The difference in concentration of molecules on either side of the membrane is called a concentration gradient. The concentration gradient is responsible for driving transport of molecules across the semi-permeable cell membrane.
Exploration 1: Understanding how concentration gradient affects rate of selective diffusion
In this exploration, you will observe how the concentration of a particular molecule, for example chloride ions, in the cell affects the rate of diffusion. The effect of presence of different molecules in the cell on the rate of diffusion of another molecule will also be examined.
Setup a simulation where chloride ions diffuse in and out of the cell. In this exercise, chloride ions will be represented by green circles. Press the “reset all” button to remove all of the molecules and channels.
1. Add 20 chloride ions inside and 5 chloride ions to the outside of the cell and set the animation speed (by moving the slider at the bottom) at the default speed.
2. Check the box 'SHOW CONCENTRATION'.
Answer Question 3 and 4
3. Click the PAUSE icon (next to the slider) to pause the simulation. Place 2 green leakage channels into the membrane.
4. Observe the bar graph on the right side of the simulation screen.
5. Begin a timer and click the PLAY icon at the same time. Keep observing the bar graph as the simulation progresses.
6. Stop the timer and note the time when you think concentration (the number) of the chloride ions is equal on both sides; that is, when equilibrium is reached.
Answer Question 5:
Earlier in the exploration, you made a hypothesis. You estimated the amount of time it will take to reach equilibrium by observing the bar graph. Now, you will use another method.
1. Press the “reset all” button to remove all of the molecules and channels.
2. Add 60 chloride ions (green circles) inside and 15 chloride ions to the outside of the cell and set the animation speed (by moving the slider at the bottom) at 170.
3. Click the PAUSE icon (next to the slider) to pause the simulation. Place 2 green leakage channels into the membrane.
4. 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 5 minutes. Complete Table 1
5. Press the “reset all” button to remove all of the molecules and channels.
6. Follow the steps mentioned earlier but add 40 chloride ions inside the cell.
7. 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 5 minutes. Complete Table 2
You will now test if the presence of different molecules in the cell affects the rate of diffusion of a particular molecule. Setup a simulation where a cell contains sodium ions in addition to chloride ions. In the model, blue diamonds will represent sodium ions.
1. Press the “reset all” button to remove all of the molecules and channels.
2. Follow the steps mentioned earlier but add 20 chloride ions (green circles) and 20 sodium ions (blue diamonds) inside the cell.
3. 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 5 minutes. Complete Table 3
