Osmosis & Osmolarity: 1. Osmosis

Osmosis

The diffusion of water across a semi-permeable membrane due to differences in solute concentrations.

How to think about Osmosis....

Let's imagine a way to think about the direction of water movement. You have two glasses filled with marbles but one is filled with a lot more marbles than the other. If you added water to both glasses to reach a certain height, then the glass with more marbles would require less water to reach the height. There is a lower concentration of water in this glass than the other glass.
However, if the two glasses were connected by a semi-permeable membrane, osmosis would cause water to flow from the glass of high water concentration to the glass with low water concentration. It is important to note that this is not the actual mechanism of osmosis, just an easy way to think about it.