"The battery died."
Toma was looking at his watch.
"Electrons stopped flowing," Rei said.
"Electrons?" Kana asked.
"Oxidation-reduction reactions. Exchange of electrons."
Milia wrote in her notebook. "Oxidation = losing electrons, reduction = gaining electrons."
"Seems backwards," Kana was confused.
"Named from reactions involving oxygen," Rei explained. "Now defined by electron movement."
"How to remember?" Toma asked.
"OIL RIG. Oxidation Is Loss, Reduction Is Gain."
"English mnemonic," Kana understood.
Rei drew a diagram on the whiteboard.
"Redox potential. An indicator of electron-accepting tendency."
"Higher potential means easier to accept electrons."
Milia gave concrete examples. "O₂/H₂O: +0.82 V, NAD⁺/NADH: -0.32 V."
"When there's a potential difference?" Kana guessed.
"Electrons flow. Toward higher potential."
"Like electric current," Toma said.
"Exactly. In living organisms, this electron flow becomes an energy source."
Rei continued. "Electron transport chain. Located in mitochondrial inner membrane."
"What's the mechanism?" Kana asked.
"NADH supplies electrons. Those electrons pass through multiple protein complexes and are finally transferred to oxygen."
Milia drew a diagram. Electrons moving down like descending stairs.
"At each step, energy is released," Rei explained.
"Where does that energy go?"
"Used to pump protons outside the membrane. A proton gradient is created."
Toma remembered. "The protons we learned about before."
"Yes. That proton gradient makes ATP."
Kana summarized. "Electron flow → proton gradient → ATP."
"Perfect," Rei acknowledged.
Milia added. "The redox potential difference is the source of energy."
"How much energy?" Kana asked.
Rei calculated. "ΔG = -nFΔE. n is electron number, F is Faraday constant, ΔE is potential difference."
"From NADH to oxygen, about 1.1 V potential difference. This yields about 220 kJ/mol of energy."
"Is that large?" Toma confirmed.
"ATP synthesis requires about 30 kJ/mol per molecule. Theoretically, you can make more than 7 molecules."
"But in reality?"
"Efficiency isn't 100 percent. Actually, about 2.5 molecules."
Kana had a question. "Why is efficiency poor?"
"Some energy becomes heat. Also, there's proton leakage."
Milia supplemented. "But about 40 percent efficiency. Higher than car engines."
"Life is efficient," Toma was impressed.
Rei continued. "Oxidation-reduction reactions are central to metabolism."
"For example?"
"Photosynthesis. Oxidizing water and reducing CO₂."
"Reverse reaction?" Kana confirmed.
"Yes. Respiration oxidizes sugars and reduces oxygen. Photosynthesis is the reverse."
Milia added to the diagram. "Ecosystems are oxidation-reduction cycles."
"Plants reduce, animals oxidize," Toma understood.
"Simplified, yes," Rei acknowledged.
Kana suddenly thought. "What about antioxidants?"
"Substances that supply electrons and prevent oxidation," Rei answered.
"Like vitamin C?"
"Yes. By being oxidized themselves, they protect other molecules."
Milia added. "Reactive oxygen species are also oxidation-reduction reactions."
"Are reactive oxygen bad?" Toma asked.
"Double-edged sword," Rei explained. "Useful for sterilization, but excessive amounts damage cells."
"That's why antioxidants are needed," Kana understood.
Rei said finally. "When redox potential fluctuates, energy is born."
"Electron flow supports life," Milia confirmed.
Toma inserted a new battery. "This is also an oxidation-reduction reaction."
"Yes. Zinc is oxidized, manganese dioxide is reduced."
"Life is like a battery," Kana murmured.
Outside the window, the sun shines. Light energy excites electrons. A massive cycle of oxidation-reduction envelops the entire Earth.
"Next, let's talk about sugars," Rei proposed.
The three nodded. The journey of oxidation-reduction is not yet over.