What principle governs the movement of substances like drugs across cell membranes by diffusion?

The principle that governs the movement of substances like drugs across cell membranes by diffusion is concentration gradients. Diffusion is the process whereby molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached. This movement occurs without the need for energy input because it relies on the natural tendency of substances to spread out and move toward areas of lesser concentration.

In the context of cells, drugs or other substances will traverse the phospholipid bilayer of the cell membrane primarily based on their concentration on either side of the membrane. If there is a higher concentration of a drug outside the cell than inside, molecules will diffuse into the cell until the concentrations equalize. This process is fundamental to pharmacology and cellular biology, as it directly impacts how drugs enter, work within, and exit from cells.

Osmotic pressure is more specifically related to the movement of water across membranes due to solute concentration differences rather than the direct diffusion of drugs. Pressure gradients generally refer to the movement of gases or fluids rather than solute molecules in the context of cellular diffusion. Thermodynamic efficiency involves the energy changes and spontaneous processes but does not specifically describe the movement mechanism for substances like drugs across cell membranes. Thus, concentration gradients are the primary factor explaining how diffusion