What does it mean for a molecule to diffuse down a concentration gradient?

it means that to diffuse a concentration gradient means that the molecule goes from an area that is high concentration to an area that is of low concentration. 

What some science videos to you

1) the mayans built pyramids just like the egyptians. the mayans also used hieroglyphics like them.2) power was passed down by family. for example, if the father dies, the son would be the next king.3)they both built pyramids for their structures.4)the main crop for the mayans was maize.5)they also grew manioc, beans,  squash,  amaranth, and chile peppers.