📚 Biological Membranes: Structure, Function and Transport | 生物膜：结构、功能与物质运输
All living cells are surrounded by a partially permeable membrane that controls the movement of substances into and out of the cell. In A-level Biology, understanding the fluid mosaic model, the roles of membrane proteins, and the mechanisms of transport is essential for explaining how cells maintain internal conditions and communicate with their environment.
所有活细胞都被一层部分通透性膜包围，这层膜控制物质进出细胞。在 A-level 生物学中，理解流动镶嵌模型、膜蛋白的作用以及运输机制，对于解释细胞如何维持内部条件并与环境进行通讯至关重要。
1. Roles of Membranes | 膜的功能
Membranes at the surface of cells and around organelles are not just passive barriers. They separate the internal environment from the external environment, compartmentalise organelles so that incompatible reactions can occur separately, provide attachment sites for enzymes and ribosomes, and control the transport of solutes, ions and signalling molecules.
细胞表面以及细胞器周围的膜并非只是被动屏障。它们将内部环境与外部环境隔开，将细胞器区室化使不相容的反应可以分开进行，为酶和核糖体提供附着位点，并控制溶质、离子和信号分子的运输。
The plasma membrane is described as partially permeable because it allows small, non-polar molecules such as oxygen and carbon dioxide to pass freely while restricting large, charged or polar molecules unless specific transport proteins are present.
质膜被描述为部分通透性，因为它允许氧气和二氧化碳等小的非极性分子自由通过，同时限制大的、带电或极性分子，除非存在特定的转运蛋白。
2. Fluid Mosaic Model | 流动镶嵌模型
The fluid mosaic model describes the membrane as a dynamic phospholipid bilayer with proteins floating or embedded within it. The term ‘fluid’ refers to the ability of phospholipids and many proteins to move laterally within the layer, while ‘mosaic’ refers to the patchwork of different proteins scattered in the bilayer.
流动镶嵌模型将膜描述为一种动态的磷脂双分子层，其中漂浮或嵌有蛋白质。”流动”是指磷脂和许多蛋白质能够在层内横向移动，”镶嵌”则是指散布在双分子层中的多种蛋白质形成的拼图样结构。
Each phospholipid consists of a hydrophilic phosphate head that faces the aqueous environments inside and outside the cell, and two hydrophobic fatty acid tails that point inwards, away from water. This amphipathic nature drives spontaneous bilayer formation in water.
每个磷脂由一个亲水的磷酸头（朝向细胞内外的水环境）和两条亲脂（疏水）的脂肪酸尾（朝内、远离水）组成。这种两亲性驱动磷脂在水中自发形成双分子层。
3. Phospholipids and Membrane Fluidity | 磷脂与膜流动性
Membrane fluidity depends on the length and saturation of fatty acid tails. Shorter tails and unsaturated tails with double bonds produce kinks that reduce packing, making the membrane more fluid. Longer, saturated tails pack closely and reduce fluidity.
膜的流动性取决于脂肪酸尾的长度和饱和度。较短的尾以及含有双键的不饱和尾会产生扭结，减少堆积，使膜更具流动性。较长、饱和的尾排列紧密，降低流动性。
Cholesterol is an important regulator of fluidity in animal cell membranes. At high temperatures it restricts phospholipid movement and reduces fluidity; at low temperatures it prevents phospholipids from packing too tightly, maintaining fluidity and preventing the membrane from becoming rigid.
胆固醇是动物细胞膜流动性的重要调节因子。在高温下，它限制磷脂运动并降低流动性；在低温下，它防止磷脂堆积过紧，维持流动性并避免膜变得僵硬。
4. Membrane Proteins | 膜蛋白
Membrane proteins are classified as intrinsic (integral) or extrinsic (peripheral). Intrinsic proteins span the whole bilayer or are embedded within it, while extrinsic proteins are attached to the surface of the membrane, often to intrinsic proteins or to phospholipid heads.
