等主题。本文提供深入复习指南，涵盖流体镶嵌模型、运输机制、影响通透性的因素及核心实验。
1. The Fluid Mosaic Model | 流体镶嵌模型
The fluid mosaic model describes the structure of cell membranes as a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It is termed ‘fluid’ because the phospholipids and proteins can move laterally within the layer, and ‘mosaic’ due to the patchwork of proteins. This model was proposed by Singer and Nicolson in 1972 and replaced the earlier Davson-Danielli model.
流体镶嵌模型将细胞膜结构描述为磷脂双分子层，其中嵌有蛋白质、胆固醇和碳水化合物。之所以称为“流体”，是因为磷脂和蛋白质可在层内横向移动；称为“镶嵌”，是因为蛋白质呈马赛克状分布。该模型由Singer和Nicolson于1972年提出，取代了早期的 Davson-Danielli 模型。
Evidence for the fluid mosaic model includes freeze-fracture electron micrographs showing proteins embedded within the bilayer, and the ability of membrane proteins to move, as demonstrated by cell fusion experiments.
支持流体镶嵌模型的证据包括：冷冻断裂电镜照片显示蛋白质嵌入双分子层，以及细胞融合实验证明膜蛋白能够移动。
2. Phospholipid Bilayer | 磷脂双分子层
The membrane is composed mainly of phospholipids arranged in a bilayer. Each phospholipid has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. In the bilayer, the heads face outward towards the aqueous environments on both sides, while the tails point inward, shielded from water. This arrangement is thermodynamically favourable and forms the basic barrier.
细胞膜主要由排列成双分子层的磷脂组成。每个磷脂具有亲水性（喜水）的磷酸头端和两个疏水性（惧水）的脂肪酸尾端。在双分子层中，头部朝外，面向两侧的水性环境，而尾部朝内，与水隔绝。这种排列在热力学上是有利的，形成了基本屏障。
The hydrophobic core prevents free passage of ions and polar molecules such as glucose, allowing the membrane to act as a selective barrier.
疏水核心阻止离子和葡萄糖等极性分子自由通过，使得细胞膜起选择性屏障作用。
3. Membrane Proteins: Intrinsic and Extrinsic | 膜蛋白：内在与外在
Proteins are scattered within the membrane and serve various functions. Intrinsic (integral) proteins span the entire bilayer, with hydrophobic regions interacting with the fatty acid tails. Many are channel proteins or carrier proteins for transport. Extrinsic (peripheral) proteins are attached to the surface of the membrane, often on the cytoplasmic side, and may act as enzymes or provide structural support.
蛋白质散布在膜中并具有多种功能。内在（整合）蛋白横跨整个双分子层，其疏水区域与脂肪酸尾端相互作用。许多是用于运输的通道蛋白或载体蛋白。外在（外周）蛋白附着在膜表面，通常位于细胞质侧，可作为酶或提供结构支撑。
Glycoproteins, which are proteins with carbohydrate chains attached, are located on the extracellular surface and play key roles in cell recognition.
糖蛋白是带有碳水化合物链的蛋白质，位于细胞外表面，在细胞识别中起关键作用。
4. Role of Cholesterol | 胆固醇的作用
Cholesterol is a lipid molecule found within the phospholipid bilayer of animal cell membranes. It fits between phospholipid molecules, with its hydroxyl group oriented towards the aqueous surface and its hydrophobic region interacting with the tails. Cholesterol modulates membrane fluidity: at low temperatures, it prevents close packing of phospholipids, increasing fluidity; at high temperatures, it restricts excessive movement, reducing fluidity. It also increases mechanical stability.
胆固醇是存在于动物细胞膜磷脂双分子层中的脂质分子。它嵌在磷脂分子之间，其羟基朝向水相表面，疏水区域与尾端相互作用。胆固醇调节膜的流动性：低温下，它防止磷脂紧密堆积，增加流动性；高温下，它限制过度运动，降低流动性。它还增强机械稳定性。
This helps maintain an optimal fluid environment for membrane protein function and permeability.
