Cell Membrane: Key Points for IGCSE WJEC Biology | 细胞膜考点精讲

📚 Cell Membrane: Key Points for IGCSE WJEC Biology | 细胞膜考点精讲

The cell membrane, also known as the plasma membrane, is a fundamental component of all living cells. It acts as a selective barrier, controlling what enters and leaves the cell. For IGCSE WJEC Biology, understanding the structure and function of the cell membrane is crucial for explaining processes like diffusion, osmosis, and active transport.

细胞膜，也称质膜，是所有活细胞的基本组成部分。它作为一个选择性屏障，控制物质进出细胞。对于IGCSE WJEC生物课程，理解细胞膜的结构和功能对于解释扩散、渗透和主动运输等过程至关重要。

1. Introduction to the Cell Membrane | 细胞膜简介

The cell membrane is found in all cells, both prokaryotic and eukaryotic. It is about 7.5 nanometres thick, which is too thin to be seen with a light microscope. The membrane surrounds the cytoplasm and separates the cell interior from the external environment. In WJEC IGCSE, you need to know that it is composed mainly of lipids and proteins, and that it is described by the fluid mosaic model.

细胞膜存在于所有细胞中，无论是原核生物还是真核生物。它的厚度约为7.5纳米，太薄以至于用光学显微镜无法观察到。细胞膜包围着细胞质，将细胞内部与外部环境隔开。在WJEC IGCSE中，你需要知道它主要由脂类和蛋白质组成，并用流动镶嵌模型来描述。

2. The Fluid Mosaic Model | 流动镶嵌模型

The fluid mosaic model was proposed by Jonathan Singer and Garth Nicolson in 1972. It describes the cell membrane as a flexible (‘fluid’) structure composed of a double layer of phospholipids, with various proteins floating like a mosaic within and on the surface. The phospholipid molecules can move laterally, giving the membrane its fluid nature. This model helps explain how the membrane is both stable and flexible, and how large molecules can be transported across it.

流动镶嵌模型由乔纳森·辛格和加斯·尼科尔森于1972年提出。它将细胞膜描述为由磷脂双分子层构成的灵活（“流动”）结构，各种蛋白质像马赛克一样漂浮在其中和表面上。磷脂分子可以侧向移动，赋予了膜的流动性。该模型有助于解释细胞膜如何既稳定又灵活，以及大分子如何穿过细胞膜进行运输。

Key features of the fluid mosaic model for WJEC: phospholipids form a bilayer; proteins are embedded (intrinsic) or on the surface (extrinsic); some phospholipids and proteins have carbohydrate chains attached, forming glycoproteins and glycolipids; cholesterol molecules are present in animal cell membranes, modulating fluidity.

WJEC考试中流动镶嵌模型的关键特征：磷脂形成双分子层；蛋白质嵌入（内在蛋白）或位于表面（外在蛋白）；一些磷脂和蛋白质附有糖链，形成糖蛋白和糖脂；胆固醇分子存在于动物细胞膜中，调节膜的流动性。

3. 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 aqueous environment of the cell, phospholipids arrange themselves into a bilayer: the hydrophilic heads face outward towards the water inside and outside the cell, while the hydrophobic tails face inward, away from water. This arrangement is fundamental to the selective permeability of the membrane.

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。在细胞的水环境中，磷脂自行排列成双分子层：亲水头端朝外，面向细胞内外两侧的水，而疏水尾端朝内，远离水。这种排列方式是细胞膜选择透过性的基础。

The phosphate head group is often represented as a circle labelled ‘hydrophilic head’, and the fatty acid tails as wavy lines labelled ‘hydrophobic tails’. In WJEC diagrams, you should be able to label these parts and explain why the bilayer forms spontaneously.

磷酸头端常被画成一个标有“亲水头”的圆圈，脂肪酸尾端则是标有“疏水尾”的波浪线。在WJEC的图表中，你应该能够标出这些部分并解释双层结构为何会自发形成。

4. Membrane Proteins | 膜蛋白质

Proteins are scattered throughout the phospholipid bilayer. They can be intrinsic (integral) proteins that span the entire membrane, or extrinsic (peripheral) proteins that are found only on one surface. Intrinsic proteins often act as channel proteins or carrier proteins for the transport of ions and large molecules. Extrinsic proteins may act as enzymes or be involved in cell signalling.

蛋白质散布在整个磷脂双分子层中。它们可以是贯穿整个膜的内在（整合）蛋白，也可以是仅存在于一侧表面的外在（周边）蛋白。内在蛋白通常作为通道蛋白或载体蛋白，用于运输离子和大分子。外在蛋白可能作为酶或参与细胞信号传递。

Carrier proteins change shape to move substances across the membrane during facilitated diffusion and active transport. Channel proteins form pores filled with water, allowing specific ions to pass through. The WJEC specification expects you to know that these proteins have specific binding sites and are complementary to the substances they transport.

