If the internalĪnd external concentrations are similar (low concentration gradient) the rate of diffusion will be low.Ĭonstant determining speed of saturation (mol/cm 4) 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. Interpretation: Notice the rate of diffusion increases as the If we graph the rate of diffusion as a function of the concentration gradient, we get a simple linear function. Notice that when A and P are constants, this equation simply describes a line where dn/dt is a function of dC/dx. P is a constant relating the ease of entry of a molecule into the cell depending on the molecule's size and lipid solubility. Where A is the membrane area and P is the permeability constant. We can describe the rate of diffusion as directly proportional to the concentration gradient by the following equation: Gradient (dC/dx) will be positive, and net movement will be into the cell (positive value of dn/dt). When the concentration outside the cell (C out) is larger than inside the cell (C in), the concentration This is equivalent to (C out - C in)/ Dx where C out and C inĪre the substrate concentrations inside and outside the cell, and Dx is the width of the cell membrane. Gradient, dC/dx, is the difference in molecule concentration inside and outside of the cellĪcross a cell membrane of width dx. The concentration gradient (dC/dx) across the membrane. Since the net movement of diffusing molecules depends on theĬoncentration gradient, the rate of diffusion is directly proportional to The rate of diffusion, dn/dt, is the change in the number of diffusing molecules inside the cell over time. Method: The rate of simple diffusion can be expressed by a modification of Fick's Law for small, nonpolar molecules. Permeability constant for a particular molecule (cm/sec)Ĭoncentration of diffusing molecule (mol/cm 3) Question: How do rates of simple and facilitated diffusion differ in response to a concentration gradient? We can also compare simple and facilitated diffusion. We can use simple equations and graphs to examine how particular molecules and their concentration affect the rate Importance: The rate of diffusion is affected by properties of the cell, the diffusing molecule, and the surrounding solution. Specific substances to pass through, that might not be able to diffuse through the cell membrane. Facilitated diffusion depends on carrier proteins imbedded in the membrane to allow Simple passive diffusion occurs when small molecules pass through the lipid bilayer of a cell membrane. Of lower concentration in order to reach an equillibrium. Diffusion is random movement of molecules but has a net direction toward regions Introduction: Substances, such as water, ions, and molecules needed for cellular processes, can enter and leave cells by a passive process such as diffusion.
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