Cell Membranes & Transport · 细胞膜与运输

What Are Cell Membranes? · 什么是细胞膜？

All living cells are surrounded by a cell membrane, also called the plasma membrane. This structure is not simply a passive barrier: it controls which substances enter and leave the cell, maintains internal conditions, and enables cell-to-cell communication. The currently accepted model for membrane structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972.

所有活细胞都被细胞膜（也称质膜）包围。这一结构不仅仅是被动的屏障：它控制物质进出细胞、维持内部环境、并实现细胞间通讯。目前公认的膜结构模型是 流动镶嵌模型，由 Singer 和 Nicolson 于 1972 年提出。

The Fluid Mosaic Model · 流动镶嵌模型

The cell membrane consists mainly of a phospholipid bilayer. Each phospholipid molecule has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty acid tails. In an aqueous environment, phospholipids spontaneously arrange into a bilayer: the hydrophilic heads face outward toward the water on both sides, while the hydrophobic tails point inward, shielded from water. This arrangement makes the membrane selectively permeable: small, non-polar molecules like oxygen and carbon dioxide can diffuse through freely, but large or charged particles require protein channels or carriers.

细胞膜主要由 磷脂双层 组成。每个磷脂分子有一个亲水（喜水）的磷酸头和两条疏水（拒水）的脂肪酸尾。在水环境中，磷脂自发排列成双层结构：亲水头朝外面向两侧的水环境，疏水尾朝内避开水分。这种排列使膜具有 选择透过性：小的非极性分子如氧气和二氧化碳可以自由扩散通过，但大分子或带电粒子需要蛋白质通道或载体。

Embedded within the phospholipid bilayer are various proteins. Intrinsic (integral) proteins span the entire width of the membrane; many of these are channel proteins or carrier proteins involved in transport. Extrinsic (peripheral) proteins are found on the inner or outer surface of the membrane and often play roles in cell signalling or maintaining the cytoskeleton. Some proteins have carbohydrate chains attached, forming glycoproteins, which act as recognition sites for hormones, neurotransmitters, and other signalling molecules. Cholesterol molecules are also present in animal cell membranes: they sit between phospholipid tails and regulate membrane fluidity, making the membrane less fluid at high temperatures and preventing it from becoming too rigid at low temperatures.

嵌入磷脂双层中的是各种 蛋白质。内在蛋白 横跨整个膜宽度，其中许多是参与运输的通道蛋白或载体蛋白。外在蛋白 位于膜的内表面或外表面，通常在细胞信号传导或维持细胞骨架中发挥作用。一些蛋白质附着有碳水化合物链，形成 糖蛋白，作为激素、神经递质和其他信号分子的识别位点。胆固醇 分子也存在于动物细胞膜中：它们位于磷脂尾之间，调节膜的流动性，使膜在高温下不易过流、低温下不易过硬。

Passive Transport: Diffusion and Osmosis · 被动运输：扩散与渗透

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 that does not require metabolic energy (ATP). The rate of diffusion across a membrane is governed by Fick’s Law: rate is proportional to (surface area × concentration difference) ÷ diffusion distance. Membranes with a larger surface area, steeper concentration gradients, and shorter diffusion distances allow faster diffusion.

扩散 是粒子沿浓度梯度从高浓度区域向低浓度区域的净移动。这是一个被动过程，不需要代谢能量（ATP）。跨膜扩散的速率由 菲克定律 决定：速率与（表面积 × 浓度差）÷ 扩散距离成正比。表面积越大、浓度梯度越陡、扩散距离越短，扩散越快。

Osmosis is a special case of diffusion involving water molecules moving across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Water potential (Ψ) is measured in kilopascals (kPa); pure water has a water potential of zero, and adding solutes makes the water potential negative. In animal cells, if the external solution has a higher water potential (hypotonic), water enters the cell by osmosis, causing it to swell and potentially burst (cytolysis). If the external solution has a lower water potential (hypertonic), water leaves the cell, causing it to shrink (crenation). In plant cells, the rigid cell wall prevents bursting: in a hypotonic solution, the cell becomes turgid, which is the normal healthy state providing support; in a hypertonic solution, the cell membrane pulls away from the cell wall (plasmolysis).

