For diffusion to occur, which condition must be met?

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...

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.