Diffusion and the Axon

Now see if you can apply diffusion to the movement of ions across the membrane of the axon. In Figure 3 you will find a diagrammatic representation of the resting state of the axon.

Figure3. A diagram of the axon during the resting potential. The symbols represent different ions and the relative number of a given symbol inside and outside of the axon indicates the relative concentration of that ion inside and outside of the axon. The gaps covered with color bars are ion channels. The color of the bar represents the type of ion channel, i.e., the rust bars are sodium channels and the dark green bars are potassium channels. Click on the figure to see a diagrammatic representation of the events at the membrane that result from diffusion and the opening of first sodium and then potassium channels.

As Figure 3 shows, during the resting phase of the axon, the concentration of sodium is greater outside and the concentration of potassium is greater on the inside of the axon. Also indicated in Figure 3 are the two most important ion channels for understanding the action potential, sodium and potassium channels. Try these questions.  See if you can answer them, using the concept of diffusion only.

Question 1.

If sodium channels were to open, as in Figure 4, what would be the general direction of flow of the ions through channel and why?

(A). Out; since the greater concentration of sodium outside would draw the ions.
(B). In; since random movements would lead to more moving in than out.
(C). In; the greater concentration outside would push the ions in.
(D). Out; the ion environment outside is more conducive to the presence of sodium ions.

 Review the role that charged ions play in generating the action potential
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