When these quantities are mentioned with what reference do we give signs to them?

Why is Osmotic Pressure positive, but Osmotic Potential, negative?

What about Turgor Pressure (Pressure exerted by the protoplasmic fluid on the cell wall) and the Wall Pressure (Pressure exerted by the Wall as a reaction to the Turgor Pressure), what signs do we assign to them?

What I understand is that it's based on the tendency of water to enter or exit the system being studied, but I lack clarity on how signs are being assigned to these different quantities. When we talk about soil having a low, does low mean more negative or does it mean a lower magnitude osmotic potential, for example?

  • I think you already know the sign convention in calculating the water potential: the addition of solutes lowers the potential (negative vector), while an increase in pressure increases the potential (positive vector).(Another way of saying the same would be more solute = more negative.) {1} This is the same everywhere.

  • Pressure potential: The pressure potential in a plant cell is usually positive. In plasmolysed cells, pressure potential is almost zero. Negative pressure potentials occur when water is pulled through an open system such as a plant xylem vessel.{1} This means that it is negative during a "suction" action, otherwise, it is positive.

  • Osmotic potential (solute potential): The relationship of solute concentration (in molarity) to solute potential is given by the Van't Hoff equation:


(The formula, note, has a -ve sign.)

For example, when a solute is dissolved in water, water molecules are less likely to diffuse away via osmosis than when there is no solute. A solution will have a lower and hence more negative water potential than that of pure water. Furthermore, the more solute molecules present, the more negative the solute potential is.

  • Turgor pressure and wall pressure: I recommend reading this for a clearer understanding.

...This actual pressure which develops within such a cell is called turgor pressure. This term can be applied with equal appropriateness to the actual pressures developing in purely physical osmotic systems such, for example, as a sucrose solution enclosed within a collodion or cellophane membrane and immersed in water. As will become clear in the following discussion, TPs in osmotic systems can sometimes be negative in value.

When the TP is negative, that is, when the cell sap is under tension (see later), the WP is likewise negative, the wall being subjected to a centripetal pull because of its adhesion to the protoplasmic layer and cell sap mass within the cell.

On the basis of fundamental physical principles it is generally considered that the WP and TP of a cell are equal in magnitude and oppositely directed. This situation is not difficult to visualize for a cell in an equilibrium condition. For dynamic states in which water is being gained or lost by a cell, however, the validity of this assumption has been questioned.



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