"How do cells talk to each other?"
Kana looked up from the microscope.
Milia smiled. "They use signaling molecules. Chemical messages."
"Messages?"
Rei explained. "Hormones, neurotransmitters, growth factors... various types."
"How are they delivered?"
"Carried by bloodstream, or passed directly to adjacent cells," Milia answered.
Kana wrote in her notebook. "The receiving side?"
"Receptors," Rei said. "Proteins in cell membrane or inside cells."
"Receptors receive?"
"When signaling molecules bind, receptor shape changes."
Milia drew a diagram. "This becomes a trigger."
"Trigger?"
"A cascade starts inside the cell," Rei continued. "A chain reaction."
"What kind of chain?"
"First, the receptor gets phosphorylated."
Kana questioned. "Phosphorylated?"
"Phosphate groups attach. This is the activation signal."
Milia supplemented. "Phosphorylated receptor phosphorylates the next protein."
"Then the next too?"
"Yes. Signal is amplified as it's transmitted."
Rei enlarged the diagram. "One signaling molecule activates thousands of proteins."
"Amplification?" Kana was surprised.
"The power of cascade. Small input becomes large output."
Milia gave another example. "cAMP is a second messenger."
"Second?"
"First messenger is hormone. Secondary transmitter inside cell is cAMP."
Rei explained. "When receptor activates, an enzyme called adenylyl cyclase works."
"And?"
"Makes cAMP from ATP. cAMP diffuses through the cell."
"After diffusing?"
"Activates protein kinase A," Milia continued.
"Kinase?"
"Enzyme that phosphorylates. Phosphorylates various proteins, changing their activity."
Kana organized. "Hormone→receptor→cAMP→kinase→effect."
"Perfect," Rei nodded.
"Other second messengers?"
"Calcium ions, IP₃, diacylglycerol..." Milia listed.
"Calcium?"
"Muscle contraction, nerve transmission, enzyme activity... diverse roles."
Rei supplemented. "Intracellular calcium concentration is strictly controlled."
"Why?"
"Too high causes unwanted reactions. Too low, signals don't transmit."
Kana questioned. "How do signals stop?"
"Phosphatases," Milia answered. "Enzymes that remove phosphate groups."
"Opposite action?"
"Yes. Kinases and phosphatases maintain equilibrium."
Rei continued. "Also, signaling molecules themselves are degraded."
"Immediately?"
"Seconds to minutes. Transient signals."
Milia drew a new diagram. "G protein-coupled receptors. Very important."
"G?"
"Guanine nucleotide-binding protein," Rei explained.
"Complex..."
"But principle is simple. Receptor activates G protein, G protein activates enzyme."
Kana thought. "Indirect?"
"Yes. Highly flexible. One receptor can activate multiple pathways."
Milia gave an example. "Adrenaline receptor. Increases heart rate."
"How?"
"Increases cAMP, opens calcium channels, contracts cardiac muscle."
Rei supplemented. "Multiple pathways coordinate. That's physiological response."
Kana was moved. "Cell conversations are so complex."
"But logical," Milia said. "Amplification, integration, termination. Same as information processing."
Rei nodded. "Cells are computers. Signal transduction is their program."
"If mistakes happen?"
"Disease occurs," Milia said quietly. "Cancer, diabetes, neurodegenerative diseases..."
"Signal transduction abnormalities?"
"Many diseases are. That's why they're drug targets."
Kana looked at the microscope. "Inside these tiny cells, such complex things..."
"Every second, thousands of conversations are exchanged," Rei said.
Milia smiled. "More like lively debates than secret conferences."
The three laughed.
Invisible, but cells converse.
In the language of chemistry, coordinating life.