📚 Movement of Substances across Membranes | 细胞膜的物质运输
Cells need a constant supply of nutrients and must remove waste products to stay alive. The plasma membrane controls which substances enter and leave the cell, using processes such as diffusion, osmosis and active transport. Understanding these mechanisms is a core part of Edexcel IGCSE Biology and is essential for explaining how organisms survive and respond to their environment.
细胞需要持续获得营养物质并及时排出废物才能维持生命。细胞膜通过扩散、渗透和主动运输来控制物质进出。理解这些机制是 Edexcel IGCSE 生物学的核心内容，也是解释生物如何生存和适应环境的基础。
1. The Structure of the Cell Membrane | 细胞膜的结构
The cell membrane is a partially permeable barrier composed mainly of a phospholipid bilayer with embedded proteins. Small, non-polar molecules like oxygen and carbon dioxide can pass straight through the bilayer, while larger or charged particles often need help from transport proteins.
细胞膜是一种具有选择透过性的屏障，主要由磷脂双分子层和镶嵌在其中的蛋白质组成。像氧气和二氧化碳这样的小分子、非极性分子可以直接穿过磷脂层，而较大的分子或带电粒子通常需要转运蛋白的帮助。
The term ‘partially permeable’ means the membrane allows some substances to cross but not others. This property is fundamental to all three transport processes you need to learn for the Edexcel IGCSE specification.
“选择透过性”是指细胞膜允许某些物质穿过而阻止另一些物质。这一特性是你需要为 Edexcel IGCSE 学习的三种运输过程的基础。
2. Diffusion – Passive Movement Down a Gradient | 扩散 – 顺浓度梯度的被动运输
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 it does not require metabolic energy (ATP).
扩散是粒子从高浓度区域向低浓度区域的净运动，沿着浓度梯度进行。这是一个被动过程，不需要代谢能量（ATP）。
The rate of diffusion can be increased by a steeper concentration gradient, a higher temperature, a larger surface area and a shorter diffusion distance. These factors are often linked to adaptations of exchange surfaces in living organisms.
扩散速率可通过增大浓度梯度、升高温度、增加表面积和缩短扩散距离来提高。这些因素常与生物体交换表面的适应性相关。
3. Factors Affecting Diffusion Rate | 影响扩散速率的因素
Temperature gives particles more kinetic energy, so they move faster and diffuse more quickly. The concentration gradient is the difference in particle numbers across a space – the bigger the difference, the faster diffusion occurs until equilibrium is reached.
温度赋予粒子更大的动能，使它们运动加快，扩散更快。浓度梯度是指空间两侧粒子数量的差异；差异越大，扩散越快，直至达到平衡。
Surface area to volume ratio is particularly important in biology. Organisms such as single-celled amoeba have a large surface area relative to their volume, allowing sufficient diffusion of oxygen and waste. Larger organisms need specialised exchange systems like lungs and gills.
表面积与体积之比在生物学中尤其重要。像变形虫这样的单细胞生物具有相对较大的表面积，足以通过扩散获得氧气并排出废物。较大的生物则需要肺和鳃等专门的交换系统。
Increasing temperature → faster diffusion.
升高温度 → 扩散加快。
Shorter diffusion distance → faster diffusion.
扩散距离越短 → 扩散越快。
Larger surface area → more particles can cross at once.
表面积越大 → 同时穿过的粒子越多。
Steeper concentration gradient → faster net movement.
浓度梯度越大 → 净运动越快。
4. Osmosis – A Special Case of Diffusion | 渗透 – 扩散的一种特殊情况
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane. Water potential is the tendency of water to move; pure water has the highest water potential, taken as zero in IGCSE contexts.
渗透是水分子通过选择透过性膜从水势较高的区域向水势较低区域的净运动。水势是水移动的趋势；在IGCSE课程中，纯水的水势最高，设为零。
It is vital to remember that osmosis only involves water, and the membrane must be partially permeable. Many students confuse osmosis with diffusion – always check if water and a membrane are both involved.
