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

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

Every living cell must exchange materials with its environment in order to survive. The cell membrane controls what enters and leaves the cell, and most of this movement happens through three important processes: diffusion, osmosis and active transport. In your IGCSE Biology exam, you need to understand the differences between these processes, the conditions that affect them, and how they are used in living organisms.

每个活细胞都必须与环境进行物质交换才能生存。细胞膜控制着进出细胞的物质，而大多数物质转移都通过三个重要过程完成：扩散、渗透和主动运输。在 IGCSE 生物考试中，你需要理解这些过程之间的区别、影响它们的条件，以及它们在生物体中的实际用途。

1. Cell Transport Overview | 细胞运输概述

Cells are surrounded by a partially permeable cell membrane. This means that small molecules such as water, oxygen and carbon dioxide can pass through easily, while larger molecules such as proteins and starch cannot pass through freely.

细胞被一层部分通透的细胞膜包围。这意味着水、氧气和二氧化碳等小分子能够轻易通过，而蛋白质和淀粉等大分子则不能自由通过。

Some transport processes require no energy and are called passive processes. Diffusion and osmosis are passive. Active transport, however, requires energy released by respiration.

有些运输过程不需要能量，称为被动过程。扩散和渗透属于被动运输。然而，主动运输需要呼吸作用释放的能量。

You can remember the overall pattern: diffusion moves small particles down a concentration gradient, osmosis moves water down a water potential gradient, and active transport moves molecules against a concentration gradient.

你可以记住总体规律：扩散使小颗粒沿浓度梯度方向移动，渗透使水沿水势梯度方向移动，而主动运输使分子逆浓度梯度移动。

2. Diffusion: The Passive Spreading of Particles | 扩散：微粒的被动扩散

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of the random movement of particles. It occurs in solutions and in gases.

扩散是指微粒因自身随机运动，从高浓度区域向低浓度区域的净移动，即沿浓度梯度向下移动。扩散发生在溶液和气体中。

One common example is gas exchange in the lungs. Oxygen diffuses from the alveoli, where its concentration is high, into the blood, where its concentration is lower. Carbon dioxide diffuses in the opposite direction.

一个常见例子是肺部的气体交换。氧气从浓度较高的肺泡扩散进入浓度较低的血液中。二氧化碳则沿相反方向扩散。

Another example is smelling perfume or food in the air. Perfume particles spread out from the bottle and move from an area of high concentration to an area of low concentration until they reach your nose.

另一个例子是闻到香水或食物的气味。香水微粒从瓶口扩散，从高浓度区域移动到低浓度区域，直到到达你的鼻子。

Diffusion is described as a net movement because particles keep moving in all directions. It is the overall movement, not the movement of a single particle, that matters.

扩散被称为“净移动”，因为微粒始终朝各个方向运动。重要的是总体的移动方向，而不是单个微粒的运动轨迹。

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

Several factors affect how quickly diffusion happens. You should be able to explain each one clearly in an exam answer.

有几个因素会影响扩散发生的速度。你应该能够在考试中清楚地解释每一个因素。

The greater the surface area of the membrane, the faster diffusion occurs. A large surface area provides more space for particles to cross.

膜的表面积越大，扩散发生得越快。大的表面积提供更多空间让微粒通过。
The steeper the concentration gradient, the faster diffusion occurs. If there is a big difference in concentration, particles will move across more quickly.

浓度梯度越大，扩散越快。如果浓度差异很大，微粒就会更迅速地移动通过。
The shorter the distance, the faster diffusion occurs. Diffusion is much faster across a thin membrane, such as the wall of an alveolus, than across a thick barrier.

距离越短，扩散越快。扩散穿过像肺泡壁这样的薄膜，比穿过厚屏障要快得多。
The higher the temperature, the faster diffusion occurs. Higher temperature gives particles more kinetic energy, so they move around more quickly.

