Advanced Placement (AP) Biology Practice Exam

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What does the "i" represent in the formula for solute potential (Ys = -iCRT)?

Ionization constant
Solute concentration
Temperature
Pressure

The correct answer is: Ionization constant

In the formula for solute potential (Ys = -iCRT), the "i" represents the ionization constant. This value indicates the number of particles into which a solute dissociates in solution. For instance, when a solute such as sodium chloride (NaCl) dissolves in water, it separates into two ions (Na+ and Cl-), resulting in an ionization constant (i) of 2. Determining the ionization constant is crucial for calculating solute potential accurately because it reflects how many contributing solute particles are present in the solution, which directly influences the osmotic pressure. The more particles there are, the greater the effect on solute potential, which ultimately impacts water movement in plant cells and other biological systems. Other options relate to different variables in the equation and play roles in calculating solute potential, but "i" specifically identifies the ionization characteristics of the solute involved.