Transport: diffusion, osmosis, and why size changes everything
Fick's laws govern the passive movement of any substance down its concentration gradient: the flux is proportional to the gradient and to the diffusion coefficient, and the diffusion time scales with the square of the distance. That square law is the single most useful fact in microscopic biology: double the distance and a molecule takes four times as long; shrink to cellular scale and diffusion becomes almost instantaneous. Oxygen crosses a 20 µm cell in milliseconds; it would take an elephant's leg minutes without a circulatory system — which is why large organisms need plumbing and small ones do not.
Osmosis is diffusion applied to water across a selectively permeable membrane. Water moves toward the higher solute concentration (hypotonic side), and the pressure required to stop it is the osmotic pressure, which for dilute solutions follows the van 't Hoff relation — it depends on the molar concentration of *particles*, not their identity. That is why 1 mol/L glucose and 1 mol/L NaCl osmotically differ by a factor of two: NaCl dissociates.
In living cells, the consequences are daily lab routine. Animal cells lyse in hypotonic media and shrink in hypertonic ones; bacteria in hypotonic environments rely on their cell wall to avoid bursting; plant cells go turgid and stiff — turgor pressure is what keeps a wilting plant upright.