"The reaction seems to have stopped."
Kana said while looking at the clock.
"It hasn't stopped," Rei corrected. "It just reached equilibrium."
"What's the difference?"
Milia drew a diagram. Arrows pointing in both directions.
"Reversible reaction," Rei explained. "The reaction of A becoming B and B returning to A happen simultaneously."
"Indecisive?"
"At equilibrium, the rates of forward and reverse reactions are equal."
Kana thought. "So concentrations don't change?"
"Apparently. But at the molecular level, conversion is constantly occurring."
Milia began moving molecular models. A to B, B to A, endlessly.
"Dynamic equilibrium," Rei confirmed.
"But how do we know when equilibrium is reached?" Kana asked.
"Equilibrium constant K," Rei wrote an equation on the whiteboard.
"K = [B]/[A]"
"Product concentration divided by reactant concentration. This value doesn't change if temperature is constant."
"If K is large?"
"Equilibrium with more products. The reaction proceeds nearly completely."
"If K is small?"
"Equilibrium with more reactants. It doesn't proceed much."
Milia wrote a number in her notebook. "If K = 1, exactly half and half."
Kana asked for a real example. "In living organisms?"
"Conversion between glucose and glucose-6-phosphate," Rei answered. "Using ATP, glucose is phosphorylated."
"Is this also equilibrium?"
"Theoretically. But inside cells, it's kept far from equilibrium."
"Why?"
Milia intervened. "Because it's alive."
Rei explained in detail. "Equilibrium state is the lowest energy state. But life constantly consumes energy, staying far from equilibrium."
"When you die, you reach equilibrium?"
"You could say that. When metabolism stops, all reactions move toward equilibrium."
Kana said a bit sadly, "Living is resisting equilibrium."
"Philosophical, but biochemically correct," Rei acknowledged.
Milia took out an enzyme model.
"Enzymes don't change equilibrium," Rei emphasized. "They just speed up the rate of reaching equilibrium."
"Because they're catalysts?"
"Yes. They lower activation energy. But the equilibrium constant K doesn't change."
Kana summarized in her notebook. "Enzyme = shortcut to equilibrium."
"Good expression," Rei acknowledged.
Milia added. "But cells avoid equilibrium."
"How?" Kana asked.
"Consume products in the next reaction. Create continuous reaction pathways."
Rei drew a diagram. "A→B→C→D. Each step is reversible, but overall proceeds in one direction."
"Products are used immediately, so B doesn't accumulate."
"Exactly. This is the cleverness of metabolic pathways."
Kana suddenly thought. "What happens if we reach equilibrium?"
"Energy production stops. Can't make ATP."
"That's a problem."
"That's why cells constantly input energy to stay away from equilibrium."
Milia said quietly. "Life is a non-equilibrium system."
Rei continued. "According to the second law of thermodynamics, closed systems increase entropy and move toward equilibrium."
"But organisms?"
"Open systems. They take in energy and matter from outside and expel entropy."
Kana took a deep breath. "Eating and breathing are also to avoid equilibrium."
"You could say that."
Milia smiled. "Slowly shifting equilibrium. But never reaching it."
"That's the proof of life," Rei concluded.
Outside the window, the seasons slowly change. But life doesn't stop. It keeps moving, always avoiding equilibrium. That's what living means.
"Next time, let's talk about proton movement," Rei proposed.
Kana and Milia nodded. The journey of biochemistry continues.