Small molecules, such as water and many ions, cross the membrane without assistance from the cell. This movement is called ________ and requires NO energy expenditure by the cell.

A) passive transport
B) facilitated diffusion
C) active transport
D) none of the above

Passive transport is the movement of molecules across a cell membrane from an area of high concentration to an area of low concentration without requiring any energy input from the cell. This process is also known as diffusion. Small molecules such as water and ions are able to diffuse across the membrane through channels or pores without the need for any assistance from the cell. Facilitated diffusion requires the use of transport proteins to move molecules across the membrane, while active transport requires the cell to expend energy to move molecules against their concentration gradient. None of the above is not a valid answer in this case.