If a solution is subjected to a pressure greater than atmospheric pressure, its water potential:

Question

Accepted Answer

The correct option is 1: Increases

Water potential ($\Psi$) is a critical concept in biology, particularly in plant and cell physiology, representing the potential energy of water per unit volume relative to pure water at standard conditions (atmospheric pressure and ambient temperature). This potential energy determines the direction in which water will move, always from a region of higher (less negative) water potential to a region of lower (more negative) water potential. The total water potential ($\Psi\_{\text{total}}$) is defined by its main components: the solute potential ($\Psi\_{\text{s}}$) and the pressure potential ($\Psi\_{\text{p}}$), expressed by the equation $\Psi\_{\text{total}} = \Psi\_{\text{s}} + \Psi\_{\text{p}}$. When a solution is subjected to a pressure greater than atmospheric pressure, this external, mechanical force registers as a *positive* pressure potential ($\Psi\_{\text{p}} \> 0$). Since a positive value is being added to the overall water potential equation, it directly increases the value of $\Psi\_{\text{total}}$. In physical terms, the applied pressure increases the free energy of the water molecules in the solution, enhancing their tendency to move out of that system. Therefore, increasing the external pressure always increases the water potential.

The incorrect option is 2: Decreases

Water potential decreases, or becomes more negative, when the concentration of solutes is increased. This is because the addition of solutes lowers the solute potential ($\Psi\_{\text{s}}$), which is a negative value. A decrease in water potential can also occur if the solution is under tension, which would result in a *negative* pressure potential (for instance, in the xylem of plants). However, subjecting a solution to a pressure *greater* than atmospheric pressure is applying positive pressure, which is physically and mathematically the opposite of tension or adding solutes, and thus will always cause the water potential to increase, not decrease.

The incorrect option is 3: Becomes zero

The total water potential ($\Psi_{	ext{total}}$) of a solution would only become zero if the positive pressure potential ($\Psi_{	ext{p}}$) exactly cancelled out the negative solute potential ($\Psi_{	ext{s}}$), so that $\Psi_{	ext{s}} + \Psi_{	ext{p}} = 0$. Pure water at atmospheric pressure has a water potential of zero by definition ($\Psi_{	ext{s}}=0$ and $\Psi_{	ext{p}}=0$). For any given solution (which has a negative $\Psi_{	ext{s}}$), the water potential is only zero under this very specific, engineered condition where the applied pressure is equal to the solution's osmotic potential, which is highly unlikely to be the result of a general application of pressure greater than atmospheric pressure. Therefore, 'becomes zero' describes a specific equilibrium state, not the general effect of increasing pressure.

The incorrect option is 4: Remains unaffected

The water potential is an absolute measure of the system's energy status and is directly influenced by the pressure component ($\Psi\_{\text{p}}$). Because the question specifies that the pressure applied is *greater* than atmospheric pressure (where $\Psi\_{\text{p}}$ is conventionally zero), the pressure potential must change from zero to a positive value. Since the overall water potential is the sum of its components, including this pressure term, the total water potential cannot remain unaffected; it will be mathematically and functionally altered by the change in pressure.