"Same molecular formula but different compounds?"
Kana compared structural formulas with a confused look.
Rei nodded. "Isomers. Atom arrangement differs."
"But same number of atoms?"
"C₆H₁₂O₆. Both glucose and fructose."
Milia assembled model. "But shape differs."
"Different shape means different properties?" Kana asked.
"Completely different. Taste, reactivity."
Rei explained. "Carbon skeleton differs. This is the source of organic compound diversity."
Kana drew in notebook. "Carbon skeleton?"
"Framework of connected carbon atoms. Can branch or form rings."
Milia continued. "Carbon has four hands."
"Four?"
"Can make four bonds. So can create complex structures."
Rei drew diagram. "Linear, branched, cyclic... infinite combinations."
Kana was surprised. "Infinite?"
"Theoretically. If carbons keep connecting."
"Life uses this diversity," Milia said.
Rei gave concrete example. "Fatty acids. Long carbon chains."
"How many?"
"Palmitic acid has 16. Stearic acid has 18."
Kana took notes. "Long..."
"But regular. CH₂ repeats."
Milia showed another example. "Steroids. Four fused rings."
"Cholesterol?"
"Yes. Complex ring structure. But all carbon and hydrogen."
Rei continued. "Functional groups determine properties."
"Functional groups?" Kana asked.
"Specific groups attached to carbon skeleton. Hydroxyl, carboxyl..."
"That changes properties?"
"Dramatically. Ethanol and acetic acid, carbon skeleton nearly same but completely different."
Milia organized. "Skeleton is foundation, functional groups are personality."
Kana understood. "Building structure and decoration?"
"Good metaphor," Rei acknowledged.
Toma changed topic. "In metabolism, how does skeleton change?"
"Converted little by little. Carbons added or removed."
Rei drew metabolic pathway. "Glucose to pyruvate. C₆ to C₃."
"Becomes half?"
"Glycolysis. Changes carbon skeleton while extracting energy."
Milia continued. "Pyruvate to acetyl-CoA. C₃ to C₂."
"Decreases again?"
"Released as CO₂. Until completely oxidized."
Kana wrote in notebook. "C₆ → C₃ → C₂ → CO₂."
"More precisely, all becomes CO₂ in TCA cycle."
Rei drew circle. "Citric acid cycle. Each round releases CO₂."
"Carbon's journey?" Kana made metaphor.
"Beautiful expression," Milia smiled.
Rei showed another pathway. "Reverse also exists. Making sugar from CO₂."
"Photosynthesis?"
"Yes. Calvin cycle. Fixes CO₂ and assembles carbon skeleton."
Kana was moved. "Making organic from inorganic."
"Plant's magic."
Milia added. "But complex maze."
"Maze?"
"Metabolic pathways are network-like. Multiple pathways from one molecule."
Rei drew diagram. "Glucose can become fatty acid, amino acid, nucleic acid."
"Versatile?" Kana was surprised.
"By rearranging carbon skeleton, can make anything."
Milia continued. "But enzymes decide the path."
"Enzymes?"
"Dedicated enzyme for each step. Guide in the maze."
Rei explained. "So can control. Make what's needed, in needed amount."
Kana had a doubt. "What if enzyme breaks?"
"Metabolic disease. Goes down wrong path."
Milia gave example. "Phenylketonuria. Abnormality in amino acid metabolism."
"What happens?"
"Phenylalanine accumulates. Brain damage occurs."
Rei continued. "So screen with newborn testing."
Kana became serious. "Serious if wrong path in carbon skeleton."
"Yes. Life walks precise maze."
Milia showed model. "But also flexible."
"Flexible?"
"Multiple pathways exist. If one unusable, can take another path."
Rei supplemented. "Metabolic redundancy. Survival strategy."
Kana looked out window. "Plants, animals, all walking carbon maze."
"Same principle," Milia said.
"So by eating, we inherit carbon," Rei continued.
Kana said thoughtfully. "My body is also collection of carbon skeletons."
"And constantly being rearranged."
Milia said finally. "We are carbon travelers."
"Carbon travelers," Rei translated.
Kana smiled. "In the maze, life continues."
The three nodded. The carbon skeleton maze is complex but beautiful.