The movement of substances across cell membranes without energy input, driven by concentration gradients or other natural forces. This includes simple diffusion, facilitated diffusion, and osmosis.
From Latin 'passivus' meaning inactive or acted upon, combined with 'transport.' The term emphasizes that no cellular energy is required - the movement happens naturally due to physical and chemical forces discovered in the 19th century.
Passive transport is like rolling downhill - no energy required because nature does the work! It's how oxygen gets into your blood and carbon dioxide gets out, simply following the natural tendency of molecules to spread from crowded areas to less crowded ones.
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