"When I raised the temperature, the membrane became transparent."
Kana reported in surprise.
Milia nodded. "Phase transition. The lipid bilayer changed from gel phase to liquid-crystalline phase."
"Phase transition? A physics topic?"
Rei explained. "Similar to water changing from ice to liquid. Lipid arrangement goes from ordered to disordered."
"Why does it become transparent?"
"In gel phase, lipids are tightly packed and scatter light. In liquid-crystalline phase, gaps form and light passes through."
Milia drew a diagram. "Lipid molecules have hydrophilic heads and hydrophobic tails. The hydrocarbon chains of the tails change their movement with temperature."
"At low temperature?"
"Tails extend and align. Molecules line up tightly."
"At high temperature?"
"Tails bend and move around. Gaps form and fluidity increases."
Kana wrote in her notebook. "Higher temperature makes the membrane softer."
"Yes. This fluidity is important for biological membrane function."
Rei continued. "Membrane proteins float in a sea of lipids. With high fluidity, proteins move easily."
"Do they need to move?"
"Yes. After receptors bind ligands, they associate to transmit signals. Without fluidity, they can't associate."
Milia added. "So organisms regulate membrane fluidity. In cold environments, they increase unsaturated fatty acids."
"Unsaturated fatty acids?"
"Fatty acids with double bonds. Double bonds make tails bend. They pack less tightly."
Rei gave an example. "Arctic fish have many unsaturated fatty acids in their membranes. To maintain fluidity even at low temperatures."
"That's clever," Kana admired.
"It's the result of evolution. Without adaptation, they can't survive."
Milia asked. "What if there are many saturated fatty acids?"
"Membranes become rigid. To adapt to high temperature environments, some bacteria increase saturated fatty acids."
Kana summarized. "Lipid types change the phase transition temperature. Organisms adjust lipid composition to match their environment."
"Perfect," Rei acknowledged.
"But what about humans? Body temperature is constant, right?"
Milia answered. "Constant, but locally it varies. Hands and feet get cold easily. Membrane fluidity must be maintained there too."
"So unsaturated fatty acids are necessary."
"Yes. Called essential fatty acids."
Rei added. "Cholesterol is also important. It intercalates into membranes and regulates fluidity."
"Does it increase or decrease fluidity?"
"Both. At low temperatures it increases fluidity, at high temperatures it decreases it. Works as a buffer."
Kana was impressed. "So cholesterol isn't a villain."
"Too much is problematic, but the right amount is necessary. Essential for membrane stability."
Milia said, "Lipids aren't just walls. They're dynamic, functional structures."
"They have a temperature where they unravel," Kana murmured.
"That flexibility makes life possible."
The three continued their experiment, imagining the invisible movement of membranes.