📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动转运
In Edexcel IGCSE Science, students must be able to explain how substances move into and out of cells. This article covers diffusion, osmosis and active transport, with definitions, examples and experimental guidance for your revision.
在爱德思 IGCSE 科学学习中，学生需要能够解释物质如何进出细胞。本文涵盖扩散、渗透作用和主动转运，为复习提供定义、示例和实验指导。
1. Cell Membranes | 细胞膜
Cell membranes are partially permeable. This means they allow some molecules to pass through while preventing others.
细胞膜是部分通透的。这意味着它允许某些分子通过，同时阻止其他分子通过。
The membrane is made of a phospholipid bilayer with proteins embedded. Small non-polar molecules such as oxygen and carbon dioxide move through easily.
细胞膜由磷脂双分子层构成，其中镶嵌着蛋白质。氧气、二氧化碳等小的非极性分子很容易穿过。
Water molecules are small enough to pass through the bilayer, while ions and larger molecules often require protein carriers or channels.
水分子足够小，能穿过双分子层，而离子和较大的分子通常需要蛋白质载体或通道。
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. It is a passive process because it does not require energy.
扩散是粒子从浓度较高区域向浓度较低区域的净移动，即沿浓度梯度方向进行。这是一个被动过程，因为不需要能量。
For example, oxygen diffuses from the air in the alveoli into the blood, where oxygen concentration is lower. Carbon dioxide diffuses in the opposite direction.
例如，氧气从肺泡中的空气扩散到血液中，因为那里氧气浓度较低。二氧化碳则朝相反方向扩散。
Diffusion can happen in gases and liquids. In solids, diffusion is extremely slow because particles are fixed in position.
扩散可以发生在气体和液体中。在固体中，扩散极慢，因为粒子位置固定。
3. Factors Affecting Diffusion | 影响扩散的因素
The greater the concentration gradient, the faster the rate of diffusion. Particles move from high to low concentration more quickly when the difference is large.
浓度梯度越大，扩散速率越快。当浓度差较大时，粒子从高浓度向低浓度移动得更快。
Higher temperature increases the kinetic energy of particles, so they move faster and diffusion speeds up.
较高温度会增加粒子的动能，使它们运动更快，从而加速扩散。
A larger surface area allows more particles to cross a membrane at the same time, increasing the rate.
更大的表面积允许更多粒子同时穿过膜，从而提高速率。
A shorter diffusion distance means particles do not have to travel as far, so the rate is higher. For example, alveolar walls are very thin.
更短的扩散距离意味着粒子不需要移动很远，因此速率更高。例如，肺泡壁非常薄。
The thinner the cell membrane, the faster diffusion occurs. A thick wall would slow the process.
细胞膜越薄，扩散发生得越快。较厚的壁会减慢该过程。
4. Osmosis | 渗透作用
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 special type of diffusion.
渗透作用是水分子通过部分通透膜，从水势较高区域向水势较低区域的净移动。这是一种特殊的扩散。
Water potential measures the tendency of water to move out of a solution. Pure water has the highest water potential, usually assigned a value of 0. Adding solute reduces the water potential and makes it more negative.
水势衡量水分子从溶液中移动出去的趋势。纯水的水势最高，通常定为0。加入溶质会降低水势，使其变得更负。
Water movement: high water potential → low water potential
水分子从高水势向低水势移动。
For example, if a cell is placed in pure water, water enters because the cell has a lower water potential than the surrounding water.
例如，将细胞放入纯水中时，由于细胞水势低于周围水，水会进入细胞。
5. Water Potential and Solute Concentration | 水势和溶质浓度
The solute concentration of a solution affects its water potential. A solution with a high solute concentration has a low water potential, because solute molecules bind to water molecules and reduce their movement.
溶液的溶质浓度影响其水势。溶质浓度高的溶液水势低，因为溶质分子与水分子
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