AP Biology · Unit 2 · Topic 2.6
An ion cannot cross an oily membrane interior on its own. Add the right protein and the flow starts, still with no energy spent.
Learning objective 2.6.A Explain how the structure of a molecule affects its ability to pass through the plasma membrane.
Narrated walkthrough of the “Find the right door” tab. Captions are on by default.
Four embedded proteins, each selective for a different substance. Drag a solute at a protein and see whether that protein will take it. An ion or a sugar cannot cross the interior on its own, so if no protein accepts it, it does not get across at all. Water is the exception: small and polar, a little seeps through the bare bilayer, and an aquaporin makes it fast.
Run the flow below from a gradient and it stops at equilibrium. Run it from equilibrium and nothing moves at all. A protein is a door, not an engine — it decides whether something can cross, never which way it goes.
A sodium ion is smaller than a potassium ion, and it still will not go through a potassium channel. If the pore were just a hole, the smaller ion would fit. It is not a hole — it is a structure whose shape and charge suit one ion and not the other.
Both of these are passive and both go down the gradient. Push the gradient up and watch what happens to each rate.
Selective permeability does more than hold concentration gradients. A resting cell keeps potassium high inside and sodium and chloride high outside. Open a channel and the ion it fits crosses down its own gradient, one at a time. If the charge crossing is not balanced, a difference in electrical charge builds up across the membrane, and that charge pulls back on the ions until the flow stops on its own. This is a different membrane from the one on the first tab: it has a chloride channel as well as the potassium and sodium channels.
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