📚 GCSE OCR Biology: Cell Membranes – Key Points Revision | GCSE OCR 生物：细胞膜 考点精讲
The cell membrane is one of the most fundamental structures in biology. It forms the boundary between the cell's interior and its external environment, controlling what enters and leaves the cell. For the OCR GCSE Biology specification, you need to understand its structure, how substances move across it, and the factors that affect this movement. Let's break down every key point you need to know.
细胞膜是生物学中最基础的结构之一。它构成了细胞内部与外部环境之间的边界，控制着物质的进出。针对 OCR GCSE 生物考纲，你需要理解它的结构、物质如何跨越它以及影响这些运动的因素。让我们逐一拆解你需要掌握的每一个关键点。
1. Introduction to Cell Membranes | 细胞膜简介
All cells, whether prokaryotic or eukaryotic, are surrounded by a cell membrane. This ultra-thin layer, about 7–10 nm in thickness, is not visible under a light microscope but can be observed with an electron microscope.
所有的细胞，无论是原核还是真核，都被细胞膜所包裹。这层极薄的膜厚约 7–10 纳米，在光学显微镜下看不见，但在电子显微镜下可以观察到。
The cell membrane is described as partially permeable (or selectively permeable) because it allows some molecules to pass through while restricting others. This property is essential for maintaining homeostasis and controlling cell contents.
细胞膜被描述为具有部分透性（或选择透过性），因为它允许某些分子通过，同时限制其他分子。这一特性对于维持稳态和控制细胞内含物至关重要。
2. Structure of the Cell Membrane | 细胞膜的结构
The widely accepted model of the cell membrane is the fluid mosaic model. It is 'fluid' because the phospholipid molecules can move laterally, and 'mosaic' because proteins are embedded in the phospholipid bilayer like tiles in a mosaic.
细胞膜被广泛接受的模型是流动镶嵌模型。之所以称为“流动”，是因为磷脂分子可以横向移动；之所以称为“镶嵌”，是因为蛋白质像马赛克瓷砖一样镶嵌在磷脂双分子层中。
The main components are phospholipids, proteins, cholesterol, and carbohydrate chains attached to proteins (glycoproteins) or lipids (glycolipids). These components each have specific roles in the membrane's function.
主要成分包括磷脂、蛋白质、胆固醇，以及附着在蛋白质上（糖蛋白）或脂质上（糖脂）的糖链。这些成分各自在膜的功能中扮演特定角色。
3. The Phospholipid Bilayer | 磷脂双分子层
A phospholipid molecule consists of a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty acid tails. In the bilayer, the hydrophilic heads face outwards towards the watery environments inside and outside the cell, while the hydrophobic tails point inwards, shielded from water.
一个磷脂分子由一个亲水（喜水）的磷酸头端和两个疏水（厌水）的脂肪酸尾端组成。在双分子层中，亲水头端朝外，面向细胞内外两侧的水环境，而疏水尾端朝内，远离水。
This arrangement forms a stable barrier to large, charged, or polar molecules. Small, non-polar molecules such as oxygen (O₂) and carbon dioxide (CO₂) can diffuse directly through the phospholipid bilayer.
这种排列形成了一道稳定的屏障，阻挡大分子、带电分子或极性分子。小的非极性分子，如氧气（O₂）和二氧化碳（CO₂），可以直接通过磷脂双分子层扩散。
4. Membrane Proteins | 膜蛋白
Proteins are scattered throughout the membrane. Some span the entire bilayer (integral proteins), while others are attached to one surface (peripheral proteins). Integral proteins often act as channels or carriers for facilitated diffusion and active transport.
蛋白质散布在整个膜中。有的贯穿整个双分子层（整合蛋白），有的附着在膜的一侧表面（外周蛋白）。整合蛋白常作为通道或载体，参与协助扩散和主动运输。
Channel proteins have a water-filled pore that allows specific ions or small polar molecules to pass through. Carrier proteins bind to a specific molecule, undergo a shape change, and shuttle it across the membrane.