渗透 是扩散的一种特殊情况，涉及水分子跨过半透膜从较高水势区域向较低水势区域移动。水势（Ψ）以千帕（kPa）为单位测量：纯水的水势为零，加入溶质使水势变为负值。在动物细胞中，如果外部溶液具有较高水势（低渗），水通过渗透进入细胞，导致细胞膨胀甚至破裂（细胞溶解）。如果外部溶液具有较低水势（高渗），水离开细胞，导致细胞皱缩。在植物细胞中，坚硬的细胞壁防止破裂：在低渗溶液中，细胞变得饱满，这是提供支撑的正常健康状态；在高渗溶液中，细胞膜从细胞壁上剥离（质壁分离）。

Facilitated Diffusion · 协助扩散

Large or charged molecules such as glucose, amino acids, and ions cannot pass directly through the phospholipid bilayer. They move down their concentration gradient through specific transport proteins in a process called facilitated diffusion. Like simple diffusion, this is a passive process requiring no ATP. Two types of proteins are involved: channel proteins form water-filled pores that allow specific ions to pass through ： for example, sodium ion channels and potassium ion channels, which are often gated and open only in response to specific stimuli; carrier proteins bind to a specific molecule on one side of the membrane, undergo a conformational change, and release the molecule on the other side ： for example, GLUT proteins that transport glucose into cells.

大的或带电荷的分子如葡萄糖、氨基酸和离子不能直接穿过磷脂双层。它们通过特定的运输蛋白沿浓度梯度移动，这一过程称为 协助扩散。与简单扩散一样，这是一个不需要 ATP 的被动过程。涉及两种类型的蛋白质：通道蛋白 形成充满水的孔道，允许特定离子通过：例如钠离子通道和钾离子通道，通常是门控的，仅在特定刺激下打开；载体蛋白 在膜的一侧与特定分子结合，发生构象变化，在另一侧释放分子：例如将葡萄糖运入细胞的 GLUT 蛋白。

Active Transport · 主动运输

Active transport moves molecules or ions against their concentration gradient ： from a region of lower concentration to a region of higher concentration. This process requires energy in the form of ATP and uses specific carrier proteins, often called pumps. The most important example is the sodium-potassium pump (Na⁺/K⁺ ATPase), which actively transports three sodium ions out of the cell and two potassium ions into the cell for each ATP molecule hydrolysed. This pump is crucial for maintaining the resting potential in nerve cells and for driving secondary active transport processes such as the co-transport of glucose in the small intestine.

主动运输 将分子或离子 逆 浓度梯度移动：从较低浓度区域到较高浓度区域。此过程需要 ATP 形式的能量，并使用特定的载体蛋白，通常称为泵。最重要的例子是 钠钾泵（Na⁺/K⁺ ATP酶），每水解一个 ATP 分子，它主动将三个钠离子运出细胞、两个钾离子运入细胞。该泵对于维持神经细胞的静息电位以及驱动次级主动运输过程（如小肠中葡萄糖的共转运）至关重要。

Bulk Transport: Endocytosis and Exocytosis · 批量运输：内吞与外排

Very large molecules or particles, such as proteins, polysaccharides, and even whole microorganisms, are transported across the membrane by bulk transport. This is an active process requiring ATP. Endocytosis involves the cell membrane folding inward to engulf material, forming a vesicle that pinches off into the cytoplasm. Phagocytosis (“cell eating”) is the uptake of solid particles; pinocytosis (“cell drinking”) is the uptake of liquid droplets containing dissolved substances. Exocytosis is the reverse process: vesicles containing materials (such as digestive enzymes or hormones) fuse with the cell membrane and release their contents outside the cell. Both endocytosis and exocytosis are essential for secretion, immune defence, and cell growth.

非常大的分子或颗粒，如蛋白质、多糖甚至整个微生物，通过 批量运输 跨膜转运。这是一个需要 ATP 的主动过程。内吞作用 涉及细胞膜向内折叠包裹物质，形成囊泡并脱离进入细胞质。吞噬作用 是摄取固体颗粒；胞饮作用 是摄取含有溶解物质的液滴。外排作用 是相反的过程：含有物质（如消化酶或激素）的囊泡与细胞膜融合，将其内容物释放到细胞外。内吞和外排对于分泌、免疫防御和细胞生长至关重要。

Factors Affecting Membrane Transport · 影响膜运输的因素

Several factors influence the rate of transport across cell membranes. Temperature has two opposing effects: higher temperatures increase the kinetic energy of molecules, speeding up diffusion, but excessively high temperatures denature transport proteins and increase membrane fluidity to the point of losing integrity. pH affects the tertiary structure of transport proteins; extreme pH values can denature these proteins and disrupt hydrogen and ionic bonds. Surface area to volume ratio is critical: cells with a larger surface area relative to their volume have more membrane available for transport. This explains why many absorptive cells, such as those lining the small intestine, have microvilli to increase surface area. Concentration gradient directly affects the rate of simple and facilitated diffusion; a steeper gradient produces a faster rate. Inhibitors such as cyanide, which blocks ATP production, will stop active transport but have no effect on passive processes.

