"How is ATP made?"
Kana stared at a diagram of mitochondria.
Milia answered. "With a rotating molecular machine."
"Rotating?"
"ATP synthase. Called F₀F₁-ATPase."
Rei took out a model. "This is its structure."
Kana was surprised. "It really rotates?"
"Over 100 times per second. Driven by proton flow."
"Protons?"
Milia explained. "Across the mitochondrial inner membrane, there's a proton gradient."
"Gradient?"
"High H⁺ on one side, low on the other. Concentration difference."
Rei drew a diagram. "The electron transport chain pumps protons outside the membrane."
"Why?"
"Using energy from electron flow. NADH → Complex I → CoQ → Complex III → Cyt c → Complex IV → O₂"
Kana wrote in her notebook. "Complex..."
"At each step, protons are pumped out," Milia continued.
"And a gradient forms?"
"Yes. Outside the membrane has excess H⁺, inside has deficit."
Rei emphasized. "This becomes potential energy."
"Potential energy?"
"Water at high places flows to low places. That energy turns waterwheels."
Kana understood. "Protons are water?"
"Exactly. When protons return through the membrane, they rotate ATP synthase."
Milia moved the model. "F₀ part is the rotor. F₁ part is the stator."
"Rotor?"
"When protons pass through, F₀ rotates. That rotation is transmitted to F₁."
Rei continued. "F₁ has three catalytic sites. Structure changes with rotation."
"When it changes?"
"ADP + Pi → ATP. Three sites synthesize in sequence."
Kana was moved. "Like a factory."
"A molecular factory," Milia smiled.
Rei showed numbers. "One rotation produces three ATP molecules."
"Efficient."
"But not perfect. Protons sometimes leak."
Kana asked. "Who discovered this mechanism?"
"Peter Mitchell. Proposed the chemiosmotic theory," Milia answered.
"Initially unbelievable."
"Why?"
"Not direct reaction, but through proton gradient. Too indirect."
Rei added. "But proven by experiments. Revolutionary discovery."
Kana murmured. "Storing energy as a gradient."
"Like a battery," Milia explained.
"And when needed, convert to ATP."
Rei gave another example. "Chloroplasts use the same mechanism. Create proton gradient with light energy."
"Photosynthesis?"
"Yes. CF₀CF₁-ATPase. Almost the same structure as mitochondria."
Kana was surprised. "The same machine in different places?"
"Evolutionarily conserved. That's how excellent the design is."
Milia said quietly. "ATP synthesis is life's core."
"Why?"
"All energy-requiring reactions depend on ATP."
Rei listed. "Muscle contraction, nerve transmission, substance synthesis, active transport."
"Without ATP, can't live."
Kana stared at the model. "This small machine moves me."
"Every day, we make and use ATP equal to our body weight," Milia said.
"That much!"
"Constantly repeating synthesis and decomposition. Dynamic equilibrium."
Rei looked outside. "Invisible, but rotating tirelessly."
Kana smiled. "The mystery of ATP synthesis. It's life that rotates."
"And that rotation moves the world."
The three quietly felt the rotation of the invisible molecular machine.