"Sugar is burning!"
Toma heated sugar in an experiment. It turned black and carbonized.
Kana watched curiously. "Does the same thing happen in the body?"
Rei shook his head. "Similar but different. Controlled combustion."
"Controlled?"
"Cells slowly oxidize glucose. Don't burn it all at once."
Toma supplemented. "Burning it all at once would destroy cells with heat."
Rei drew a diagram. "Glycolysis. The first stage where glucose is broken down into pyruvate."
"Broken down?"
"A six-carbon sugar becomes two three-carbon molecules."
Kana wrote in her notebook. "C6H12O6 → 2 C3H4O3"
"Simplified, yes," Rei admitted. "But actually ten reaction steps."
"Ten steps?" Toma was surprised.
"Each step extracts energy bit by bit. That's the secret of control."
Kana asked. "How much energy comes out?"
"From glycolysis alone, two ATP. Small amount."
"Small?"
Rei explained. "Complete glucose oxidation theoretically yields 38 ATP. Glycolysis is just preparation."
Toma asked next. "Where's the remaining energy?"
"In pyruvate," Rei answered. "This is oxidized further."
"How?"
"Citric acid cycle. Occurs in mitochondria."
Kana showed interest. "Citric acid cycle?"
Rei drew a circle. "A cycle. Going around the same pathway repeatedly."
"Pyruvate first becomes acetyl-CoA. That enters the cycle."
Toma added. "One cycle produces two CO2."
"Carbon dioxide?"
"Carbon is completely oxidized. C-C bonds are broken, becoming C=O."
Kana understood. "That's why we exhale CO2 in respiration."
"Right. All glucose carbons eventually become CO2."
Rei wrote the equation. "C6H12O6 + 6O2 → 6CO2 + 6H2O"
"This is complete oxidation. Same chemical formula as combustion."
"But you said it's different from combustion?" Kana confirmed.
"The process is different," Rei explained. "Combustion is all at once. Cells are stepwise."
Toma pointed to the experiment's flame. "This produces heat and light. Cells?"
"Chemical energy called ATP."
Kana reviewed her notes. "2 from glycolysis, from citric acid cycle...?"
"Little from the cycle itself. But NADH and FADH2 are made."
"Those make ATP in the electron transport chain," Toma continued.
Rei calculated. "About 2.5 ATP per NADH, about 1.5 per FADH2."
"From one glucose, 10 NADH and 2 FADH2."
Kana was surprised. "That's a lot."
"That's why the electron transport chain is the main ATP source. Glycolysis and citric acid cycle are for making electron carriers."
Toma asked. "Why stepwise? Isn't that inefficient?"
"Opposite," Rei answered. "Because it's stepwise, efficiency is high."
"Oxidizing all at once, most energy escapes as heat. Little by little, it can be captured as ATP."
Kana used a metaphor. "Going down stairs?"
"Good example. Jumping down creates big impact. With stairs, you can control at each step."
Toma looked at the carbonized sugar. "This is the result of loss of control."
"Right. Life has tamed combustion."
Kana stared at glucose's structural formula. "So much energy in this small molecule."
Rei said quietly. "Energy in C-H bonds. What plants stored through photosynthesis."
"We're extracting it stepwise."
Toma murmured. "Controlled combustion. The flame of life."
Kana looked out the window. The sun was setting.
"Sun's energy becomes glucose, becomes ATP."
Rei nodded. "The flow of energy. That's life."
The three fell silent. An invisible flame burns quietly inside cells.