"Water splits into oxygen and hydrogen."
Milia said quietly, gazing at the potted plant by the window.
Kana asked curiously. "That's electrolysis, right?"
"No, photosynthesis," Rei supplemented. "Plants split water using light energy."
Milia wrote a chemical formula in her notebook. "2H₂O → O₂ + 4H⁺ + 4e⁻"
"Such a simple formula, but the actual reaction is extremely complex."
Kana thought. "Water seems so stable though."
"Exactly. That's why a special catalyst is needed," Rei began explaining. "It happens in a giant protein complex called photosystem II."
"Photosystem?"
"The heart of photosynthesis. Chlorophyll absorbs light and excites electrons. To fill the electron hole, it takes electrons from water."
Milia began drawing a diagram. "Manganese cluster. A catalytic center composed of four manganese atoms and one calcium atom."
"Manganese?" Kana was surprised.
"Metal ions play an important role. Mn₄CaO₅ cluster. This structure tears apart water molecules."
Rei continued. "Water splitting proceeds in four steps. Going through states from S₀ to S₄. One electron is removed at each step."
"S states?"
"Oxidation states of photosystem II. With four light absorptions, four electrons are removed, and finally oxygen molecules are generated."
Kana looked at her notebook. "Four electrons... four lights?"
"Exactly. Called the Kok cycle. Named after the proposer."
Milia quietly added. "This reaction brought oxygen to Earth. From 2.7 billion years ago."
"Before plants appeared?"
"Cyanobacteria. The first oxygen-evolving photosynthetic organisms."
Rei pointed to the diagram. "What's important is the cooperativity of the reaction. While removing four electrons one by one, it stabilizes intermediates."
"If intermediates are unstable?"
"Dangerous radicals form. So the manganese cluster precisely controls electrons and oxidation states at each step."
Kana asked. "Why manganese?"
"Manganese can take multiple oxidation states. Mn²⁺, Mn³⁺, Mn⁴⁺. This flexibility suits multi-step electron transfer."
Milia put down the chalk. "And calcium contributes to structural stabilization."
"Each atom has meaning," Kana was impressed.
Rei continued. "In the final step, two water molecules become an oxygen molecule. O-O bond formation is the most energetically difficult part."
"How?"
"Not fully understood yet. Probably, the protein forcibly creates an arrangement where two oxygen atoms are close together."
Milia looked out the window. "Thanks to this reaction, we can breathe."
"Plants make oxygen, we use it," Kana said quietly.
"And water, the most familiar molecule, is decomposed by the most sophisticated mechanism," Rei concluded.
Kana touched the potted plant. "Even now, inside these leaves?"
"Every second, thousands of times water is split. As long as there's light."
"More than fate, it's a mission," Kana smiled.
Milia nodded. "Water's mission. Creating oxygen, supporting life."
Rei said quietly. "I think it's one of the most beautiful chemical reactions."
The three felt deep respect for the water-splitting reaction that the plant by the window quietly continues.