Which process describes the movement of substances across a cell membrane without the use of energy?

The movement of substances across a cell membrane without the use of energy is referred to as passive transport. This process relies on the natural kinetic energy of molecules and their tendency to move from areas of higher concentration to areas of lower concentration, a concept known as diffusion. In passive transport, substances can diffuse directly through the lipid bilayer, or via specific channels or carriers in the membrane that facilitate their movement, but still without requiring any external energy input.

Facilitated diffusion, which is indeed another form of passive transport, involves specific proteins that help larger or polar molecules cross the membrane more easily. However, it is still classified under passive transport because it does not require energy. Active transport, on the other hand, specifically requires energy (usually in the form of ATP) to move substances against their concentration gradient, while endocytosis involves the cell membrane engulfing substances to bring them into the cell, which also does not fall under the category of passive transport. Therefore, passive transport represents the broad category that includes all energy-independent movement of substances across cell membranes, making it the correct answer.