"What's inside a cell?"
Kana asked while looking through the microscope.
Milia quietly answered. "Crowded."
"Crowded?"
Rei supplemented. "Cytoplasm isn't just water. Proteins, RNA, metabolites. The concentration is very high."
"How much?"
"Protein concentration is 200-300 mg/mL. Quite viscous."
Kana was surprised. "But reactions still happen, right?"
"Yes. Molecules are constantly moving," Milia explained.
"How?"
"Diffusion," Rei answered. "Through random thermal motion, they move along concentration gradients."
"Concentration gradient?"
"From high concentration areas to low concentration areas. Passive transport without using energy."
Milia drew a diagram. Molecules moving randomly. But overall flowing in one direction.
"But if it's crowded, how can they move?" Kana questioned.
"Brownian motion," Rei explained. "They proceed while changing course through collisions with other molecules."
"Seems inefficient."
"Indeed. Diffusion is slow. Especially for large molecules."
Milia added. "That's why cells are small."
"Smaller is better?"
"Diffusion distance becomes shorter. Time is proportional to the square of distance."
Rei calculated. "In a cell 10 times larger, diffusion takes 100 times longer."
"That's why cells don't grow beyond a certain size," Kana understood.
"Exactly. Diffusion constraints determine cell size."
Milia drew a new diagram. Molecules crossing the cell membrane.
"But sometimes we transport against the concentration gradient," Rei continued.
"How?"
"Active transport. Using ATP energy."
"Concrete example?" Kana asked.
"Sodium-potassium pump," Milia answered.
"This keeps K⁺ high inside cells and Na⁺ low," Rei explained.
"Opposite to natural diffusion direction."
"Yes. That's why energy is needed."
Kana took notes. "Passive transport = going downhill, active transport = going uphill."
"Good analogy," Rei acknowledged.
Milia added. "Cytoplasm is a dynamic environment."
"Dynamic?"
"Constantly changing. Metabolites are produced and consumed."
Rei continued. "Transport is important to maintain steady state."
"Steady state is different from equilibrium?" Kana confirmed.
"Different. Steady state has constant concentration despite flow. Equilibrium has no flow."
"Difficult..."
Milia gave an example. "Filling a bathtub with water while pulling the plug. If water level is constant, it's steady state."
"But water is flowing," Kana understood.
"Living cells are in steady state," Rei emphasized.
"Cytoplasmic flow is also a kind of steady state," Milia confirmed.
Kana suddenly thought. "What about the cytoskeleton?"
"Supports cytoplasmic structure," Rei answered. "Actin filaments, microtubules, intermediate filaments."
"These determine cell shape," Milia supplemented.
"But don't they hinder flow?"
"On the contrary, they direct flow," Rei explained. "Molecular motors transport materials along the cytoskeleton."
"Molecular motors?"
"Kinesin, dynein, myosin. Proteins that move using ATP."
Kana was impressed. "Inside cells is so organized."
"Crowded but orderly," Milia confirmed.
Rei said finally. "The gentle flow of cytoplasm. That's the foundation of life."
"Invisible, but certainly exists," Kana murmured.
Milia smiled. "Can't see with microscope. But can understand."
Outside the window, a river flows. Cytoplasmic flow is similar. Gentle but ceaseless. An invisible river supporting life.
"Next, let's talk about oxidation-reduction," Rei proposed.
Kana and Milia nodded. The journey of cytoplasm continues.