温度越高，扩散越快。较高的温度赋予微粒更多动能，因此它们运动得更快。
Smaller particles diffuse faster than larger particles because smaller particles move more quickly at the same temperature.

相同温度下，较小微粒比大微粒扩散得更快，因为小微粒运动得更快。

These relationships can be summarised in the following expression:

这些关系可以用下面的表达式概括：

rate of diffusion ∝ (surface area × concentration gradient) ÷ membrane thickness

扩散速率 ∝ (表面积 × 浓度梯度) ÷ 膜厚度

4. Osmosis: The Special Case of Water Diffusion | 渗透：水的特殊扩散

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. It is a type of diffusion, but it involves only water molecules.

渗透是指水分子通过部分通透膜，从水势较高的区域向水势较低的区域进行的净移动。它是一种扩散，但只涉及水分子。

A partially permeable membrane allows water molecules to pass through, but not larger solute molecules such as sugar. This is why osmosis depends on solute concentration on each side of the membrane.

部分通透膜允许水分子通过，但糖等较大的溶质分子不能通过。这就是为什么渗透取决于膜两侧的溶质浓度。

In osmosis, water moves from a dilute solution to a concentrated solution. More simply, it moves from where there is more water to where there is less water, as long as the membrane allows water through.

在渗透中，水从稀溶液移动到浓溶液。更简单地说，只要膜允许水通过，水就会从水较多的地方移动到水较少的地方。

Osmosis is a passive process because it does not require metabolic energy. The water molecules move randomly, but the overall net movement is down the water potential gradient.

渗透是被动过程，因为它不需要代谢能量。水分子在随机运动，但总的净移动方向是沿水势梯度向下。

5. Water Potential and Solute Concentration | 水势与溶质浓度

Water potential is a measure of the tendency of water molecules to move from one place to another. Pure water has the highest possible water potential, which is usually given the value 0.

水势是衡量水分子从一个位置移动到另一个位置的趋势的指标。纯水具有最高的水势，通常取值为 0。

When a solute is dissolved in water, the water potential becomes more negative than 0. The more concentrated the solution, the lower the water potential becomes.

当溶质溶解在水中时，水势变得比 0 更负。溶液越浓，水势就越低。

For example, pure water has a water potential of 0. A dilute sugar solution might have a water potential of -200 kPa, and a concentrated sugar solution might have a water potential of -600 kPa.

例如，纯水的水势为 0。稀糖溶液的水势可能是 -200 kPa，而浓糖溶液的水势可能是 -600 kPa。

Water always moves from a region of less negative water potential to a region of more negative water potential. In other words, water moves from high water potential to low water potential.

水总是从负值较小（较高）的水势区域流向负值较大（较低）的水势区域。换句话说，水从高水势流向低水势。

You can also think in terms of solute concentration: water moves from a dilute solution to a concentrated solution across a partially permeable membrane.

你也可以从溶质浓度来理解：在部分通透膜两侧，水从稀溶液移动到浓溶液。

6. Osmosis in Animal Cells | 渗透在动物细胞中的作用

Animal cells do not have a cell wall. As a result, they are more vulnerable to changes in the surrounding solution.

动物细胞没有细胞壁。因此，它们更容易受到周围溶液变化的影响。

When an animal cell is placed in pure water or a very dilute solution, water enters the cell by osmosis because the cell cytoplasm has a lower water potential than the outside solution. The cell swells and may eventually burst. In red blood cells, this process is called haemolysis.

当动物细胞放入纯水或非常稀的溶液中时，由于细胞质的水势低于外部溶液，水通过渗透进入细胞。细胞会膨胀，最终可能破裂。在红细胞中，这个过程称为溶血。

When an animal cell is placed in a concentrated solution, water leaves the cell by osmosis. The cell shrinks and becomes wrinkled. In red blood cells, this shrinking is called crenation.

