In other words, there is some maximum rate of diffusion Vmax when all the carrier pro teins are saturated. Therefore, we can not use a simple linear equation to describe the rate of diffusion. The rate of diffusion will increase with increasing solute concentration, but must asymptotically approach the saturation rate, V max.
K and Vmax depend on properties of the diffusing molecule, such as its permeability P , as well as the surface area A of the cell, but for simplification we give the equation as:. Interpretation: By graphing this equation, we see that at low concentrations of solute, the rate of diffusion into a cell occurs almost linearly, like simple diffusion.
Notice that at low solute concentrations, the slope is much steeper than that of simple diffusion. Facilitated diffusion can increase the rate of diffusion of particular molecules at low concentrations. However, the rate of facilitated diffusion levels off with increasing solute concentration. Additional increases in external solute concentration cannot increase the rate of diffusion once carrier proteins are saturated.
Conclusion: Passive diffusion of solute into a cell is linearly related to the concentration of solute outside the cell. Carrier proteins increase the rate of diffusion by allowing more solute to enter the cell.
As well as diffusion occurring between different regions, it also occurs across membranes, between the outside and inside of cells. The rate of diffusion can be affected by a number of factors:. For a bacterium , substances diffuse into and out of the bacterial cell across its surface.
For simple multicellular organisms , such as small plants like mosses, substances diffuse into the leaves and roots over their surface.
Again, once inside the plant, they don't need to move far. Substances move into and around the moss plants by diffusion and osmosis. Simple organisms take in substances over their body surface. Their needs are determined by their volume. 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.
Key Points Substances diffuse according to their concentration gradient; within a system, different substances in the medium will each diffuse at different rates according to their individual gradients. 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, a state known as dynamic equilibrium.
If there is not sufficient energy to move a larger molecule, it resists the effects of diffusion and is unable to move from one area to the next. Increases in environmental heat or other extraneous factors also have an impact on the energy involved in the diffusion process and the rate at which a large molecule diffuses. World View.
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