If the lipid membrane insulates, how do ions cross it at all? · Lobeworks/17

Through proteins that make holes in the insulator: ion channels and transporters.

If the lipid membrane insulates, how do ions cross it at all?

Through proteins that make holes in the insulator: ion channels and transporters. The oily core of the bilayer keeps ions out because leaving water for a hydrocarbon would cost them far more energy than the membrane voltage supplies, so the membrane's resistance is set by how many channels are open. Opening voltage-gated sodium channels is, in electrical terms, punching thousands of tiny holes in the insulation for a millisecond.