IGCSE Edexcel Biology: Cell Membranes – Key Points | IGCSE Edexcel 生物：细胞膜考点精讲

📚 IGCSE Edexcel Biology: Cell Membranes – Key Points | IGCSE Edexcel 生物：细胞膜考点精讲

The cell membrane is a vital structure that surrounds all cells, controlling the movement of substances in and out. In IGCSE Edexcel Biology, understanding its structure and function is key to explaining processes like diffusion, osmosis, and active transport. This article provides a detailed, bilingual breakdown of the cell membrane, tailored to the Edexcel specification.

细胞膜是包围所有细胞的重要结构，控制物质的进出。在 IGCSE Edexcel 生物学中，理解其结构和功能是解释扩散、渗透和主动运输等过程的关键。本文根据 Edexcel 考试大纲，提供细胞膜考点的中英双语详解。

1. What is a Cell Membrane? | 什么是细胞膜？

The cell membrane, also called the plasma membrane, is a thin, flexible barrier around every cell. It separates the cell’s interior from the external environment, maintaining homeostasis and enabling communication with other cells.

细胞膜，也称质膜，是包围每个细胞的薄而柔韧的屏障。它将细胞内部与外界环境分开，维持稳态，并使细胞能够与其他细胞通信。

All cell membranes share a common basic structure: they are composed mainly of lipids and proteins. The IGCSE syllabus emphasises the fluid mosaic model, which describes how these components are arranged.

所有细胞膜都有一个共同的基本结构：主要由脂类和蛋白质组成。IGCSE 大纲强调流动镶嵌模型，该模型描述了这些成分是如何排列的。

The membrane is partially permeable (semipermeable), meaning it allows only certain molecules to pass through. This property is critical for the selective transport of substances.

细胞膜具有部分通透性（半透性），意味着它只允许某些分子通过。这种性质对于物质的选择性运输至关重要。

2. The Fluid Mosaic Model | 流动镶嵌模型

The fluid mosaic model is the accepted description of cell membrane structure. The term ‘fluid’ refers to the ability of lipid and protein molecules to move laterally within the layer, while ‘mosaic’ describes the patchy arrangement of proteins embedded in the lipid bilayer.

流动镶嵌模型是公认的细胞膜结构描述。“流动”指的是脂质和蛋白质分子在层内横向移动的能力，“镶嵌”则描述了蛋白质嵌在脂质双分子层中的斑驳排列。

According to this model, the membrane is not rigid; phospholipids and some proteins can drift freely. This fluidity is essential for functions like endocytosis and cell signalling.

根据该模型，膜不是刚性的；磷脂和一些蛋白质可以自由漂移。这种流动性对于内吞作用和细胞信号转导等功能至关重要。

The mosaic appearance comes from protein molecules that float in or on the bilayer. These proteins have diverse roles, including transport, enzymatic activity, and cell recognition.

镶嵌外观来源于漂浮在双分子层内或上的蛋白质分子。这些蛋白质具有多种作用，包括运输、酶活性和细胞识别。

Cholesterol molecules are also present in animal cell membranes, wedged between phospholipids. They contribute to membrane fluidity and stability, preventing the membrane from becoming too fluid or too rigid at different temperatures.

动物细胞膜中还含有胆固醇分子，插在磷脂之间。它们有助于膜的流动性和稳定性，防止膜在不同温度下变得过软或过僵硬。

3. Phospholipid Bilayer Structure | 磷脂双分子层结构

A phospholipid molecule has a hydrophilic (water-loving) head and two hydrophobic (water-hating) fatty acid tails. This dual nature is described as amphipathic.

一个磷脂分子有一个亲水（喜水）的头部和两条疏水（憎水）的脂肪酸尾部。这种双重性质被称为两亲性。

In water, phospholipids naturally arrange into a bilayer: the hydrophilic heads face outwards, towards the watery environments inside and outside the cell, while the hydrophobic tails point inwards, shielded from water. This arrangement forms a stable barrier.

