What is the term for the movement of molecules across cell membranes without the use of energy?

The movement of molecules across cell membranes without the use of energy is referred to as passive transport. This process allows substances to move along their concentration gradient, which means they travel from areas of higher concentration to areas of lower concentration. Because this movement does not require cellular energy, it is an efficient way for cells to maintain homeostasis and regulate the internal environment.

Passive transport includes various methods, such as simple diffusion, where small and nonpolar molecules can directly pass through the lipid bilayer of the membrane, and facilitated diffusion, where larger or polar molecules move through specific protein channels embedded in the membrane. While both facilitated diffusion and simple diffusion fall under the umbrella of passive transport, the foundational characteristic that defines passive transport is the lack of energy usage.

Energy-dependent processes, such as active transport, involve moving substances against their concentration gradient and require ATP or another energy source. Endocytosis, on the other hand, involves engulfing substances to take them into the cell and is also an energy-requiring process. Thus, the defining trait of passive transport is its energy independence, making this answer accurate.