The movement of substances from higher to lower concentration across a semipermeable membrane that must have specific protein carriers but no energy expenditure is called facilitated diffusion.

Here's why:

* Facilitated diffusion is a type of passive transport. This means it doesn't require energy from the cell.

* Specific protein carriers are required. These proteins bind to the substance being transported, helping it cross the membrane.

* Movement down the concentration gradient is the key. The substance moves from an area of high concentration to an area of low concentration, following the natural flow of the gradient.

Let me know if you have any other questions about cell transport mechanisms!