"I'm tired..."
Kana slumped over the desk.
Milia laughed. "Not enough ATP?"
"ATP?"
"Adenosine triphosphate. The cell's energy currency."
Rei drew a diagram. "Three phosphate groups connected by high-energy bonds."
"Bonds are energy?" Kana asked.
"Precisely, energy is released when hydrolyzed."
Milia supplemented. "ATP → ADP + Pi. About 30 kJ/mol of energy."
"That moves muscles?"
"Muscles, nerves, synthesis reactions, active transport... almost everything needs ATP."
Kana took notes. "How is it made?"
"Mainly three pathways," Rei answered. "Glycolysis, citric acid cycle, oxidative phosphorylation."
"Sounds complex..."
"Mechanism to extract energy stepwise."
Milia began explaining. "First glycolysis. Breaks down glucose to pyruvate."
"That makes ATP?"
"A little. Net 2 ATP from one glucose molecule."
"So little..." Kana was surprised.
"Glycolysis doesn't need oxygen. It's an ancient metabolic pathway."
Rei continued. "With oxygen, pyruvate enters mitochondria."
"Mitochondria?"
"The cell's power plant. Has citric acid cycle and electron transport chain."
Milia drew in detail. "Citric acid cycle. Pyruvate is completely oxidized."
"Becomes CO₂?"
"Yes. But doesn't make much ATP directly."
"Then what for?" Kana asked.
"Makes NADH and FADH₂. These are electron carriers."
Rei explained. "They pass electrons to electron transport chain."
"Electron transport chain?"
"Series of protein complexes. Electrons move stepwise."
Milia showed an animation. "Each time electrons move, protons are pumped outside membrane."
"Protons meaning H⁺?"
"Yes. Creates concentration gradient."
Rei continued. "Those protons return through ATP synthase."
"ATP is made when they return?"
"Correct. Chemiosmotic theory. Won Nobel Prize."
Kana calculated. "From one glucose molecule, total how many ATP?"
"Theoretically, about 30 to 32 ATP," Milia answered.
"Compared to 2 from glycolysis..."
"Overwhelmingly efficient. That's why oxygen-breathing organisms prospered."
Rei organized. "But ATP is always in short supply."
"Why?" Kana asked.
"Because usage rate is fast. Especially brain and muscles."
Milia added. "Brain is 2% of body weight but consumes 20% of ATP."
"Huge consumption..."
"So constantly needs oxygen and glucose. Just minutes without damages brain."
Rei continued. "Muscles store creatine phosphate."
"That's also energy?"
"ATP reserve. Can instantly regenerate ATP. But only lasts seconds."
Kana reviewed her notes. "Why ATP is insufficient... because it's hard to make?"
"And because it's easy to use," Milia said.
Rei smiled. "Efficient but always walking tightrope."
Kana murmured. "That I'm tired..."
"Cells are running out of ATP."
"Will rest restore it?"
"Yes. Even during sleep, ATP is continuously made."
Milia opened the window. "Every breath makes ATP."
Kana took a deep breath. "Are mitochondria working now?"
"Trillions of mitochondria, simultaneously."
Rei said quietly. "Being alive is continuous ATP synthesis and consumption."
Kana closed her notebook. "I think I understand why ATP is insufficient."
"Life constantly seeks energy," Milia said.
"But that's why we can move," Rei added.
The three fell silent. Invisible molecules continuously make energy without rest.
"Thank you, ATP," Kana murmured.
Milia and Rei laughed. "Grateful for energy."