📚 Cell Membranes and Transport Across Cell Membranes | 细胞膜与跨膜运输
Cell membranes control what enters and leaves every living cell. Understanding their structure and transport mechanisms is central to A-level Biology Unit 2. This article covers the fluid mosaic model, passive and active transport, osmosis, co-transport, bulk transport, and experimental methods used to investigate membrane permeability.
细胞膜控制着每个活细胞中物质的进出。理解其结构与运输机制是 A-level 生物第二单元的核心内容。本文将涵盖流动镶嵌模型、被动运输与主动运输、渗透作用、协同运输、批量运输以及研究膜通透性的实验方法。
1. Membrane Structure: Phospholipid Bilayer | 膜结构：磷脂双分子层
Cell membranes are made mainly of phospholipids arranged in a bilayer. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails. The bilayer forms a stable barrier because the hydrophobic tails face each other inside, away from water, while the hydrophilic heads face the aqueous cytoplasm and extracellular fluid.
细胞膜主要由排列成双分子层的磷脂组成。每个磷脂分子有一个亲水的磷酸头部和两条疏水的脂肪酸尾部。双分子层形成稳定的屏障，因为疏水尾部朝内相互靠拢、远离水，而亲水头部面向细胞质和细胞外液等水环境。
Small, non-polar molecules such as oxygen and carbon dioxide can pass through the phospholipid bilayer easily. However, large polar molecules and ions cannot cross the hydrophobic core without help from membrane proteins.
氧气和二氧化碳等小分子、非极性分子可以轻松穿过磷脂双分子层。然而，大型极性分子和离子在没有膜蛋白帮助的情况下无法穿过疏水核心。
2. Fluid Mosaic Model: Proteins and Cholesterol | 流动镶嵌模型：蛋白质与胆固醇
The fluid mosaic model describes the membrane as a dynamic structure. Phospholipids can move laterally within the layer, giving the membrane fluidity. Proteins embedded in the bilayer form a mosaic pattern. Some proteins span the whole membrane, called integral proteins, while others sit on one surface, called peripheral proteins.
流动镶嵌模型将膜描述为一种动态结构。磷脂可以在层内横向移动，从而赋予膜流动性。嵌入双分子层中的蛋白质形成镶嵌图案。有些蛋白质贯穿整个膜，称为内在蛋白；另一些位于膜的一侧表面，称为外在蛋白。
Cholesterol is another important component. It sits between phospholipids and regulates membrane fluidity. At high temperatures it reduces fluidity by restraining phospholipid movement, while at low temperatures it prevents tight packing and maintains flexibility. Glycoproteins and glycolipids on the outer surface function in cell recognition and adhesion.
胆固醇是另一个重要成分。它位于磷脂之间，调节膜的流动性。高温时，它通过限制磷脂移动来降低流动性；低温时，它防止磷脂紧密堆积并保持膜的柔韧性。外表面的糖蛋白和糖脂在细胞识别和黏附中发挥作用。
| Component | Role | 成分 | 作用 |
|---|---|---|---|
| Phospholipid bilayer | Forms the basic barrier | 磷脂双分子层 | 构成基本屏障 |
| Cholesterol | Regulates fluidity and stability | 胆固醇 | 调节流动性与稳定性 |
| Integral proteins | Transport, enzymes, receptors | 内在蛋白 | 运输、酶、受体 |
| Glycoproteins | Cell recognition and signalling | 糖蛋白 | 细胞识别与信号传导 |
3. Membrane Functions: Barrier and Recognition | 膜的功能：屏障与识别
The cell membrane is selectively permeable. It allows some substances to cross while restricting others. This is essential for maintaining different internal conditions from the external environment. For example, cells keep a high potassium ion concentration inside and a high sodium ion concentration outside.
细胞膜具有选择透过性。它允许某些物质通过，同时限制其他物质。这对于维持内部环境与外部环境的差异至关重要。例如，细胞内部保持高钾离子浓度，外部保持高钠离子浓度。
Membranes also provide a surface for chemical reactions. In mitochondria, the inner membrane contains electron carriers and ATP synthase for oxidative phosphorylation. In chloroplasts, thylakoid membranes house the photosynthetic machinery. Receptor proteins allow cells to respond to hormones and other signals.
膜还为化学反应提供表面。在线粒体中，内膜含有电子载体和 ATP 合酶，用于氧化磷酸化。在叶绿体中，类囊体膜承载光合作用机制。受体蛋白使细胞能够对激素和其他信号作出反应。
4. Simple Diffusion: Passive Transport | 简单扩散：被动运输
Simple 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 no metabolic energy from ATP is required. Particles move because they have kinetic energy.
