Definition of diffusion
Diffusionis the net passive movement ofmoleculesor particles from regions of higher to regions of lower concentration. For diffusion to occur there must be aconcentration gradient. The dissimilarity in the amounts ofsolutes, particles, or molecules between the two regions will cause them to move between the two regions. To define diffusion in the biological context, imagine the ions, respiratory gases, glucose, or other particles that move randomly between the crowded and the less-crowded regions (fluids), often across a semi-permeable membrane. The unequal concentration of the particles between two fluids will generate a gradient that will incite them to move in order toequalizethe disparity in concentrations. Nevertheless, the movement of solutes during the diffusion process is not a one-way route. There could be movements to and fro. However, the movement is inclined towards the area of lower concentration. Thus, diffusion is characterized by anet movementof the particle down their concentration gradient — or in other words, from the area of greater concentration to an area of lower concentration. The net movement ofoxygenandcarbon dioxideacross the alveolar-capillary membrane of mammalianlungsand the net movement of glucose down the concentration gradient are diffusion examples in biological systems.
The worddiffusion来自拉丁语diffusionem,diffusio, meaning “a pouring forth”.
Diffusion in Physics, Chemistry, and Biology
扩散物理学is the movement of particles from an area of higher concentration to an area of lower concentration as driven by thermal energy.(1)This definition is affirmed as well in chemistry. The particles suspended in liquids and gases, for instance, struck each other resulting in their random constant motion. This motion due to the collisions of particles is calledpedesisor the Brownian movement. When these particles increase in number, they become compacted and the Brownian movement is seemingly lost as what can be observed in a concentrated solution. When provided with an augmented space they will start to move but in a rather organized manner from an area of high concentration to an area of low concentration, followingFick’s laws.(2)
The factors that affect diffusion rates are temperature, concentration, distance, and material.(3)In particular, a hot temperature causes the particles to acquire more kinetic energy (i.e. the energy possessed by the particles in motion), and therefore will move and push other particles more. Thus, the diffusion of particles becomes faster when the temperature is higher. Conversely, it is slower at a lower temperature. As for particle concentration, the greater the number of particles in a solution, the rate of diffusion will also be faster. As for the distance, the shorter the distance for the diffusing particles to travel, the faster the rate will be. The kind of material in the solution also affects the rate of diffusion. Smaller and lighter particles can spread more easily than larger and heavier ones. Thus, gaseous particles diffuse faster than the particles in liquids or solids. Liquids, in turn, diffuse quicker than solids.Diffusion in chemistry是由食品染料例举S IN的水溶液丢弃。染料颗粒可以看作移动，因为它们在整个溶液展开。另一个例子是香水喷洒时，香味会有所回升，因为它扩散到整个房间。
In physics and chemistry, diffusion is basically defined as the“spreading out”of the objects from the initial area of higher concentration.扩散生物学applies the same principle but the process involves a biological system, such as a semipermeable membrane. Furthermore, it is defined as anetmovement of particles or molecules. In biological systems, it is a type ofpassive transport. Passive transport refers to the type of cellular transport wherein the net movement of substances is down the concentration gradient. In contrast, cellular transport wherein substances have to be moved to an area that is already saturated or high in concentration is calledactive transport. Because the movement of substances in passive transport does not go against the concentration gradient it, therefore, does not require chemical energy (e.g. ATP) to proceed. Rather, it is driven by kinetic and natural energy. Other examples of passive transport arefiltrationandosmosis.
Types of Diffusion
扩散是两大类。向下移动的浓度梯度时，第一个不需要援助。这种类型被称为简单扩散. In contrast,facilitated diffusion，顾名思义，是一个在需要援助。援助来了，例如，从嵌入在生物膜中的蛋白质。一个发生无援助和与转运蛋白的帮助下发生的另一个 - 基本上，这两种类型的扩散的，通过该物质移动机构不同。因此，在易化扩散，传输只有当分子能够结合到膜蛋白转运发生。他们都导致物质的净下坡运动，不需要化学能继续进行。
Diffusion vs. Active Transport
While diffusion is the movement of particles down their concentration gradient, active transport is the movement of particles against the concentration gradient. Since the movement is characteristically uphill this type of transport requires energy often in the form of adenosine triphosphate (ATP).
Diffusion and Osmosis
Diffusion and osmosis are both types of passive transport. Thus, both of them occur in a downhill manner and without energy expenditure. The difference is the diffusing molecules or particles. In diffusion, the diffusing particles are the solutes of a solution. In osmosis, the diffusing particles are the solvent of the solution, i.e. water molecules. In osmosis, water molecules diffuse from an area of high water concentration to an area of low water concentration across a biological membrane. Water that is drawn towards a concentrated solution but does not involve a biological membrane is not osmosis.
Diffusion in Cells
In this table, the mechanism employed by certain types of molecules and ions in order to pass across the plasma membrane down their concentration gradient is shown.
|Molecules for transport||Type of diffusion|
|Small nonpolar molecules
(e.g. oxygen, carbon dioxide)
|通过简单扩散,即分子可以很容易地diffuse across the membrane|
(e.g. glucose and amino acids)
(e.g. sodium ions and chloride ions)
|By facilitated diffusion, i.e. the molecules require membrane proteins such aspermeasesto move along their concentration gradient|
|Large nonpolar molecules
|By facilitated diffusion, i.e. the molecules require membrane proteins such asretinol-binding proteinto move along their concentration gradient|
Diffusion in Plants and Animals
In plants and animals, diffusion is exemplified by the diffusion of gasses. In plants, the openings formed by the guard cells calledstomata是二氧化碳进入和氧气离开哪里the plant. The plant takes in carbon dioxide since it is one of the major reactants in photosynthesis. Oxygen, in turn, is a byproduct of photosynthesis and is then released as it diffuses into the environment through the stomata.
Similarly, in animals, respiratory gases are transported generally by simple diffusion. In humans, the diffusion of respiratory gases takes place at the capillary beds that separate blood from the tissue fluid. In the lungs, carbon dioxide is released by diffusing from the blood into the alveoli and then breathed out. Oxygen, in turn, is breathed in, and then diffuses from the alveoli of the lungs into the blood. The oxygen then diffuses from the circulating blood to the various tissues of the body.
- Anomalous diffusion
- Bohm diffusion
- Diffusion equilibrium
- Diffusion shell
- Facilitated diffusion
- Least diffusion circle
- Neoclassical diffusion
- Population diffusion coefficient
- Simple diffusion
- Lin, Y.C., Phua, S. C., Lin, B., & Inoue, T. (2013). Visualizing molecular diffusion through passive permeability barriers in cells: conventional and novel approaches.Current Opinion in Chemical Biology, 17（4），663-671。https://doi.org/10.1016/j.cbpa.2013.04.027
- Lecture 3: Diffusion: Fick’s first law. (n.d.). Retrieved from https://my.eng.utah.edu/~lzang/images/lecture-3.pdf
- Vinzant, A. (2017, September 7). DIY: Diffusion Science Experiment – Astrocamp School. Retrieved from Astrocamp School website: https://astrocampschool.org/diy-diffusion/
- Pratt, C. A., Voet, D., Voet, J. G. (2002). Fundamentals of biochemistry upgrade. New York: Wiley. pp. 264–266.
© Biology Online. Content provided and moderated byBiology Online Editors