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Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration. It requires no cellular energy because molecules move spontaneously down their concentration gradient, driven by random molecular motion. This process continues until concentrations equalize, reaching a state called dynamic equilibrium where molecules continue moving randomly but produce no net change in concentration.

Small nonpolar molecules like oxygen and carbon dioxide easily pass through the lipid bilayer via simple diffusion. Small uncharged polar molecules such as water cross more slowly. Larger uncharged molecules like glucose cannot pass directly through the membrane and instead require facilitated transport across cell membrane through specialized transport proteins. Charged ions cannot cross the hydrophobic core regardless of size.

During respiration, oxygen diffuses from the alveoli into the blood because oxygen concentration is higher in the lungs than in blood. Oxygen then diffuses into body tissues where its concentration is lower. Metabolic waste carbon dioxide diffuses from tissues into capillaries, then into alveoli for exhalation. This concentration-gradient-driven exchange enables organisms to obtain oxygen and eliminate carbon dioxide without expending energy.

Diffusion rates depend primarily on molecular hydrophobicity, size, and the magnitude of the concentration gradient. Lipid-soluble, nonpolar molecules diffuse faster than polar or charged molecules. A larger concentration difference between compartments accelerates diffusion, while as a system approaches equilibrium, the diffusion rate slows. Temperature, molecular mass, and solvent density also influence how quickly substances move.

Simple diffusion allows substances to move directly across membranes along concentration gradients without assistance or energy. Facilitated diffusion requires membrane-embedded transport proteins to help substances cross membranes, also without expending energy. Both are passive processes, but facilitated diffusion enables larger or charged molecules that cannot pass through the lipid bilayer to move down their concentration gradients.

Charged ions cannot cross the hydrophobic core of the lipid bilayer because they are repelled by the nonpolar lipid environment. Regardless of their size, ions require specialized membrane proteins like ion channels to traverse the membrane. These proteins provide a hydrophilic pathway through the membrane, allowing charged particles to move across while maintaining the membrane's barrier function against other charged substances.

Dynamic equilibrium is a state where concentrations stabilize and no net movement of molecules occurs, yet particles continue moving randomly between compartments at equal rates in both directions. In living cells, this bidirectional exchange is essential for maintaining homeostasis. Even at equilibrium, molecules remain in constant motion, ensuring that cells can respond to changing conditions and sustain vital biological processes.