Which passive process involves the movement of molecules from an area of higher concentration to an area of lower concentration?

Simple diffusion is the process being described as it refers to the movement of molecules from an area of higher concentration to an area of lower concentration. This movement occurs through a semipermeable membrane and does not require any energy input, making it a passive process.

In simple diffusion, small, nonpolar molecules like oxygen and carbon dioxide can pass freely across cell membranes because they can easily move through the lipid bilayer. The driving force behind simple diffusion is the concentration gradient, which is the difference in concentration of the molecules on either side of the membrane. Molecules will naturally tend to spread out evenly in a given space, moving down their concentration gradient until equilibrium is reached.

Osmosis specifically refers to the movement of water across a semipermeable membrane, while facilitated diffusion involves the use of protein channels or carriers to help larger or polar molecules pass through the membrane, also down their concentration gradient. Filtration, on the other hand, is typically driven by pressure differences rather than concentration gradients. Therefore, simple diffusion is specifically identified by its mechanism of moving molecules driven solely by concentration differences without the need for additional assistance or energy.