What type of diffusion occurs without the use of ATP?

Diffusion that occurs without the use of ATP is primarily characterized by passive transport mechanisms, which include osmosis and facilitated diffusion. Facilitated diffusion, in particular, allows specific molecules to cross biological membranes with the help of transport proteins, like channel or carrier proteins, but does not require energy input from the cell.

This process relies on the concentration gradient of the molecules, moving them from areas of higher concentration to areas of lower concentration until equilibrium is reached. Unlike active transport, which uses ATP to move substances against their concentration gradient, facilitated diffusion works along the gradient, making it a passive process. It is an essential method for cells to regulate the intake of necessary substances, such as glucose and ions, without expending energy.

In contrast, active transport would consume ATP to move substances uphill against their concentration gradient, endocytosis involves engulfing substances into the cell, which also requires energy, and osmosis, while a passive process, specifically refers to the movement of water rather than solutes in general. Therefore, facilitated diffusion aptly fits the definition of diffusion that occurs without ATP.