We can also compare simple and facilitated diffusion. Question: How do rates of simple and facilitated diffusion differ in response to a concentration gradient? Method: The rate of simple diffusion can be expressed by a modification of Fick's Law for small, nonpolar molecules.
We can describe the rate of diffusion as directly proportional to the concentration gradient by the following equation:. P is a constant relating the ease of entry of a molecule into the cell depending on the molecule's size and lipid solubility.
If we graph the rate of diffusion as a function of the concentration gradient, we get a simple linear function.
Interpretation: Notice the rate of diffusion increases as the concentration gradient increases. If the concentration of molecules outside the cell is very high relative to the internal cell concentration, the rate of diffusion will also be high.
If the internal and external concentrations are similar low concentration gradient the rate of diffusion will be low. Method: Unlike simple diffusion, facilitated diffusion involves a limited number of carrier proteins. At low concentrations, molecules pass through the carrier proteins in a way similar to that of simple diffusion.
At high solute concentrations, however, all the proteins are occupied with the diffusing molecules. Increasing the solute concentration further will not change the rate of diffusion. In other words, there is some maximum rate of diffusion Vmax when all the carrier pro teins are saturated.
The mass transfer of a species is the evolution of its concentration in space and time. If the concentration of a species is initially not uniform the concentration might be greater in one region of a vessel than another, for example then, over time, diffusion causes mass transfer in favor of a more uniform concentration. The driving force for diffusion is the thermal motion of molecules. At temperatures above absolute zero, molecules are never at rest. Their kinetic energy means that they are always in motion, and when molecules collide with each other frequently, the direction of the motion becomes randomized.
In most cases, these collisions are common; even in air at atmospheric pressure, which hardly seems a "dense" fluid, each molecule collides with a neighbor every few nanoseconds.
When molecules are moving but also constantly changing direction, diffusion occurs because of the statistics of this movement. The image below shows a volume of a solution in which there is a nonuniform concentration. The red color indicates a high concentration of solute, whereas the blue color indicates nearly pure solvent. In the figure below, the initial condition is shown with arrows whose size and direction show the number of molecules moving in a particular direction at one time — remember that their motion is random, so they'll be moving equally in all directions from any point:.
At most points in the system, the uniform concentration means that the number of molecules moving in opposite directions is the same. As you can see, however, close to the boundary between the region of high and low concentration, there will be many more molecules moving to the right than moving to the left:.
This is not because the molecules "prefer" to move in one direction, but just because there are more of them on one side of the boundary than the other. As a consequence, there is a net flux of material from left to right. This is diffusion. In this case, mass moves from left to right so that the concentration becomes globally more uniform. Crossing a membrane by simple diffusion can be distinguished from facilitated diffusion because:. If the particles can move through the lipid bilayer by simple diffusion, then there is no limit to the number that can fit through the membrane.
The rate of diffusion increases linearly as we add more particles to one side of the membrane. If the particles can only pass through protein channels, then the rate of diffusion is determined by the number of channels as well as the number of particles. Once the channels operate at their maximal rate, a further increase in particle numbers no longer increases the apparent rate of diffusion.
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