Which process involves the movement of particles across a membrane without energy input from the cell?

The correct answer is passive transport. This process refers to the movement of particles across a biological membrane without the need for cellular energy, specifically ATP. The driving force behind passive transport is the concentration gradient—the natural tendency of particles to move from an area of higher concentration to an area of lower concentration until equilibrium is achieved.

Passive transport encompasses various mechanisms, including simple diffusion, where small uncharged particles move freely across the lipid bilayer, and facilitated diffusion, which involves specific transport proteins aiding the movement of larger or polar substances. In both cases, no energy is required from the cell.

On the other hand, active transport, endocytosis, and facilitated diffusion (which is a subset of passive transport) do not fully align with the concept of movement without energy input. Active transport specifically requires energy to move substances against their concentration gradient, while endocytosis entails the engulfing of particles into the cell, which also requires energy. Thus, passive transport is uniquely characterized by its reliance on natural concentration gradients without additional input of energy.