Two in the morning. The library lights were still on.
"Sleepy..." Kana collapsed onto her notebook.
"But your cells aren't sleeping," Rei said quietly.
"Huh?"
Toma drank coffee. "Mitochondria work 24 hours."
"Mitochondria?"
Rei drew a diagram. "The cell's power plant. Continuously making ATP."
"ATP..." Kana recalled. "Energy currency."
"Yes. Moving muscles, transmitting nerves, everything needs ATP."
Toma added. "You're using it to think right now."
"In my head?"
"The brain is 2% of body weight but consumes 20% of all ATP," Rei explained.
Kana was surprised. "That much?"
"Every time neurons send signals, ion pumps use ATP."
Toma looked out the window. "Even when you're sleepy, mitochondria don't rest."
"How do they make ATP?" Kana showed interest.
Rei wrote pathways in the notebook. "First, the citric acid cycle. Also called the TCA cycle."
"Cycle?"
"Glucose breaks down into acetyl-CoA. That enters the cycle."
Toma continued. "Each time it goes around, CO₂ comes out and NADH or FADH₂ is made."
"NADH?"
"Electron carrier. Carries high-energy electrons," Rei explained.
"Where do those electrons go?"
"Electron transport chain. Located in the mitochondrial inner membrane."
Toma got excited. "Here's where it gets interesting."
Rei drew a diagram. "Complexes I, II, III, IV. Electrons are passed like a relay baton."
"Why?" Kana asked.
"As electrons are passed, energy is released. That energy pumps H⁺ out of the membrane."
"Pumps out?"
"Active transport. Using ATP to... no, in this case using electron energy."
Toma supplemented. "When H⁺ accumulates outside the membrane, a concentration gradient forms."
"Gradient?"
Rei smiled. "Like water flows from high to low, H⁺ wants to return."
"But can't cross the membrane?"
"There's one place they can. ATP synthase."
Kana wrote in her notebook. "Synthase?"
"A rotating molecular machine," Toma said. "When H⁺ passes through, it rotates like a turbine."
"Rotates?" Kana looked incredulous.
Rei acknowledged. "Really rotates. Over 100 times per second."
"That rotation makes ATP?"
"It bonds ADP and phosphate. Mechanical energy becomes chemical energy."
Toma admired. "Nature's finest nanomachine."
Kana calculated. "So how much ATP is made?"
"From one glucose, maximum 38 ATP. But actually around 30," Rei answered.
"Amazing efficiency..."
"But heat is also produced. That becomes body temperature."
Toma said. "That's why when you're cold, you shiver. Moving muscles to use ATP and produce heat."
Kana gripped her coffee cup. "Mitochondria are warming me?"
"24 hours, 365 days," Rei said quietly.
"They never rest?"
"Efficiency varies. When oxygen is low, it switches to anaerobic glycolysis."
Toma added. "That's why marathon runners emphasize aerobic exercise."
Kana asked. "Where did mitochondria come from?"
"Interesting question," Rei's eyes lit up. "They're thought to have once been bacteria."
"Bacteria?"
"Endosymbiotic theory. Long ago, eukaryotic cells took in bacteria and formed a symbiosis."
Toma explained. "Mitochondria have their own DNA. Inherited from the mother."
"From mother?" Kana was surprised.
"Eggs have many mitochondria, but sperm have almost none."
Rei looked out the window. "Your mitochondria come from your mother's mother's mother's... continuing back."
Kana put her hand on her chest. "An inherited power plant."
"Yes. Right this moment, trillions of mitochondria are working."
Toma drained his coffee. "So we can keep going a bit longer."
Kana smiled. "Mitochondria are cheering us on."
Rei closed the notebook. "The never-sleeping story of never-sleeping cells."
The three returned to studying in the midnight library. Inside their bodies, mitochondria quietly continued spinning.