"How are substances transported inside cells?"
Kana asked Milia.
"There are little mailmen," Milia smiled.
"Mailmen?"
"Vesicles. Small bags made of lipid bilayers. They wrap and transport proteins and other molecules."
Rei added. "The cell's logistics system. Very efficient and accurate."
Milia displayed an animation on her tablet. Small spheres moving through the cell.
"These are vesicles. They bud from the cell membrane and are carried to their destinations."
"How do they bud?" Kana asked with interest.
"Coat proteins are the key," Rei explained. "Clathrin and COP bend the membrane to form vesicles."
Milia drew a diagram. "Clathrin has a tripod structure. When they gather, they form a shape like a soccer ball."
"Cute," Kana laughed.
"Also functional. This structure efficiently bends the membrane."
Rei continued. "Endocytosis. The process of taking in extracellular substances."
"Receptors recognize specific molecules. When ligands bind, the membrane invaginates inward."
Milia advanced the animation. "Then the vesicle is pinched off inside the cell."
"With substances inside," Kana observed.
"Yes. This becomes an endosome, the initial sorting station."
Rei added explanation. "Endosomes are acidified. When pH drops, receptors and ligands dissociate."
"Why acidic?"
"For efficient recycling. Receptors return to the membrane, ligands are degraded or sent elsewhere."
Milia displayed a new diagram. "There's also a reverse process. Exocytosis."
"Releasing substances made inside the cell to the outside."
Kana thought. "For example?"
"Hormones, neurotransmitters, digestive enzymes," Rei listed.
"At synapses, exocytosis happens ultra-fast. Under a millisecond."
Milia got excited. "Calcium ions are the trigger. When Ca²⁺ flows in, vesicles fuse with the membrane."
"Fusion?"
"Two lipid bilayers become one. SNARE proteins mediate it."
Rei explained in detail. "v-SNARE and t-SNARE. Each on vesicles and target membranes. They bind and pull membranes together."
"Like a zipper," Milia added.
Kana asked. "But how do they reach the right place?"
"Excellent question," Milia nodded. "Rab proteins act as address labels."
"Each vesicle has specific Rabs. The destination membrane has corresponding receptors."
Rei drew a diagram. "In the Golgi apparatus, even more complex sorting occurs."
"Golgi apparatus?"
"The center for protein processing and sorting. Like a postal sorting center."
Milia showed an animation. "Proteins are modified as they pass through the Golgi. Sugar chains are added, or proteins are cleaved."
"Then they're placed in different vesicles according to their final destinations."
Kana was impressed. "Amazing system."
"Happens thousands of times a day," Rei said.
Milia added. "There are errors. Sometimes delivered to wrong places, or vesicles get jammed."
"That can cause diseases."
Rei gave an example. "Lysosomal storage diseases. Enzymes don't reach lysosomes correctly, undigested substances accumulate."
Kana became serious. "Vesicular transport is essential for life."
"Yes. A cell is a small city. Without logistics, it can't function."
Milia replayed the animation from the start. Vesicles busily moving through the cell.
"Invisible mailmen are working even now."
Kana placed her hand on her body. "Inside me too?"
"In trillions of cells, every second."
Rei said quietly. "Life is the flow of information and matter."
The three stared at the screen. Little mailmen supporting life.
"Thank you, vesicles," Kana said softly.
Milia and Rei laughed. But that gratitude was correct.