"I don't want to move anymore... but I'm hungry."
Toma collapsed onto the bench.
Kana sat beside him. "Why does metabolism continue after exercise ends?"
Rei explained. "Basal metabolism. Energy consumed just by being alive."
"Even when not moving?"
"The heart beats, lungs breathe, brain is active."
Toma nodded. "Working without being aware of it."
"Just maintaining body temperature requires enormous energy," Rei continued.
Kana took notes. "What's it used for?"
"Ion pumps. Na+/K+-ATPase maintains concentration gradients."
"All the time?"
"Always. If gradients collapse, neither nerve transmission nor muscle contraction work."
Toma asked. "Even while sleeping?"
"Of course. About 20% of basal metabolism is consumed by the brain."
Kana was surprised. "The brain uses that much?"
"Despite being only 2% of body weight. Extremely energy-intensive."
Rei wrote in his notebook. "Protein synthesis never stops. Degradation and synthesis constantly occur."
"Why break them down?"
"To remove old, damaged proteins."
Toma understood. "So need to keep making them."
"Yes. Cellular remodeling."
Kana continued. "What else?"
"Lipid synthesis, nucleic acid synthesis, preparing for cell division," Milia joined in.
"Cells are always preparing?"
"Maintaining homeostasis. Keeping internal environment constant despite environmental changes."
Rei explained. "This demands enormous energy."
Toma murmured. "Living means continuously consuming energy?"
"Thermodynamically, yes. Maintaining order requires fighting entropy."
Kana asked. "But how is it regulated?"
"Feedback control," Rei answered. "When ATP levels drop, metabolism accelerates."
"There's a sensor?"
"AMPK. AMP-activated protein kinase."
Milia supplemented. "Activated when ATP/AMP ratio drops."
"And then?"
"Promotes catabolism to make ATP. Simultaneously suppresses anabolism."
Toma took notes. "When energy is low, focus on making it."
"Exactly. There are priorities."
Kana thought. "Conversely, when ATP is abundant?"
"Anabolism is activated. Glycogen synthesis, fat synthesis."
"Storage?"
"Yes. Saving surplus energy for the future."
Rei continued. "So without eating, we start breaking down stores."
Toma got it. "Muscle loss too?"
"In extreme starvation, proteins are also degraded. As material for gluconeogenesis."
Kana worried. "The body eats itself?"
"For survival. The brain can only use glucose."
Milia corrected. "Can also use ketone bodies. Starvation adaptation."
Rei summarized. "Metabolism is always in dynamic equilibrium. It never stops."
Toma murmured. "Until death?"
"Death is when metabolism stops."
Kana said quietly. "Why metabolism never stops. Because we're alive."
Rei acknowledged. "Tautological but true."
Milia added. "Life is a flow of energy."
Toma stood up. "Then let's go eat. To support metabolism."
The three laughed. As long as metabolism doesn't stop, hunger won't either.