Q. 13

[andrew.bra3483]

I'd like some help understanding Table 1 which is used to work question #13.

Looking at the K+ row, if K+ is more abundant inside the cell than outside, shouldn't that give an equilibrium potential of +90 mV, rather than -90 mV?

Similarly, summing up the Na+ K+ and Cl- charges gives a net charge of +156 mEq/L inside the cell, and only +30 mEq/L outside the cell. Yet I also know that the cell membrane potential is -70 mV, i.e. that the inside of the cell is negatively charged relative to the outside of the cell, so that conventional current (positive charge) wants to flow from outside to inside the cell. How do these numbers go together?

Finally, if 3 Na+ ions are booted out for every 2 K+ ions taken in, AND if K+ ions follow their concentration gradient and passively diffuse outside the cell, how come the K+ concentration gradient (144.5 mmol/L) is slightly LARGER than the Na+ concentration gradient (135 mmol/L)?

Any help is appreciated!

[admin]

You will find a complete response to your various questions here: http://en.wikipedia.org/wiki/Cell_potential

However, it's important for you to realize that for the purposes of the new MCAT, the actual information that you require is far less than the answers that your questions would produce. In other words, the question based on the passage/table is really asking about your understanding of simple diffusion as opposed to your understanding of the dynamics involved in developing a membrane potential.

[admin]

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