On the contrary, concentration gradients are a form of potential energy, dissipated as the gradient is eliminated. Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient.
Within a system, there will be different rates of diffusion of the different substances in the medium. Molecules move constantly in a random manner at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature.
This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. After a substance has diffused completely through a space removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another.
This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion:. A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly.
This occurs in the kidney where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules. The rate of diffusion in this instance is almost totally dependent on pressure. Learning Objectives Describe diffusion and the factors that affect how materials move across the cell membrane.
This movement is affected by the size of the particle and the environment the particle is in. Particles will always move around in a medium but the overall rate of diffusion can be affected by many factors. Concentration : Diffusion of molecules is entirely dependent on moving from an area of higher concentration to an area of lower concentration.
In other words, diffusion occurs down the concentration gradient of the molecule in question. If the difference in concentration is higher, then the molecules will go down the concentration gradient faster.
If there is not as great of a difference in concentration, the molecules will not move as quickly and the rate of diffusion will decrease. Temperature: Particles move due to the kinetic energy associated with them. As temperature increases, the kinetic energy associated with each particle also increases.
As a result, particles will move faster. If they can move faster, then they can also diffuse faster. Conversely, when the kinetic energy associated with the molecules decreases so does their movement.
As a result, the rate of diffusion will be slower. Mass of Particle: Heavier particles will move more slowly and so will have a slower rate of diffusion. Smaller particles on the other hand will diffuse faster because they can move faster. As is key with all factors affecting diffusion, movement of the particle is paramount in determining if diffusion is slowed down or sped up. Solvent Properties: Viscosity and density greatly affect diffusion.
If the medium that a given particle has to diffuse through is very dense or viscous, then the particle will have a harder time diffusing through it. It originates with the atoms which move of themselves [i.
Many scientists have explored this random molecular motion in a variety of contexts, most famously by the Scottish botanist Robert Brown in the 19 th century. However, it would take nearly a century for scientists to mathematically quantify Brownian motion and demonstrate that this random movement of molecules dictates diffusion.
When molecules move from an area of high concentration to an area of low concentration,. About the same time that Brown was making his observations , a group of scientists including the French engineer Sadi Carnot and German physicist Rudolph Clausius were establishing a whole new field of scientific study: the field of Thermodynamics see our Thermodynamics I module for more information.
The molecules of a solid are generally considered to be locked in place though they vibrate ; however, the molecules of a liquid or a gas are free to move around, and they do: bumping in to one another or the walls of their container like balls on a pool table. As molecules in a liquid or gas move through space, they bump into one another and follow random paths — moving in a straight line until something blocks their way and then bouncing off of that thing.
It is this spontaneous and random motion that leads to diffusion. For example, as the scent molecules from baking cookies move into the air, they interact with air molecules — crashing into them and changing direction.
Over time, these random processes will cause the scent molecules to disperse throughout the room. Diffusion is presented as a process in which a substance moves down a concentration gradient — from an area of high concentration to an area of low concentration.
However, it is important to recognize that there is no directional force at play — the scent molecules are not pushed to the edge of the room because the concentration is lower there. It is the random movement of these molecules within the roomful of moving air molecules that causes them to evenly spread out throughout the entire space — bouncing off walls, moving through doors, and eventually moving through the whole house.
In this way, it appears to move along a concentration gradient — from the kitchen oven to the most distant rooms of the house. A simple illustration of this process can be seen using a glass of water and food coloring.
When a drop of food coloring enters the water, the food coloring molecules are highly concentrated at the location where the dye molecules meet the water molecules, giving the water in that area a very dark color Figure 2. The bottom of the glass initially has few or no food coloring molecules and so remains clear. As the food coloring molecules begin to interact with the water molecules, molecular collisions cause them to move randomly around the glass.
As collisions continue, the molecules spread out, or diffuse , over space. Eventually, the molecules spread throughout the entire glass, becoming evenly distributed and filling the space. At this point, the molecules have reached a state of equilibrium in which no net diffusion is taking place and the concentration gradient no longer exists.
Once equilibrium has been reached, the probability that a molecule will move from the top to the bottom is equal to the probability a molecule will move from the bottom to the top. We know that diffusion involves the movement of particles from one place to another; thus, the speed at which those particles move affects diffusion.
Since molecular motion can be measured by the heat of an object, it follows that the hotter a substance is the faster diffusion will take place in that substance.
Click the animation below to see how temperature affects diffusion. If you were to repeat your food coloring and water experiment comparing a glass of cold to a glass of hot water, you would see that the color disperses much more quickly in the hot water.
But what other factors influence the speed, or rate, at which diffusion takes place? In , the Scottish physical chemist Thomas Graham first quantified diffusion behavior before the idea of atoms and molecules was widely established. One of his experiments , detailed in Figure 3, used an apparatus with the open end of a tube sitting in a beaker of water and the other end sealed with a plaster stopper containing holes large enough for gases to enter and leave the tube.
Graham filled the open end of the tube with various gases as indicated by the red tube in Figure 3 , and observed the rate at which the gases effused , or escaped through the plaster plug. If the gas effused from the tube faster than the air outside of the tube moved in, the water level in the tube would rise. On the other hand, if the outside air moved through the plaster faster than the gas in the tube escaped to the outside, the water level in the tube would go down.
He used the rate of change in the water level to determine the relative rate at which the different gases diffused into air.
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