这有助于维持膜蛋白功能和通透性的最佳流体环境。
5. Glycoproteins and Glycolipids: Cell Recognition | 糖蛋白与糖脂：细胞识别
Carbohydrates are present on the outer surface of the membrane, covalently attached to proteins (glycoproteins) or lipids (glycolipids). The carbohydrate chains form the glycocalyx, which acts as recognition sites. These molecules are crucial in cell-cell recognition, immune responses (e.g., ABO blood group antigens), and cell adhesion.
碳水化合物存在于膜外表面，与蛋白质（糖蛋白）或脂质（糖脂）共价结合。糖链形成糖萼，作为识别位点。这些分子在细胞间识别、免疫应答（如ABO血型抗原）和细胞黏附中至关重要。
6. Functions: Selective Permeability | 细胞膜的功能：选择性通透性
The primary function of the cell membrane is to control the passage of substances into and out of the cell. It is selectively permeable, allowing small, non-polar molecules (e.g., O₂, CO₂) to diffuse freely, while restricting large, charged, or polar molecules (e.g., ions, glucose). This property is due to the hydrophobic core of the bilayer and the presence of specific transport proteins.
细胞膜的主要功能是控制物质进出细胞。它具有选择性通透性，允许小的非极性分子（如O₂、CO₂）自由扩散，同时限制大的带电或极性分子（如离子、葡萄糖）。这一特性源于双分子层的疏水核心和特定运输蛋白的存在。
Additionally, the membrane maintains electrochemical gradients and compartments within the cell, essential for processes like oxidative phosphorylation and nerve impulse transmission.
此外，细胞膜维持电化学梯度和细胞内区室化，这对于氧化磷酸化和神经冲动传递等过程至关重要。
7. Passive Transport: Simple and Facilitated Diffusion | 被动运输：简单扩散与易化扩散
Passive transport does not require ATP and occurs down a concentration gradient. Simple diffusion is the movement of small, non-polar molecules directly through the phospholipid bilayer. Facilitated diffusion involves channel proteins or carrier proteins for larger or charged molecules. Channel proteins form aqueous pores; carrier proteins undergo conformational changes to transport specific molecules.
被动运输不需要ATP，顺浓度梯度发生。简单扩散是小分子非极性分子直接通过磷脂双分子层的运动。易化扩散涉及通道蛋白或载体蛋白，用于较大或带电分子。通道蛋白形成水性孔道；载体蛋白发生构象变化以运输特定分子。
Key examples include the diffusion of oxygen and carbon dioxide, and the facilitated diffusion of glucose via GLUT carriers and ions through gated channels.
主要示例包括氧气和二氧化碳的扩散，以及葡萄糖通过GLUT载体和离子通过门控通道的易化扩散。
The rate of facilitated diffusion reaches a maximum (Vmax) when all carrier proteins are saturated, unlike simple diffusion which is directly proportional to concentration gradient.
当所有载体蛋白饱和时，易化扩散速率达到最大值(Vmax)，而简单扩散则与浓度梯度成正比。
8. Osmosis and Water Potential | 渗透与水势
Osmosis is the net movement of water molecules through a selectively permeable membrane from a region of higher water potential to lower water potential. Water potential (ψ) is the pressure exerted by water molecules and is measured in kilopascals (kPa). Pure water has a water potential of zero; the addition of solutes lowers water potential to negative values.
渗透是水分子通过选择性通透膜从水势较高区域向水势较低区域的净移动。水势(ψ)是水分子施加的压力，单位为千帕(kPa)。纯水的水势为零；加入溶质会降低水势至负值。
Water potential is given by the formula:
水势公式为：
ψ = ψₛ + ψₚ
where ψₛ is the solute potential (always negative or zero) and ψₚ is the pressure potential (can be positive or zero). In plant cells, the cell wall exerts a pressure potential that counteracts further water uptake.
