Diffusion

扩散

Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. If the partition is removed as in the lower illustration, the gases will mix because of the random velocities of their molecules. In time a uniform mixture of A and B molecules will be produced in the container.

扩散是指由于分子的随机运动所产生的动能，使分子相互混合的过程。考虑两个分别装有气体A和B的容器，它们由一个隔板隔开。两种气体的分子都在不断运动，并与隔板频繁碰撞。如果像下图所示那样移除隔板，由于分子的随机速度，气体将会混合。随着时间的推移，容器中将形成均匀的A和B分子混合物。

The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles.

在室温下，由于粒子的热能所导致的高分子运动速度，扩散倾向非常强烈。

Rate of diffusion

扩散速率

Osmosis

渗透

Thermal energy

热能

Energetics of solute diffusion

溶质扩散的能学

Index

Kinetic theory concepts

Applications of kinetic theory

Fluid concepts

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Rate of Diffusion

扩散速率

Since the average kinetic energy of different types of molecules (different masses) which are at thermal equilibrium is the same, then their average velocities are different. Their average diffusion rate is expected to depend upon that average velocity, which gives a relative diffusion rate

由于不同种类（不同质量）的分子在热平衡时的平均动能相同，因此它们的平均速度不同。它们的平均扩散速率预计取决于这个平均速度，从而给出相对的扩散速率。

where the constant K depends upon geometric factors including the area across which the diffusion is occuring. The relative diffusion rate for two different molecular species is then given by

其中常数K取决于包括扩散发生区域在内的几何因素。两种不同分子物种的相对扩散速率则由以下公式给出：

Index

Kinetic theory concepts

Applications of kinetic theory

Fluid concepts

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Osmosis

渗透

If two solutions of different concentration are separated by a semi-permeable membrane which is permeable to the smaller solvent molecules but not to the larger solute molecules, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis.

如果两种浓度不同的溶液被半透膜分隔，该膜允许较小的溶剂分子通过，但不允许较大的溶质分子通过，那么溶剂会倾向于从浓度较低的溶液扩散到浓度较高的溶液。这个过程称为渗透。

Osmosis is of great importance in biological processes where the solvent is water. The transport of water and other molecules across biological membranes is essential to many processes in living organisms. The energy which drives the process is usually discussed in terms of osmotic pressure.

渗透作用在生物过程中具有重要意义，其中溶剂是水。水和其他分子通过生物膜的运输对许多生命过程至关重要。驱动这一过程的能量通常以渗透压来讨论。

Index

Kinetic theory concepts

Applications of kinetic theory

Fluid concepts

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Osmotic Pressure

渗透压

Osmosis is a selective diffusion process driven by the internal energy of the solvent molecules. It is convenient to express the available energy per unit volume in terms of "osmotic pressure". It is customary to express this tendency toward solvent transport in pressure units relative to the pure solvent.

If pure water were on both sides of the membrane, the osmotic pressure difference would be zero. But if normal human blood were on the right side of the membrane, the osmotic pressure would be about seven atmospheres! This illustrates how potent the influence of osmotic pressure is for membrane transport in living organisms.

如果纯水在膜的两侧，渗透压差将为零。但若正常的人类血液在膜的右侧，渗透压将约为七大气压！这说明渗透压对生物体内膜运输的影响有多么显著。

The decision about which side of the membrane to call "high" osmotic pressure is a troublesome one. The choice made here is the opposite of that made in many biology texts, which attribute "high" osmotic pressure to the solution and zero osmotic pressure to pure water. The rationale for the choice is that the energy which drives the fluid transfer is the thermal energy of the water molecules, and that energy density is higher in the pure solvent since there are more water molecules. The thermal energy of the solute molecules does not contribute to transport, presuming that the membrane is impermeable to them. The choice is also influenced by the observed direction of fluid movement, since under this choice the fluid transport is from high "pressure" to low, congruent with normal fluid flow through pipes from high pressure to low. The final rationale has to do with the measurement of osmotic pressure by determining how much hydrostatic pressure on the solution is required to prevent the transport of water from a pure source across a semi-permeable membrane into the soluton. A positive pressure must be exerted on the solution to prevent osmotic transport, again congruent with the concept that the osmotic pressure of the pure solvent is relatively "high".

关于将膜的哪一侧称为“高”渗透压的问题是一个棘手的问题。此处的选择与许多生物学教材中的选择相反，这些教材将“高”渗透压归于溶液，而将纯水的渗透压视为零。选择的理由是，驱动流体转移的能量是水分子的热能，而由于纯溶剂中水分子更多，因此单位体积内的能量密度更高。假设膜对溶质分子是不通透的，溶质分子的热能不参与传输。这一选择也受到观察到的流体流动方向的影响，因为在这一选择下，流体传输是从高“压力”向低“压力”方向进行，这与通过管道从高压向低压方向流动的正常流体流动方向一致。最后，这一选择还与通过测定使溶液所受的静压强来确定渗透压有关，即确定需要多大的静压强才能阻止水从纯源通过半透膜进入溶液。必须对溶液施加正压以阻止渗透传输，这再次与渗透压为“高”的纯溶剂的概念一致。

Nevertheless, the dialog continues on this issue since the discussion of osmosis is most relevant to the biological and life sciences and perhaps the logic stated above should yield to the conventions of the field in which the phenomena are most relevant.

然而，关于这个问题的讨论仍将继续，因为渗透压的讨论最相关于生物科学和生命科学，也许上述的逻辑应让位于这些领域中现象最相关的惯例。

渗透是一种由溶剂分子内部能量驱动的选择性扩散过程。方便地，可以将单位体积中的可用能量表示为“渗透压”。通常，这种溶剂运输的趋势可以以相对于纯溶剂的压强单位来表达。

Measuring osmotic pressure

测量渗透压

Calculating osmotic pressure

计算渗透压

Osmotic pressure example: egg in syrup

渗透压示例：鸡蛋在糖浆中

Index

Kinetic theory concepts

Applications of kinetic theory

Fluid concepts

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Measuring Osmotic Pressure

测量渗透压

One approach to the measurement of osmotic pressure is to measure the amount of hydrostatic pressure necessary to prevent fluid transfer by osmosis.

测量渗透压的一种方法是测量阻止渗透作用下流体转移所需的静液压力。

Membrane transport

膜运输

Index

Kinetic theory concepts

Applications of kinetic theory

Fluid concepts

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Membrane Transport

膜运输

The transport of water and other types of molecules across membranes is the key to many processes in living organisms. Many of these transport processes proceed by diffusion through membranes which are selectively permeable, allowing small molecules to pass but blocking larger ones. These processes, including osmosis and dialysis, are sometimes called passive transport since they do not require any active role for the membrane. Other types of transport, called active transport, involve properties of a cell membrane to selectively "pump" certain types of molecules across the membrane.

水和其他类型分子跨膜的运输是生物体内许多过程的关键。许多这些运输过程通过选择性渗透的膜进行扩散，允许小分子通过但阻止较大的分子。这些过程，包括渗透和透析，有时被称为被动运输，因为它们不需要膜的任何主动作用。其他类型的运输，称为主动运输，涉及细胞膜的特性，能够有选择地“泵”某些类型的分子穿过膜。

The transport of gases across membranes depends upon diffusion and the solubility of the gases involved. In life science applications such transport is characterized by Graham's Law and Fick's Law.

气体通过膜的运输取决于扩散和所涉及气体的溶解性。在生命科学应用中，这种运输特性由格雷厄姆定律和菲克定律描述。