A cell is a city with gates. Each gate is a protein channel in the membrane: through it, ions, sugars, and signaling molecules pass in and out. Some gates are almost always open, while others open and close randomly, like subway doors that let a passenger through one moment and slam shut in their face the next.

Previously, physicists calculated the flow through a channel piecemeal: they accounted for the channel's shape separately, random openings separately, and the heterogeneity of the medium where molecules move sometimes fast, sometimes slow. The authors of the new work combined all three factors into a single formula. It predicts how many molecules will pass per second, given the channel's geometry, the frequency of openings, and the nature of molecular motion.

The result differs from some previous estimates in the physics literature. This means that old calculations could have overestimated or underestimated the flow in certain cases. The new formula gives biologists a more reliable way to predict how a drug will enter a cell or how a nerve impulse will pass through a channel.

So, the cell's gates are no longer a black box. Their capacity can now be calculated in advance, just as an engineer calculates the flow of cars through a tunnel, accounting for traffic lights, road width, and congestion.