"The pH test paper turned blue!"
Toma shouted.
"It's alkaline," Kana confirmed.
"But what is pH?" Toma asked simply.
Rei began explaining. "An indicator of proton concentration. Precisely, the negative logarithm of hydrogen ion H⁺ concentration."
"Logarithm?"
"pH = -log[H⁺]. When concentration increases 10-fold, pH decreases by 1."
Milia wrote in her notebook. "pH 7 = neutral, pH < 7 = acidic, pH > 7 = alkaline."
Kana had a question. "Why are protons important?"
"Protons are the smallest ions," Rei answered. "So reaction rates are very fast."
Toma vigorously poured water. "Do protons also move fast in water?"
"Precisely, protons are passed like a baton from water molecule to water molecule."
"Like a baton?"
Milia drew a diagram. A chain of water molecules. Protons moving from one to the next.
"Grotthuss mechanism," Rei explained. "Transfer through bond rearrangement is faster than the proton itself moving."
"Interesting," Kana was impressed.
"In living organisms, pH control is extremely important," Rei continued.
"Why?"
"Enzyme activity depends on pH. Outside the optimal pH, the enzyme shape changes and it loses function."
Toma started an experiment. "What if we add acid?"
The pH test paper turned red.
"pH decreased. But not that much," Kana noticed.
"Because it's a buffer solution," Rei explained. "A solution that suppresses pH changes."
"How?"
Milia took notes. "Pair of weak acid and conjugate base."
"For example, acetic acid and sodium acetate," Rei gave a concrete example.
"When acid is added, acetate ions accept protons and become acetic acid. When base is added, acetic acid releases protons and becomes acetate ions."
Toma understood. "There's a receiver and supplier of protons."
"Exactly. Can be calculated with the Henderson-Hasselbalch equation."
Rei wrote the equation.
"pH = pKa + log([A⁻]/[HA])"
"What's pKa?" Kana asked.
"The negative logarithm of the acid dissociation constant. Represents acid strength."
Milia added. "When pKa = pH, buffering capacity is maximum."
"Why?"
"Because [A⁻] = [HA]. Both forms are sufficiently present."
Toma said excitedly, "Is blood also a buffer solution?"
"Exactly," Rei acknowledged. "Carbonic acid-bicarbonate system. Pair of CO₂ and HCO₃⁻."
"What's blood pH?"
"About 7.4. Maintained in a very narrow range."
"If it deviates?" Kana asked worriedly.
"Acidosis or alkalosis. Life-threatening."
Milia said with a serious expression. "Below pH 7.0 or above 7.8, loss of consciousness."
"That strict," Toma was surprised.
"Respiration and kidneys regulate pH," Rei continued.
"Respiration?"
"By exhaling CO₂, acid is expelled. Hyperventilation shifts toward alkaline."
Kana took notes. "Respiration = pH control device."
"Kidneys adjust by reabsorbing or excreting HCO₃⁻."
Toma suddenly thought. "Protons are quite hardworking."
"Central players in biochemistry," Rei acknowledged. "Protons are also important in oxidation-reduction reactions."
Milia supplemented. "ATP synthesis also uses proton gradients."
"When protons run through, energy is created," Rei explained.
"In mitochondria?" Kana confirmed.
"Yes. Proton pumps create a concentration difference across the membrane. Using that gradient, ATP synthase synthesizes ATP."
Toma was impressed. "Protons are amazing."
"Just one proton," Rei said. "But one of the fundamental particles supporting life."
Milia said finally. "Understanding the path of protons is understanding life."
Outside the window, rain is falling. Within water molecules, countless protons run through. Invisible to the eye, but certainly existing, messengers of life.
"Next, let's talk about chemical bonds," Rei proposed.
The three nodded. The journey of protons has only just begun.