Which of the following describes a spontaneous movement of molecules due to constant random motion?

Simple diffusion refers to the process by which molecules move spontaneously from an area of higher concentration to an area of lower concentration, driven by their constant random motion. This movement occurs without the need for energy input because it is a passive transport mechanism. The driving force behind simple diffusion is the concentration gradient, which is the difference in concentration of molecules across a membrane or within a space. Molecules will continue to move until they reach an equilibrium state, where concentrations are equal on both sides of the barrier.

In biological systems, small nonpolar molecules, like oxygen and carbon dioxide, typically undergo simple diffusion across cell membranes. Since this process does not require energy and occurs naturally due to the kinetic energy of the molecules, it embodies the concept of spontaneous movement.

Other processes mentioned, such as active transport and secondary active transport, require energy to move molecules against their concentration gradients, while facilitated diffusion uses specific transport proteins but still operates passively. Understanding these distinctions helps clarify that simple diffusion is solely based on the random motion of molecules without external energy influences.