Simple diffusion: Movement of oxygen and carbon dioxide across the alveolar epithelium., Small, non-polar molecules are typically transported this way., Transport rate increases in direct proportion to the increase in concentration gradient, with no saturation point (at biological concentrations)., Transport is non-specific, as the rate is proportional to the lipid solubility of the molecule., The overall rate is primarily determined by the steepness of the concentration gradient and the surface area/thickness of the exchange surface, Facilitated diffusion: Rate of transport can be limited by the number of transport proteins in the membrane., Involves the use of carrier proteins or channel proteins., Used to transport large molecules like glucose and charged particles like sodium ions (Na+)., The rate of transport will level off when all specific membrane proteins are saturated., Both types of diffusion: The movement of molecules or ions is down a concentration gradient, A passive process that does not require metabolic energy (ATP), Molecules dissolve in and pass directly through the phospholipid bilayer.,

Types of diffusion

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