Biology – 3.2 Osmosis | e-Consult

Water potential is a measure of the free energy of water per unit volume, relative to pure water. It's often described as the tendency of water to move from one area to another. Pure water has a water potential of 0. Water potential can be positive (less negative than pure water), negative (more negative than pure water), or zero. Solutes in water generally lower the water potential of the solution.

Water moves from an area of high water potential to an area of low water potential via osmosis. This is driven by the tendency of water to move from where it is more free (higher water potential) to where it is less free (lower water potential), aiming to equalize the water potential on both sides of the membrane.

For plant uptake of water, the water potential of the soil is typically higher than that of the plant's root cells. This creates a water potential gradient. As a result, water moves into the root cells by osmosis. This influx of water increases the water potential inside the root cells, and the water then moves through the xylem towards the rest of the plant. The transpiration stream, driven by the water potential gradient, is essential for water transport in plants.