Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输

📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输

In this revision article, we explore the three fundamental processes by which substances move across cell membranes: diffusion, osmosis, and active transport. These concepts are essential for understanding how organisms absorb nutrients, exchange gases, and maintain homeostasis.

本复习文章将探讨物质穿过细胞膜的三种基本过程：扩散、渗透和主动运输。理解这些概念对于掌握生物体如何吸收营养、交换气体以及维持稳态至关重要。

1. Diffusion: The Passive Movement of Particles | 扩散：粒子的被动运动

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. This process is entirely passive, meaning it does not require energy from respiration.

扩散是粒子从高浓度区域向低浓度区域沿浓度梯度进行的净移动。这一过程完全是被动的，意味着不需要呼吸作用提供的能量。

For example, when a drop of ink is placed in water, the ink particles spread out until they are evenly distributed. In biology, oxygen diffuses into the blood from the alveoli because the concentration of oxygen is higher in the alveoli than in the blood.

例如，将一滴墨水滴入水中，墨水粒子会扩散直到均匀分布。在生物学中，氧气从肺泡扩散进入血液，因为肺泡中氧气的浓度高于血液中的氧气浓度。

Factors affecting the rate of diffusion include: concentration gradient, temperature, surface area, and distance.
英语：影响扩散速率的因素包括：浓度梯度、温度、表面积和距离。

In the human body, diffusion is responsible for gas exchange in the lungs and in respiring tissues. The thin walls of alveoli and capillaries, combined with their large surface area, maximise the efficiency of diffusion.

在人体中，扩散负责肺部和呼吸组织中的气体交换。肺泡和毛细血管的薄壁，加上巨大的表面积，使扩散效率最大化。

2. Osmosis: Diffusion of Water Across a Membrane | 渗透：水通过膜扩散

Osmosis is a special type of diffusion that involves the movement of water molecules. It is defined as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.

渗透是涉及水分子运动的一种特殊扩散。它的定义是水分子通过部分透性膜，从水势较高（稀溶液）区域向水势较低（浓溶液）区域进行的净移动。

The partially permeable membrane allows water molecules to pass through, but prevents larger solute particles from passing. This is why plant roots can absorb water from the soil, even when the soil is not very wet.

部分透性膜允许水分子通过，但阻止较大的溶质粒子通过。这就是为什么植物根能从土壤中吸收水分，即使土壤不是非常湿润。

water potential (high) → water potential (low)

When a plant cell is placed in pure water, water enters the cell by osmosis, causing the vacuole to swell and the cell to become turgid. However, if placed in a concentrated solution, water leaves the cell and the cell becomes plasmolysed.

当植物细胞放入纯水中时，水通过渗透进入细胞，导致液泡膨胀，细胞变得充盈。然而，如果放入浓溶液中，水离开细胞，细胞会发生质壁分离。

3. Active Transport: Moving Against the Gradient | 主动运输：逆浓度梯度移动

Active transport is the movement of particles 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, which is produced by respiration.

主动运输是粒子从低浓度区域向高浓度区域逆浓度梯度的移动。这个过程需要以ATP形式提供的能量，ATP由呼吸作用产生。

Carrier proteins in the membrane bind to the specific molecule and change shape using ATP, allowing the molecule to move across.
膜上的载体蛋白与特定分子结合，利用ATP改变形状，使分子能够跨膜移动。

A classic example is the absorption of glucose by the villi in the small intestine. The concentration of glucose inside the villi is already higher than in the gut lumen, so diffusion alone cannot absorb enough glucose. Active transport allows the remaining glucose to be absorbed.

一个经典例子是小肠绒毛对葡萄糖的吸收。绒毛内部葡萄糖浓度已经高于肠腔，因此仅靠扩散无法吸收足够的葡萄糖。主动运输使剩余的葡萄糖被吸收。

Another example is the uptake of mineral ions by plant root hairs from the soil, where ion concentrations are much lower than in the root cells.

另一个例子是植物根毛从土壤中吸收矿质离子，土壤中离子浓度远低于根细胞内的浓度。

4. Comparing the Three Processes | 三种过程的比较

It is important to distinguish between diffusion, osmosis, and active transport. The table below summarises their key differences.

区分扩散、渗透和主动运输非常重要。下表总结了它们的主要区别。

Feature	Diffusion	Osmosis	Active transport
Substance	All particles	Water only	Ions / molecules
Direction	Down gradient	Down water potential gradient	Up gradient
Energy (ATP)	Not required	Not required	Required
Membrane protein	No	No (only pores)	Yes (carrier proteins)

In IGCSE Biology, you must be able to explain these differences and apply them to unfamiliar situations, such as explaining why salt can preserve meat.

在IGCSE生物学中，你必须能够解释这些差异，并将其应用于陌生情境，例如解释为什么盐能保存肉类。

5. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素

The rate of diffusion depends on several physical factors. A higher concentration gradient leads to a faster rate of diffusion because more particles move from the high-concentration region to the low-concentration region per unit time.

