When a substance moves across the plasma membrane along a concentration gradient at a rate faster than would be expected by simple diffusion alone but without the expenditure of metabolic energy, the process is best described as:

The correct answer is: Facilitated Diffusion.

Explanation:

Facilitated Diffusion can be defined as the process by which certain solutes, molecules and substances are transported across the plasma membrane  down their concentration gradient (from a region where their concentration is high to a region where their concentration is low) with the help of carrier proteins.Carrier proteins are trans-membrane integral membrane proteins, those remain interspersed in the lipid bilayer of the plasma membrane and spans across the entire length of the plasma membrane such that they open up both in the cell interior as well as the cell exterior.Polar molecules like glucose, charged ions, amino acids are transported across the membrane by the process of facilitated diffusion. This is because due to their charge the lipid bilayer is impermeable to allow their direct simple diffusion.The process takes place along the concentration gradient and so it does not require the expenditure of metabolic energy.

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