Later that night, ATP sat by the membrane, exhausted but content. A young vesicle floated by, curious. “So active transport is just moving things against the flow?” it asked.

“Yes,” ATP said. “But the types depend on how you pay for it. is direct—I burn my own cash (ATP). Secondary is indirect—I create a traffic jam of ions (like sodium) and then use their rush to drag other things along, either in the same direction (Symporter) or opposite (Antiporter).”

ATP burned one of his own phosphate groups, releasing a burst of energy. Click, whirr. The Uniporter spun, and Gus was whisked inside, moving against his natural gradient. This, ATP explained to a passing mitochondrion, was the first type of active transport: .

“Nat!” ATP called out. “I have a deal for you. You’re going to flow inside anyway. But if you do it through this special door, you can drag Suzy along with you.”