必须记住，渗透只涉及水分子，且膜必须是选择透过性的。许多学生将渗透与扩散混淆——务必检查是否同时涉及水和膜。
5. Water Potential and Tonicity | 水势与渗透压
A solution with a higher water potential than the cell is hypotonic. Water enters the cell by osmosis. A solution with a lower water potential is hypertonic; water leaves the cell. When water potentials are equal, the solution is isotonic and there is no net movement of water.
水势高于细胞的溶液为低渗溶液，水会通过渗透进入细胞。水势低于细胞的为高渗溶液，水会离开细胞。当水势相等时，溶液为等渗，没有水的净移动。
In Edexcel IGCSE, you might be given concentrations of sucrose or salt solutions. Higher solute concentration means lower water potential. You should be able to predict the direction of water movement based on these values.
在 Edexcel IGCSE 考试中，你可能会遇到蔗糖或盐溶液的浓度。溶质浓度越高，水势越低。你应能根据数值预测水的运动方向。
6. Osmosis in Plant Cells | 植物细胞中的渗透
When a plant cell is placed in a hypotonic solution, water enters by osmosis. The cell swells and becomes turgid; the cell wall prevents bursting. Turgor pressure pushes the cell membrane against the wall, supporting the plant.
当植物细胞置于低渗溶液中时，水通过渗透进入细胞，细胞膨胀并变得坚挺；细胞壁可以阻止细胞破裂。膨压将细胞膜推向细胞壁，起到支持植物的作用。
In a hypertonic solution, water leaves the plant cell. The cytoplasm shrinks and the cell membrane pulls away from the cell wall – this is called plasmolysis. A plasmolysed plant cell becomes flaccid and the plant wilts.
在高渗溶液中，水离开植物细胞。细胞质收缩，细胞膜与细胞壁分离——这称为质壁分离。发生质壁分离的植物细胞变得松弛，植物萎蔫。
7. Osmosis in Animal Cells | 动物细胞中的渗透
Animal cells do not have a cell wall. In a hypotonic solution, water rushes in and the cell may burst (lysis). In a hypertonic solution, water leaves and the cell shrinks (crenation). This is why the internal environment of animals must be kept isotonic to their cells.
动物细胞没有细胞壁。在低渗溶液中，水涌入细胞，可能导致细胞破裂（溶血）。在高渗溶液中，水离开细胞，细胞皱缩（皱缩）。因此，动物的内环境必须与细胞保持等渗。
Edexcel questions often ask you to compare the outcomes of osmosis in plant and animal cells. Always highlight the role of the cell wall in preventing lysis.
Edexcel 考题经常要求比较渗透在植物和动物细胞中的结果。一定要强调细胞壁阻止细胞破裂的作用。
8. Active Transport – Movement Against the Gradient | 主动运输 – 逆浓度梯度的运动
Active transport is the movement of particles from a lower concentration to a higher concentration against the concentration gradient. Unlike diffusion and osmosis, active transport requires energy from respiration in the form of ATP.
主动运输是粒子从低浓度向高浓度、逆浓度梯度的运动。与扩散和渗透不同，主动运输需要呼吸作用提供的能量，以 ATP 的形式。
Carrier proteins in the cell membrane bind to the specific substance, change shape using energy, and release the substance on the other side. Active transport allows cells to absorb ions and nutrients even when they are already at higher concentration inside the cell.
细胞膜上的载体蛋白与特定物质结合，利用能量改变形状，并在另一侧释放该物质。主动运输使细胞即使在内部浓度已经较高时，仍然能够吸收离子和营养物质。
Example: uptake of nitrate ions by root hair cells from the soil.
例如：根毛细胞从土壤中吸收硝酸根离子。
Example: absorption of glucose by kidney tubule cells after filtration.
例如：肾小管细胞在滤过后重吸收葡萄糖。
9. Comparing the Three Transport Mechanisms | 三种运输机制的比较
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Particles moved | Any small particles | Water only | Ions, glucose, amino acids |
| Concentration gradient | Down | Down water potential gradient | Against |
| Membrane required | No (but can occur across membranes) | Yes, partially permeable | Yes, with carrier proteins |
| Energy (ATP) needed | No | No | Yes |
掌握这三种机制的区别对于 Edexcel IGCSE 生物学至关重要。务必使用像“顺浓度梯度”、“需能”、“水势”和“载体蛋白”这样的精确术语。
10. Practical Investigation: Osmosis in Potato Cylinders | 实验探究：马铃薯圆柱体的渗透作用
A classic Edexcel IGCSE practical involves placing potato cylinders of equal mass and length into a range of sucrose or salt solutions. After a fixed time, changes in mass and length are recorded. The results reveal the water potential of the potato tissue.
