"Isn't mitochondria overworked?"
Kana stared at the microscope image.
Milia nodded. "24/7. The cell's energy factory."
"Doesn't it get tired?"
Rei explained. "Let's think from mitochondria's perspective."
Toma laughed. "Mitochondria's complaints?"
"Constantly demanded. ATP, ATP, and more ATP," Rei continued.
Kana took notes. "How does it make it?"
"First, citric acid cycle," Milia drew a diagram. "Starts with acetyl-CoA."
"Acetyl?"
"Breakdown product of sugars and fatty acids. Two-carbon unit."
Rei continued. "One turn of the cycle produces CO2, NADH, FADH2."
"And then?"
"NADH and FADH2 are electron carriers."
Toma asked. "Electrons go where?"
"Electron transport chain. A series of protein complexes in the inner membrane," Milia answered.
Kana looked at the diagram. "Complex I, II, III, IV?"
"Yes. Electrons are relayed while releasing energy."
Rei explained. "That energy pumps protons into the intermembrane space."
"A proton gradient forms?"
"Exactly. Concentration difference and potential difference. Electrochemical gradient."
Milia added. "And finally, at complex IV, oxygen is reduced."
"Oxygen?" Toma was surprised.
"Final electron acceptor. O2 + 4H+ + 4e- → 2H2O. That's why we need oxygen for breathing."
Kana understood. "What happens to the proton gradient?"
"Flows back through ATP synthase," Rei answered.
"And then?"
"That flow spins a turbine. Physically rotates."
Toma took notes. "Mechanical?"
"Molecular machine. When protons flow, the F0 part rotates."
Milia showed an animation. "In the F1 part, ADP and phosphate combine to become ATP."
Kana was moved. "Like hydroelectric power."
"Chemiosmotic theory," Rei said. "Peter Mitchell's theory."
"Wasn't believed at the time?"
"Too beautiful."
Toma asked. "From one glucose, how much ATP?"
"Theoretically 38, but actually about 30-32," Milia answered.
"Why less?"
"There's leakage. Protons return on their own."
Rei continued. "Plus, transporting ATP to cytoplasm also requires energy."
Kana murmured. "Mitochondria work hard."
"Furthermore," Milia added. "Reactive oxygen species are also generated."
"Harmful?"
"Byproduct of electron transport. Superoxide, hydrogen peroxide."
Rei explained. "That's why antioxidant enzymes are needed. SOD, catalase."
Toma took notes. "Mitochondria also self-defend."
"They have their own DNA too," Milia said.
"What?" Kana was surprised.
"Once independent organisms. Endosymbiotic theory."
Rei continued. "That's why they have double membrane and their own ribosomes."
Kana said with feeling. "Mitochondria were originally separate life."
"Now symbiotic. Mutually dependent."
Toma summarized in his notebook. "Citric acid cycle, electron transport, proton gradient, ATP synthesis."
"Everything works together to efficiently produce energy."
Milia said quietly. "But mitochondria don't complain."
Kana laughed. "Even while grumbling, they keep working."
Rei smiled. "That's the kindness of life."
The four fell silent. Mitochondria's complaints support the cell.