Advanced Placement (AP) Biology Practice Exam

When solute concentration is balanced across a membrane, the term that best describes this condition is isotonic. In an isotonic solution, the concentration of solutes is the same inside the cell as it is outside the cell. This balance means there is no net movement of water into or out of the cell, which maintains cell size and shape.

Considering hypotonic and hypertonic conditions, these terms describe situations where there is an imbalance in solute concentrations, leading to water movement either into the cell (hypotonic) or out of the cell (hypertonic). The concept of equilibrium, while related to balance, is more general and does not specifically pertain to solute concentrations; it typically refers to a state of rest or stability in a system where opposing forces or influences are balanced. Therefore, isotonic specifically captures the idea of equal solute concentration, validating the choice as correct in this context.