Learning fascinating stuff.
- Potassium channels are ‘selectively permeable’ to only potassium because they actually have a filter (carbonyl oxygens) on their outer surface that dehydrates ions. Only dehydrated ions can pass through the channel protein. Since, sodium ions are smaller than potassium ions, and the filter has a fixed width which fits just for potassium, the sodium ions pass through without touching both sides of the filter (cause they are smaller) and do not get dehydrated by the filter. Hence, they cannot pass through potassium channels. Think about why selective permeability would not be possible if the potassium pump used just size of ion as a selecting criteria ; )
- Enzymes (and similarly proteins) can run in reverse! Like the sodium potassium pump, pumps sodium out and potassium into the cell against their concentration gradient using ATP. However, if we were to increase their electrochemical gradients to such an extent that the pump can’t really push against such an overly ‘steep’ gradient, the ions will flow in down their gradient (in opposite direction to sodium potassium pump). So the pump runs in reverse, and what’s interesting is; while it runs in reverse; it (ATP synthase in the pump) generates ATP from ADP and phosphate! So, it’s kind of like a dynamo running down a hill, utilizing the reverse flow of ions.
- Only 1/100000000 of the total positive charges inside the cell membrane need to go out of change the +35 mv to – 90mv (and the other way around) which takes place in a matter of 1/10000th of a second. Hence, transfer of incredibly small number of ions can cause big changes in the resting potential of membrane. That’s the reason why nerve impulses can conduct so quickly.
- The ‘Voltage Clamp’ method of measuring the effect of voltage on opening and closing of voltage gated channels, was such an ingenious experiment by Hodgkin and Huxley that they won a Nobel prize for it. It’s a really cool method but it will take some time to explain it all and it’s midnight already hehe, so google it if you want to.
- Apart from the much-discussed sodium and potassium ions, there are other ions that also play a part in maintaining resting potential. These are negative anions inside the cells and calcium ions. Calcium ions particularly are interesting because they play a very similar role to sodium ions for maintaining membrane potential. What’s more, if there is a deficiency of calcium ions in the body, the voltage gated sodium ion gates become more sensitive to stimuli than normal which can lead to spontaneous discharge of nerve impulses which causes spontaneous muscular contraction throughout the body, a condition called Tetany, which can be life threatening because of contraction of respiratory muscles. Btw ‘Tetany’ and ‘Tetanus’ are not the same thing.
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