Passive transport does not require that the cell expends any energy for the movement of molecules across the cell membrane. Rather, a concentration gradient exists between outside and inside the cell membrane. Molecules move across the membrane from high to low concentration until an equilibrium on both sides of the membrane is established. An example of passive transport is the process of diffusion. If there is a higher concentration of oxygen outside the cell versus inside, oxygen will diffuse into the cell. Osmosis refers to when water diffuses across a membrane along the concentration gradient.
Passive transport is the form of cellular transport that does not require the energy produced by the cell in cellular respiration.