"What does it mean that cells respire?"
Kana voiced a simple question.
Milia answered. "Using oxygen to make energy. Same purpose as human respiration."
"But cells don't have lungs."
Rei supplemented. "Lungs are organs for taking in oxygen. Cellular respiration is the chemical reaction using that oxygen."
Kana opened her notebook. "What kind of reaction?"
"Glucose + oxygen → carbon dioxide + water + energy," Milia explained concisely.
"Sounds simple."
"Actually, over 30 reactions occur step by step," Rei corrected.
"That many?"
"Three major stages," Milia drew a diagram. "Glycolysis, citric acid cycle, electron transport chain."
Kana asked about the first stage. "What's glycolysis?"
"Breaking down glucose. Occurs in the cytoplasm," Rei explained.
"Glucose has 6 carbons. It splits into two pyruvate molecules."
"Pyruvate?"
"A 3-carbon molecule. From here on, to the mitochondria."
Milia continued. "Mitochondria are the cell's power plants."
"I've heard that," Kana nodded.
"Pyruvate is converted to acetyl-CoA. Then to the citric acid cycle."
Rei drew the cycle. "Eight reactions connected in a ring."
"Why a cycle?"
"The starting material is regenerated. Begins with oxaloacetate, returns to oxaloacetate."
Kana followed. "In between?"
"Carbon dioxide is released. And electron carriers are reduced."
"Electron carriers?"
"NADH and FADH2. Molecules that carry electrons," Milia explained.
"Carry electrons where?"
"Electron transport chain. This is the climax," Rei emphasized.
Kana became interested. "What happens?"
"Electrons pass through protein complexes lined up in the mitochondrial inner membrane."
Milia supplemented. "Complex I, II, III, IV. Electrons are passed in sequence."
"Bucket brigade?" Kana made a metaphor.
"Good example. Each time electrons move, energy is released."
Rei continued. "That energy pumps protons outside the membrane."
"Protons?"
"Hydrogen ions. H+."
Kana tried to understand. "Why pump them out?"
"To create a concentration gradient," Milia answered. "High concentration outside, low inside."
"Then what?"
Rei explained. "Protons want to return. Following the concentration gradient."
"When returning, they pass through ATP synthase."
"ATP synthase?"
"Rotary motor," Milia said excitedly. "Rotates with proton flow, making ATP from ADP."
Kana was surprised. "It rotates?"
"Actually rotates. A nanomachine."
Rei showed numbers. "About 30-32 ATP molecules from one glucose molecule."
"What's the efficiency?"
"About 40%. The rest becomes heat."
Kana thought. "That's why bodies are warm."
"Yes. A byproduct of cellular respiration," Milia acknowledged.
"Where is oxygen used?" Kana asked.
"At the end of the electron transport chain," Rei answered. "Final electron acceptor."
"Electrons, protons, and oxygen combine to make water."
Kana summarized. "That's why oxygen is needed for respiration."
"Yes. Without oxygen, electron transport stops."
Milia added. "Then NADH isn't oxidized. Glycolysis stops too."
"Everything's connected."
Rei nodded. "Cellular respiration is a precisely regulated system."
Kana asked. "When exercising, breathing speeds up because?"
"ATP consumption increases. So production increases too," Milia explained.
"Taking in more oxygen, breaking down glucose."
Rei continued. "Balance of supply and demand. Feedback control."
Kana took a deep breath. "Right now, my cells are too?"
"Trillions of cells, breathing simultaneously."
Milia said quietly. "Living means breathing."
Outside the window, wind swayed the trees. Invisible, but felt. Cellular respiration is like that too.