Answer: A concentration gradient must exist
Explanation: Think about it this way. If you want movement in a tub of water, the water can't be flat. This would mean it was very stable. But if there is an inequality on the water levels of the tub, meaning that one side was much higher than the other, then the water would "diffuse" and try to get flat. Same with cells. I hope that explanation somehow made sense, it made more sense in my head then written out...