通道蛋白含有一个充满水的孔道，允许特定的离子或小极性分子通过。载体蛋白与特定分子结合，发生形状变化，将其运送到膜的另一侧。
5. Cholesterol in Cell Membranes | 膜中的胆固醇
In animal cell membranes, cholesterol molecules are found between phospholipids. They bind to the hydrophobic tails, regulating membrane fluidity. At moderate temperatures, cholesterol reduces fluidity by restraining phospholipid movement; at low temperatures, it prevents the membrane from becoming too rigid by disrupting close packing.
在动物细胞膜中，胆固醇分子存在于磷脂之间。它们与疏水尾端结合，调节膜的流动性。在适中温度下，胆固醇通过限制磷脂移动来降低流动性；在低温下，它通过破坏紧密排列来防止膜变得过于僵硬。
Cholesterol also contributes to the mechanical stability of the membrane. Plant cell membranes contain similar sterols, but cholesterol is specifically an animal cell component.
胆固醇还有助于维持膜的机械稳定性。植物细胞膜含有类似的固醇类物质，但胆固醇特指动物细胞成分。
6. Selectively Permeable Barrier | 选择透过性屏障
The term 'selectively permeable' means the membrane discriminates between substances primarily based on size, charge, and lipid solubility. Very small uncharged molecules (e.g. O₂, CO₂, H₂O) diffuse freely; larger polar molecules (e.g. glucose, amino acids) require transport proteins.
“选择透过性”意味着膜主要根据分子大小、电荷和脂溶性来区分物质。极小的不带电分子（如 O₂、CO₂、H₂O）自由扩散；较大的极性分子（如葡萄糖、氨基酸）则需要转运蛋白。
Ions and charged particles cannot pass through the hydrophobic core unaided; they must move through channel proteins using facilitated diffusion or active transport. This selective control is vital for nerve impulses, muscle contraction, and nutrient uptake.
离子和带电粒子无法自行通过疏水核心；它们必须利用协助扩散或主动运输，经由通道蛋白移动。这种选择性控制对神经冲动、肌肉收缩和营养吸收至关重要。
7. Diffusion | 扩散
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).
扩散是粒子从高浓度区域向低浓度区域、沿浓度梯度的净移动。这是一个被动过程，不需要代谢能量（ATP）。
In the context of the cell membrane, simple diffusion occurs when small, non-polar molecules slip through the phospholipid bilayer. Facilitated diffusion uses channel or carrier proteins for larger or charged molecules, still down the concentration gradient and without energy input.
在细胞膜中，简单扩散发生在小的非极性分子直接穿过磷脂双分子层时。协助扩散则利用通道蛋白或载体蛋白来运输较大或带电的分子，仍然是顺着浓度梯度，不消耗能量。
8. Osmosis | 渗透
Osmosis is a special case of diffusion: the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.
渗透是扩散的一种特殊情况：水分子从水势较高的区域（稀溶液）通过部分透性膜向水势较低的区域（浓溶液）的净移动。
Water potential is the tendency of water to move out of a solution. Pure water has the highest water potential (set as 0 kPa); adding solutes lowers the water potential (more negative). Water moves from a less negative to a more negative water potential.
水势是水从溶液中移出的趋势。纯水具有最高的水势（定为 0 kPa）；加入溶质会降低水势（变得更负）。水会从较不负的水势区域向较负的水势区域移动。
9. Active Transport | 主动运输
Active transport is the movement of molecules or ions against their concentration gradient (from low to high concentration). This process requires energy in the form of ATP and the use of specific carrier proteins, often referred to as pumps.
主动运输是分子或离子逆着浓度梯度（从低浓度到高浓度）的移动。此过程需要以 ATP 形式存在的能量，并使用特定的载体蛋白，常被称为泵。
A classic example is the sodium-potassium pump in animal cells, which moves Na⁺ out of the cell and K⁺ into the cell, both against their gradients. In plants, active transport enables root hair cells to absorb mineral ions from the soil, where their concentration is lower.
