"Why do we need oxygen?"
Kana asked while opening a window.
Toma answered immediately. "For breathing, right?"
"But what is breathing?"
Rei answered quietly. "The process of extracting energy. Oxygen is the final electron acceptor."
"Electron acceptor?" Kana tilted her head.
"A molecule that accepts electrons. Oxygen loves electrons."
Toma laughed. "Oxygen smiles?"
"Metaphorically. When it receives electrons, oxygen is satisfied," Rei explained.
Kana wrote in her notebook. "What's the structure of O₂?"
"Double bond. But not a complete double bond."
"What do you mean?"
"Two unpaired electrons. So oxygen is in a triplet state."
Toma was confused. "Triplet?"
"Quantum mechanical talk. Simply put, oxygen has radical-like properties."
Kana was surprised. "Dangerous?"
"In a way. That's why oxygen is highly reactive."
Rei continued. "Consider combustion. Wood burns because oxygen reacts with carbon."
"C + O₂ → CO₂," Toma wrote.
"Right. At this time, large amounts of energy are released."
Kana thought. "So respiration in the body is also combustion?"
"Slow combustion. Oxidizing glucose to obtain energy."
Rei drew a diagram. "C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP"
"ATP?"
"Energy currency. The form of energy cells can use."
Toma asked. "Why do we need oxygen? Can't other molecules work?"
"Oxygen has high electron affinity. Strong power to extract electrons."
"So energy comes out?"
"Yes. Electrons fall from high energy state to low energy state. That difference becomes ATP."
Kana understood. "Oxygen is the final destination of electrons?"
"Beautiful expression," Rei acknowledged. "In mitochondria, electrons descend the electron transport chain, and finally oxygen waits."
"Then it becomes water?"
"H⁺ + e⁻ + O₂ → H₂O. Oxygen receives electrons and protons, becoming water."
Toma murmured. "Water is like combustion residue?"
"In a way. The final product of respiration."
Kana looked outside. "Twenty percent of air is oxygen. We breathe it to live."
"But ancient Earth had no oxygen," Rei said.
"None?"
"Until cyanobacteria started photosynthesis, oxygen was nearly zero."
Toma was surprised. "Then organisms?"
"Only anaerobic organisms. Lived without oxygen."
"When oxygen increased, what changed?"
"Aerobic organisms were born. Using oxygen, energy efficiency jumped."
Rei showed numbers. "Anaerobic glycolysis yields 2 ATP from one glucose. Aerobic respiration yields about 36 ATP."
"18 times!" Kana was surprised.
"That's why complex life could evolve. Oxygen was life's revolution."
Toma thought. "But oxygen is also poison, right?"
"Sharp. Reactive oxygen species damage cells."
"Radicals?"
"Superoxide, hydrogen peroxide, hydroxyl radical. Oxygen's dangerous face."
Kana worried. "Then how do we protect ourselves?"
"Antioxidant enzymes. Superoxide dismutase, catalase."
"We use oxygen but also protect ourselves," Toma understood.
"Life learned the balance between risk and return."
Rei said quietly. "When oxygen smiles, energy is born. But too much smiling is dangerous."
Kana smiled. "So we maintain just the right smile."
"That's life's wisdom."
The three took deep breaths, feeling oxygen's invisible smile.