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Diffusion is the passive movement of substances from areas of higher to lower concentration without requiring energy. Molecules in constant random motion collide and disperse until evenly distributed. In cells, diffusion moves materials across the plasma membrane and through the cytosol, driven by concentration gradients that represent potential energy dissipating naturally.

Simple diffusion allows gases and lipid-soluble molecules like steroids and fatty acids to move directly through the lipid bilayer. Facilitated diffusion enables water-soluble molecules such as glucose and charged ions to cross the membrane via transmembrane carriers or channels, since these substances cannot pass through the lipid bilayer alone.

The greater the difference in concentration between areas, the more rapid the diffusion occurs. As the material distribution approaches equilibrium, the diffusion rate slows. This concentration difference represents potential energy that drives molecular movement from high to low concentration until balance is reached.

Diffusion rate depends on molecular mass, temperature, and solvent density. Larger molecules diffuse more slowly through denser mediums. Higher temperatures increase molecular energy and movement, accelerating diffusion. Lower solvent density also increases diffusion speed since molecules move more easily through less dense substances.

During respiration, oxygen-rich air with low carbon dioxide enters the alveoli, which are surrounded by carbon dioxide-rich blood capillaries. This concentration difference allows gases to exchange between the alveoli and blood via diffusion, moving oxygen into the bloodstream and carbon dioxide out for exhalation.

Diffusion is passive transport driven by concentration gradients, which represent potential energy. Molecules naturally move from high to low concentration as this potential energy dissipates. No ATP or active cellular processes are needed; the concentration difference itself provides the driving force for molecular movement.

Each substance in a medium, such as extracellular fluid, maintains its own independent concentration gradient separate from other materials. Understanding how cells regulate these gradients is essential to comprehending tonicity in animals and how osmotic balance is maintained across cell membranes.