What does the term tonicity describe?

1. Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. 2. It works entirely through water movement, so it is a statement about cell volume rather than about solute traffic. 3. A solution's tonicity often directly correlates with the osmolarity of that solution. 4. ATP spending and membrane thickness are separate properties and neither is what tonicity measures. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 5 "Structure and Function of Plasma Membranes", section 5.2 Passive Transport_