在水中，磷脂自然排列成双分子层：亲水头部朝外，面向细胞内外两侧的水环境，而疏水尾部朝内，避开水。这种排列形成了一个稳定的屏障。

The bilayer is a fluid structure, with individual phospholipids constantly moving sideways. Small, non-polar molecules like oxygen and carbon dioxide can diffuse directly through the bilayer, but larger or polar molecules cannot easily pass through this hydrophobic core.

双分子层是一种流动结构，单个磷脂不断侧向移动。小而非极性的分子（如氧气和二氧化碳）可以直接通过双分子层扩散，但较大或极性的分子难以通过这个疏水核心。

The phospholipid bilayer therefore forms the fundamental selectively permeable barrier of the membrane. Its properties are essential for compartmentalisation within cells.

因此，磷脂双分子层构成了细胞膜的基本选择透过性屏障。它的性质对于细胞内的区室化至关重要。

4. Membrane Proteins: Channels and Carriers | 膜蛋白：通道蛋白与载体蛋白

Proteins embedded in the cell membrane perform most of the specific functions. There are two main types involved in transport: channel proteins and carrier proteins.

嵌入细胞膜的蛋白质执行大部分特定功能。参与运输的主要有两类：通道蛋白和载体蛋白。

Channel proteins form hydrophilic pores that allow specific ions or small molecules to pass through by facilitated diffusion, down their concentration gradient. These channels can be gated, opening or closing in response to signals.

通道蛋白形成亲水孔道，允许特定离子或小分子通过促进扩散沿浓度梯度下降方向通过。这些通道可以是门控的，根据信号开启或关闭。

Carrier proteins bind to a specific solute and undergo a shape change to shuttle the molecule across the membrane. Like channels, they can function in facilitated diffusion, but they are also essential for active transport when energy is supplied.

载体蛋白与特定的溶质结合，并发生形状变化以将该分子转运过膜。与通道一样，它们可以在促进扩散中发挥作用，但当提供能量时，它们对于主动运输也必不可少。

Other membrane proteins act as enzymes, such as those in mitochondrial membranes, or as receptors for hormones and neurotransmitters. These are not directly tested in the basic membrane transport section but provide context.

其他膜蛋白充当酶，例如线粒体膜中的那些，或作为激素和神经递质的受体。这些在基本的膜运输部分不直接考查，但提供了背景。

5. The Role of Cholesterol | 胆固醇的作用

Cholesterol is a type of lipid found in animal cell membranes. It is positioned between phospholipid molecules, with its hydroxyl group near the hydrophilic heads and its rigid ring structure interacting with the hydrophobic tails.

胆固醇是动物细胞膜中的一种脂类。它位于磷脂分子之间，其羟基靠近亲水头部，刚性的环状结构与疏水尾部相互作用。

Cholesterol modulates membrane fluidity. At high temperatures, it restrains phospholipid movement, reducing fluidity and preventing the membrane from becoming too leaky. At low temperatures, it prevents phospholipids from packing too closely together, maintaining fluidity and preventing the membrane from solidifying.

胆固醇调节膜的流动性。在高温下，它限制磷脂运动，降低流动性，防止膜变得过于渗漏。在低温下，它防止磷脂过于紧密堆积，保持流动性，防止膜凝固。

Cholesterol also increases the mechanical stability of the membrane. This is why animal cells, which lack a rigid cell wall, rely on cholesterol to maintain membrane integrity under stress.

胆固醇还增加膜的机械稳定性。这就是为什么缺乏刚性细胞壁的动物细胞依赖胆固醇在压力下维持膜的完整性。

Plant cell membranes generally do not contain cholesterol but have other sterols. However, the IGCSE specification focuses mainly on animal cells for this concept.

