Which of the following cellular processes does not include an ATP requirement?

Facilitated diffusion is a process that allows molecules to cross cell membranes with the assistance of special proteins, known as transport proteins. This process occurs down the concentration gradient, meaning it moves substances from an area of higher concentration to an area of lower concentration. Since it relies on the natural kinetic energy of the molecules rather than consuming energy in the form of ATP, facilitated diffusion does not require any ATP.

In contrast, active transport and endocytosis both necessitate ATP. Active transport involves moving substances against their concentration gradient, which requires energy input, typically in the form of ATP. Endocytosis, where cells engulf materials from outside their membrane, also consumes ATP to facilitate the cellular machinery involved in membrane dynamics and vesicle formation.

Cell signaling often does not directly require ATP; however, many signal transduction pathways may utilize ATP in downstream processes to propagate the signal.

Thus, the unique characteristic of facilitated diffusion is its reliance on passive transport mechanisms that utilize existing concentration gradients, making it the correct answer as the process that does not require ATP.