载体蛋白在易化扩散和主动运输中改变形状以移动物质穿过细胞膜。通道蛋白形成充满水的孔道，允许特定离子通过。WJEC考纲要求你了解这些蛋白质具有特定的结合位点，并与它们运输的物质互补。

5. Cholesterol in Membranes | 细胞膜中的胆固醇

Cholesterol is a type of lipid found in the cell membranes of animal cells. It is not present in plant cell membranes. Cholesterol molecules fit between the phospholipids, restricting their movement. This reduces the fluidity of the membrane, making it more stable. Cholesterol also prevents the membrane from becoming too rigid at low temperatures by breaking up the regular packing of phospholipid tails. In WJEC, you may be asked to explain the role of cholesterol in maintaining membrane structure.

胆固醇是一种存在于动物细胞膜中的脂类。植物细胞膜中不含胆固醇。胆固醇分子嵌在磷脂之间，限制其运动。这降低了膜的流动性，使其更加稳定。胆固醇还能防止膜在低温下变得过于僵硬，因为它打乱了磷脂尾端的规则排列。在WJEC考试中，你可能需要解释胆固醇在维持膜结构中的作用。

6. Glycoproteins and Glycolipids | 糖蛋白与糖脂

Glycoproteins are membrane proteins with short carbohydrate chains attached on the outer surface. Glycolipids are phospholipids with attached carbohydrate chains. These carbohydrate chains form the glycocalyx, which acts as a recognition site for other cells and molecules. They are important in cell adhesion, cell signalling, and immunity. For WJEC, you should know that glycoproteins act as antigens, allowing the immune system to distinguish self from non-self, and as receptors for hormones and neurotransmitters.

糖蛋白是附着有短糖链的膜蛋白，位于外表面。糖脂是附着有糖链的磷脂。这些糖链构成糖萼，作为其他细胞和分子的识别位点。它们在细胞黏附、细胞信号和免疫中起重要作用。对于WJEC，你应该知道糖蛋白充当抗原，使免疫系统能够区分自身和非自身，并作为激素和神经递质的受体。

7. Selective Permeability | 选择透过性

The cell membrane is selectively permeable, meaning it allows some molecules to pass through but not others. Small, non-polar molecules such as oxygen (O₂) and carbon dioxide (CO₂) can diffuse freely through the phospholipid bilayer. Water (H₂O), though polar, is small enough to move through slowly or via channel proteins called aquaporins. Large polar molecules like glucose and charged ions such as Na⁺, K⁺, and Cl⁻ require transport proteins to cross the membrane.

细胞膜具有选择透过性，意味着它允许某些分子通过，而不允许其他分子通过。小的非极性分子，如氧气（O₂）和二氧化碳（CO₂），可以自由扩散通过磷脂双分子层。水（H₂O）虽然是有极性的，但分子足够小，可缓慢通过或通过名为水通道蛋白的通道蛋白穿过。大的极性分子如葡萄糖，以及带电离子如Na⁺、K⁺、Cl⁻需要转运蛋白才能穿过细胞膜。

8. Diffusion and Osmosis | 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 and does not require cellular energy (ATP). In WJEC, you must be able to describe how factors like temperature, concentration gradient, and surface area affect the rate of diffusion across the cell membrane.

扩散是粒子从较高浓度区域向较低浓度区域的净移动，沿着浓度梯度进行。这是一个被动过程，不需要细胞能量（ATP）。在WJEC中，你必须能够描述温度、浓度梯度和表面积等因素如何影响物质穿过细胞膜的扩散速率。

Osmosis is a special type of diffusion involving the movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane. Water potential is affected by solute concentration: more solute means lower water potential. WJEC questions often involve explaining what happens to animal cells in pure water (they swell and burst) and plant cells in concentrated salt solution (they become plasmolysed).

渗透是一种特殊的扩散，指水分子通过选择性透过膜从水势较高的区域向水势较低的区域移动。水势受溶质浓度影响：溶质越多，水势越低。WJEC考题经常要求解释动物细胞在纯水中（膨胀并破裂）和植物细胞在浓盐溶液中（发生质壁分离）会发生什么。

9. Active Transport | 主动运输

Active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient. This process requires energy in the form of ATP and involves carrier proteins. In WJEC IGCSE, the example of the sodium-potassium pump in nerve cells is commonly used. Three sodium ions (Na⁺) are pumped out of the cell while two potassium ions (K⁺) are pumped in, both against their gradients.

主动运输是分子或离子穿过细胞膜从较低浓度区域向较高浓度区域的移动，逆浓度梯度进行。该过程需要ATP形式的能量，并涉及载体蛋白。在WJEC IGCSE中，神经细胞中的钠钾泵是一个常用例子。三个钠离子（Na⁺）被泵出细胞，两个钾离子（K⁺）被泵入，两者均逆浓度梯度进行。

Another key example is the uptake of mineral ions by root hair cells from the dilute soil solution. The concentration of nitrate ions (NO₃⁻) is higher inside the root hair cell than in the soil, so active transport is needed to absorb them. This requires respiration and oxygen, so waterlogged soils with low oxygen reduce nutrient uptake.

