📚 IB Biology: Cell Membrane Key Points | IB 生物：细胞膜 考点精讲
Mastering the cell membrane is essential for IB Biology, as it underpins topics from transport to cell communication. This article breaks down every key concept you need for the exam, with clear explanations in both English and Chinese.
掌握细胞膜是 IB 生物学的关键，它贯穿物质运输到细胞通讯等多个主题。本文以中英双语逐项拆解你必须掌握的考点，助你高效备考。
1. Structure and Function Overview | 结构与功能概述
The cell membrane is a selectively permeable barrier that surrounds the cell, separating internal contents from the external environment. It maintains homeostasis by controlling the movement of substances and facilitates cell recognition and signalling.
细胞膜是包围细胞的半透性屏障，将细胞内部与外部环境分隔开。它通过控制物质的进出维持稳态，并参与细胞识别和信号传导。
2. Phospholipid Bilayer | 磷脂双分子层
The fundamental structure of the membrane is a phospholipid bilayer. Each phospholipid has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. In an aqueous environment, they arrange themselves with heads facing outward and tails inward, forming a stable boundary.
细胞膜的基本结构是磷脂双分子层。每个磷脂分子有一个亲水的磷酸头端和两个疏水的脂肪酸尾端。在水环境中，磷脂排列成头部朝外、尾部朝内的双层，形成一个稳定的边界。
Phospholipids are amphipathic – they possess both hydrophilic and hydrophobic regions.
磷脂是双亲性分子，同时具有亲水区和疏水区。
The bilayer is fluid, allowing lateral movement of phospholipids and proteins.
双分子层具有流动性，允许磷脂和蛋白质侧向移动。
3. Membrane Proteins | 膜蛋白
Proteins are embedded within or attached to the phospholipid bilayer. Integral proteins span the entire membrane, while peripheral proteins are located on one side only. These proteins serve as channels, carriers, receptors, or enzymes.
蛋白质镶嵌或附着在磷脂双分子层上。整合蛋白贯穿整个膜，周边蛋白只位于膜的一侧。这些蛋白质充当通道、载体、受体或酶。
| Protein Type 蛋白类型 | Function 功能 |
|---|---|
| Channel proteins | Provide a hydrophilic pore for passive transport of specific ions or molecules. |
| Carrier proteins | Bind to a specific solute and undergo a conformational change to transport it. |
| Receptor proteins | Bind to signalling molecules (ligands) and trigger intracellular responses. |
| Glycoproteins | Proteins with carbohydrate chains; key for cell–cell recognition and adhesion. |
Channel proteins and carrier proteins are involved in facilitated diffusion and active transport. Glycoproteins form the glycocalyx, which protects the cell and enables immune recognition.
通道蛋白和载体蛋白参与协助扩散和主动运输。糖蛋白形成糖萼，保护细胞并实现免疫识别。
4. Fluid Mosaic Model | 流动镶嵌模型
The currently accepted model of membrane structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972. It describes the membrane as a mosaic of phospholipid molecules, proteins, and cholesterol, all moving fluidly within the layer.
目前公认的膜结构模型是流动镶嵌模型，由 Singer 和 Nicolson 于 1972 年提出。该模型将膜描述为磷脂、蛋白质和胆固醇分子动态排列的镶嵌结构，各组分可在层内流动。
'Fluid' refers to the lateral movement of phospholipids and many proteins; 'mosaic' refers to the patchwork of different proteins embedded in the bilayer.
“流动”指磷脂和许多蛋白质的侧向运动；“镶嵌”指不同蛋白质嵌入双分子层形成的不规则图案。
Experimental evidence: freeze-fracture electron microscopy revealed particles (proteins) embedded within the lipid bilayer.
实验证据：冷冻断裂电镜显示脂双层内嵌有颗粒（蛋白质）。
5. Cholesterol and Membrane Fluidity | 胆固醇与膜的流动性
In animal cell membranes, cholesterol molecules are positioned between phospholipids. Cholesterol moderates membrane fluidity: at high temperatures it restrains excessive movement, while at low temperatures it prevents tight packing of phospholipids, maintaining fluidity.