当动物细胞放入浓溶液中时，水通过渗透离开细胞。细胞会缩小并变得皱缩。在红细胞中，这种皱缩称为皱缩现象。

In the body, animal cells are normally surrounded by tissue fluid that has the same water potential as the cell contents. This is called an isotonic solution, and it prevents net water movement into or out of the cells.

在体内，动物细胞周围通常有组织液，其水势与细胞内容物相同。这称为等渗溶液，可以防止水大量进出细胞。

7. Osmosis in Plant Cells | 渗透在植物细胞中的作用

Plant cells have a rigid cell wall made of cellulose. This cell wall prevents plant cells from bursting when water enters by osmosis.

植物细胞有由纤维素构成的坚硬细胞壁。这个细胞壁能防止植物细胞因渗透进入水分而爆裂。

When a plant cell is placed in pure water or a dilute solution, water enters the vacuole and cytoplasm. The cell swells and becomes firm, but the cell wall stops it from bursting. The cell is described as turgid. This turgidity provides support for plants.

当植物细胞放入纯水或稀溶液中时，水进入液泡和细胞质。细胞膨胀并变得坚挺，但细胞壁阻止其破裂。这种情况称为细胞膨胀（turgid）。这种膨胀为植物提供支撑。

When a plant cell is placed in a concentrated solution, water leaves the vacuole and cytoplasm. The protoplast shrinks away from the cell wall. This is called plasmolysis. A plant that loses too much water becomes wilted because its cells are no longer turgid.

当植物细胞放入浓溶液中时，水离开液泡和细胞质。原生质体收缩并与细胞壁分离。这称为质壁分离。失去过多水分的植物会萎蔫，因为其细胞不再膨胀。

The cell wall is fully permeable, so it does not control which substances enter the cell. The partially permeable cell membrane is the structure that controls movement.

细胞壁是完全通透的，因此它不控制哪些物质进入细胞。细胞膜是部分通透的，它才是控制物质进出的结构。

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

Active transport is the movement of substances from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration. This process requires specific carrier proteins in the cell membrane.

主动运输是指物质从低浓度区域向高浓度区域移动，即逆浓度梯度移动，并且需要利用呼吸作用释放的能量。这个过程需要细胞膜上特定的载体蛋白。

Because active transport works against the concentration gradient, it needs energy in the form of ATP. ATP is produced during respiration. If oxygen or glucose is unavailable, active transport slows down or stops.

由于主动运输逆浓度梯度进行，它需要 ATP 形式的能量。ATP 通过呼吸作用产生。如果氧气或葡萄糖不足，主动运输就会减慢或停止。

Carrier proteins in the membrane bind to the specific substance being transported. The protein then changes shape to move the substance across the membrane. Each carrier protein is specific to one type of molecule or ion.

膜上的载体蛋白与所运输的特定物质结合。随后蛋白质改变形状以将物质运过膜。每种载体蛋白只针对一种分子或离子。

One common example is the uptake of mineral ions by root hair cells in plants. The concentration of mineral ions in the soil is usually lower than inside the root cells, so active transport is needed to absorb them. Examples include potassium ions K⁺ and magnesium ions Mg²⁺.

一个常见例子是植物根毛细胞吸收矿质离子。土壤中矿质离子的浓度通常低于根细胞内部，因此需要主动运输来吸收它们。例子包括钾离子 K⁺ 和镁离子 Mg²⁺。

Another important example is the reabsorption of glucose in the kidney tubules. Glucose is filtered out of the blood, but almost all of it is reabsorbed back into the blood by active transport.

另一个重要例子是肾小管对葡萄糖的重吸收。葡萄糖从血液中被滤出，但几乎全部通过主动运输重新吸收回血液。

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

The table below summarises the key similarities and differences between diffusion, osmosis and active transport. You should be able to reproduce this comparison in an exam.

