2.4—Osmosis and water potential gradients

Osmosis is the net movement of water across a partially permeable membrane from higher water potential to lower water potential. Water potential combines pressure effects and solute effects, so it is not simply “water concentration”.

A concentrated solution has a more negative solute potential. Water leaves a cell placed in a lower-water-potential solution, reducing turgor; water enters when the external solution has higher water potential.

If potato cylinders lose mass in a sucrose solution, the solution had a lower water potential than the potato cells. The concentration at which there is no net mass change estimates the tissue’s initial water potential.

Use equal cylinders, blot them consistently, control time and temperature, repeat at several concentrations and plot percentage mass change rather than relying on one sample.

Water still moves in both directions at the microscopic level; “no net movement” means equal opposing fluxes. Do not equate a mass change directly with solute entering the cells.