Which process involves substances crossing the membrane without energy input from the cell?

The process of passive transport involves substances crossing the cell membrane without the need for energy input from the cell. This movement occurs along the concentration gradient, meaning that substances move from an area of higher concentration to an area of lower concentration. This natural tendency of molecules to move and achieve equilibrium allows for essential functions such as gas exchange in the lungs and nutrient absorption in the intestines.

Passive transport can occur through diffusion, where small nonpolar molecules like oxygen and carbon dioxide freely pass through the lipid bilayer of the cell membrane due to their size and solubility. It can also include facilitated diffusion, in which larger or polar molecules, such as glucose, require specific transport proteins in the membrane to assist their passage, also without using energy.

In contrast, processes like active transport require energy (usually from ATP) to move substances against their concentration gradient, endocytosis involves the cell membrane enveloping substances to bring them into the cell, and each of these involves energy expenditure or specific mechanisms that do not fall under the umbrella of passive transport.