Cell Membranes and Transport Across Membranes | 细胞膜与跨膜运输

📚 Cell Membranes and Transport Across Membranes | 细胞膜与跨膜运输

Understanding how substances move across cell membranes is a central topic in Edexcel A-Level Combined Science and underpins later work on physiology, homeostasis and cell signalling. Module 128 focuses on membrane structure, passive and active transport, and practical measurement of water potential.

理解物质如何跨细胞膜移动是 Edexcel A-Level 综合科学中的一个核心主题，也为后续的生理学、体内稳态和细胞信号传导打下基础。模块 128 重点学习膜结构、被动和主动运输以及水势的实际测量。

1. Membrane Structure and the Fluid Mosaic Model | 膜结构与流动镶嵌模型

The cell surface membrane is described by the fluid mosaic model. A phospholipid bilayer forms the basic framework: hydrophilic phosphate heads face the aqueous cytoplasm and exterior, while hydrophobic fatty acid tails face inward, creating a barrier to ions and large polar molecules. Membrane proteins float in or on this bilayer, giving the structure a ‘mosaic’ appearance, and the components can move laterally, making it fluid.

细胞表面膜用流动镶嵌模型描述。磷脂双分子层构成基本骨架：亲水的磷酸头部朝向细胞质和外部水环境，而疏水的脂肪酸尾部朝内，形成一个阻挡离子和大极性分子的屏障。膜蛋白漂浮或镶嵌在双分子层中，使结构呈现’镶嵌’外观，成分可以横向移动，因此具有流动性。

Cholesterol sits between phospholipids and regulates membrane fluidity: it restricts movement at high temperatures and prevents close packing at low temperatures. Glycoproteins and glycolipids on the outer surface are involved in cell recognition, adhesion and immune response. Intrinsic proteins span the bilayer, while extrinsic proteins are partially embedded or attached to the surface.

胆固醇位于磷脂之间，调节膜的流动性：在高温下限制运动，在低温下防止紧密堆积。外表面的糖蛋白和糖脂参与细胞识别、黏附和免疫反应。内在蛋白贯穿双分子层，外在蛋白部分嵌入或附着在表面上。

2. Simple Diffusion and Fick’s Law | 简单扩散与菲克定律

Simple diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It is passive and does not require ATP; particles move down a concentration gradient until equilibrium is reached. Only small, non-polar molecules such as O₂, CO₂ and urea (partially) can cross the phospholipid bilayer directly.

简单扩散是粒子从浓度较高区域向浓度较低区域的净运动。它是被动的，不需要 ATP；粒子沿浓度梯度移动，直到达到平衡。只有小分子、非极性分子如 O₂、CO₂ 和尿素（部分）可以直接穿过磷脂双分子层。

The rate of simple diffusion is described by Fick’s law, which links the rate to surface area, concentration gradient and diffusion distance. This explains why cells with large surface-area-to-volume ratios and thin membranes exchange materials more efficiently, as seen in alveolar epithelial cells and root hair cells.

简单扩散速率可用菲克定律描述，该定律将速率与表面积、浓度梯度和扩散距离联系起来。这解释了为什么表面积与体积比大、膜薄的细胞能够更高效地交换物质，例如肺泡上皮细胞和根毛细胞。

Rate of diffusion ∝ (SA × ΔC) / d

If surface area doubles, the rate doubles; if diffusion distance doubles, the rate halves. Temperature also increases the kinetic energy of particles, so diffusion becomes faster up to the point where membrane proteins denature.

如果表面积加倍，速率加倍；如果扩散距离加倍，速率减半。温度也会增加粒子的动能，因此扩散在膜蛋白变性前会加快。

3. Facilitated Diffusion | 协助扩散

Facilitated diffusion assists ions and polar molecules such as glucose and amino acids. It still moves down a concentration gradient and does not require metabolic energy, but it uses two types of membrane proteins: channel proteins and carrier proteins. Channel proteins form hydrophilic pores, often gated, while carrier proteins bind the solute and undergo a conformational change.

协助扩散帮助离子和极性分子（如葡萄糖和氨基酸）过膜。它仍然沿浓度梯度移动，不需要代谢能量，但使用两种膜蛋白：通道蛋白和载体蛋白。通道蛋白形成亲水孔，通常有门控；载体蛋白结合溶质并发生构象变化。

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