Lipid membrane · Lobeworks/17

The lipid membrane is the thin double sheet of phospholipids, about 5 nm thick, that encloses every cell and its compartments, and it matters to the brain because its oily core is almost impassable to ions, so a neuron can hold different concentrations on its two sides and only lets charge through where it places a pro

Lipid membrane. The lipid membrane is the thin double sheet of phospholipids, about 5 nm thick, that encloses every cell and its compartments, and it matters to the brain because its oily core is almost impassable to ions, so a neuron can hold different concentrations on its two sides and only lets charge through where it places a protein.

Each phospholipid has a head that likes water and two fatty tails that avoid it, so in water they assemble by themselves into two layers with the tails facing inwards. That core has a dielectric constant near 2 against water's 80: pulling a charged ion out of water into it costs so much energy that sodium and potassium cross bare lipid about a billion times more slowly than water does, while oxygen, carbon dioxide and small fat-soluble molecules slip through. Everything else crosses on proteins set into the sheet: ion channels, pumps such as the sodium-potassium pump, transporters and receptors.

The barrier is what makes the battery. Because ions cannot leak through the lipid, the pump can build gradients and the channels can spend them, which gives the resting membrane potential and every spike.

It is also a capacitor. Two conducting solutions separated by a thin insulator store charge, about 1 µF per cm² of membrane, the membrane capacitance that sets how fast the voltage can change.

It is a fluid. Lipids and many proteins drift sideways within the sheet, and its makeup (cholesterol, the length of the tails) changes how leaky and how stiff it is.

The membrane is an insulator with doors.

The lipid keeps the ions apart, and the brain's whole electrical life happens at the proteins that let them through.

Questions: Why can sodium and potassium not cross the cell membrane without a channel? Because the middle of the membrane is oil. Its fatty core has a dielectric constant near 2 against water's 80, so taking a charged ion out of water and into it costs a great deal of energy; bare lipid lets sodium and potassium through about a billion times more slowly than water. A channel is a protein that lines a water-filled path through the oil, which is why the brain can decide exactly where and when ions cross.