"Are molecules standing still?"
Kana stared at the clear liquid in the beaker.
Rei shook his head. "Always moving. Brownian motion."
"Brownian motion?"
"Thermal motion. Molecules constantly move irregularly."
Milia set up a microscope. "Want to see?"
Kana looked in. Fine particles were flickering.
"They're moving!"
"Collided by water molecules, moving randomly," Rei explained.
Kana asked curiously. "Why can't we see molecules themselves?"
"Too small. But we can see the effect of collisions."
Milia continued. "Molecular dance floor."
"Molecular dance floor?" Kana laughed.
"All molecules are dancing. In water, in air, in the body."
Rei showed calculations. "At room temperature, water molecules move at about 500 meters per second."
"Faster than bullet trains!" Kana was surprised.
"But immediately collide with other molecules. So can't move forward."
"Constantly colliding?"
"Yes. Creates phenomenon called diffusion."
Kana took notes. "Diffusion?"
Rei dropped ink in water. It spread out.
"Tries to become uniform concentration. Result of molecules' disordered motion."
Milia supplemented. "But no directionality. Random walk."
"Random walk?"
"Drunk person's stagger. Direction unpredictable."
Rei drew a diagram. "But statistically predictable. Mean square displacement proportional to time."
Kana was confused. "Individual unpredictable, but whole predictable?"
"Exactly. This is the beauty of statistical mechanics."
Milia continued the experiment. "What about reactions?"
"For reactions to occur, molecules must collide," Rei answered.
"Aren't they always colliding?"
"But not all collisions cause reactions."
Kana asked. "Why not?"
"Energy and orientation are important. Only collisions with appropriate conditions cause reactions."
Rei continued. "Collision theory. Reaction rate is product of collision frequency and reaction probability."
"Probability?"
"Proportion of collisions with energy above activation energy."
Milia showed concrete example. "Enzyme and substrate. Move randomly and meet."
"By chance?" Kana was surprised.
"Yes. But higher concentration means easier to meet."
Rei calculated. "Diffusion inside cells is faster than expected. Because it's small space."
"How fast?"
"Can diffuse from cell end to end in seconds."
Kana admired. "So reactions inside cells are efficient?"
"But crowded," Milia warned.
"Crowded?"
"Molecular crowding. Full of proteins, hard to move."
Rei supplemented. "Over 30 percent of cytoplasm is protein. Packed state."
"Can still react?" Kana worried.
"Rather, crowding sometimes promotes reactions."
"Why?"
"Molecules are close to each other. Easy to meet."
Milia showed another aspect. "But specificity is important."
"Specificity?"
"Among countless molecules, finding the right partner."
Rei explained. "Molecular recognition. Complementarity of shape and charge."
Kana made a metaphor. "On dance floor, finding the right partner?"
"Perfect analogy," Rei acknowledged.
Milia continued. "Key and lock. But key and lock keep moving."
"Match while moving?"
"Yes. Induced fit. When binding, shape changes slightly."
Rei drew a diagram. "Enzyme and substrate. When binding, active site closes."
Kana was moved. "Dynamic recognition."
"Life isn't static. Always fluctuating."
Milia added. "But fluctuation creates function."
"Fluctuation?"
"Small structural changes. They promote reactions."
Rei gave an example. "Hemoglobin cooperativity. When one binds oxygen, others also bind easily."
"Why?"
"Structural change propagates. Protein moves."
Kana understood. "Molecules aren't fixed. Dancing."
"Yes. Molecular dynamics," Milia smiled.
Rei said finally. "Computer simulation can reproduce molecular dance."
"Can see it?" Kana got excited.
"Can track motion of each atom."
Milia showed tablet. Protein was fluctuating.
"Really dancing..."
"This is the foundation of life," Rei said.
Kana looked out the window. "All over the world, countless molecules are dancing."
"Every second, countless collisions and reactions."
Milia said quietly. "We are the dance."
"We are the dance," Rei translated.
The three smiled. On the molecular dance floor, life continues.