"Is a membrane just a wall?"
Kana asked while looking at the cell membrane model.
Milia shook her head. "Not just a wall. More like intelligent gate."
"Intelligent gate?"
Rei explained. "Selective permeability. Chooses what passes and what doesn't."
Kana asked curiously. "How does it choose?"
"First, the basic structure," Rei drew on the whiteboard. "Phospholipid bilayer."
"Phospholipid?"
"Molecule with hydrophilic head and hydrophobic tail. Naturally forms bilayer in water."
Milia assembled the model. "Heads outside, tails inside."
"Why?" Kana asked.
"Hydrophobic tails escape from water. So they hide inside."
Rei continued. "This structure naturally forms closed bags. That's the cell membrane."
Kana tried to touch it. "Is it hard?"
"Fluid. Lipids can move laterally," Milia answered.
"Moves?"
"Fluid mosaic model. Both lipids and membrane proteins swim in the membrane."
Rei supplemented. "That's why it's flexible, and cells can change shape."
Kana had a doubt. "Then doesn't everything pass through?"
"That's the important point," Rei emphasized. "Lipid-soluble small molecules can pass. But ions and large molecules can't."
"Why not?"
"Hydrophobic layer becomes a barrier. Ions dissolved in water can't pass through the oil layer."
Milia gave examples. "Oxygen and carbon dioxide can pass. But sodium ions can't."
"Then how do necessary things get in?"
"Membrane proteins," Rei added to the model. "Channels, pumps, carriers... various types."
Kana took notes. "Channels?"
"Like waterways made of protein. Pass specific ions."
Milia showed concrete examples. "Sodium channels, potassium channels, chloride channels."
"Each is dedicated?"
"Yes. They select by shape and charge," Rei explained.
Kana was surprised. "So precise..."
"Nerve transmission depends on this. Ion flow becomes electrical signals."
Milia continued. "But channels are passive transport."
"Passive?"
"Follows concentration gradient. From high to low."
Rei drew a diagram. "Same as water flowing from high to low."
"What about the opposite direction?" Kana asked.
"Active transport. Needs energy."
"Energy?"
"Uses ATP. Sodium-potassium pump is famous."
Milia explained. "Sodium out, potassium in. Both against concentration gradient."
"Why do that?"
Rei answered. "To maintain concentration difference. This is the basis of nerves and muscles."
Kana understood. "Like charging a battery?"
"Perfect metaphor. Membrane potential stores energy."
Milia showed another example. "Glucose transporter. Takes sugar into cells."
"This is active transport too?"
"Carries with sodium. Called cotransport."
Rei supplemented. "Uses sodium's concentration gradient to pull in glucose."
"Smart..." Kana admired.
"Precision machinery created by evolution," Rei said.
Kana had another question. "What about large proteins?"
"Endocytosis," Milia answered. "Membrane wraps around and takes it inside."
"Like eating?"
"Phagocytosis. Also called cellular eating."
Rei drew a diagram. "Membrane deforms to make vesicles. Reverse is exocytosis."
"Reverse?"
"From inside cells to outside. Used in neurotransmitter release and such."
Kana looked at the model again. "Membranes are so busy."
"Always controlling substance entry and exit," Milia nodded.
Rei added. "Also have receptors. Receive signals from outside."
"Signals?"
"Hormones, growth factors... messages between cells."
Kana was moved. "Membranes are the cell's windows."
"Window, gate, and sensor," Rei summarized.
Milia said quietly. "Life is defined by boundaries."
"Life is defined by boundaries," Rei translated.
Kana nodded deeply. "But not completely closed. Selectively open."
"That's the wisdom of life," Rei smiled.
Wind blows outside the window. Countless cell membranes continue controlling substance entry and exit even now.