"It's rotating!"
Kana was watching a video of ATP synthase.
"It's a molecular motor," Rei explained. "Proton flow makes it rotate and produce ATP."
Milia showed her notebook. "100 rotations per second"
"That fast?" Kana was surprised.
"Embedded in the mitochondrial inner membrane. The engine of life."
Rei drew a diagram. "The electron transport chain pumps protons into the intermembrane space."
"Like a pump."
"Exactly. Proton pump. Using energy to create concentration gradient."
Kana thought. "Normally it flows from high to low, but you're pumping it up the opposite way."
"Yes. But that gradient becomes the energy source."
Milia wrote an equation. "ΔG = RTln(c2/c1) + zFΔΨ"
"Chemical potential and electrical potential," Rei explained. "The difference between both is the proton-motive force."
"Difficult..." Kana murmured.
"Simply put, it's like a dam," Rei compared. "The electron transport chain pumps up water. ATP synthase is the waterwheel."
"The water drop generates power."
"Correct. Proton flow makes it rotate. That rotational energy synthesizes ATP."
Kana began to understand. "It's indirect."
"Yes. You don't make ATP directly from nutrients. First create proton gradient, then use it to make ATP."
Milia showed a note. "Chemiosmotic theory: Peter Mitchell, 1978 Nobel Prize"
"At first it wasn't believed," Rei said. "Why bother using protons?"
"Why?"
"Control and coupling," Rei explained. "Different metabolic pathways can be integrated through proton gradient."
Kana wrote in her notebook. "Like a common currency."
"Good metaphor. Proton gradient is energy's common currency."
"Same mechanism in chloroplasts?"
"Exactly. Pump protons across thylakoid membrane to make ATP."
Milia supplemented. "Bacteria too, prokaryotes too."
"All life uses this principle," Rei said. "Universal mechanism."
Kana suddenly thought. "What if the pump breaks?"
"Death," Rei said quietly. "Without ATP, life activities stop."
"That important."
"Cyanide is poison because it stops the electron transport chain. Can't create proton gradient."
Milia nodded sadly.
"But what's the efficiency?" Kana asked. "Is it perfect?"
"About 40%," Rei answered. "The rest becomes heat."
"That's wasteful."
"But more efficient than car engines. And it works at room temperature."
Kana was impressed. "Amazing."
Rei continued. "ATP synthase structure is also beautiful. F1 part and F0 part."
"F1 is the head, F0 is the shaft," Milia drew.
"F0 is the proton pathway. When protons pass through, it rotates."
"That rotation transfers to F1, joining ADP and phosphate."
Kana watched the video again. "Like a machine."
"Biological machine," Rei smiled. "But more precise than artificial ones."
"It works as a single molecule?"
"Yes. One ATP synthase functions independently."
Milia showed a note. "About 300 ATP per second"
"From one molecule, that many?" Kana was surprised.
"In the whole cell, hundreds of millions per second," Rei said. "Working continuously without rest."
"The mission of proton pumps," Kana murmured.
"The mission that supports life," Rei answered.
Milia looked outside the window. Breathing. That's proof that in mitochondria throughout the body, protons are flowing and ATP synthase is rotating.
"Invisible, but I can feel it," Kana said.
"Yes. Life moves through invisible rotations."
The three sat quietly. Inside their bodies, at this very moment, trillions of proton pumps fulfill their mission. To live is to keep rotating.