Which process requires no cellular energy and allows substances to move across the cell membrane?

The process that requires no cellular energy and allows substances to move across the cell membrane is diffusion. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient. This process occurs naturally as molecules spontaneously spread out to reach equilibrium without needing any energy input from the cell.

In contrast, active transport requires energy, typically in the form of ATP, because it involves moving substances against their concentration gradient. Endocytosis is also an energy-dependent process, where the cell engulfs substances by folding its membrane around them. Facilitated transport, while it does not require energy, involves specific transport proteins to assist in the movement of substances across the membrane. However, diffusion stands out as the simplest form of movement that directly utilizes the natural kinetic energy of molecules to achieve balance across the membrane.