物质进出细胞的运动
生物学· 2.15–2.17（第3版大纲）· 25 分钟阅读
1. 核心细胞转运过程（2.15）★★☆☆☆⏱ 8 min
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扩散
粒子从高浓度区域向低浓度区域（沿浓度梯度）发生的净移动，无需消耗能量，属于被动过程
渗透作用
水分子穿过半透膜，从高水浓度区域（稀溶液）向低水浓度区域（浓溶液）发生的净移动，属于被动过程
主动运输
物质逆浓度梯度（从低浓度到高浓度）发生的移动，需要消耗呼吸作用释放的能量，依赖细胞膜上的载体蛋白完成
某学生将渗透作用定义为「水从稀溶液到浓溶液的扩散」。请写出该定义中缺失的一个满分必备要素。
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步骤1：回忆爱德思对渗透作用定义的强制要求。
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步骤2：判断该定义提到了水的移动和浓度方向，但遗漏了半透膜这一关键要求。
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答案：该定义没有说明渗透作用是穿过半透膜发生的。
2. 影响转运速率的因素（2.16）★★★☆☆⏱ 7 min
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共有4个核心因素会影响物质进出细胞的速率，所有因素都是考试中的高频考点：
Concentration gradient steepness: The steeper the difference in concentration between two regions, the faster the rate of diffusion and osmosis.
Temperature: Higher temperatures give particles more kinetic energy, so they move faster, increasing transport rate.
Diffusion pathway distance: The shorter the distance particles have to travel, the faster the rate of transport.
Surface area to volume ratio (SA:V): Smaller cells/organisms have a larger surface area relative to their volume, so exchange of substances happens faster. Larger organisms require specialized exchange surfaces to compensate for their small SA:V.
请解释为什么小鼠每克体重的气体交换速率比大象更快。
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步骤1：将生物体大小与表面积体积比（比表面积）关联起来。
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步骤2：小鼠的体型远小于大象，因此它的表面积体积比要高得多。
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步骤3：更高的表面积体积比意味着每克体重可用于气体交换的表面积更大，因此交换速率更快。
3. 必修实验：扩散与渗透作用（2.17）★★★☆☆⏱ 7 min
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考试会考察你对扩散与渗透作用两类实验的理解：一类使用活体系（例如马铃薯组织），另一类使用非活体系（例如琼脂、维金斯透析管）。
关于常见的土豆渗透作用实验：将土豆块茎切为大小相等的试样，吸干表面水分后称重，随后放入不同糖/盐浓度的溶液中静置固定时长，再次吸干表面水分并称重，计算质量的百分比变化。
Mass/length increase: Net movement of water into potato cells, so the external solution is more dilute than potato cell sap. Cells become turgid.
Mass/length decrease: Net movement of water out of potato cells, so the external solution is more concentrated than potato cell sap. Cells become flaccid or plasmolysed.
No change in mass/length: The external solution has the same concentration as the potato cell sap, so there is no net movement of water.
置于0.5M蔗糖溶液中的土豆试样质量损失了8%，请解释该实验结果。
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步骤1：将质量损失与渗透作用引发的水分移动关联起来。
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步骤2：0.5M蔗糖溶液的浓度高于土豆的细胞液，因此水分会穿过土豆细胞的半透性细胞膜，从细胞内流出。
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步骤3：土豆细胞的水分净损失导致土豆试样的总质量下降。
4. 被动运输与主动运输的对比★★☆☆☆⏱ 3 min
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Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
Energy required? | No (passive) | No (passive) | Yes (from respiration) |
Gradient direction | Down (high→low) | Down (high→low water) | Against (low→high) |
Partially permeable membrane required? | No | Yes | No (uses carrier proteins) |
Substances transported | Small soluble particles (O₂, glucose) | Water only | Ions, large molecules (minerals) |
Which process requires energy from respiration to move substances against a concentration gradient?
Diffusion
Osmosis
Active transport
显示答案
2 —Active transport is the only active process listed, using energy from respiration to move substances up a concentration gradient.
5. 常见陷阱
错误做法:
Defining osmosis as 'diffusion of water' without mentioning a partially permeable membrane
原因:
Edexcel marking schemes explicitly require reference to the membrane for full marks, this is a common discriminator question
正确做法:
Always include 'across a partially permeable membrane' in any osmosis definition
错误做法:
Stating active transport uses energy from the concentration gradient
原因:
Active transport moves against the gradient, so it cannot use gradient energy
正确做法:
Explicitly state active transport uses energy released from aerobic respiration
错误做法:
Mixing up water movement direction in osmosis practicals
原因:
Many students mix up dilute/concentrated solutions and mass change, losing marks in practical interpretation questions
正确做法:
Remember water moves to the more concentrated solution: if external solution is more concentrated than cells, water leaves and mass decreases
错误做法:
Stating larger organisms have a larger surface area to volume ratio
原因:
SA:V decreases as organism size increases, a common multiple choice misconception
正确做法:
Smaller cells/organisms have higher SA:V, larger ones have lower SA:V
错误做法:
Using absolute mass change instead of percentage change for practical comparisons
原因:
Starting masses of potato cores are never identical, so absolute change does not allow fair comparison
正确做法:
Calculate percentage change in mass using % change = ((final mass - initial mass)/initial mass) × 100
6. 速查表
Term | Exam-Ready Key Definition |
|---|---|
Diffusion | Net movement of particles down concentration gradient, passive, no energy |
Osmosis | Net movement of water down water concentration gradient, across partially permeable membrane, passive |
Active Transport | Movement against concentration gradient, uses energy from respiration |
Rate Factors | SA:V, temperature, concentration gradient, diffusion distance |
Practical Rule | Mass ↑ = water in (external dilute), mass ↓ = water out (external concentrated) |
7. 常见问题
考试中我可以将渗透作用称为「水的扩散」吗？
不可以。爱德思评分标准要求你必须明确说明渗透作用穿过半透膜发生，同时提及水分子沿水的浓度梯度发生净移动，才能拿到满分。
主动运输使用的能量来源是什么？
主动运输消耗细胞内有氧呼吸释放的能量，而非像被动转运过程那样利用浓度梯度的势能。
深入阅读
- syllabus爱德思IGCSE生物4BI1官方大纲可参考第2.15–2.17节获取官方指导要求
- practical guide爱德思IGCSE生物实验手册包含渗透作用马铃薯核心实验的完整操作流程
下一步
Now that you have mastered core cell transport processes, you can apply this foundational knowledge to specialized exchange surfaces and transport systems across living organisms, a frequent topic in Edexcel IGCSE Biology exam papers. You will encounter diffusion in gas exchange systems in animals (alveoli) and plants (leaf stomata), osmosis in root hair cell water uptake, and active transport in mineral ion absorption in plant roots and nutrient uptake in the small intestine villi. Be sure to practice past paper questions on this topic to familiarize yourself with strict marking scheme requirements, especially for definition and practical interpretation questions, which are common high-mark questions across both Paper 1 and Paper 2.