Diffusion

The Kinetic Molecular Theory explains that all particles are in constant, random motion due to their thermal energy. While individual particle movements are unpredictable (Brownian motion), the statistical outcome of millions of random movements results in a predictable net flow toward areas of lower density.

Fick's First Law of Diffusion mathematically describes this process, stating that the flux ($J$) is proportional to the concentration gradient. The formula is expressed as $J = -D \frac{dc}{dx}$, where $D$ is the diffusion coefficient and $\frac{dc}{dx}$ is the change in concentration over distance.

The Diffusion Coefficient ($D$) is a constant that depends on the nature of the substance, the medium through which it is diffusing, and the temperature. A higher $D$ value indicates that particles spread more rapidly through the medium.

Check the Gradient Direction: When solving problems, always identify the high and low concentration zones first. Net movement must always be directed toward the lower concentration unless external energy (active transport) is mentioned.

Units of the Diffusion Coefficient: Pay close attention to the units of $D$, typically expressed in $m^2/s$ or $cm^2/s$. Ensure all other variables in Fick's Law equations are converted to matching units before performing calculations.

Equilibrium Misconception: Remember that at dynamic equilibrium, particle movement does NOT stop. The net movement is zero because particles move in both directions at equal rates, but individual molecules remain in constant motion.

Distance Squared Relationship: In many exam scenarios, you must recognize that the time required for diffusion increases with the square of the distance ($t \propto x^2$). This explains why diffusion is highly efficient over cellular distances but ineffective for transport across large organisms.