"After eating breakfast, what happens?"
Kana asked. Morning sunlight streamed through the classroom window.
Milia smiled. "Metabolism begins. Quietly, but surely."
"Metabolism?"
Rei answered. "A series of chemical reactions that convert food into energy."
Kana wrote in her notebook. "Where?"
"Inside cells. Especially in mitochondria."
Milia drew a diagram. "First, digestion. Bread breaks down into glucose."
"Then?"
"Glucose is taken up by cells. Glycolysis begins," Rei explained.
"Glycolysis?"
"A pathway that breaks glucose into pyruvate. Happens in cytoplasm."
Kana questioned. "What about energy?"
"A little is produced. Two ATP molecules," Milia answered.
"ATP?"
"Adenosine triphosphate. Energy currency," Rei said.
"Currency?"
"Energy in a form cells can use. Energy is stored in phosphate bonds."
Milia continued. "But glycolysis alone isn't enough. Pyruvate goes to mitochondria."
"There?"
"Citric acid cycle, or TCA cycle," Rei drew a diagram.
"Cycle?"
"A circular reaction system. Pyruvate is completely broken down into CO₂."
Kana was surprised. "Completely?"
"Yes. Carbon leaves as carbon dioxide."
Milia supplemented. "In the process, electrons are extracted."
"Electrons?"
"Stored in molecules called NADH and FADH₂. These go to the next stage."
Rei continued explaining. "Electron transport chain. Happens in mitochondrial inner membrane."
"What does it do?"
"Relays electrons. Carries them to oxygen."
Kana thought. "And then?"
"Energy from electron movement pumps protons outside the membrane."
Milia drew a diagram. "A proton concentration gradient forms across the membrane."
"Concentration gradient?"
"Yes. That becomes an energy source."
Rei continued. "When protons return, they turn ATP synthase."
"Turn?"
"Literally. It's a molecular motor. Rotation creates ATP."
Kana was moved. "Molecules rotate?"
"Nanomachines. Hundreds of rotations per second."
Milia said quietly. "This is oxidative phosphorylation. About 30 ATP from one glucose molecule."
"Thirty! Glycolysis was only 2."
"Efficiency is completely different," Rei nodded.
Kana questioned. "What is ATP used for?"
"Everything," Milia answered. "Muscle contraction, nerve transmission, substance synthesis."
"Making proteins too?"
"Yes. ATP supplies energy."
Rei supplemented. "There's also reverse process. Anabolism."
"Anabolism?"
"Making large molecules from small ones. Protein synthesis, DNA replication."
"That requires energy too?"
"Lots. That's why we need to constantly make ATP."
Milia looked at the clock. "Right now, hundreds of billions of ATP are being made and consumed in your body."
Kana looked at her hand. "Invisible but..."
"Invisible, but surely progressing," Rei said.
"If metabolism stopped?"
"That's death," Milia said quietly. "Life is the flow of metabolism itself."
Kana looked out the window. Morning sun illuminated the schoolyard.
"Is photosynthesis metabolism too?"
"Yes," Rei answered. "Makes ATP with light energy, synthesizes glucose from CO₂."
"Plants and animals do opposite things?"
"It circulates," Milia said. "Animals break down glucose plants made, plants use CO₂ animals released."
Kana was moved. "The whole world is one metabolic system."
Rei nodded. "Earth's ecosystem is a giant chemical factory."
The three sat quietly.
Invisible, but metabolism progresses inside the body.
Quietly, ceaselessly.
That is what it means to be alive.