"How is it decided which way electrons flow?"
Toma posed a simple question.
Rei picked up an electrode. "Determined by potential difference. From high to low."
"Same as electricity?"
"Exactly. Chemical reactions can be viewed electrically."
Kana opened her notebook. "Potential difference... how to measure?"
Rei began assembling equipment. "Using reference electrode. Standard hydrogen electrode is the reference."
"Hydrogen?"
"Yes. Equilibrium of hydrogen ion and hydrogen gas. Defined as 0 volts."
Toma became interested. "So all potentials are compared to hydrogen?"
"Precisely. For example, copper ion and copper electrode is +0.34 volts."
"Plus?"
Rei explained. "More likely to accept electrons than hydrogen. Easily reduced."
Kana understood. "If potential is negative?"
"More likely to release electrons than hydrogen. Easily oxidized."
Toma wrote on whiteboard. "High potential = wants electrons. Low potential = wants to give electrons."
"Good summary," Rei acknowledged.
"Then," Kana thought, "when mixing two substances, which gets oxidized and which gets reduced?"
"Determined by potential difference," Rei wrote equation. "ΔE° = E°(reduction) - E°(oxidation)"
"If this is positive, reaction proceeds."
Toma asked. "Why does positive mean it proceeds?"
"Related to Gibbs energy," Rei continued. "ΔG° = -nFΔE°"
"n is number of electrons, F is Faraday constant."
"If ΔE° is positive, ΔG° is negative. Spontaneous reaction."
Kana was convinced. "Thermodynamics and electrochemistry connected."
Rei gave example. "Mix iron and copper ions, iron gets oxidized and copper ions reduced."
"Why?"
"Iron potential is -0.44 volts, copper is +0.34 volts. Potential difference is 0.78 volts."
"Big difference," Toma said.
"Yes. So reaction proceeds strongly."
Kana recorded in lab notebook. "This principle works in living organisms too?"
"Exactly," Rei nodded. "Electron transport chain descends potential difference stairs."
"Stairs?"
"Multiple molecules arranged in order of potential. Electrons flow toward lower."
Milia entered. "Talking about respiration?"
"Talking about potential," Kana answered.
"Same thing," Milia smiled. "Respiration is electron flow from NADH to oxygen."
Rei continued. "NADH potential is about -0.32 volts, oxygen is +0.82 volts."
"Over 1 volt difference!" Toma was surprised.
"So large amounts of energy are released. Used to make ATP."
Kana asked. "What if potential difference is small?"
"Energy is also small," Milia answered. "So life chooses high potential difference reactions."
"Oxygen is optimal electron acceptor."
"Yes. High potential, so efficient."
Toma thought. "In oxygen-free environments?"
Rei explained. "Some microbes use sulfate or nitrate. Lower potential but usable."
"Life's diversity," Kana was impressed.
Milia added. "But basic principle is same. Potential difference is energy source."
Rei said quietly. "Reference electrode isn't just measurement standard. It tells thermodynamic truth."
"What truth?"
"Electrons naturally flow to lower potential. That generates energy."
Toma summarized. "Electrochemistry is life's energy theory."
"Perfect understanding," Milia nodded.
Kana looked at the equipment. "This electrode teaches us universal laws."
"Exaggerated but correct," Rei smiled.
The three continued measuring electrode potentials. Invisible electron flow moves the world.