扩散速率取决于几个物理因素。浓度梯度越高，扩散速率越快，因为单位时间内有更多粒子从高浓度区域移向低浓度区域。

Temperature: Higher temperatures provide particles with more kinetic energy, causing them to move faster and increasing the rate of diffusion.
温度： 更高的温度赋予粒子更多动能，使其运动更快，从而增加扩散速率。
Surface area: A larger surface area offers more space for particles to cross, speeding up diffusion.
表面积： 更大的表面积提供了更多粒子穿过的空间，加速扩散。
Distance: Shorter distances allow particles to reach equilibrium more quickly, as seen in single-celled organisms.
距离： 距离越短，粒子能更快达到平衡，如单细胞生物中所示。
Concentration gradient: Steeper gradients push more particles across a membrane per second.
浓度梯度： 梯度越陡，每秒穿过膜的粒子越多。

For example, the alveoli in the lungs are extremely thin (one cell thick) and have a huge total surface area, making gas exchange highly efficient.

例如，肺部的肺泡极薄（仅一个细胞厚）且总表面积巨大，使气体交换效率极高。

6. Osmosis in Animal Cells | 动物细胞中的渗透

Animal cells do not have a cell wall, so they are more vulnerable to changes in the surrounding solution concentration. When an animal cell is placed in a hypotonic solution (higher water potential than the cell), water enters by osmosis. The cell swells and may burst, a process known as lysis.

动物细胞没有细胞壁，因此更容易受到周围溶液浓度变化的影响。当动物细胞置于低渗溶液（水势高于细胞）中时，水通过渗透进入。细胞膨胀并可能破裂，这个过程称为裂解。

Conversely, in a hypertonic solution (lower water potential than the cell), water leaves the cell, causing it to shrink and become crenated. In an isotonic solution, there is no net movement of water, and the cell maintains its normal shape.

相反，在高渗溶液（水势低于细胞）中，水离开细胞，导致细胞收缩并变成锯齿状。在等渗溶液中，水的净移动为零，细胞保持正常形态。

This is why medical drips contain saline solution with a concentration that matches blood plasma, so red blood cells are not damaged.

这就是为什么医用点滴含有与血浆浓度匹配的生理盐水，以免红细胞受损。

7. Osmosis in Plant Cells | 植物细胞中的渗透

Plant cells have a rigid cellulose cell wall, which prevents them from bursting even in a hypotonic solution. Instead, they become turgid, with the cytoplasm pressing against the cell wall. This turgor pressure provides structural support for plants.

植物细胞有坚硬的纤维素细胞壁，即使在高渗溶液中也不会破裂。相反，它们会变得充盈，细胞质紧贴细胞壁。这种膨压为植物提供结构支持。

When placed in a hypertonic solution, water leaves the plant cell. The cell membrane pulls away from the cell wall, a condition called plasmolysis. If the plant is watered again, the cell can recover.

当置于高渗溶液中时，水离开植物细胞。细胞膜从细胞壁上脱离，这种情况称为质壁分离。如果再次给植物浇水，细胞可以恢复。

Farmers need to understand osmosis to manage irrigation. If crops are watered with seawater or over-fertilised soil, the water potential outside the roots becomes lower, making it difficult for roots to absorb water.

农民需要理解渗透来管理灌溉。如果作物用海水或过度施肥的土壤浇水，根部外水势变得更低，使根难以吸收水分。

8. Active Transport in the Small Intestine | 小肠中的主动运输

The inner surface of the small intestine is covered with millions of villi, which increase the surface area for absorption. Glucose and amino acids are absorbed into the blood through a combination of diffusion and active transport.

小肠内表面覆盖着数百万个绒毛，增加吸收表面积。葡萄糖和氨基酸通过扩散和主动运输的组合被吸收到血液中。

When the concentration of glucose in the intestinal lumen is lower than in the blood, active transport is required. The carrier proteins use ATP to move glucose against its concentration gradient into the epithelial cells.

当肠腔中葡萄糖浓度低于血液时，需要主动运输。载体蛋白利用ATP将葡萄糖逆浓度梯度移入上皮细胞。

This process is vital for ensuring that almost all glucose is absorbed from food. People with defects in these carrier proteins may suffer from malabsorption.

这个过程对于确保几乎所有葡萄糖从食物中吸收至关重要。载体蛋白有缺陷的人可能会出现吸收不良。

9. Active Transport in Plant Roots | 植物根中的主动运输

Plants need mineral ions such as nitrate, phosphate and potassium for healthy growth. These ions are often present in very low concentrations in the soil. Root hair cells have specialised carrier proteins that actively transport these ions into the cell, even when the soil concentration is much lower than inside the root.

