📚 Edexcel A-Level Biology | Cell Membranes and Transport (Combined 103) | 细胞膜与物质运输（综合科学103）
Cell membranes are selectively permeable barriers that separate the cell interior from the external environment. In A-Level Biology, understanding how substances cross membranes is essential for explaining nutrient uptake, waste removal, nerve impulse generation, muscle contraction and homeostasis. This article covers the fluid mosaic model, simple and facilitated diffusion, osmosis, active transport, co-transport and bulk transport, with practical investigations and exam tips.
细胞膜是一种选择性透过屏障，将细胞内部与外部环境分隔开来。在 A-Level 生物学中，理解物质如何跨膜运输对于解释营养吸收、废物排出、神经冲动产生、肌肉收缩和稳态维持至关重要。本文涵盖流动镶嵌模型、简单扩散与易化扩散、渗透作用、主动运输、协同运输和大分子运输，并提供实验研究与备考建议。
1. The Fluid Mosaic Model | 流动镶嵌模型
The fluid mosaic model describes the cell membrane as a flexible phospholipid bilayer studded with proteins, glycoproteins, glycolipids and cholesterol. 'Fluid' refers to the ability of phospholipids and many proteins to move laterally within the bilayer, while 'mosaic' reflects the patchwork of different molecules embedded in the membrane.
流动镶嵌模型将细胞膜描述为柔性的磷脂双分子层，其中镶嵌着蛋白质、糖蛋白、糖脂和胆固醇。"流动"是指磷脂和许多蛋白质能够在双分子层内进行侧向移动；"镶嵌"则反映不同分子嵌入膜中形成的拼图状结构。
Membrane proteins are not fixed rigidly in place; they float within the phospholipid environment. Cholesterol molecules fit between phospholipids, restricting excessive movement at high temperatures and preventing packing at low temperatures, thereby stabilising membrane fluidity.
膜蛋白并非被刚性固定在某一位置，而是漂浮在磷脂环境中。胆固醇分子插入磷脂之间，在高温时限制过度流动，在低温时防止磷脂紧密堆积，从而稳定膜的流动性。
2. Phospholipid Bilayer and Membrane Proteins | 磷脂双分子层与膜蛋白
Each phospholipid molecule has a hydrophilic phosphate head and two hydrophobic fatty acid tails. In an aqueous environment, phospholipids spontaneously arrange into a bilayer: the heads face outward towards water, while the tails face inward, creating a hydrophobic core.
每个磷脂分子具有一个亲水的磷酸头部和两条疏水的脂肪酸尾部。在水性环境中，磷脂自发排列成双分子层：头部朝外接触水环境，尾部朝内，形成疏水核心。
This structure allows small, non-polar molecules such as oxygen and carbon dioxide to pass through the hydrophobic core, but it blocks large polar molecules and ions. Therefore, most transport functions depend on membrane proteins.
这种结构允许氧气和二氧化碳等小的非极性分子穿过疏水核心，但阻挡大的极性分子和离子。因此，大多数运输功能依赖膜蛋白来完成。
- Channel proteins form hydrophilic pores for specific ions or water molecules.
通道蛋白形成亲水孔道，供特定离子或水分子通过。 - Carrier proteins change shape to move molecules across the membrane.
载体蛋白通过改变构象将分子转运过膜。 - Receptor proteins bind hormones and trigger intracellular responses.
受体蛋白结合激素并触发细胞内的反应。 - Glycoproteins and glycolipids act as recognition markers.
糖蛋白和糖脂充当细胞识别标志。
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