一个经典的例子是动物细胞中的钠钾泵，它将 Na⁺ 运出细胞、将 K⁺ 运入细胞，两者都是逆浓度梯度。在植物中，主动运输使根毛细胞能够从浓度较低的土壤中吸收矿质离子。
10. Factors Affecting Membrane Transport | 影响跨膜运输的因素
Several factors influence the rate of diffusion, osmosis, and active transport across cell membranes. These are important to recall when explaining experimental results or real-life scenarios.
有几个因素会影响物质跨越细胞膜的扩散、渗透和主动运输速率。在解释实验结果或现实生活情境时，记住这些因素很重要。
| Factor | Effect on diffusion/osmosis | Effect on active transport |
|---|---|---|
| Temperature | Higher temperature increases kinetic energy, speeding up movement. Very high temperatures denature membrane proteins, increasing permeability but damaging the cell. | Rate increases with temperature up to optimum because enzyme-driven ATP production is enhanced; too high denatures carrier proteins. |
| Concentration gradient | The steeper the gradient, the faster the rate of diffusion/osmosis. | Not directly dependent on gradient; limited by number of carrier proteins and ATP availability. |
| Surface area to volume ratio | A larger surface area relative to volume increases the exchange rate. | More membrane surface allows more carrier proteins, increasing potential transport rate. |
| Distance (diffusion pathway) | The thinner the exchange surface, the shorter the diffusion distance, the faster the rate. | Not directly applicable; active transport is protein-mediated and less distance-dependent. |
Factors affecting diffusion rate can be summarised by Fick' sion is proportional to (surface area × concentration difference) ÷ thickness of membrane.
影响扩散速率的因素可以用菲克定律概括：扩散速率正比于（表面积 × 浓度差）÷ 膜厚度。
11. Endocytosis and Exocytosis | 胞吞与胞吐
Very large molecules, such as proteins or polysaccharides, cannot cross the membrane through carrier proteins. Instead, they are transported in bulk via endocytosis (entering the cell) and exocytosis (exiting the cell). These require ATP.
非常大的分子，如蛋白质或多糖，无法通过载体蛋白穿过膜。它们通过胞吞吐作用进行批量运输：胞吞（进入细胞）和胞吐（离开细胞）。这些过程需要 ATP。
During endocytosis, the cell membrane invaginates to form a vesicle around the material, which is then pinched off inside. Exocytosis is the reverse: a vesicle inside the cell fuses with the cell membrane and releases its contents outside.
在胞吞过程中，细胞膜内陷形成包裹物质的囊泡，然后脱离进入细胞内部。胞吐则相反：细胞内的囊泡与细胞膜融合，将其内容物释放到细胞外。
12. Required Practical: Osmosis Investigation | 实验：探究渗透作用
A key OCR practical involves investigating the effect of a range of solute concentrations on the mass of potato cylinders. Potato discs are placed in sucrose solutions of varying molarity (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³) and left for a set time. Their change in mass is recorded.
OCR 重要的实验包括探究一系列蔗糖浓度对土豆条质量的影响。将土豆条放入不同摩尔浓度的蔗糖溶液（如 0.0、0.2、0.4、0.6、0.8、1.0 mol/dm³）中，放置一段时间后记录质量变化。
If the potato gains mass, water has moved in by osmosis from a region of higher water potential (the solution) to a lower water potential (the potato cells). If it loses mass, water has moved out of the cells into the surrounding solution with a lower water potential.
如果土豆质量增加，说明水通过渗透从水势较高的区域（溶液）进入水势较低的细胞中。如果质量减少，说明水从细胞流出到水势更低的周围溶液中。
From a graph of percentage change in mass against concentration, the solute concentration where the line crosses zero change is an estimate of the osmotic potential inside the potato cells (isotonic point).
通过绘制质量变化百分比对浓度的关系图，曲线穿过零变化线对应的浓度即可估算土豆细胞内的渗透势（等渗点）。
Controlling variables such as temperature, surface area of cylinders, and blotting technique is essential for valid results.
控制变量如温度、土豆条表面积和吸干技术，对于获取有效结果至关重要。
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