一项经典的 Edexcel IGCSE 实验是将质量和长度相等的马铃薯圆柱体放入不同浓度的蔗糖或盐溶液中。经过固定时间后，记录质量和长度的变化。结果可以揭示马铃薯组织的水势。
If the potato gains mass, the solution was hypotonic to the cells; water entered by osmosis. If the potato loses mass, the solution was hypertonic. By plotting percentage change in mass against concentration, students can find the concentration where mass does not change – this is the point where the water potential of the solution equals that of the potato cells.
如果马铃薯质量增加，说明溶液相对于细胞是低渗的，水通过渗透进入。如果马铃薯质量减少，说明溶液是高渗的。将质量变化百分比与浓度作图，学生可以找到质量不变的浓度点——此时溶液的水势与马铃薯细胞的水势相等。
% change in mass = (final mass − initial mass) ÷ initial mass × 100
质量变化百分比 = （最终质量 − 初始质量）÷ 初始质量 × 100
Controlled variables include surface area of potato pieces, temperature, time and blotting method. This practical often appears in Edexcel IGCSE questions, so you should be able to describe the method, graph the results and draw conclusions.
需要控制的变量包括马铃薯切块的表面积、温度、时间和吸干方法。该实验常出现在 Edexcel IGCSE 考题中，因此你应能描述方法、绘制结果图并得出结论。
11. Common Exam Mistakes and How to Avoid Them | 常见考试错误及避免方法
Many students lose marks by writing “diffusion” when the question clearly involves water and a partially permeable membrane – the correct term is “osmosis”. Always check the key words in the question stem.
许多学生因为写“扩散”而丢分，但题目明确涉及水和选择透过性膜——正确的术语是“渗透”。务必审题，留意题干中的关键词。
Another common error is stating that active transport uses “energy” without specifying that this energy comes from respiration in the form of ATP. Examiners look for precise biological vocabulary.
另一个常见错误是说主动运输使用“能量”而不指明该能量来自呼吸作用并以 ATP 形式。考官看重精准的生物学词汇。
Be careful when explaining plasmolysis: the cell membrane pulls away from the cell wall, not the cell wall shrinking. Also, when comparing animal and plant cells in osmosis, remember that plant cells burst only if the cell wall is weakened or absent – in normal conditions the cell wall protects them.
在解释质壁分离时要小心：是细胞膜与细胞壁分离，而不是细胞壁收缩。另外，在比较动物细胞和植物细胞的渗透时，请记住植物细胞只有在细胞壁被削弱或缺失时才会破裂——正常情况下细胞壁具有保护作用。
Exam questions often ask for “describe and explain”. Describing means saying what happens; explaining means giving a scientific reason using concepts like water potential or kinetic energy. Always address both parts.
考题常要求“描述并解释”。描述是说出发生了什么；解释是利用水势或动能等概念给出科学理由。务必两部分都作答。
12. Summary and Link to the Edexcel IGCSE Syllabus | 总结与 Edexcel IGCSE 大纲的衔接
To master this topic, make sure you can define diffusion, osmosis and active transport, list the factors that influence their rates, and apply your knowledge to practical contexts. This topic links directly to enzyme activity, transpiration, kidney function and nutrient uptake in plants – all high-mark areas in the exam.
要掌握本专题，确保你能定义扩散、渗透和主动运输，列出影响其速率的因素，并在实验情境中应用这些知识。该专题与酶活性、蒸腾作用、肾功能和植物养分吸收等高分考点直接相关。
Revisit the practical investigation regularly and be ready to interpret graphs of mass change against concentration. Success in Edexcel IGCSE Biology depends on both factual recall and the ability to apply concepts to unfamiliar scenarios.
定期复习实验探究，并准备好解读显示质量随浓度变化的图表。Edexcel IGCSE 生物学的成功既取决于对事实的记忆，也取决于将概念应用于陌生情境的能力。
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