膜蛋白分为内在蛋白（整合蛋白）和外在蛋白（外周蛋白）。内在蛋白贯穿整个双分子层或嵌入其中，而外在蛋白附着在膜表面，通常与内在蛋白或磷脂头部结合。
Channel proteins and carrier proteins are two major types of intrinsic transport proteins. Channel proteins form hydrophilic pores for passive movement of ions or water, while carrier proteins change shape to transport specific molecules across the membrane, either passively or actively.
通道蛋白和载体蛋白是两种主要的内在转运蛋白。通道蛋白形成亲水孔道，供离子或水被动通过；载体蛋白则通过改变构象来跨膜运输特定分子，既可以是被动也可以是主动运输。
Glycoproteins and glycolipids on the extracellular surface act as recognition sites, antigens, and receptors. Their carbohydrate chains form a glycocalyx that protects the cell and enables cell-cell communication.
细胞外表面的糖蛋白和糖脂充当识别位点、抗原和受体。其糖链形成糖萼，保护细胞并实现细胞间通讯。
5. Cell Signalling and Receptors | 细胞信号与受体
Many hormones and drugs are hydrophilic and cannot cross the phospholipid bilayer, so they bind to specific receptor proteins on the cell surface. This binding causes a conformational change in the receptor, which triggers a cascade of intracellular signals, such as activation of enzymes or transcription factors.
许多激素和药物是亲水性的，无法穿过磷脂双分子层，因此它们与细胞表面的特异性受体蛋白结合。这种结合引起受体构象变化，触发细胞内信号级联，例如激活酶或转录因子。
Lipid-soluble signalling molecules, such as steroid hormones, can diffuse through the membrane and bind to intracellular receptors. The hormone-receptor complex often moves into the nucleus and regulates gene expression directly.
脂溶性信号分子，如类固醇激素，可以扩散通过膜并与胞内受体结合。激素-受体复合物通常进入细胞核并直接调控基因表达。
6. Diffusion and Facilitated Diffusion | 扩散与易化扩散
Simple diffusion is the net movement of small, non-polar molecules from a region of higher concentration to a region of lower concentration across the phospholipid bilayer. It is a passive process that does not require ATP or transport proteins.
简单扩散是小的非极性分子从高浓度区域穿过磷脂双分子层向低浓度区域的净移动。这是一种被动过程，不需要 ATP 或转运蛋白。
Facilitated diffusion uses channel proteins or carrier proteins to move ions and polar molecules down their concentration gradient. It is still passive because no ATP is used, but the rate depends on the number of available transport proteins and can become saturated at high concentrations.
易化扩散利用通道蛋白或载体蛋白使离子和极性分子顺浓度梯度移动。这一过程仍是被动的，因为不消耗 ATP，但速率取决于可利用的转运蛋白数量，并在高浓度时可达到饱和。
For simple diffusion, the rate is proportional to the surface area of the membrane and the concentration difference, and inversely proportional to the thickness of the exchange surface.
对于简单扩散，速率与膜的表面积和浓度差成正比，与交换表面的厚度成反比。
rate ∝ (surface area × concentration difference) ÷ thickness
7. Osmosis and Water Potential | 渗透作用与水势
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Water potential is the pressure exerted by water molecules and is measured in kilopascals (kPa); pure water has a water potential of 0 kPa under standard conditions.
渗透作用是水分子通过部分通透性膜从水势较高的区域向水势较低区域的净移动。水势是水分子产生的压力，以千帕（kPa）为单位；在标准条件下，纯水的水势为 0 kPa。
Adding solutes lowers the water potential because solute particles reduce the proportion of water molecules that can move freely. Therefore, a concentrated solution has a more negative water potential than pure water and water moves towards it.
加入溶质会降低水势，因为溶质颗粒减少了可自由移动的水分子比例。因此，浓溶液的水势比纯水更负，水分会向它移动。
In plant cells, water potential is the sum of solute potential and pressure potential. The cell wall exerts pressure as water enters, causing turgor pressure and preventing excessive water uptake; animal cells lack a cell wall and may burst in hypotonic solutions.
