📚 GCSE CCEA Biology: Cell Membrane | GCSE CCEA 生物：细胞膜考点精讲
The cell membrane is a fundamental component of all living cells, acting as a selectively permeable barrier that controls the movement of substances into and out of the cell. For GCSE CCEA Biology, understanding the structure of the fluid mosaic model and how it facilitates diffusion, osmosis, and active transport is essential. This revision guide will cover key concepts, common exam questions, and practical investigations related to the cell membrane.
细胞膜是所有活细胞的基本组成部分，作为一层选择透过性屏障，控制物质进出细胞。在 GCSE CCEA 生物学中，理解流动镶嵌模型的结构以及它如何促进扩散、渗透和主动运输至关重要。本考点精讲将涵盖关键概念、常见考试题型以及与细胞膜相关的实验探究。
1. Structure and Components of the Cell Membrane | 细胞膜的结构与组成
The cell membrane is described by the fluid mosaic model, where a phospholipid bilayer forms the basic fabric. Embedded within this bilayer are proteins, cholesterol molecules (in animal cells), and carbohydrate chains attached to proteins or lipids.
细胞膜被描述为流动镶嵌模型，其中磷脂双分子层构成基本框架。嵌在双分子层中的有蛋白质、胆固醇分子（动物细胞中）以及附着在蛋白质或脂质上的糖链。
The term ‘fluid’ refers to the ability of phospholipids to move laterally within their layer, giving the membrane flexibility. ‘Mosaic’ describes the patchwork of proteins floating in the phospholipid sea.
“流动”一词指磷脂分子可在其单层内横向移动，使细胞膜具有柔韧性。“镶嵌”则形容蛋白质像漂浮在磷脂海洋中的补丁。
2. The Phospholipid Bilayer | 磷脂双分子层
Each phospholipid molecule consists of a hydrophilic (water‑loving) phosphate head and two hydrophobic (water‑fearing) fatty acid tails. The heads face outward toward the aqueous environments inside and outside the cell, while the tails hide in the interior, away from water.
每个磷脂分子由一个亲水（喜水）的磷酸头部和两条疏水（厌水）的脂肪酸尾部组成。头部朝向细胞内外两侧的水环境，尾部则藏在内部，避开水分。
This spontaneous arrangement forms a stable, self‑sealing bilayer that is the foundation of all cell membranes. Small, non‑polar molecules such as O₂ and CO₂ can pass directly through the bilayer, but polar or charged substances cannot.
这种自发排列形成了一个稳定、能自我修复的双分子层，是所有细胞膜的基础。小而非极性的分子（如 O₂ 和 CO₂）可以直接穿过双分子层，但极性或带电的物质则不能。
3. Membrane Proteins | 膜蛋白
Proteins embedded in the membrane serve many functions. Intrinsic (integral) proteins span the whole bilayer, while extrinsic (peripheral) proteins are found on the surface. Channel proteins and carrier proteins are intrinsic proteins that assist the movement of specific substances across the membrane.
嵌在膜中的蛋白质有许多功能。内在蛋白（整合蛋白）贯穿整个双分子层，而外在蛋白（周边蛋白）位于表面。通道蛋白和载体蛋白都是内在蛋白，它们协助特定物质穿过细胞膜。
Channel proteins form pores that allow ions or water to diffuse through. Carrier proteins bind to specific solutes, change shape, and release them on the other side; this is crucial for facilitated diffusion and active transport.
通道蛋白形成孔道，让离子或水分子扩散通过。载体蛋白与特定溶质结合，改变形状，并在另一侧将其释放；这对应促进扩散和主动运输至关重要。
4. Cholesterol in the Membrane | 胆固醇在细胞膜中的作用
In animal cell membranes, cholesterol molecules are inserted between phospholipid tails. Cholesterol regulates membrane fluidity: it prevents the fatty acid tails from packing too closely at low temperatures, keeping the membrane fluid, and restricts excessive movement at high temperatures, maintaining stability.
在动物细胞膜中，胆固醇分子嵌入在磷脂尾部之间。胆固醇可以调节膜的流动性：低温时防止脂肪酸尾部聚集过密，从而保持膜的流动性；高温时限制过度运动，维持稳定性。
Plant cell membranes generally lack cholesterol but contain other sterols. The presence of cholesterol helps animal cells maintain a consistent barrier function across a range of temperatures.
植物细胞膜通常缺乏胆固醇，但含有其他固醇。胆固醇的存在有助于动物细胞在一定温度范围内维持稳定的屏障功能。
5. Glycoproteins and Glycolipids | 糖蛋白与糖脂
Short carbohydrate chains attach to proteins (forming glycoproteins) or to lipids (forming glycolipids) on the outer surface of the cell membrane. These carbohydrate projections form the glycocalyx and play a key role in cell recognition and communication.
短糖链附着在蛋白质上（形成糖蛋白）或脂质上（形成糖脂），位于细胞膜外表面。这些糖类突起构成糖萼，在细胞识别与通讯中发挥关键作用。
Glycoproteins act as receptors for hormones and other signalling molecules, and they can serve as antigens that allow the immune system to distinguish ‘self’ from ‘non‑self’. For example, the ABO blood group system is determined by different carbohydrate structures on the surface of red blood cells.
糖蛋白充当激素和其他信号分子的受体，并且可作为抗原，让免疫系统区分“自身”与“非自身”。例如，ABO 血型系统就是由红细胞表面不同的糖类结构决定的。
6. Selectively Permeable Nature | 选择透过性
Cell membranes are selectively permeable (partially permeable), meaning they allow some substances to cross but not others. This property is essential for maintaining the internal environment of the cell.