简单扩散是粒子从浓度较高的区域向浓度较低的区域净移动，沿浓度梯度进行。这是一个被动过程，不需要 ATP 提供代谢能量。粒子因为具有动能而移动。
The rate of simple diffusion depends on the concentration gradient, the surface area of the membrane, the thickness of the membrane, temperature, and the size and polarity of the diffusing molecule. Oxygen, carbon dioxide, and steroid hormones diffuse directly through the phospholipid bilayer.
简单扩散的速率取决于浓度梯度、膜的表面积、膜的厚度、温度以及扩散分子的大小和极性。氧气、二氧化碳和类固醇激素可直接穿过磷脂双分子层扩散。
5. Facilitated Diffusion: Channel and Carrier Proteins | 促进扩散：通道蛋白与载体蛋白
Facilitated diffusion allows larger polar molecules and ions to cross the membrane down their concentration gradient without using ATP. It relies on two types of membrane proteins: channel proteins and carrier proteins. Channel proteins form water-filled pores for specific ions, for example aquaporins for water and ion channels for sodium or potassium.
促进扩散使较大的极性分子和离子能够沿浓度梯度穿过膜，且不需要消耗 ATP。它依赖两类膜蛋白：通道蛋白和载体蛋白。通道蛋白为特定离子形成充满水的孔道，例如水通道蛋白用于水分子，离子通道用于钠或钾离子。
Carrier proteins change shape to shuttle specific molecules across the membrane. For example, GLUT1 transports glucose into red blood cells. Facilitated diffusion shows saturation because the number of protein channels or carriers is limited.
载体蛋白改变形状以将特定分子运载过膜。例如，GLUT1 将葡萄糖运输进入红细胞。促进扩散表现出饱和现象，因为通道蛋白或载体蛋白的数量有限。
6. Osmosis: Water Potential and Tonicity | 渗透作用：水势与渗透压
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 denoted by the Greek letter psi, ψ, and is measured in kilopascals, kPa. Pure water under standard conditions has a water potential of zero.
渗透作用是水分子通过部分透性膜从水势较高的区域向水势较低的区域净移动。水势用希腊字母 ψ 表示，单位为千帕斯卡 (kPa)。标准条件下纯水的水势为零。
Adding solute lowers the water potential because solute particles reduce the free energy of water. A solution with a more negative water potential is more concentrated. Plant cells swell and become turgid in hypotonic solutions, while in hypertonic solutions they undergo plasmolysis. Animal cells may burst in hypotonic solutions and shrink in hypertonic solutions.
加入溶质会降低水势，因为溶质粒子降低了水的自由能。水势越负，溶液越浓。植物细胞在低渗溶液中会膨胀并变得坚挺，在高渗溶液中会发生质壁分离。动物细胞在低渗溶液中可能破裂，在高渗溶液中会皱缩。
Water potential (ψ) = Solute potential (ψₛ) + Pressure potential (ψₚ)
7. Active Transport: ATP-Driven Pumps | 主动运输：ATP 驱动的泵
Active transport moves substances against their concentration gradient, from lower to higher concentration. It requires energy in the form of ATP and specific carrier proteins called pumps. The sodium-potassium pump is a classic example found in animal cell membranes.
主动运输将物质逆浓度梯度移动，从低浓度向高浓度运输。它需要 ATP 形式的能量和称为泵的特定载体蛋白。钠钾泵是动物细胞膜中的典型例子。
For every ATP molecule hydrolysed, the Na⁺/K⁺ pump transports three sodium ions out of the cell and two potassium ions into the cell. This creates concentration gradients that are vital for nerve impulses, muscle contraction, and secondary active transport.
每水解一个 ATP 分子，钠钾泵会将三个钠离子运出细胞，并将两个钾离子运入细胞。这建立了对神经冲动、肌肉收缩和次级主动运输至关重要的浓度梯度。
8. Co-transport: Coupling Ion Gradients | 协同运输：偶联离子梯度
Co-transport, or secondary active transport, uses the energy stored in an ion gradient to drive the movement of another substance against its own concentration gradient. In the small intestine, glucose absorption depends on co-transport with sodium ions.
协同运输，或称次级主动运输，利用储存在离子梯度中的能量来驱动另一种物质逆自身浓度梯度移动。在小肠中，葡萄糖的吸收依赖于与钠离子的协同运输。
Sodium ions enter intestinal epithelial cells down their concentration gradient through a sodium-glucose co-transporter protein. This simultaneously pulls glucose into the cell against its concentration gradient. The sodium gradient is maintained by the Na⁺/K⁺ pump on the opposite membrane.
