What is the primary movement characteristic of diffusion?

The primary movement characteristic of diffusion is solute movement from high to low concentration. This process is driven by the principle that molecules naturally move from areas where they are more concentrated to areas where they are less concentrated, in an effort to achieve equilibrium. This movement occurs without the need for any energy input, making diffusion a passive process.

In biological contexts, diffusion is crucial for many physiological processes, such as the movement of oxygen and carbon dioxide in the lungs. As gas molecules circulate, they diffuse based on their concentration gradients, ensuring that oxygen enters the bloodstream while carbon dioxide exits. This fundamental idea of concentration gradient driving movement is why solute movement from high to low concentration correctly describes the primary characteristic of diffusion.

Other options present different concepts; fluid movement from low to high concentration refers instead to active transport which requires energy input. Gas exchange across membranes is a consequence of diffusion but does not encapsulate the broader characteristic of the process. Pressure-driven fluid movement, as seen in filtration, is an entirely different mechanism unrelated to concentration gradients. Understanding these distinctions is essential for grasping how diffusion operates in various biological systems.