细胞膜具有选择透过性（部分通透性），即允许某些物质穿过而阻止另一些物质。这一特性对于维持细胞内部环境至关重要。
Small, non‑polar molecules and water can diffuse freely through the bilayer, while ions and larger polar molecules require transport proteins. The membrane’s hydrophobic core acts as a barrier to most water‑soluble particles.
小而非极性的分子和水可以自由扩散穿过双分子层，而离子和较大的极性分子则需要转运蛋白。细胞膜的疏水核心对大多数水溶性颗粒起到了屏障作用。
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 energy from the cell.
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。这是一种被动过程，不需要细胞提供能量。
Substances such as oxygen and carbon dioxide cross the alveolar and capillary walls by diffusion. The rate of diffusion is affected by several factors, described by Fick’s law.
氧气和二氧化碳等物质通过扩散穿过肺泡壁和毛细血管壁。扩散速率受多个因素影响，可用菲克定律描述。
Rate of diffusion ∝ (surface area × concentration difference) ÷ distance
扩散速率 ∝ (表面积 × 浓度差) ÷ 距离
8. Osmosis | 渗透作用
Osmosis is the net movement of water molecules through a selectively permeable membrane from a region of higher water potential to a region of lower water potential. Water potential (ψ) is a measure of the tendency of water to leave a solution; pure water has the highest water potential (0 kPa), and solutions have negative ψ values.
渗透作用是水分子通过选择透过性膜，从较高水势区域向较低水势区域的净移动。水势 (ψ) 是衡量水离开溶液趋势的指标；纯水的水势最高 (0 kPa)，溶液的水势为负值。
In a hypotonic solution, water enters an animal cell, causing it to swell and possibly lyse (burst). In a hypertonic solution, water leaves, causing the cell to shrivel (crenation).
在低渗溶液中，水进入动物细胞，使其膨胀甚至破裂（溶血）。在高渗溶液中，水离开，导致细胞皱缩（皱缩）。
A summary of osmotic effects on cells is shown below.
不同渗透压对细胞的影响总结如下。
| Solution | Animal Cell (e.g. red blood cell) | Plant Cell (e.g. epidermal cell) |
|---|---|---|
| Hypotonic (low solute, high water potential) | Water enters, cell swells and may burst (lysis). | Water enters, vacuole fills, cell becomes turgid. No bursting due to cell wall. |
| Isotonic | No net water movement; cell normal. | No net movement; cell flaccid (may become plasmolyzed if water lost). |
| Hypertonic (high solute, low water potential) | Water leaves, cell shrinks (crenation). | Water leaves, cytoplasm shrinks, cell membrane pulls away from cell wall (plasmolysis). |
The table summarises the effects of different solutions on animal and plant cells. In a hypotonic solution, animal cells risk lysis, while plant cells become turgid, which is essential for support. In a hypertonic solution, animal cells crenate and plant cells undergo plasmolysis, where the plasma membrane detaches from the cell wall.
上表总结了不同溶液对动物和植物细胞的影响。在低渗溶液中，动物细胞有破裂风险，而植物细胞变得硬挺，这对支撑很重要。在高渗溶液中，动物细胞皱缩，植物细胞发生质壁分离，即细胞膜与细胞壁分离。
9. Active Transport | 主动运输
Active transport is the movement of substances against a concentration gradient, from a lower to a higher concentration. This process requires energy, released from ATP, and is carried out by specific carrier proteins.
主动运输是物质逆浓度梯度、从较低浓度向较高浓度的移动。这一过程需要能量（来自 ATP），并由特定的载体蛋白执行。
Examples of active transport include the uptake of mineral ions (e.g. nitrate, K⁺) by root hair cells from the dilute soil solution, and the absorption of glucose and amino acids in the small intestine against a concentration gradient.
主动运输的例子包括根毛细胞从稀薄的土壤溶液中吸收矿物质离子（如硝酸盐、K⁺），以及小肠逆浓度梯度吸收葡萄糖和氨基酸。
Inhibiting respiration (e.g. with a metabolic poison or lack of oxygen) stops active transport, because ATP is not produced.
抑制呼吸作用（例如使用代谢抑制剂或缺氧）会阻止主动运输，因为无法产生 ATP。
10. Factors Affecting the Rate of Movement | 影响运输速率的因素
Several factors influence how quickly substances can diffuse or be transported across the cell membrane. A greater surface area, steeper concentration gradient, and higher temperature (within limits) all increase the rate of diffusion.
多种因素影响物质穿过细胞膜的扩散或运输速率。表面积更大、浓度梯度更陡、温度更高（在一定范围内）都会提高扩散速率。
For osmosis, the water potential gradient is the driving force. In active transport, the rate depends on the number of available carrier proteins and the supply of ATP.
对于渗透作用，水势梯度是驱动力。主动运输的速率取决于可用载体蛋白的数量和 ATP 的供应。
Increasing the temperature initially raises kinetic energy and speeds up diffusion, but excessively high temperatures can denature membrane proteins and damage the phospholipid bilayer, making the membrane fully permeable.
升高温度起初会增加动能、加快扩散，但温度过高会使膜蛋白变性、破坏磷脂双分子层，导致细胞膜完全通透。
11. Investigating Diffusion and Osmosis | 探究扩散和渗透作用的实验
A classic practical is using a Visking (dialysis) tube to model a selectively permeable membrane. The tube is
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