AP Biology help. What is the concentration gradient? ?

OK--so a concentration gradient is a condition where, for example, the concentration of sodium ions is higher on one side of the membrane than on the other.  This represents potential energy because ions (all molecules, actually) move **down** their concentration gradient--from higher to lower concentration, in an attempt to "even things out".  This movement can be used by cells as an energy source (think of water falling down a waterfall making a turbine spin).

Movement *against* a concentration gradient, therefore, requires work--such as active transport.  Active transport takes molecules from a *lower* to a *higher* concentration (like when cells take in nutrients), but since it's against the normal flow in a gradient, it takes work--energy--to make this happen.

usually a concentration gradient is created because there is a difference in the environments. could be caused bya  difference in the levels of solute dissolved or by ions.

i dont really understand the "how it represent potential energy" part...but theres either active transportation which require cellular energy, ATP and usually carrier proteins to help transport particles, for example, across or against the gradient - like into a cell.
then theres passive transport which doesnt require ATP, particle move along the concentration gradient.

active transportation goes against

hope some of this has helped you!