In addition to concentration gradient, rate of facilitated diffusion also depends on the number of specific carrier protein molecules in the membrane. Considering the previously stated example of diffusion of glucose: No matter how much the cell “needs” the glucose- no matter how great the concentration gradient- the rate at which the glucose can diffuse into the cell has a limit because of the limited number of glucose carrier protein molecules in the lipid bi-layer.
To understand how number of protein channels available for diffusion affect rate of diffusion, again setup the previous simulation where glucose molecules are diffusing into the cell with the help of the carrier proteins present in the cell membrane.
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Add 5 glucose molecules (green circles) inside the cell. Add 20 glucose molecules outside of the cell.
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Set the animation speed (by moving the slider at the bottom) at 170.
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Check the box 'SHOW CONCENTRATION'.
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Click the PAUSE icon (next to the slider) to pause the simulation. Place 2 green gated channels into the membrane.
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Begin a timer for 20 seconds and click the PLAY icon at the same time. After every 20 seconds, pause the simulation and count the number of green circles on each side of the cell membrane. Do this for about 3 minutes. Complete Table 6
Complete Table 7 for the previous scenario with glucose molecules having 4 channels for diffusion.
Complete Table 8 for the previous scenario with glucose molecules having 8 channels for diffusion.
From tables 6, 7 and 8 find out how much time was needed in each case to reach equilibrium and fill in table 6. If you have a time range, take the value at the middle of that range as the required time.
