Final answer:
The plant cell's pressure potential (Yp) is 3.5 bars, as it must compensate for the difference in solute potential between the plant cell (-7.5 bars) and the surrounding solution (-4 bars) to maintain a constant volume.
Step-by-step explanation:
The question is asking to determine the plant cell's pressure potential (Yp) given that the cell is maintaining a constant volume in a solution with a known solute potential (Ys). Using the water potential equation Ysystem = Ytotal = Ys + Yp, where Ys and Yp are the solute and pressure potentials, we can calculate the cell's Yp. Since the plant cell's Ys is -7.5 bars and it maintains constant volume in a solution of -4 bars Ys, we can infer that the Yp of the plant cell must be equal to the difference between its internal Ys and the external Ys of the solution; hence, Yp = 3.5 bars.
Pressure potential, which represents the mechanical pressure on water, can be positive (compression) increasing the water potential, or negative (tension) decreasing the water potential. The cell wall in plant cells can sustain a positive pressure, leading to turgor pressure, which is crucial for the plant's structural integrity. When the total water potential inside the plant cell matches the surrounding solution, there is no net movement of water, indicating equilibrium.