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

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

Diffusion, osmosis and active transport are three fundamental processes by which substances move into and out of cells. Together, they explain how organisms obtain nutrients, remove wastes and maintain internal balance.

扩散、渗透和主动运输是物质进出细胞的三种基本过程。它们共同解释了生物体如何获取营养、排除废物以及维持内部平衡。

1. What is Diffusion? | 什么是扩散？

Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient. This movement is passive, meaning it does not require energy from respiration.

扩散是粒子（分子或离子）从高浓度区域沿浓度梯度向低浓度区域进行的净移动。这种移动是被动的，意味着它不需要来自呼吸作用的能量。

The word ‘net’ is important: individual particles move randomly in all directions, but the overall result is that more particles travel from high to low concentration. Net movement continues until the concentrations are equalised — this state is called equilibrium.

“净”这个词很重要：单个粒子会向各个方向随机运动，但总体结果是从高浓度向低浓度移动的粒子更多。净移动一直持续到浓度相等为止——这种状态称为平衡。

A simple example is the smell of perfume spreading across a room, or oxygen dissolving and diffusing into red blood cells across the alveoli in the lungs.

一个简单的例子是香水的气味在房间中扩散，或者氧气在肺部溶解并通过肺泡扩散进入红细胞。

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

Four main factors control how quickly diffusion happens. The first is the concentration gradient: the steeper the difference in concentration, the faster the rate of diffusion. The second is temperature: higher temperature gives particles more kinetic energy so they move faster and collide more often.

有四个主要因素决定扩散发生的速度。第一个是浓度梯度：浓度差越大，扩散速率越快。第二个是温度：温度越高，粒子获得更多动能，因而运动更快、碰撞更频繁。

The third factor is the surface area available: a larger surface area allows more particles to cross at the same time. The fourth is the distance the particles must travel: shorter distances give faster diffusion, which is why the alveoli and villi are so thin.

第三个因素是可用表面积：更大的表面积允许更多粒子同时通过。第四个因素是粒子需要移动的距离：距离越短，扩散越快，这就是肺泡和绒毛如此薄的原因。

These relationships can be summarised by Fick’s law:

这些关系可以用菲克定律来概括：

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