"I'm lost."
Toma stared at the metabolic map. Arrows running in all directions.
"It's a labyrinth," Rei acknowledged. "But there are rules."
"Rules?" Kana asked.
"Destinations are decided. ATP and NADH."
Milia explained. "The ultimate goal of sugar metabolism is creating energy currency."
"Energy currency?"
"ATP. Energy form cells can use."
Toma pointed. "This is glycolysis?"
"Yes. Breaking down glucose to pyruvate. Ten steps."
Kana wrote in her notebook. "Ten steps?"
"Each step has an enzyme. Extracts energy bit by bit."
Rei added. "If you break down all at once, it becomes heat and wastes."
"That's why stepwise?"
"For efficient energy recovery."
Milia drew a diagram. "Glycolysis produces two ATPs."
"Only two?"
"From one glucose molecule. But without oxygen, that's all."
Toma asked. "With oxygen?"
"Pyruvate enters mitochondria. To citric acid cycle."
Rei continued. "First converted to acetyl-CoA. CO2 is removed."
"CO2?"
"Decarboxylation. Reaction where carbon decreases."
Kana was confused. "What's citric acid cycle?"
"Eight steps that completely oxidize acetyl-CoA to CO2," Milia explained.
"Completely?"
"All carbons become CO2. In the process, NADH and FADH2 are produced."
Toma asked. "What's NADH used for?"
"Electron transport chain. Where large amounts of ATP are made."
"Large amounts?"
"About 30 ATPs from one glucose molecule."
Kana was surprised. "Compared to two from glycolysis!"
"Overwhelmingly more efficient with oxygen," Rei emphasized.
Milia showed another pathway. "There's also gluconeogenesis."
"Gluconeogenesis?"
"Reverse direction. Making glucose from pyruvate."
Toma was confused. "Reverse? What if both happen at once?"
"Won't happen. There are regulatory mechanisms," Rei answered.
"Regulation?"
"Allosteric control, hormones, phosphorylation... complex regulatory network."
Kana asked. "Why is reverse direction needed?"
"Brain can only use sugar. To maintain blood glucose," Milia explained.
"Gluconeogenesis occurs in liver."
Rei added. "Regenerates glucose from amino acids and lactate."
Toma murmured. "Going around in circles?"
"Metabolic cycles. Materials circulate."
Milia emphasized. "Citric acid cycle is actually a cycle."
"Cycle?"
"Starts with oxaloacetate, ends with oxaloacetate. Rotates."
Kana was moved. "Like perpetual motion?"
"No," Rei corrected. "Energy and matter are input. It's an open system."
"Open system?"
"Nutrients enter from food, CO2 and water exit. There's flow."
Toma looked at the metabolic map again. "Everything connected?"
"Everything. Sugar, lipid, amino acid metabolism intersect."
Milia drew a diagram. "Acetyl-CoA is central. Many pathways converge here."
"Hub?"
"Yes. Metabolic crossroads."
Rei continued. "That's why sugar can become fat, and glucose can be made from amino acids."
Kana understood. "Convertible?"
"Within bounds of energy conservation."
Milia added. "But there are also unidirectional reactions."
"Unidirectional?"
"Irreversible steps. Become control points."
Toma got excited. "A labyrinth, but there's a map!"
"Yes. Metabolic map is life's map," Rei said.
Kana murmured. "Sugar metabolism, more complex than I thought."
"But beautiful," Milia smiled.
"Optimized pathway network," Rei added.
Toma laughed. "The labyrinth is fun."
"Exploration has just begun," Milia said.
The four stared at the metabolic map. In the labyrinth, there is order.