"Tired..."
Kana collapsed on her desk.
Toma looked worried. "You okay?"
"Maybe I exercised too much."
Rei said quietly. "Might be oxidative stress."
"Oxidative stress?" Kana lifted her face.
"A state where reactive oxygen species increase," Rei explained.
"Reactive oxygen?"
Toma opened his notebook. "Oxygen more reactive than normal O₂."
"For example, superoxide O₂⁻, hydrogen peroxide H₂O₂," Rei continued.
Kana asked. "Why are they in the body?"
"Made in mitochondria. Byproduct of the electron transport chain."
Toma added. "When electrons leak, they react with oxygen."
"Leak?"
"No system is perfect. About 2% becomes reactive oxygen."
Rei drew a diagram. "Reactive oxygen are free radicals. Have unpaired electrons."
"Unpaired electrons?" Kana tilted her head.
"Electrons without pairs. So they try to steal electrons from other molecules."
Toma said. "That's oxidation. Attacking the target."
"What do they attack?"
"Lipids, proteins, DNA... everything," Rei said quietly.
Kana was surprised. "Dangerous!"
"So there's a defense system."
Toma continued. "Antioxidant enzymes. SOD, catalase, glutathione peroxidase."
"SOD?"
"Superoxide dismutase. Converts O₂⁻ to H₂O₂," Rei explained.
"But isn't H₂O₂ harmful too?"
"So catalase breaks down H₂O₂ into water and oxygen."
Kana wrote in her notebook. "Two-stage detoxification?"
"Yes. Coordinated defense."
Toma gave another example. "Glutathione is also important."
"Glutathione?"
Rei answered. "A tripeptide. Protects others by being oxidized itself."
"Sacrifices itself?"
"But enzymes reduce it. Can be reused."
Kana asked. "So why oxidative stress when tired?"
"Exercise increases oxygen consumption. Mitochondria become active," Toma explained.
"So reactive oxygen also increases."
Rei continued. "When antioxidant systems can't keep up, it becomes stress."
"Vitamins C and E also help," Toma added.
"Vitamins?"
"Antioxidants. Provide electrons to stabilize free radicals."
Kana understood. "That's why we're told to eat fruit?"
"Yes. Polyphenols too."
Rei looked out the window. "But reactive oxygen aren't all bad."
"Huh?" Kana was surprised.
"White blood cells use them. Deliberately make reactive oxygen to kill bacteria."
Toma acknowledged. "Poison or medicine depends on dose."
"Low-level reactive oxygen work as signaling molecules," Rei explained.
"Signals?"
"Convey information to cells. Activate stress responses."
Kana pondered. "Balance is important?"
"Redox balance. Redox homeostasis."
Toma drank water. "Hydration helps a bit too."
"Why?"
"Helps metabolism, excretes waste."
Rei continued. "Sleep is important too. Repair systems work."
Kana looked at her notes. "Antioxidants, enzymes, rest..."
"Comprehensive defense," Toma acknowledged.
"What happens with age?" Kana asked.
"Antioxidant capacity decreases," Rei said quietly.
"So aging?"
"Oxidative stress accumulates. That's one cause of aging."
Toma added. "Can't prevent completely, but can slow it down."
Kana stood up. "Then I'll rest today."
"Wise," Rei acknowledged.
"Let the antioxidant system recover for tomorrow."
Toma laughed. "While thanking our cells."
Kana looked out the window. "My cells are fighting right now."
"Trillions of cells, every day."
Rei closed the notebook. "That's what living means."
The three quietly left the room.
Kana murmured. "Thank you, antioxidant enzymes."
Toma and Rei laughed.
"Thanking molecules again."
Under the night sky, cells quietly continue fighting oxidative stress.