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So many metaphors

Here is a little physics/biology with a couple metaphors. Good/bad? Mixed? (This is for a class I'm teaching to college seniors, so I might take this down later.)

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The semipermeable cell membrane of a neuron is mainly a phospholipid bilayer. Because the hydrophobic tails of the phospholipids are in the middle, it's difficult for ions to cross the membrane. The outer part of the membrane is polar on both sides, but the nonpolar inside is enough to scare all the ions away. (Imagine the nonpolar center is the Pacific Ocean, separating California from Hawaii. The ions would be happy in California or Hawaii, but they don't have any transportation ... unless they find a boat or a plane).

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Ions are able to cross the membrane because of ion channels (boats and planes). We will speak more about channels, but you should be aware that there are channels of various selectivity that allow ions to cross the membrane. It's like a police escort through shark-infested waters: each escort may pick up a group of people, or may choose only members of the same group. This is (lack of) selectivity.

Because ion channels are few and far between compared to the rest of the cell membrane, the cell membrane plays the role of a capacitor, resistor, and battery.

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Without ion channels, the cell membrane is impermeable, so there's a lot of pressure building up (aka the potential difference/voltage across the membrane). Adding ion channels is like poking tiny holes in the membrane, because suddenly ions can flow across the membrane. The more holes you have, the easier it is for ions to flow. The easier it is for ions to flow across the membrane, the more resistance decreases and conductance increases.

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