下表总结了扩散、渗透和主动运输之间的主要异同。你应该能够在考试中重现这个比较。

Feature	Diffusion	Osmosis	Active Transport
Direction of movement	High concentration → low concentration	High water potential → low water potential	Low concentration → high concentration
Energy required	No	No	Yes, from ATP
Membrane proteins required	Usually no	No	Yes, carrier proteins
Substances moved	Small molecules and gases	Water only	Ions and molecules against a gradient
Example	O₂ entering the blood from alveoli	Water absorbed by root hairs from soil	Glucose reabsorbed in kidney tubules

10. Experimental Investigations | 实验探究

In the IGCSE course, you need to understand how diffusion and osmosis can be investigated in the laboratory.

在 IGCSE 课程中，你需要了解如何在实验室中研究扩散和渗透。

To investigate osmosis with potato strips, cut equal-sized potato strips, measure their mass and length, then place them in different concentrations of sugar or salt solution. After a set time, remove the strips, dry them gently, and measure their mass and length again.

要用土豆条探究渗透，可将土豆切成等长等大的条，测量质量和长度，然后放入不同浓度的糖或盐溶液中。一段时间后取出，轻轻吸干表面水分，再次测量质量和长度。
If the potato strip becomes heavier and firmer, water has entered by osmosis. This happens in dilute solutions. If the strip becomes lighter and softer, water has left by osmosis. This happens in concentrated solutions.

如果土豆条变得更重更硬，说明水通过渗透进入细胞，这发生在稀溶液中。如果土豆条变得更轻更软，说明水通过渗透离开细胞，这发生在浓溶液中。
To make results more reliable, calculate the percentage change in mass rather than just the absolute change. This makes it easier to compare strips that started with slightly different initial masses.

为了使结果更可靠，应计算质量变化百分比，而不是仅计算绝对变化量。这样更容易比较初始质量略有不同的土豆条。
To investigate diffusion, a Visking tubing bag containing starch and glucose can be placed in a beaker of water. After some time, glucose can be detected outside the bag using Benedict’s solution, but starch cannot pass through the membrane. This shows that glucose molecules are smaller than starch molecules.

要研究扩散，可将装有淀粉和葡萄糖的玻璃纸袋放入烧杯水中。一段时间后，外部水中可用本尼迪克特溶液检测到葡萄糖，而淀粉不能通过膜。这说明葡萄糖分子比淀粉分子小。
Always use a control experiment and keep temperature and time constant so that the only variable is the concentration of the solution.

一定要设置对照实验，并保持温度和浸泡时间不变，使唯一变量是溶液浓度。

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

Many students lose marks in this topic by using unclear or incomplete definitions. Learn the definitions word by word, and always include the phrase “net movement”.

许多学生在这个话题上因为定义不清或不完整而丢分。请逐字记住定义，并且一定要写出“净移动”这个短语。

Do not say that particles “try” to spread out. Particles move randomly, and diffusion is the result of this random movement.

不要认为微粒“试图”散开。微粒是随机运动的，扩散正是这种随机运动的结果。
Do not say that osmosis is “movement of water from a dilute solution to a concentrated solution” without mentioning a partially permeable membrane. The membrane is essential for osmosis.

不要只说渗透是“水从稀溶液向浓溶液移动”而不提部分通透膜。膜对渗透至关重要。
Do not confuse water potential with solute concentration. High water potential means a dilute solution; low water potential means a concentrated solution.

不要混淆水势与溶质浓度。高水势意味着稀溶液；低水势意味着浓溶液。
For active transport, remember to mention energy from respiration and carrier proteins. The key idea is that it moves substances against the concentration gradient.

对于主动运输，记得提到呼吸作用释放的能量和载体蛋白。关键是它使物质逆浓度梯度移动。
When describing plant cells, use the terms turgid and plasmolysis confidently. When describing animal cells, use haemolysis and crenation.

描述植物细胞时，请自信使用 turgid（细胞膨胀）和 plasmolysis（质壁分离）。描述动物细胞时，使用 haemolysis（溶血）和 crenation（皱缩）。

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