"Why does ATP have three phosphate groups attached?"
Kana asked while looking at the model.
Rei answered. "To store energy. The bonds between phosphates are high-energy."
"But they're both negatively charged, yet stuck together. Isn't that unstable?"
"That instability is the source of energy."
Toma interrupted. "So they want to come off?"
"You could say that. When hydrolyzed, ATP becomes ADP and phosphate. At that moment, large amounts of energy are released."
Kana wrote in her notebook. "ATP → ADP + Pi, ΔG = -30.5 kJ/mol"
"Accurate. These -30.5 kilojoules move muscles, synthesize materials, and transport ions."
"But why adenine? Why not other bases?"
Rei thought. "Good question. Actually, GTP and CTP have similar roles. But ATP is overwhelmingly more abundant."
"Why?"
"In the evolutionary process, adenine nucleotides were selected. The reason isn't fully understood, but probably because they were easier to use in early metabolic systems."
Toma asked. "What happens if all ATP is used up?"
"The cell dies. But normally it's regenerated in seconds."
"Seconds?"
"Yes. ATP's lifespan is very short. Made and broken down, broken down and resynthesized."
Kana was surprised. "It's recycled."
"Every day, ATP equivalent to your body weight is made and used."
"Body weight?" Toma's eyes widened.
"Yes. But the amount of ATP in the body stays constant. Rotating at tremendous speed."
Rei continued. "Resynthesis from ADP to ATP mainly occurs in mitochondria. A process called oxidative phosphorylation."
"So that's what mitochondria as energy factories means."
"More precisely, power plants. Using the proton gradient created by the electron transport chain, ATP synthase synthesizes ATP."
Kana became interested. "ATP synthase?"
"A rotating molecular machine. It rotates with proton flow and uses that force to combine ADP and phosphate."
"Rotating? A molecule?"
Rei showed a video. "This is the structure of ATP synthase. It really spins."
On the screen was a protein rotating like a motor.
"Unbelievable..." Toma muttered.
"It rotates hundreds of times per second. Each rotation produces 3 ATP molecules."
Kana summarized. "Adenine nucleotides are energy currency. They move between ATP, ADP, and AMP, carrying energy."
"Perfect," Rei acknowledged.
"But what is AMP used for?"
"AMP also serves as a signal for energy shortage. An enzyme called AMPK senses AMP concentration and regulates metabolism."
"Notifying that energy is insufficient."
"Yes. Then breakdown of sugar and fat is promoted."
Toma said, "Adenine doesn't just carry energy, it monitors energy status."
"Exactly. It's also a sensor."
Kana said quietly, "Even now, countless adenines are whispering inside our bodies. Energy is needed, it's sufficient, make more."
"Poetic but accurate," Rei smiled.
"I feel like I'm hearing molecules' voices."
The three continued their experiment, sensing the invisible flow of energy.