"Do we just eat amino acids?"
Kana stared at the supplement bottle.
Rei answered. "Body can make them too. But not all."
"Some can be made and some can't?"
"Essential and non-essential amino acids," Milia explained.
Kana opened her notebook. "How many are essential?"
"Nine in humans. Valine, leucine, isoleucine..."
"The rest?"
"Can be synthesized in the body. But 'non-essential' is misleading," Rei added.
"Why?"
"Doesn't mean unnecessary. Rather essential. Just don't need to obtain externally."
Milia drew a diagram. "From glutamic acid, proline, arginine, glutamine are made."
"They're converted."
"Enzyme called transaminase. Transfers amino groups."
Rei explained. "Adding amino group to α-keto acid makes amino acid."
Kana asked. "Then why can't we make essential amino acids?"
"Synthesis pathway too complex. Or lost the enzyme."
Milia supplemented. "During evolution, if obtainable from food, no need to make it."
"Cost reduction?"
"Energy and resource saving. Rational choice."
Rei showed another diagram. "But there are conditionally essential amino acids."
"Conditionally?"
"Arginine. During growth or illness, synthesis can't keep up."
Kana thought. "So where do amino acids come from?"
"Ultimately nitrogen fixation," Milia answered. "Converting atmospheric N₂ to ammonia."
"Plants and microbes do it," Rei continued. "Animals can't."
"So we?"
"Obtain as organic nitrogen compounds from plants and other animals."
Kana organized. "What happens to eaten amino acids?"
"Digested, absorbed. Then reused in body."
Milia explained. "Protein synthesis, energy source, conversion to other molecules."
"Other molecules?"
"Glycine to heme, tryptophan to serotonin."
Rei added. "Tyrosine to dopamine, noradrenaline."
Kana was surprised. "Neurotransmitters from amino acids too?"
"Yes. Amino acids are multifunctional precursors."
Milia drew another pathway. "Excess amino acids used for gluconeogenesis or fat synthesis."
"Not discarded."
"Life dislikes waste. Recycling is fundamental."
Rei warned. "But amino groups are problematic."
"Why?"
"Become ammonia. Toxic."
Kana worried. "What to do?"
"Urea cycle," Milia answered. "Liver converts ammonia to urea."
Rei drew a diagram. "Complex pathway. Five enzymes involved."
"So complex?"
"Because ammonia toxicity is high. Strictly processed."
Kana asked. "After becoming urea?"
"Filtered by kidneys, excreted as urine."
Milia said quietly. "Nitrogen's journey. From atmosphere to plants, animals, then excreted again."
Rei nodded. "Part of nitrogen cycle. Amino acids are its carriers."
Kana drew pathways in her notebook. "Amino acids keep transforming."
"Central to metabolism," Milia expressed.
Rei added. "Energy, structure, signals. Amino acids involved in everything."
Kana looked at the supplement. "Strange journey indeed."
Milia smiled. "Starting from atmosphere, circulating through life, back to environment."
Rei said quietly. "Nitrogen's story. Told by amino acids."
The three fell silent. Small molecules on grand journeys.