在动物细胞膜中，胆固醇分子位于磷脂之间。胆固醇调节膜的流动性：在高温下它限制过度运动，在低温下它阻止磷脂紧密堆积，保持膜的流动。
Cholesterol reduces membrane permeability to small water-soluble molecules.
胆固醇降低膜对小分子水溶性物质的通透性。
Plant cell membranes rely on phytosterols instead of cholesterol for a similar role.
植物细胞膜则依靠植物甾醇发挥类似作用。
6. Passive Transport: Diffusion | 被动运输：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 ATP. Small, nonpolar molecules (e.g., O₂, CO₂) and very small polar molecules (e.g., urea) can cross the membrane by simple diffusion.
扩散是指粒子从高浓度区域向低浓度区域的净运动，顺浓度梯度进行。它是一个被动过程，不需要 ATP。小分子非极性物质（如 O₂、CO₂）和极小的极性分子（如尿素）可通过简单扩散穿过细胞膜。
Rate of diffusion ∝ (surface area × concentration difference) / membrane thickness
扩散速率 ∝ (表面积 × 浓度差) / 膜厚度
Factors affecting diffusion rate include temperature, particle size, concentration gradient, and membrane surface area.
影响扩散速率的因素包括温度、粒子大小、浓度梯度和膜表面积。
7. Osmosis | 渗透作用
Osmosis is the passive movement of water molecules across a partially permeable membrane from a region of lower solute concentration (high water potential) to a region of higher solute concentration (low water potential). Water moves through aquaporin channels or directly through the bilayer.
渗透是水分子通过半透膜从低溶质浓度（高水势）区域向高溶质浓度（低水势）区域的被动运动。水分子可通过水通道蛋白或直接穿越磷脂双分子层。
Water potential (Ψ) measures the tendency of water to move; pure water has a water potential of zero under standard conditions.
水势 (Ψ) 衡量水分运动的趋势；在标准条件下纯水的水势为零。
In plant cells, turgor pressure builds when water enters the vacuole, pushing the membrane against the cell wall. Plasmolysis occurs when water leaves the cell in a hypertonic solution.
在植物细胞中，水分进入液泡产生膨压，将细胞膜推向细胞壁。当细胞处于高渗溶液中失水时，发生质壁分离。
8. Facilitated Diffusion | 协助扩散
Facilitated diffusion allows larger or charged molecules (e.g., glucose, amino acids, ions) to cross the membrane via channel proteins or carrier proteins. Like simple diffusion, it is passive and follows the concentration gradient.
协助扩散通过通道蛋白或载体蛋白，使较大的或带电荷的分子（如葡萄糖、氨基酸、离子）穿过细胞膜。与简单扩散一样，它是顺浓度梯度的被动过程。
Channel proteins form a hydrophilic tunnel; some are gated, opening in response to stimuli (voltage-gated, ligand-gated).
通道蛋白形成亲水通道；有些是门控的，响应刺激而开放（电压门控、配体门控）。
Carrier proteins bind the specific solute and undergo a conformational change to translocate it across the membrane.
载体蛋白与特定溶质结合并发生构象变化，将其转运过膜。
The rate of facilitated diffusion is limited by the number of transport proteins available; once all are saturated, the rate plateaus.
协助扩散的速率受限于可用的转运蛋白数量；一旦全部饱和，速率便达到平台期。
9. Active Transport | 主动运输
Active transport moves substances against their concentration gradient, from low to high concentration. It requires energy in the form of ATP and specific carrier proteins (pumps). A classic example is the Na⁺/K⁺ pump, which transports 3 Na⁺ out of the cell and 2 K⁺ in for each ATP hydrolysed.
