Passive membrane transport processes include ________.

A) consumption of ATP
B) movement of water from a low concentration to an area of high concentration.
C) movement of a substance up its concentration gradient
D) movement of a substance down its concentration gradient

Passive membrane transport processes refer to the movement of molecules across the membrane without consuming ATP. Such processes are driven by the concentration gradient of the substance, and therefore, molecules move down their concentration gradient from an area of high concentration to an area of low concentration. This includes diffusion and facilitated diffusion. Choice D describes the movement of molecules down their concentration gradient, which is a passive process.