其中 ψₛ 是溶质势（总是负值或零），ψₚ 是压力势（可为正值或零）。在植物细胞中，细胞壁施加的压力势可抵消进一步吸水。
Understanding water potential helps explain turgidity in plants and haemolysis/crenation in animal cells.
理解水势有助于解释植物的膨胀现象以及动物细胞的溶血/皱缩。
9. Active Transport | 主动运输
Active transport moves molecules or ions against their concentration gradient, requiring energy in the form of ATP. It uses carrier proteins (pumps) that bind the solute and undergo a conformational change driven by ATP hydrolysis. The sodium-potassium pump (Na⁺/K⁺-ATPase) is a classic example, pumping 3 Na⁺ out and 2 K⁺ in per ATP, maintaining resting potential in neurons.
主动运输逆浓度梯度移动分子或离子，需要ATP形式的能量。它利用载体蛋白（泵），该蛋白结合溶质并在ATP水解驱动下发生构象变化。钠钾泵(Na⁺/K⁺-ATP酶)是典型例子，每消耗一个ATP泵出3个Na⁺并泵入2个K⁺，维持神经元的静息电位。
Active transport allows cells to take up essential nutrients even when extracellular concentrations are low, and to remove waste products.
主动运输使细胞即使在胞外浓度低时也能摄取必需营养，并排出废物。
10. Endocytosis and Exocytosis | 胞吞与胞吐
Large molecules or particles are transported across the membrane via vesicles in bulk transport processes requiring ATP. Endocytosis invaginates the membrane to form a vesicle bringing substances into the cell; phagocytosis is for solid particles, pinocytosis for liquids. Exocytosis involves vesicles fusing with the membrane to release contents outside, e.g., secretion of digestive enzymes or neurotransmitters.
大分子或颗粒通过需要ATP的囊泡批量运输过程穿过细胞膜。胞吞作用使膜内陷形成囊泡，将物质带入细胞；吞噬作用针对固体颗粒，胞饮作用针对液体。胞吐作用涉及囊泡与膜融合，将内容物释放到外部，例如消化酶或神经递质的分泌。
These processes rely on the fluidity of the membrane to allow vesicle formation and fusion.
这些过程依赖膜的流动性以实现囊泡的形成和融合。
11. Factors Affecting Membrane Permeability | 影响细胞膜通透性的因素
Temperature: Increasing temperature generally increases membrane permeability because phospholipids gain kinetic energy and become more fluid. However, at very high temperatures, proteins denature, creating holes and drastically increasing permeability. At low temperatures, the membrane becomes rigid and less permeable due to decreased fluidity and the possibility of ice crystal damage.
温度：升高温度通常增加膜通透性，因为磷脂获得动能而变得更流动。然而，在极高温度下，蛋白质变性，形成孔洞，通透性急剧升高。低温下，流动性降低，膜变僵硬，通透性下降，并可能因冰晶造成损伤。
Solvents: Organic solvents such as ethanol dissolve lipids, disrupting the bilayer and increasing permeability. Higher concentrations of ethanol cause greater leakage of pigments from beetroot cells in the core practical.
溶剂：有机溶剂如乙醇溶解脂质，破坏双分子层，增加通透性。在核心实验中，较高浓度的乙醇导致甜菜根细胞更多的色素泄漏。
pH: Extreme pH can denature membrane proteins, altering the structure and increasing permeability.
pH：极端pH可导致膜蛋白变性，改变结构并增加通透性。
12. Core Practical: Beetroot Membrane Permeability | 核心实验：甜菜根膜通透性
This practical investigates the effect of temperature or solvent concentration on membrane permeability using beetroot. Beetroot contains betalain, a red pigment that leaks out when the membrane is damaged. The intensity of the leaked pigment can be measured using a colorimeter, with absorbance proportional to permeability.
本实验利用甜菜根研究温度或溶剂浓度对膜通透性的影响。甜菜根
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