另一个重要例子是根毛细胞从稀薄的土壤溶液中吸收矿物质离子。根毛细胞内硝酸根离子（NO₃⁻）的浓度高于土壤，因此需要主动运输来吸收。这需要呼吸作用提供氧气，所以缺氧的水涝土壤会降低养分吸收。

10. Factors Affecting Transport | 影响运输的因素

Several factors influence the rate of movement across the cell membrane. For diffusion and osmosis: a larger concentration gradient, higher temperature, greater surface area, and shorter diffusion distance all increase the rate. For WJEC, you might be asked to interpret graphs showing how concentration difference affects rate, and explain the plateau when carrier proteins become saturated in facilitated diffusion.

若干因素影响物质穿过细胞膜的速率。就扩散和渗透而言：更大的浓度梯度、更高的温度、更大的表面积和更短的扩散距离都会提高速率。在WJEC中，你可能需要解读显示浓度差如何影响速率的图表，并解释在易化扩散中载体蛋白饱和时速率趋于平缓的现象。

Active transport can be inhibited by anything that reduces cellular respiration, such as cyanide (a respiratory poison) or lack of oxygen. Temperature affects active transport because enzyme-controlled steps in respiration are temperature-sensitive. At extremes, membrane proteins denature and the phospholipid bilayer breaks down, increasing permeability. A practical investigation often uses beetroot to show how temperature or alcohol disrupts membranes, releasing red pigment.

主动运输会受到任何降低细胞呼吸作用因素的影响，如氰化物（呼吸毒物）或缺氧。温度影响主动运输，因为呼吸作用中酶控步骤对温度敏感。在极端条件下，膜蛋白会变性，磷脂双分子层被破坏，通透性增加。常用的实验探究是用甜菜根展示温度或酒精如何破坏细胞膜，导致红色素释放。

11. Practical Investigations | 实验探究

WJEC IGCSE may include questions on practicals related to the cell membrane. One classic experiment demonstrates osmosis using potato cylinders placed in different concentrations of sucrose solution. The change in mass or length shows whether water has entered or left the cells. Another practical uses Visking tubing, an artificial selectively permeable membrane, to model the gut. The tube filled with starch and glucose solution is placed in water; glucose diffuses out but starch does not, illustrating size-based selectivity.

WJEC IGCSE可能包含与细胞膜相关的实验问题。一个经典实验是使用马铃薯条放置在不同浓度的蔗糖溶液中来演示渗透作用。质量或长度的变化显示了水进入还是离开细胞。另一个实验使用Visking管（一种人工选择性透过膜）来模拟肠道。管内装满淀粉和葡萄糖溶液，放入水中；葡萄糖扩散出来，而淀粉不能，这展示了基于大小的选择性。

To investigate the effect of temperature on membrane permeability, beetroot discs are washed and placed in water at different temperatures. The absorbance of the water is measured after a set time using a colorimeter. Higher temperatures increase permeability, resulting in more pigment leakage and higher absorbance. This is because the phospholipids become more fluid and membrane proteins denature. You should be able to describe and explain this investigation for the WJEC exam.

为了研究温度对膜通透性的影响，将甜菜根圆片清洗后放入不同温度的水中。设定时间后使用比色计测量水的吸光度。温度越高，通透性越大，导致更多色素渗出和更高的吸光度。这是因为磷脂流动性增强，膜蛋白变性。你应该能够为WJEC考试描述并解释这一探究。

12. Summary and Key Points | 总结与考点

In summary, the cell membrane is a dynamic, selectively permeable barrier composed of a phospholipid bilayer with embedded proteins, cholesterol (in animals), and carbohydrate chains. The fluid mosaic model is the accepted description of its structure. Transport mechanisms include passive diffusion, facilitated diffusion (via channel or carrier proteins), osmosis, and active transport (requiring ATP). WJEC exam questions will often ask you to compare these processes, explain how structure relates to function, and interpret results from membrane-related experiments.

总之，细胞膜是一种动态的、选择性通透的屏障，由磷脂双分子层和嵌入的蛋白质、胆固醇（动物细胞）以及糖链组成。流动镶嵌模型是对其结构的公认描述。运输机制包括被动扩散、易化扩散（通过通道或载体蛋白）、渗透和主动运输（需要ATP）。WJEC考题经常会要求你比较这些过程，解释结构如何与功能相关，并解读与膜相关的实验结果。

Process	Diffusion	Osmosis	Facilitated Diffusion	Active Transport
Gradient	Down	Down water potential	Down	Against
Energy required	No	No	No	Yes (ATP)
Proteins involved	No	No (or aquaporins)	Yes (channel/carrier)	Yes (carrier)
Examples	O₂, CO₂	Water in/out of cells	Glucose, ions	Na⁺/K⁺ pump, root hair uptake

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