"Huh? The color changed!"
Toma shouted while shaking a test tube.
"What did you do this time?" Kana peered in.
"I just added a little acid. Then it went from blue to red..."
Rei pushed up their glasses. "pH indicator. When hydrogen ion concentration changes, the molecular structure changes too."
"Hydrogen ions?" Kana tilted her head.
"H⁺. Acidic solutions have a lot of these," Toma explained.
Rei supplemented. "More precisely, pH is the negative logarithm of hydrogen ion concentration. pH = -log[H⁺]"
"Logarithm?"
"Meaning, if pH decreases by 1, H⁺ increases 10-fold."
Kana wrote in her notebook. "So pH 7 to pH 5 is 100 times?"
"Exactly. That's why small changes in pH have big impacts."
Toma took out another test tube. "Then let's add acid to this one too."
"Wait," Rei stopped him. "That's an enzyme reaction experiment, right? If you change the pH, the reaction might stop."
"Stop?" Kana was surprised.
"Enzymes are pH-sensitive. Move away from the optimal pH, and activity drops."
Toma showed interest. "Why?"
Rei drew a diagram on the whiteboard. "Enzymes are proteins. Amino acid residues become positively or negatively charged."
"Charged?"
"When pH changes, H⁺ concentration changes. Then -COOH becomes COO⁻, or -NH₂ becomes NH₃⁺."
Kana understood. "When charge changes, shape changes too?"
"Yes. Electrostatic interactions change, altering the protein's three-dimensional structure."
Toma murmured. "So that's why enzymes stop working."
"Pepsin in the stomach works at pH 2, but trypsin in the intestine works at pH 8. They've adapted to their environments."
Kana asked. "So what's the pH inside cells?"
"Cytoplasm is around pH 7.4. But stomach is pH 2, blood is also pH 7.4," Rei answered.
"Why is it constant when there's such variation?"
"Thanks to buffers," Rei said quietly.
"Buffers?"
Toma grabbed a bottle from the reagent shelf. "This. Phosphate buffer, PBS."
Rei explained. "A mixture of weak acid and its salt. pH doesn't change much whether you add acid or base."
"How come?" Kana asked.
"Because equilibrium shifts. When acid is added, the base component neutralizes it. When base is added, the acid component neutralizes it."
Toma experimented. "Let's try adding acid to this."
Even after adding several drops, the pH meter value barely moved.
"Amazing... it's protected," Kana admired.
Rei nodded. "In living organisms, there's the bicarbonate buffer system, phosphate buffer system, and protein buffer system."
"Proteins can be buffers too?"
"Because amino acid residues can accept or release H⁺."
Toma looked out the window. "Just a 0.1 change in blood pH is dangerous, right?"
"Acidosis or alkalosis. When pH goes below 7.35 or above 7.45, it's life-threatening."
Kana was surprised. "Just 0.1?"
"That's how delicate biochemical reactions are," Rei said quietly.
Toma held the test tube up to the light. "When pH changes, the world changes too."
"Enzymes, proteins, DNA... everything depends on pH."
Kana murmured. "An invisible balance in an invisible world."
Rei smiled. "That's why buffers support life."
The three stood before the lab bench, gazing at the color-changed test tube.
Toma said. "From now on, I'll measure pH before experimenting."
"That's wise," Rei acknowledged.
Kana closed her notebook. "pH determines the world."
Inside the test tube, molecules quietly maintained equilibrium.