钠离子通过钠-葡萄糖同向转运蛋白沿其浓度梯度进入小肠上皮细胞。这同时将葡萄糖逆其浓度梯度拉入细胞。钠离子梯度由另一侧膜上的钠钾泵维持。
9. Endocytosis and Exocytosis: Bulk Transport | 胞吞与胞吐：批量运输
Very large substances, such as proteins and polysaccharides, cannot cross the membrane through channel or carrier proteins. They are transported by endocytosis, in which the membrane invaginates to form a vesicle that brings material into the cell. Phagocytosis is the uptake of large particles, while pinocytosis is the uptake of fluid droplets.
非常大的物质，如蛋白质和多糖，无法通过通道蛋白或载体蛋白穿过膜。它们通过胞吞作用运输，即膜内陷形成囊泡将物质带入细胞。吞噬作用是大颗粒的摄取，而胞饮作用是液滴的摄取。
Exocytosis is the reverse process, where vesicles fuse with the plasma membrane and release their contents outside the cell. This is how cells secrete digestive enzymes, hormones, and neurotransmitters. Both processes require ATP and involve membrane remodelling.
胞吐是相反的过程，囊泡与细胞质膜融合并将其内容物释放到细胞外。细胞就是这样分泌消化酶、激素和神经递质的。这两个过程都需要 ATP 并涉及膜的重塑。
10. Factors Affecting Membrane Permeability and Transport Rate | 影响膜通透性和运输速率的因素
Temperature has a major effect on membranes. As temperature rises, phospholipids gain kinetic energy and the membrane becomes more fluid. This increases permeability, so substances leak across more easily. If the temperature is too high, membrane proteins may denature, causing loss of selective permeability.
温度对膜有重大影响。随着温度升高，磷脂获得动能，膜变得更加流动。这增加了通透性，因此物质更容易泄漏穿过。如果温度过高，膜蛋白可能变性，导致选择透过性丧失。
Solvent concentration and pH can also alter membrane structure. Organic solvents such as ethanol dissolve lipids, disrupting the bilayer. Extreme pH can denature membrane proteins. The surface area and number of transport proteins also limit transport rates, especially for facilitated diffusion and active transport.
溶剂浓度和 pH 值也会改变膜结构。乙醇等有机溶剂会溶解脂质，破坏双分子层。极端 pH 值会使膜蛋白变性。膜表面积和转运蛋白数量也会限制运输速率，尤其是促进扩散和主动运输。
- Higher temperature increases kinetic energy.
温度升高会增加动能。 - A larger concentration gradient speeds up diffusion.
更大的浓度梯度会加快扩散。 - More channel or carrier proteins allow faster facilitated diffusion.
更多的通道或载体蛋白可加快促进扩散。 - More ATP supply supports active transport.
更多的 ATP 供应可支持主动运输。
11. Experimental Approaches and Exam Tips | 实验方法与考试技巧
One common practical is the beetroot permeability experiment. Pieces of beetroot are placed in water at different temperatures or in alcohol solutions of different concentrations. The leakage of red pigment, betalain, is measured using a colorimeter. Higher absorbance indicates greater membrane damage and permeability.
一个常见的实验是甜菜根通透性实验。将甜菜根块放入不同温度的水或不同浓度的酒精溶液中。使用比色计测量红色素甜菜红素的泄漏量。吸光度越高，表明膜损伤和通透性越大。
Another practical uses Visking tubing as a model of a partially permeable membrane. Starch and glucose can be tested inside and outside the tubing with iodine and Benedict's solution. The results show that small molecules like glucose diffuse through, while large starch molecules cannot.
另一个实验使用 Visking 管作为部分透性膜的模型。可以用碘液和班氏试剂检测管内外的淀粉和葡萄糖。结果表明，葡萄糖等小分子可以扩散通过，而淀粉大分子不能通过。
In exam questions, link the structure of the membrane to the type of transport. Always state whether energy is required, whether movement is down or against a gradient, and whether a protein is involved. Use terms such as 'net movement', 'selectively permeable', and 'water potential' precisely.
在考试题中，要将膜结构与运输类型联系起来。始终说明是否需要能量、移动是顺梯度还是逆梯度，以及是否涉及蛋白质。准确使用“净移动”、“选择透过性”和“水势”等术语。
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