植物细胞膜通常不含胆固醇，而含有其他甾醇。不过，IGCSE 大纲在这一概念上主要关注动物细胞。

6. Glycoproteins and Glycolipids | 糖蛋白与糖脂

Glycoproteins and glycolipids are molecules with carbohydrate chains attached to membrane proteins or phospholipids. They are found on the extracellular side of the membrane only, forming a sugar coating known as the glycocalyx.

糖蛋白和糖脂是带有与膜蛋白或磷脂相连的碳水化合物链的分子。它们仅存在于细胞膜外侧，形成称为糖萼的糖衣。

These carbohydrate chains play a vital role in cell recognition and cell-cell adhesion. They act as identification tags, allowing cells of the immune system to distinguish ‘self’ from ‘non-self’—a crucial concept in organ transplantation and blood grouping.

这些碳水化合物链在细胞识别和细胞间黏附中起着至关重要的作用。它们充当识别标签，使免疫系统细胞能够区分“自己”和“非己”——这是器官移植和血型中的关键概念。

Glycoproteins also serve as receptors for certain substances. For example, the binding of hormones to glycoprotein receptors triggers cellular responses.

糖蛋白还可作为某些物质的受体。例如，激素与糖蛋白受体的结合会触发细胞反应。

While not a huge focus in the core IGCSE transport topics, their role in immunity links to later chapters. Exam questions may ask you to relate structure to function.

虽然它们在 IGCSE 核心运输主题中不是重点，但其在免疫中的作用与后续章节相关。考题可能会要求你将结构与功能联系起来。

7. Selective Permeability Explained | 选择透过性解析

Selective permeability (or semi-permeability) means the membrane allows some substances to cross but not others. This property results directly from the structure of the phospholipid bilayer and the transport proteins embedded in it.

选择透过性（或半透性）意味着膜允许某些物质通过，而不允许其他物质通过。这一性质直接源于磷脂双分子层的结构以及嵌入其中的运输蛋白。

Small, non-polar molecules such as O₂, CO₂, and steroid hormones can dissolve in the hydrophobic core and diffuse freely. Very small polar molecules like water and urea can also pass through, albeit slowly, possibly through transient gaps in the bilayer.

小型非极性分子，如 O₂、CO₂ 和类固醇激素，可以溶解在疏水核心中并自由扩散。非常小的极性分子，如水和尿素，也能通过，尽管较慢，可能通过双分子层中的瞬时缝隙。

Larger polar molecules (e.g., glucose, amino acids) and ions (e.g., Na⁺, K⁺, Cl⁻) cannot easily cross the hydrophobic core. They require the help of specific channel or carrier proteins, a process known as facilitated diffusion or active transport.

较大的极性分子（如葡萄糖、氨基酸）和离子（如 Na⁺、K⁺、Cl⁻）无法轻易穿过疏水核心。它们需要特定的通道或载体蛋白的帮助，这一过程称为促进扩散或主动运输。

This selective nature ensures the cell can control its internal composition—maintaining different concentrations of ions and molecules inside compared to the outside. This is fundamental for nerve impulses, muscle contraction, and metabolic reactions.

这种选择性质确保了细胞能够控制其内部组成——维持内外不同的离子和分子浓度。这对于神经冲动、肌肉收缩和代谢反应是基础性的。

8. Diffusion – Passive Movement | 扩散——被动运动

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, meaning no metabolic energy (ATP) is required.

扩散是粒子从较高浓度区域向较低浓度区域的净移动，沿浓度梯度下降。这是一个被动过程，意味着不需要代谢能量（ATP）。

Substances that can cross the membrane by simple diffusion include oxygen and carbon dioxide. These diffuse directly through the phospholipid bilayer. The rate of diffusion depends on factors like concentration gradient, temperature, distance, and surface area.

可以通过简单扩散穿过膜的物质包括氧气和二氧化碳。它们直接通过磷脂双分子层扩散。扩散速率取决于浓度梯度、温度、距离和表面积等因素。

Facilitated diffusion uses channel or carrier proteins to transport substances that cannot diffuse through the bilayer, such as glucose and ions. It is still passive, as movement is down the concentration gradient, but it depends on the presence and specificity of the proteins.

