Tonicity, osmoregulation and water potential

What the exam asks

• Classify and predict: Identify whether a solution is hypotonic, hypertonic, or isotonic relative to a cell, and predict the direction of net water movement.
• Calculate: Use $\Psi = \Psi_P + \Psi_S$ and $\Psi_S = -iCRT$ from the AP Equations and Formulas sheet to find solute, pressure, or total water potential with proper units ($\text{bars}$).
• Analyze experimental data: Determine the isotonic concentration of plant tissue from a graph or table of percent change in mass versus sucrose molarity.
• Explain osmoregulation: Connect specialized structures (contractile vacuoles, cell walls, central vacuoles) to survival in extreme osmotic environments.

Core ideas

Tonicity and osmosis

Osmosis is the passive diffusion of free water across a selectively permeable membrane (often facilitated by aquaporins). Water moves from regions of lower total solute concentration (higher free water concentration) to regions of higher total solute concentration (lower free water concentration).

Tonicity is always a relative comparison of total non-penetrating solute concentrations between two compartments:

Water potential ($\Psi$)

Water always moves by osmosis from an area of higher water potential ($\Psi$) to lower (more negative) water potential ($\Psi$) until dynamic equilibrium ($\Psi_{\text{inside}} = \Psi_{\text{outside}}$) is reached.