Which of the following statements is true regarding passive diffusion?

Passive diffusion is a fundamental process in cellular biology that involves the movement of molecules across a cell membrane without the expenditure of energy. The defining characteristic of passive diffusion is that it occurs down a concentration gradient, meaning molecules will naturally move from an area of higher concentration to an area of lower concentration until equilibrium is reached. This movement relies on the random motion of particles and does not require any input of energy or assistance from proteins, which is why it is termed "passive."

The misconception that passive diffusion requires ATP or a carrier protein contributes to a misunderstanding of how substances traverse cell membranes. In fact, many small nonpolar molecules, like oxygen and carbon dioxide, can readily diffuse through the lipid bilayer of cell membranes. Additionally, while passive diffusion can occur in various states of matter, including liquids and gases, it is not restricted solely to liquid solutions.

Thus, the true aspect of passive diffusion is its reliance on the concentration gradient and its inherent ability to facilitate molecular movement from regions of higher concentration to regions of lower concentration without any energy expenditure.