Identify one way in which organisms maintain concentration gradients across membranes

I think the answer is by use of ion channels. The ion channels that are selective to only a certain ions, such as sodium and potassium ions. Transmembrane proteins actively push ions across the membrane and establish concentration gradients across the membrane, and ion channels allow ions to move across the membrane down those concentration gradients. 

Active transport by membrane pumps

Explanation:

Membrane pumps are the transmembrane proteins that span the membrane and are involved in the active transport of substances across the membrane against their concentration gradient using the energy. These membrane pumps have two binding regions. One binds to the substance to be transported and the other serves as a binding site for ATP. ATP serves as an energy source to trigger the conformational change in the membrane pump to facilitate the transport of the bound substance across the membrane.  

For example, the sodium-potassium pump in the membrane of neurons pumps to three sodium ions out of the axon and allows entry of two potassium ions into the cell. It serves to maintain the resting membrane potential with the outside of the membrane being positive as compared to the inside which is negative.  

Ok so in this experiment what we want to find is the catalyst for the reaction. Our original reaction is the following. A + B --> C, therefore to find which of the unknown substances (X, Y, and Z) is the catalyst reaction we simply have to do the following:

A + B + X --> C

A + B + Y --> C

A + B + Z --> C

The reaction that produces C much faster than our original reaction A + B --> C will be our catalyst. The reason being that one of the substances will increase the rate of the chemical reaction.

I hope it helps, Regards.