"Mitochondria are complaining."
Toma looked up from the microscope.
Kana laughed. "Complaining? An organelle?"
Milia nodded seriously. "If they could talk, they'd surely complain."
"Why?"
"They were originally independent organisms," Milia explained.
"Independent?"
"Endosymbiotic theory. Two billion years ago, bacteria were engulfed by another cell."
Kana was surprised. "Mitochondria were bacteria?"
"Yes," Toma pointed at the model. "Evidence is the double membrane."
"Double?"
"Outer membrane is from host cell. Inner membrane is from original bacterium."
Milia supplemented. "Plus, they have their own DNA."
"Separate DNA?"
"Small circular DNA. Like bacteria."
Rei entered the club room. "Talking about mitochondria?"
"About their complaints," Toma laughed.
"Well, they're overworked," Rei sat down.
Kana questioned. "What do they do?"
"Make ATP," Milia answered. "Cell's power plant."
"How?"
Rei drew a diagram. "Electron transport chain. Enzymes embedded in inner membrane."
"Transport electrons?"
"Yes. From NADH and FADH₂ to oxygen."
Toma continued. "In the process, pumps protons outside the membrane."
"Proton concentration gradient forms?"
"Yes. This becomes energy source."
Milia explained. "When protons return, they turn ATP synthase."
"Turn?" Kana was surprised.
"Molecular motor. Actually rotates," Rei showed the model.
"Unbelievable..."
"Over 100 rotations per second. Synthesizes ATP."
Toma added. "Makes ATP equal to body weight per day."
"Body weight worth?" Kana was amazed.
"But constantly recycled."
Milia questioned. "Why are mitochondria unhappy?"
"Because," Toma laughed. "Work nonstop without appreciation."
"True," Rei nodded. "When oxygen runs out, cells complain."
"But too much oxygen is also dangerous."
Kana asked. "Why?"
"Reactive oxygen species are born," Milia explained. "Byproduct of electron transport."
"Harmful?"
"Damage DNA and proteins."
Rei supplemented. "Mitochondria themselves are victims."
"Poor things..." Kana murmured.
"Also," Toma continued. "Numbers are reduced or increased arbitrarily."
"Cell decides?"
"Yes. Divide or fuse as needed."
Milia said. "Muscle cells have thousands. But sperm have almost none."
"Unfair?"
Rei laughed. "According to function. But from mitochondria's view, maybe unfair."
Kana questioned. "Is it true they're only inherited from mother?"
"True," Milia nodded. "Only egg mitochondria remain."
"Why?"
"Sperm mitochondria are degraded after fertilization."
Toma added. "That's why mitochondrial DNA can track maternal lineage."
"Strange," Kana said.
Rei continued. "Descendants of ancient bacteria live inside us."
"Two billion year journey?"
"Yes. Gave up independence, chose symbiosis."
Milia said quietly. "But not completely integrated."
"Still separate?"
"Have their own ribosomes. Do their own protein synthesis."
Toma laughed. "That's why antibiotics work."
"Antibiotics?"
"Target bacterial ribosomes. Also affect mitochondria."
Rei supplemented. "One cause of side effects."
Kana looked at the mitochondria diagram. "Hard workers but also have complaints."
"Life's contradiction," Milia smiled.
"But," Toma said. "That's also evidence of evolution."
Rei nodded. "No perfect integration. Historical traces remain."
Kana was moved. "Inside cells, different creatures."
"Symbiosis," Milia said. "Mutually dependent, mutually dissatisfied."
"That's relationship?"
"Yes. Not perfect, but functional."
The three looked through the microscope.
Tiny mitochondria moving inside cells.
While keeping promises from two billion years ago.
Continue working while complaining.
That is mitochondria.
Life's mysterious roommate.