Molecules like water and lipids diffuse freely through the cell membrane. However, most molecules cannot diffuse similarly because of their size or electric charge and require protein channels that facilitate their transport across the cell membrane. As shown in Fig. 3, certain types of protein channels present on the surface of red blood cells facilitate the exchange of glucose molecules. Similarly, other types of protein channels facilitate the transport of ATP molecules in and out of the cell. Like glucose, ATP molecules are a source of energy for the cell. Fig. 5A and 5B show how red blood cells look like when observed under a scanning electron microscope.

Fig. 5A shows how red blood cells appear when they are immersed in a solution in which the concentration of ATP is equal on either side of the membrane. In this case, there is adequate ATP for the cell to carry out its functions and the cell is healthy and retains its donut like shape. Fig. 5B shows the red blood cells depleted of ATP. The cell, deprived of energy fails to carry out its functions and loses its shape and it shrinks. Eventually it dies.

In this exploration, you will observe how concentration of glucose molecules affects its rate of diffusion. Setup a simulation where glucose molecules are diffusing into the cell. In this exercise, glucose molecules will be represented by green circles.

1.     Press the “reset all” button to remove all of the molecules and channels.

2.     Add 5 glucose molecules inside and 20 glucose molecules outside of the cell and set the animation speed (by moving the slider at the bottom) at 170.

3.     Check the box 'SHOW CONCENTRATION'.

4.     Click the PAUSE icon (next to the slider) to pause the simulation. Place 2 green gated channels into the membrane.

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 4

Answer Questions 10 and 11

5.     Press the “reset all” button to remove all of the molecules and channels.

6.     Follow the steps mentioned earlier but add 40 glucose molecules 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 3 minutes.