多种因素影响跨膜运输的速率。温度 有两种相反的作用：较高温度增加分子动能，加速扩散，但过高温度会使运输蛋白变性并增加膜流动性直至失去完整性。pH 值 影响运输蛋白的三级结构；极端 pH 值可使这些蛋白质变性并破坏氢键和离子键。表面积与体积之比 至关重要：相对于体积具有较大表面积的细胞有更多膜可用于运输。这解释了为什么许多吸收性细胞（如排列在小肠的细胞）具有微绒毛以增加表面积。浓度梯度 直接影响简单扩散和协助扩散的速率；更陡的梯度产生更快的速率。抑制剂 如氰化物，它阻断 ATP 产生，会停止主动运输但对被动过程无影响。

Practical Investigations · 实验探究

A classic practical investigation for membrane permeability uses beetroot (Beta vulgaris). Beetroot cells contain a red pigment called betalain, which is normally contained within the vacuole. When the cell membrane is damaged, the pigment leaks out into the surrounding solution. By cutting beetroot into equal-sized discs, placing them in water baths at different temperatures (e.g., 0°C, 20°C, 40°C, 60°C, 80°C), and measuring the absorbance of the resulting solution using a colorimeter, students can quantify the effect of temperature on membrane permeability. The key control variables include the volume of water, the size and surface area of the beetroot discs, and the incubation time.

一个经典的膜通透性实验探究使用 甜菜根。甜菜根细胞含有一种称为甜菜红素的红色色素，通常储存在液泡中。当细胞膜受损时，色素渗漏到周围溶液中。通过将甜菜根切成大小相等的圆片，置于不同温度的水浴中（如 0°C、20°C、40°C、60°C、80°C），并使用比色计测量所得溶液的吸光度，学生可以量化温度对膜通透性的影响。关键控制变量包括水的体积、甜菜根圆片的大小和表面积以及孵育时间。

Another common investigation explores the effect of solvent concentration on membrane permeability. Organic solvents such as ethanol dissolve phospholipids, disrupting the membrane structure. Increasing ethanol concentration produces progressively greater pigment leakage from beetroot cells, demonstrating that the integrity of the phospholipid bilayer is essential for selective permeability. These practical investigations are frequently assessed in the A-Level Biology practical endorsement and appear in exam questions requiring students to evaluate methodology, identify limitations, and suggest improvements.

另一种常见探究探讨 溶剂浓度 对膜通透性的影响。有机溶剂如乙醇溶解磷脂，破坏膜结构。增加乙醇浓度会导致甜菜根细胞色素逐渐更多地渗漏，证明磷脂双层的完整性对于选择透过性至关重要。这些实验探究经常在 A-Level 生物实验考核中评估，并出现在要求学生评价方法、识别局限性和提出改进的考题中。

Key Bilingual Terms · 关键双语术语

Cell membrane · 细胞膜 | Phospholipid bilayer · 磷脂双层 | Fluid mosaic model · 流动镶嵌模型 | Selective permeability · 选择透过性 | Intrinsic protein · 内在蛋白 | Glycoprotein · 糖蛋白 | Cholesterol · 胆固醇 | Diffusion · 扩散 | Concentration gradient · 浓度梯度 | Osmosis · 渗透 | Water potential · 水势 | Facilitated diffusion · 协助扩散 | Channel protein · 通道蛋白 | Carrier protein · 载体蛋白 | Active transport · 主动运输 | Endocytosis · 内吞 | Exocytosis · 外排 | Phagocytosis · 吞噬作用

Exam Tips · 考试技巧

When answering A-Level Biology exam questions on cell membranes, always use precise scientific terminology. Distinguish carefully between diffusion (any particles, down a gradient), osmosis (water only, across a partially permeable membrane, down a water potential gradient), and active transport (against a gradient, requires ATP). For osmosis questions, use the term water potential rather than “concentration of water” ： exam boards expect this technical language. When explaining the fluid mosaic model, link structure to function: explain why the hydrophobic core of the bilayer prevents ions from passing through directly. In practical-based questions, always discuss control variables, the use of a colorimeter (or a simple color chart as an alternative), and the importance of replicates for reliability.

在回答 A-Level 生物关于细胞膜的考题时，始终使用准确的科学术语。仔细区分扩散（任何粒子，沿梯度）、渗透（仅水分子，跨过半透膜，沿水势梯度）和主动运输（逆梯度，需要 ATP）。对于渗透问题，使用 水势 一词而非”水的浓度”：考试局期望使用这一技术语言。解释流动镶嵌模型时，将结构与功能联系起来：解释为什么双层的疏水核心阻止离子直接通过。在实验类问题中，始终讨论控制变量、比色计的使用（或简易色卡作为替代方案），以及重复实验对于可靠性的重要性。