促进扩散利用通道或载体蛋白运输不能通过双分子层扩散的物质，如葡萄糖和离子。它依然是被动的，因为移动是顺浓度梯度的，但它依赖于蛋白质的存在和特异性。

In IGCSE exams, you may be asked to plot or interpret graphs showing the effect of concentration difference on rate of uptake, distinguishing between simple and facilitated diffusion by noting the saturation point of protein carriers.

在 IGCSE 考试中，你可能需要绘制或解读显示浓度差对吸收速率影响的图表，并通过注意蛋白质载体的饱和点来区分简单扩散和促进扩散。

9. Osmosis – A Special Case of Diffusion | 渗透——扩散的特殊情况

Osmosis is the net movement of water molecules from a region of higher water potential (less concentrated solution) to a region of lower water potential (more concentrated solution) through a partially permeable membrane.

渗透是水分子通过部分通透膜从较高水势区域（较稀溶液）向较低水势区域（较浓溶液）的净移动。

Water potential is a measure of the tendency of water to leave a system. Pure water has the highest water potential (zero, by convention). Adding solutes lowers the water potential, making it more negative. Water always moves towards the more negative water potential.

水势是衡量水离开系统的趋势的量度。纯水具有最高的水势（按惯例为零）。添加溶质会降低水势，使其变得更负。水总是向更负的水势方向移动。

In plant cells, osmosis causes cells to become turgid when placed in a dilute solution, giving support to the plant. In a concentrated solution, plant cells become flaccid, and the cytoplasm pulls away from the cell wall—a process called plasmolysis.

在植物细胞中，当置于稀溶液中时，渗透使细胞变得硬挺，为植物提供支持。在浓溶液中，植物细胞变得松软，细胞质从细胞壁拉开——这一过程称为质壁分离。

Animal cells lack a cell wall, so in a dilute solution they may swell and burst (lysis), while in a concentrated solution they shrink (crenation). Osmosis is vital for water uptake in roots and for maintaining cell turgor.

动物细胞没有细胞壁，因此在稀溶液中可能膨胀并破裂（溶解），而在浓溶液中会皱缩（皱缩）。渗透对于根部吸水和维持细胞硬挺至关重要。

10. Active Transport – Against the Gradient | 主动运输——逆浓度梯度

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.

主动运输是分子或离子逆浓度梯度从较低浓度区域向较高浓度区域穿过细胞膜的移动。此过程需要 ATP 形式的能量。

Carrier proteins involved in active transport are often called pumps. A well-known example is the sodium-potassium pump, which moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.

参与主动运输的载体蛋白常被称为泵。一个著名的例子是钠钾泵，它将钠离子移出细胞，将钾离子移入细胞，两者都逆浓度梯度。

In the IGCSE syllabus, you must know the example of active uptake of mineral ions by root hair cells. The concentration of minerals is usually higher inside the root than in the soil, yet cells continue to absorb ions using active transport.

在 IGCSE 大纲中，你必须知道根毛细胞主动吸收矿物离子的例子。根内部的矿物质浓度通常高于土壤中，但细胞仍然利用主动运输继续吸收离子。

Active transport is also critical in the human gut for the absorption of glucose from the lumen of the small intestine into the blood, even when glucose concentration in the blood is already higher.

主动运输在人体肠道中也至关重要，用于从小肠管腔将葡萄糖吸收到血液中，即使血液中的葡萄糖浓度已经更高。

Because active transport uses energy, any factor that stops respiration (such as lack of oxygen or cyanide poisoning) will inhibit it. This distinguishes it sharply from diffusion and osmosis.

