Final answer:
Passive mediated transport, or facilitated diffusion, is the energy-free movement of substances across the cell membrane with the help of membrane proteins along a concentration gradient.
Step-by-step explanation:
The definition of passive mediated transport (facilitated diffusion) is the movement of substances across the cell membrane without the use of energy. This process occurs via membrane proteins that facilitate the movement of specific molecules across the cell membrane by providing a passage through which they can travel. This type of transport is driven by a concentration gradient where molecules move from an area of higher concentration to an area of lower concentration, thus not requiring additional energy input.
In facilitated diffusion, proteins such as carrier proteins or channel proteins are involved. Carrier proteins change shape to transport molecules across the membrane, while channel proteins form pores that allow substances to pass through. Facilitated diffusion is ideal for polar molecules and ions that are repelled by the hydrophobic components of the cell membrane's lipid bilayer and thus cannot easily cross it without assistance.