"Why do I get sleepy after lunch?"
Toma yawned in the classroom.
Milia smiled. "Because blood sugar rises and insulin is secreted."
"Insulin?"
"Hormone that lowers blood sugar. Makes cells take in glucose."
Kana opened her notebook. "Glucose is an energy source?"
"Yes. Broken down in glycolysis," Rei answered.
"Glycolysis?"
"Pathway converting glucose to pyruvate. Ten enzymatic steps."
Toma was surprised. "Ten steps? That's tedious."
"It's efficient," Milia explained. "Extracts energy little by little."
Rei drew a diagram. "First uses ATP. Investment phase."
"Costs money?" Kana asked.
"Uses energy to phosphorylate glucose. Recover later."
"Invest and profit," Toma understood.
"But," Milia continued, "not all pathways run simultaneously."
"Why?"
"They're regulated. Supply changes according to demand."
Rei supplemented. "There's a rate-limiting step. The slowest reaction determines overall speed."
"Traffic bottleneck?" Kana made a metaphor.
"Perfect example," Milia acknowledged. "Like highway toll gates."
Toma showed interest. "So where's the traffic jam?"
"In glycolysis, phosphofructokinase," Rei answered.
"Long name."
"Abbreviated PFK. This enzyme controls everything."
Kana made notes. "How?"
"Allosteric regulation," Milia explained. "Regulatory molecules bind at a site separate from the active site."
"Separate site?"
"Yes. When ATP is abundant, PFK activity decreases."
Toma understood. "If energy is sufficient, no need to make more."
"Exactly. Negative feedback."
Rei continued. "Conversely, when AMP is abundant, activity increases."
"AMP?"
"Breakdown product after ATP is used. Sign that energy is insufficient."
Kana murmured. "Balance of supply and demand."
"Same as economics," Milia laughed.
"There's also the citric acid cycle," Rei drew the next diagram. "Pyruvate is further broken down."
"Another long pathway?" Toma made a tired face.
"Eight steps. But also regulated."
"Who regulates it?" Kana asked.
"Isocitrate dehydrogenase," Milia answered. "When NADH is abundant, activity decreases."
"NADH?"
"Electron carrier. When abundant, it's a sign there's enough."
Rei drew the electron transport chain diagram. "Finally, ATP is mass-produced here."
"Finally the goal?"
"Yes. But traffic jams here too."
Toma laughed. "Traffic jams everywhere."
"When oxygen is insufficient, electron transport stops," Milia explained.
"Why we get out of breath during exercise?" Kana confirmed.
"Yes. Cells are seeking oxygen."
Rei summarized. "Metabolism is a highly regulated network. No waste, efficient."
"But it jams," Toma pointed out.
"Traffic jams aren't bad," Milia said. "Evidence of control."
"What do you mean?"
"Running full speed continuously, you'll tire quickly. Pacing matters."
Kana understood. "Metabolic traffic jams are life's wisdom."
"Yes. Conserve energy, use when needed."
Rei added. "Cancer cells have broken regulation. Consume sugar without limit."
"That's why metabolic abnormality."
The four looked outside. Students running in the schoolyard. Inside their bodies, countless metabolic reactions being regulated.
"Perfect traffic control," Toma murmured.
"Life's symphony," Milia smiled.
Traffic jams were part of order.