Only small neutral molecules can pass unaided through cell membranes. Other kinds of particles—ions that mediate neural signaling, sugars that provide energy, amino acids that form proteins, and so on—enter and leave cells through specialized molecular portals known collectively as membrane transport proteins. To do their vital jobs, these proteins must recognize and grant passage, when required, to only one kind of molecule. And if getting that molecule into or out of a cell involves pushing against an electrical or concentration gradient, the transport protein has to marshal the necessary energy.

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