植物需要硝酸盐、磷酸盐和钾等矿质离子才能健康生长。这些离子在土壤中通常浓度很低。根毛细胞具有特化的载体蛋白，能主动将这些离子运输进入细胞，即使土壤浓度远低于根内浓度。

The energy for active transport in roots comes from aerobic respiration in the mitochondria. If a plant is deprived of oxygen, active transport stops, and the plant cannot absorb minerals effectively.

根系主动运输所需的能量来自线粒体中的有氧呼吸。如果植物缺氧，主动运输停止，植物无法有效吸收矿物质。

This explains why waterlogged soil harms plants: the lack of oxygen in the soil reduces respiration and hence reduces active transport.

这就解释了为什么水涝土壤对植物有害：土壤中缺氧减少呼吸作用，从而减少主动运输。

10. Experimental Investigations | 实验探究

In the laboratory, osmosis can be investigated using Visking tubing, potato strips, or egg membranes. A common experiment involves placing potato cylinders in solutions of different concentrations and measuring their change in mass.

在实验室中，渗透可以使用玻璃纸袋（Visking tubing）、马铃薯条或蛋膜来研究。一个常见实验是将马铃薯圆柱体放入不同浓度的溶液中，测量其质量变化。

Cut equal-sized potato cylinders and record their initial masses.
将马铃薯圆柱体切成相同大小，记录初始质量。
Place each cylinder in a labelled beaker containing a solution of known concentration (e.g., 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M sucrose).
将每个圆柱体放入贴有标签的烧杯中，烧杯中含有已知浓度的溶液（例如0.0 M、0.2 M、0.4 M、0.6 M、0.8 M、1.0 M蔗糖溶液）。
After 30 minutes, remove the cylinders, blot them dry, and record the final masses.
30分钟后，取出圆柱体，用纸巾吸干表面水分，记录最终质量。
Calculate the percentage change in mass and plot a graph of percentage change against concentration.
计算质量变化百分比，并绘制质量变化百分比对浓度的曲线图。

The concentration at which no mass change occurs is the isotonic point, where the water potential inside the potato tissue equals the surrounding solution.

质量无变化时所对应的浓度即为等渗点，此时马铃薯组织内的水势与周围溶液的水势相等。

11. Common Exam Questions and How to Answer Them | 常见考题及应答技巧

In the Edexcel IGCSE Science exam, you may be asked to explain observations of osmosis in different solutions, or to compare diffusion and osmotic water movement. Always use the terms “water potential”, “concentration gradient”, and “partially permeable membrane” in your answers.

在Edexcel IGCSE科学考试中，你可能会被要求解释不同溶液中渗透现象的观察结果，或比较扩散和渗透水的移动。回答时务必使用”水势”、”浓度梯度”和”部分透性膜”等术语。

A typical question: “Explain why a lettuce leaf loses water and becomes limp when salad dressing containing high salt concentration is added.”

一个典型问题：”解释为什么当添加含高浓度盐分的沙拉酱时，莴苣叶会失水并变软。”

Suggested structure: The salt solution has a lower water potential than the lettuce cells. Water molecules leave the cells by osmosis through the partially permeable membrane, down the water potential gradient. As the cells lose water, they lose turgor pressure and become limp.

建议的回答结构：盐溶液的浓度高，水势低于莴苣细胞。水分子通过部分透性膜沿水势梯度离开细胞。随着细胞失水，细胞失去膨压而变软。

12. Summary and Revision Checklist | 总结与复习清单

Here is a quick overview of the essential points for your revision:

以下是复习要点快速概览：

Diffusion is passive, moves particles down a concentration gradient, and does not require energy.
扩散是被动的，粒子沿浓度梯度移动，不需要能量。
Osmosis is the diffusion of water through a partially permeable membrane, down the water potential gradient.
渗透是水通过部分透性膜沿水势梯度进行的扩散。
Active transport requires ATP from respiration and moves particles against the concentration gradient.
主动运输需要呼吸作用产生的ATP，并逆浓度梯度移动粒子。
Animal cells can lyse or shrink, while plant cells become turgid or plasmolysed.
动物细胞会裂解或收缩，植物细胞会充盈或质壁分离。
Experiments on potato strips help determine isotonic concentration.
马铃薯条实验有助于确定等渗浓度。

Remember to practise drawing simple diagrams of cells in different solutions, and label the direction of water movement with arrows. Good luck with your revision!

记得练习绘制细胞在不同溶液中的简图，并用箭头标出水的移动方向。祝你复习顺利！

This article covers the core knowledge you need for IGCSE Science (Edexcel) on diffusion, osmosis, and active transport. Understanding these processes will help you succeed in both multiple-choice and extended-answer questions.

这篇文章涵盖了IGCSE科学（Edexcel）中关于扩散、渗透和主动运输所需的核心知识。理解这些过程将帮助你在选择题和论述题中取得好成绩。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输