The direction of transport in facilitated diffusion is always down a concentration gradient, from one side of the membrane where the substrate concentration is high to the other side of the membrane where the concentration is lower.
Define concentration gradient.
When the concentration of particles is higher in one area than another, a concentration gradient occurs.
Molecules travel passively along the concentration gradient through a process known as facilitated diffusion. The membrane only permits certain molecules and ions to pass across it, making it a selective process. But it blocks other molecules from getting past the barrier.
Since the movement is downhill (i.e., from higher to lower concentrations), chemical energy is not directly needed. Like other forms of passive transport, facilitated diffusion is propelled by kinetic energy. However, what distinguishes facilitated diffusion from the other passive transport mechanisms is the requirement for help from a transport protein anchored in the plasma membrane.
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