Why does passive transport not require the cell to use energy to move molecules across the membrane?

Passive transport not require the cell to use energy to move molecules across the membrane because in passive transport the molecules move down their concentration gradient.

Explanation:

passive transport is an important type of membrane transport process which helps in the transport of various molecules such as oxygen,carbon dioxide ,water etc.

                 Passive transport involves diffusion,osmosis and facilitated diffusion.No additional energy is required to carry out passive transport because in passive transport the molecules move from region of high concentration to the low concentration region until equilibrium is reached.

i believe it would be the lungs.