主动运输逆浓度梯度，将物质从低浓度运向高浓度。它需要 ATP 形式的能量和特异的载体蛋白（泵）。经典例子是钠钾泵，每次水解一个 ATP，将 3 个 Na⁺ 运出细胞、2 个 K⁺ 运入细胞。
3 Na⁺(out) + 2 K⁺(in) + ATP → 3 Na⁺(in) + 2 K⁺(out) + ADP + Pi
Active transport enables cells to maintain concentration gradients of ions, essential for nerve impulse transmission and nutrient uptake. Indirect active transport (co-transport) uses the energy stored in an ion gradient to move another substance.
主动运输使细胞维持离子浓度梯度，对神经冲动传导和营养吸收至关重要。间接主动运输（协同运输）利用离子梯度中储存的能量来转运另一种物质。
10. Endocytosis and Exocytosis | 胞吞与胞吐
Large macromolecules and particles cross the membrane via vesicular transport. Endocytosis brings substances into the cell by infolding of the plasma membrane to form vesicles. Phagocytosis ('cell eating') involves solid particles, while pinocytosis ('cell drinking') involves liquids. Exocytosis releases materials from the cell when vesicles fuse with the membrane.
大分子和颗粒通过囊泡运输穿过细胞膜。胞吞通过质膜内陷形成囊泡将物质带入细胞。吞噬作用（细胞进食）针对固体颗粒，胞饮作用（细胞饮液）则涉及液体。胞吐是囊泡与细胞膜融合，将物质释放出细胞。
Both processes require energy (ATP) and involve dynamic remodelling of the membrane.
两种过程都需要 ATP 并提供膜结构的动态重塑。
Example: insulin secretion from pancreatic β-cells occurs via exocytosis.
例子：胰岛素从胰岛 β 细胞的分泌通过胞吐作用实现。
11. Membrane Function in Cell Signalling | 细胞信号传导中的膜功能
The cell membrane plays a vital role in communication. Receptor proteins on the membrane bind specific signal molecules (hormones, neurotransmitters) and initiate a signal transduction pathway, often involving second messengers (cyclic AMP, Ca²⁺). This allows the cell to respond to environmental changes without the signal entering the cell.
细胞膜在通讯中发挥关键作用。膜上的受体蛋白结合特定信号分子（激素、神经递质），启动信号转导途径，常涉及第二信使（环腺苷酸、Ca²⁺）。这使得细胞能在信号不进入细胞的情况下响应环境变化。
G-protein coupled receptors and tyrosine kinase receptors are two major classes of membrane receptors.
G 蛋白偶联受体和酪氨酸激酶受体是两类主要的膜受体。
Cell–cell recognition via glycoproteins is crucial in immune response and tissue formation.
通过糖蛋白的细胞间识别在免疫反应和组织形成中至关重要。
12. Exam Tips and Common Misconceptions | 考试技巧与常见误区
When answering IB questions, always use precise terminology: 'partially permeable membrane' not 'semi-permeable'. Distinguish clearly between active and passive processes by mentioning ATP and concentration gradients. Avoid stating that water moves towards a higher concentration; instead, say water moves from a region of lower solute concentration to a region of higher solute concentration (or higher water potential to lower water potential).
回答 IB 问题时，始终使用精确术语：“partially permeable membrane”而不是“semi-permeable”。通过提及 ATP 和浓度梯度，明确区分主动和被动过程。避免说水向高浓度方向移动；应该说水从低溶质浓度区域向高溶质浓度区域移动（或从高水势向低水势移动）。
Misconception: 'The cell wall provides the main barrier for transport.' Correction: The cell wall is fully permeable, while the cell membrane is the selectively permeable barrier.
误区：“细胞壁是物质运输的主要屏障。”纠正：细胞壁是全透性的，细胞膜才是半透性屏障。
Ensure you can draw and label the fluid mosaic model, showing phospholipids, integral and peripheral proteins, cholesterol, glycoproteins and glycolipids.
确保你能绘制并标注流动镶嵌模型，标出磷脂、整合蛋白与周边蛋白、胆固醇、糖蛋白和糖脂。
Use comparative language: 'Whereas simple diffusion does not involve proteins, facilitated diffusion requires channel or carrier proteins.'
使用比较性语言：“简单扩散不涉及蛋白质，而协助扩散需要通道蛋白或载体蛋白。”
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