由于主动运输消耗能量，任何停止呼吸作用的因素（如缺氧或氰化物中毒）都会抑制它。这使其明显区别于扩散和渗透。

11. Factors Affecting Membrane Permeability | 影响膜通透性的因素

The permeability of cell membranes can be altered by environmental conditions. Common IGCSE experiments investigate the effect of temperature and organic solvents like ethanol on membrane permeability, often using beetroot cells.

细胞膜的通透性可以被环境条件改变。常见的 IGCSE 实验研究温度和有机溶剂（如乙醇）对膜通透性的影响，常使用甜菜根细胞。

As temperature increases, kinetic energy of phospholipids and proteins increases, making the membrane more fluid and more permeable. However, very high temperatures cause proteins to denature and phospholipid bilayers to become too leaky, allowing pigments or contents to escape.

随着温度升高，磷脂和蛋白质的动能增加，使膜更具流动性且通透性更高。然而，非常高的温度会引起蛋白质变性，磷脂双分子层变得过于渗漏，导致色素或内容物泄漏。

Organic solvents like ethanol dissolve lipids, disrupting the phospholipid bilayer structure and increasing permeability. This effect is concentration-dependent: higher ethanol concentrations cause more damage.

乙醇等有机溶剂能溶解脂类，破坏磷脂双分子层结构，增加通透性。这种效应与浓度有关：乙醇浓度越高，造成的损害越大。

pH extremes can also denature membrane proteins and affect lipid structure, altering permeability. In practical assessments, you may be asked to plan an investigation into one of these factors, controlling variables and measuring absorbance of leaked pigment.

极端 pH 值也会使膜蛋白变性，并影响脂质结构，从而改变通透性。在实验考核中，你可能会被要求设计一个探究上述某一因素的实验，控制变量并测量泄漏色素的吸光度。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

Always use precise terminology: ‘partially permeable membrane’ or ‘selectively permeable’ rather than vague terms. Distinguish clearly between diffusion, facilitated diffusion, osmosis, and active transport based on energy requirement and gradient direction.

务必使用精确的术语：“部分通透膜”或“选择透过性”，而非模糊的用语。根据能量需求和梯度方向，清楚地区分扩散、促进扩散、渗透和主动运输。

When explaining osmosis, mention water potential, not just concentration. Say ‘from high water potential to low water potential’ rather than ‘from weak to strong solution’, the latter is less precise for IGCSE level.

解释渗透时，要提及水势，而不仅仅是浓度。说“从高水势到低水势”，而不是“从稀溶液到浓溶液”，后者在 IGCSE 水平上不够精确。

In active transport descriptions, always state that energy from respiration (ATP) is used, and that movement is against the concentration gradient. Do not confuse carrier proteins used in facilitated diffusion with those used in active transport—both are specific, but only one uses energy.

在描述主动运输时，始终要说明使用了来自呼吸作用的能量（ATP），并且移动是逆浓度梯度的。不要将促进扩散中使用的载体蛋白与主动运输中使用的载体蛋白混淆——两者都具有特异性，但只有一种消耗能量。

For experiments, remember the key variables: temperature (controlled with a water bath), ethanol concentration, time, and the use of a colorimeter to measure the release of pigment. Practice plotting and interpreting graphs showing a threshold temperature beyond which permeability sharply increases.

对于实验，记得关键变量：温度（用水浴控制）、乙醇浓度、时间，以及使用比色计测量色素释放量。练习绘制和解读显示超出某阈值温度后通透性急剧上升的图表。

Finally, connect structure to function: the hydrophobic core prevents free passage of most solutes, proteins provide routes for specific substances, and cholesterol stabilises the membrane. Such links often gain full marks in longer questions.

最后，将结构与功能联系起来：疏水核心阻止大多数溶质自由通过，蛋白质为特定物质提供通道，胆固醇稳定膜。这种联系在较长的题目中往往能获得满分。

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IGCSE Edexcel Biology: Cell Membranes – Key Points | IGCSE Edexcel 生物：细胞膜 考点精讲