Transport Across Membranes: Diffusion and Osmosis

One of the most fundamental functions of the cell membrane is to control the movement of substances into and out of the cell. The Edexcel A-Level Biology specification (9BI0) requires a thorough understanding of passive transport mechanisms — diffusion, facilitated diffusion and osmosis — including the factors that affect their rates and how to investigate them experimentally.

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Diffusion

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process, meaning it does not require metabolic energy (ATP).

Diffusion occurs because all molecules in a liquid or gas are in constant random motion due to their kinetic energy. Although individual molecules move randomly, the overall net movement is from high to low concentration until dynamic equilibrium is reached (where molecules continue to move, but there is no net movement in either direction).

Simple Diffusion

Simple diffusion refers to the direct movement of small, non-polar molecules through the phospholipid bilayer. No membrane proteins are required.

Molecules that can diffuse directly through the bilayer include: