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The best answer to fill in the blank in the question: Active transport is the protein-mediated movement of substances , would be: Against a concentration gradient.

Explanation:

In cellular activity there are several means by which a cell may either introduce, or expell, molecules, ions, gases, waste, or water to survive. While some of these elements may enter a cell´s intracellular environment, and leave it, relatively easily, some require the help of proteins present in a cell membrane in order to either enter or exit the cell. Ions, especially, given that they are vital to maintain the cell´s capability of reacting to different stimulus, have to be particularly balanced out and controlled. In order to do so, the cells membrane possesses a series of proteins inserted between the layers that work as both gated channels and also as transporters that will allow the ions to move as they are required by the cell. Active transport then is the active passage by these proteins on the cell´s membrane of ions against their concentration gradients.