"Sweet."
Kana was eating a cookie.
"You're sensing sugar," Milia said.
"Sugar, like table sugar?"
Rei began explaining. "Table sugar is sucrose. Glucose and fructose bonded together."
"Glucose?"
"The most basic monosaccharide. One of life's energy currencies."
Milia drew a structural formula. A hexagonal ring structure.
"C₆H₁₂O₆," Rei added. "Just carbon, hydrogen, and oxygen."
"So simple," Kana was surprised.
"But functions are diverse."
"For example?"
"Energy source, structural material, cell recognition," Milia listed.
Rei continued. "Glucose is broken down in glycolysis."
"Glycolysis?"
"A metabolic pathway converting glucose to pyruvate. Ten steps of enzyme reactions."
"Long," Kana thought.
"But efficient," Rei explained. "Extracts energy bit by bit."
Milia wrote in her notebook. "1 glucose → 2 pyruvate + 2 ATP + 2 NADH."
"ATP is produced," Kana confirmed.
"Yes. But this is just the beginning," Rei continued. "Pyruvate goes further into the citric acid cycle."
"More energy there?"
"Precisely, the citric acid cycle makes NADH and FADH₂. Those become ATP in the electron transport chain."
"The oxidation-reduction we learned before," Kana recalled.
"Yes. Overall, about 30-32 molecules of ATP from 1 glucose."
"Great efficiency."
Milia added. "But oxygen is required."
"Without oxygen?" Kana asked.
"Anaerobic glycolysis. Convert pyruvate to lactic acid and regenerate NADH."
"The cause of muscle soreness," Rei explained.
"Because lactic acid accumulates?"
"Not strictly accurate. But during intense exercise when oxygen is insufficient, we rely on anaerobic glycolysis."
Kana looked at the cookie. "The energy from this cookie also comes from glucose."
"Partly," Milia corrected. "It also contains lipids."
Rei continued. "Sugars are immediately usable energy. Lipids are for long-term storage."
"Why the difference?"
"The amount of sugar that can be stored as glycogen is limited. About 500 grams."
"More than that?"
"Converted to fat. Fat holds more than twice the energy per gram."
Kana took notes. "Sugar = quick effect, fat = long-term storage."
"Perfect," Rei acknowledged.
Milia drew another diagram. Polysaccharide structure.
"Starch, glycogen, cellulose. All polymers of glucose."
"Same material but different functions?" Kana was surprised.
"Different bonding patterns," Rei explained. "Whether α-1,4 linkage or β-1,4 linkage."
"Small differences, big impact."
"Humans can't digest cellulose," Milia added.
"Why?"
"No enzyme to break β-1,4 linkages. Cows have symbiotic bacteria that can."
Kana was impressed. "Sugars are profound."
Rei continued. "There are also complex sugar chains called glycans. On cell surfaces, they become recognition signals."
"Blood types too?"
"Yes. Differences in glycans on red blood cell surfaces."
Milia supplemented. "The immune system also recognizes glycans."
"Sugars also convey information," Kana understood.
"Energy, structure, information. Sugars are multifunctional," Rei confirmed.
Kana suddenly thought. "Is diabetes a disease with too much sugar?"
"Precisely, glucose regulation dysfunction," Rei explained.
"Insufficient insulin or it doesn't work well. Blood glucose rises."
"Why is that dangerous?"
"High blood sugar damages blood vessels and nerves. Protein glycation occurs."
Milia said with a serious expression. "Sugar is a friend, but control is necessary."
"Balance is important," Kana understood.
Rei said finally. "The warm story sugars create. It's an energy story."
"Fuel that powers life," Milia confirmed.
Outside the window, plants are photosynthesizing. Making glucose from CO₂ and water. Solar energy stored in sugar bonds. And those sugars become animal energy.
"Next, let's talk about mineral ions," Rei proposed.
Kana and Milia nodded. The story of sugars continues.