IGCSE Science 524: Diffusion, Osmosis and Active Transport | IGCSE科学524：扩散、渗透与主动运输

📚 IGCSE Science 524: Diffusion, Osmosis and Active Transport | IGCSE科学524：扩散、渗透与主动运输

In this revision guide, we look at Topic 524 from the Edexcel IGCSE Science specification, which focuses on how substances enter and leave cells. Understanding diffusion, osmosis and active transport is essential for answering questions about cell biology, human physiology and plant physiology.

本复习指南围绕Edexcel IGCSE科学课程中的主题524，重点讨论物质如何进入和离开细胞。理解扩散、渗透与主动运输，对于回答细胞生物学、人体生理学和植物生理学相关题目至关重要。

1. The Cell Membrane and Selective Permeability | 细胞膜与选择透过性

Every living cell is surrounded by a cell membrane that controls the movement of substances in and out.

每个活细胞都被一层细胞膜包围，控制着物质进出细胞。

The cell membrane is made mainly of lipids and proteins. Its structure is often described as a fluid mosaic model.

细胞膜主要由脂质和蛋白质组成，其结构常称为流动镶嵌模型。

Because the membrane allows some molecules to pass through while blocking others, it is called selectively permeable.

由于细胞膜允许某些分子通过，同时阻止另一些分子，因此被称为选择透过性膜。

Small molecules such as oxygen and carbon dioxide can pass freely. Larger molecules such as glucose may need specific carriers. Charged particles such as sodium ions also require special transport proteins.

氧气、二氧化碳等小分子可以自由通过。葡萄糖等较大分子可能需要特定载体。钠离子等带电粒子也需要特殊的转运蛋白。

2. Diffusion | 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.

扩散是粒子从浓度较高区域向浓度较低区域（沿浓度梯度）的净移动。

Diffusion is a passive process, so it does not require energy from respiration.

扩散是被动过程，不需要呼吸作用提供能量。

Particles move constantly because they have kinetic energy. Even when there is no net movement, particles still move randomly – this is called dynamic equilibrium.

粒子因为具有动能而不断运动。即使没有净移动，粒子仍在随机运动——这称为动态平衡。

Common examples include oxygen moving from the alveoli of the lungs into the blood, and carbon dioxide moving from the blood into the alveoli. In plants, carbon dioxide diffuses into leaves through stomata, and oxygen diffuses out during photosynthesis.

常见实例包括氧气从肺部肺泡进入血液，二氧化碳从血液进入肺泡。在植物中，二氧化碳通过气孔扩散进入叶片，而氧气在光合作用中扩散出去。

Diffusion also occurs in simple organisms such as amoeba, where gas exchange happens directly through the cell surface.

在变形虫等单细胞生物中，扩散也发生在体表，通过细胞的直接表面积为细胞进行气体交换。

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

You need to be able to explain why some conditions speed up or slow down diffusion.

你需要能够解释为什么某些条件会加快或减慢扩散。

First, the concentration gradient: a bigger difference in concentration between two areas leads to a faster net movement.

第一，浓度梯度：两个区域间浓度差越大，净移动越快。

Second, temperature: higher temperatures give particles more kinetic energy, so they move faster and diffusion happens more quickly.

第二，温度：

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