What mechanism allows substances to cross the plasma membrane without energy input?

Facilitated diffusion is the mechanism that allows substances to cross the plasma membrane without the requirement of energy input. This process involves the movement of molecules along their concentration gradient, meaning they move from an area of higher concentration to an area of lower concentration. In facilitated diffusion, specific transport proteins embedded in the plasma membrane assist in the movement of polar and charged substances, such as ions and certain sugars, which cannot easily pass through the lipid bilayer due to their size or polarity.

Unlike active transport, which requires energy (usually in the form of ATP) to move substances against their concentration gradient, facilitated diffusion operates purely based on the natural kinetic motion of molecules and their concentration differences. This passive process is essential for maintaining cellular homeostasis and allows for the efficient uptake or removal of specific molecules essential for cellular function.