Osmosis
the movement of water molecules through a semi-permeable membrane.
Osmosis is considered a mode of passive transport because it does not require any energy.
Concentration Gradient
fewer molecules on one side of a membrane than on the other side of a membrane
Image:
When there is a higher concentration of water molecules on one side of a membrane.
More water molecules move from the side with a high concentration to the area with a low concentration of water.
Source: Concepts in Biology Twelfth Edition Eldon Enger, et al.
