"The inside of a cell is like a factory."
Kana was looking at a schematic diagram of a cell.
"Good metaphor," Rei said. "Each organelle has a specific role."
Milia pointed at the diagram. Round particles scattered everywhere.
"Ribosomes?" Toma asked.
"The factory that makes proteins," Rei explained. "They read mRNA blueprints and link amino acids."
Kana wrote in her notebook. "Translation happens here."
"Yes. mRNA transcribed in the nucleus reaches the ribosomes."
"But ribosomes are small," Toma pointed out.
"About 20 nm diameter. A complex of proteins and rRNA."
Milia supplemented. "Large subunit and small subunit."
"Two parts combine to function," Rei continued. "Like a puzzle."
Kana looked at another structure. "Is this a maze?"
"Rough endoplasmic reticulum," Rei smiled. "Ribosomes attached to the surface."
"Why?"
"To make secretory and membrane proteins. While synthesized at ribosomes, they're transported into the ER."
Toma became interested. "Simultaneous?"
"Yes. Translation and transport are linked. Efficient."
Milia drew a diagram. Ribosome, ER membrane, lumen.
"Signal peptide is the marker," Rei explained. "A short sequence at the protein's beginning."
"It's the signal to 'send here.'"
Kana understood. "Like an address."
"Exactly. Like a zip code."
Toma looked at the next structure. "Layers stacked up."
"Golgi apparatus," Rei said. "The protein finishing factory."
"Finishing?"
"Proteins made in the ER are modified. Adding sugar chains, cutting."
Milia wrote an example. "Glycosylation: adding sugars"
"What changes when sugar is added?" Kana asked.
"Stability, solubility, recognition. Sugar chains determine molecular character."
Rei continued. "Blood types are also differences in sugar chains. Same protein with different sugars attached."
"Wow!" Toma was surprised.
"The Golgi also sorts proteins. Decides where to send them."
Kana asked. "How does it decide?"
"Tags on proteins. Mannose-6-phosphate, specific sequences."
Milia drew. "Cis face → medial → trans face"
"Progresses through the Golgi in order," Rei explained. "Different modifications at each layer."
"Assembly line," Toma said.
"Yes. Balancing efficiency and accuracy."
Kana looked at the overall diagram. "Ribosome → ER → Golgi → destination"
"The journey of proteins," Rei said. "But what if they take the wrong path?"
"Disease occurs," Milia said quietly.
"Transport errors are serious," Rei continued. "Cystic fibrosis is abnormal CFTR transport. I-cell disease is transport failure to lysosomes."
Toma looked serious. "Small mistakes, big impact."
"Yes. That's why cells have quality control systems."
Kana asked. "Quality control?"
"Proteins not properly folded are detected in the ER. They're repaired or degraded."
"ER-associated degradation, ERAD," Milia supplemented.
"Harsh," Toma said.
"But necessary," Rei acknowledged. "If defective products accumulate, cells die."
Kana gazed at the diagram. "Like a secret base."
"Secret base?"
"Each room has a special mission. They cooperate to protect life."
Rei smiled. "Good expression. Organelles are the secret base of biomolecules."
Milia nodded too. Toma started coloring the diagram.
"Ribosomes are blue, ER is green, Golgi is yellow."
"Colorful secret base," Kana laughed.
"But they're actually transparent," Rei said. "Colors are a device for human understanding."
"Colors for understanding."
"Yes. Science is an attempt to make the invisible visible."
The four stared at the diagram. The secret base of biomolecules. In this tiny world, great dramas unfold.