在植物细胞中，水势等于溶质势与压力势之和。当水分进入时，细胞壁产生压力，形成膨压，阻止过多的水分进入；动物细胞没有细胞壁，在低渗溶液中可能胀破。
ψ = ψₛ + ψₚ
The equation ψ = ψₛ + ψₚ is used to calculate water potential in plant cells, where ψ is water potential, ψₛ is solute potential and ψₚ is pressure potential.
公式 ψ = ψₛ + ψₚ 用于计算植物细胞中的水势，其中 ψ 为水势，ψₛ 为溶质势，ψₚ 为压力势。
8. Active Transport | 主动运输
Active transport is the movement of molecules or ions from a region of lower concentration to a region of higher concentration across a membrane, using energy from ATP. This process is carried out by specific carrier proteins, often called pumps.
主动运输是分子或离子从低浓度区域穿过膜向高浓度区域的移动，需要消耗 ATP 提供的能量。这一过程由特定的载体蛋白（常称为泵）完成。
The sodium-potassium pump is a classic example. It actively transports three Na⁺ ions out of the cell and two K⁺ ions into the cell against their concentration gradients. ATP is hydrolysed to ADP and inorganic phosphate, causing conformational changes in the carrier protein.
钠钾泵是一个典型例子。它逆浓度梯度主动将三个 Na⁺ 离子运出细胞，同时将两个 K⁺ 离子运入细胞。ATP 水解为 ADP 和无机磷酸，引起载体蛋白构象变化。
Active transport is selective, energy-dependent, and can be inhibited by respiratory inhibitors such as cyanide that reduce ATP production. It enables cells to absorb ions from dilute solutions, such as root hair cells taking up nitrate ions from soil.
主动运输具有选择性、依赖能量，并可被氰化物等呼吸抑制剂抑制，因为这些抑制剂减少 ATP 的产生。它使细胞能够从稀溶液中吸收离子，例如根毛细胞从土壤中吸收硝酸根离子。
9. Endocytosis and Exocytosis | 胞吞与胞吐
Large molecules and particles are transported across membranes by bulk transport. Endocytosis involves the plasma membrane folding inwards to form vesicles that bring substances into the cell; phagocytosis is the uptake of large particles, while pinocytosis is the uptake of fluids.
大分子和颗粒通过批量运输跨膜转运。胞吞作用涉及质膜向内凹陷形成囊泡，将物质带入细胞；吞噬作用摄取大颗粒，胞饮作用摄取液体。
Exocytosis is the reverse process, in which vesicles fuse with the plasma membrane and release their contents outside the cell. This is how cells secrete proteins, hormones and neurotransmitters, and it also requires ATP for vesicle movement and membrane fusion.
胞吐作用则是相反的过程，囊泡与质膜融合，将其内容物释放到细胞外。细胞通过这种方式分泌蛋白质、激素和神经递质，囊泡运动与膜融合同样需要 ATP。
10. Factors Affecting Membrane Permeability | 影响膜通透性的因素
Temperature affects membrane permeability because high temperatures increase the kinetic energy of phospholipids and proteins. Above a critical temperature, the phospholipid bilayer becomes more fluid and membrane proteins denature, causing the membrane to lose its selective permeability and allowing pigments or ions to leak out.
温度影响膜通透性，因为高温会增加磷脂和蛋白质的动能。高于临界温度时，磷脂双分子层变得更流动，膜蛋白变性，导致膜失去选择通透性，使色素或离子泄漏出来。
Organic solvents, such as ethanol, dissolve lipids in the membrane and disrupt the phospholipid bilayer. Higher solvent concentrations increase permeability and can cause severe damage to membrane structure.
乙醇等有机溶剂会溶解膜中的脂质，破坏磷脂双分子层。溶剂浓度越高，通透性越大，并可能严重损害膜结构。
pH extremes can denature membrane proteins and alter the charges on phospholipid heads, disrupting membrane integrity. The type and concentration of detergent also affect permeability by solubilising lipids.
极端 pH 会使膜蛋白变性并改变磷脂头部的电荷，破坏膜的完整性。去污剂的类型和浓度也会通过溶解脂质影响通透性。
11. Practical: Beetroot Membrane Permeability | 实验：甜
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