Diffusion

Calculating Diffusion Efficiency

• To determine how size affects diffusion, calculate the Surface Area to Volume Ratio. For a cube with side length $s$, $SA = 6s^2$ and $V = s^3$.

• Flux Calculation: The net movement of a substance across a unit area per unit time is known as flux ($J$). It is mathematically expressed as $J = -D \frac{dc}{dx}$, where $D$ is the diffusion coefficient and $\frac{dc}{dx}$ is the concentration gradient.

• Experimental Modeling: Diffusion can be visualized using agar cubes containing pH indicators or by observing the movement of solutes through synthetic membranes like Visking tubing.

• Simple vs. Facilitated: Simple diffusion occurs directly through the phospholipid bilayer (e.g., $O_2$, $CO_2$), while facilitated diffusion requires transmembrane proteins for large or polar molecules (e.g., glucose, ions).

• Diffusion vs. Active Transport: Unlike diffusion, active transport moves substances against the concentration gradient and requires metabolic energy (ATP).

• Terminology Precision: Always use the term 'net movement' rather than just 'movement' to acknowledge that individual particles move in all directions.

• Gradient Direction: Ensure you specify that movement is 'down' or 'along' a concentration gradient, never 'up' (unless discussing active transport).

• SA:V Calculations: In exam questions involving different sized organisms or cells, always calculate the numerical SA:V ratio to justify why smaller structures are more efficient at exchange.

• Units Check: When calculating rates, ensure time units (e.g., $mg/s$) and distance units are consistent across all variables.

• Equilibrium Myth: A common mistake is thinking that molecular motion stops once equilibrium is reached. In reality, motion continues, but the net change is zero.

• Polarity Confusion: Students often forget that the hydrophobic core of the cell membrane prevents polar molecules from diffusing simply; they must use facilitated pathways.

• Rate vs. Time: Do not confuse the 'rate of diffusion' (speed) with the 'time taken'. A higher rate results in a shorter time.

• Respiratory Physiology: Diffusion is the primary mechanism for gas exchange in the alveoli, where oxygen enters the blood and carbon dioxide leaves it.

• Pharmacology: The rate at which a drug diffuses into tissues depends on its lipid solubility and the concentration gradient established after administration.

• Industrial Chemistry: Diffusion principles are used in dialysis machines to remove waste products from the blood by creating a concentration gradient against a dialysate solution.