Name______
Homework #3, water potential (DUE, October 19)
Water potential is determined by the following formula: Ψ = Ψp + Ψs, where Ψp is due to pressure and Ψs is due to solute molecule concentration.
1. If Ψs in a plant cell is -0.7 MPa and Ψp is +0.5 MPa; and if the surrounding solution has a Ψ value of -0.3 MPa, will water move into or out of the cell?
2. In problem 1, at what value of Ψp will net water movement stop?
3. If a cell with Ψp = 0 MPa and Ψs = -0.7 MPa is placed in distilled water, will water molecules move into or out of the cell?
4. In problem 3, at what value of Ψp is required for the cell to be at equilibrium with the external solution?
5. If a cell with Ψp = 0.1 MPa and a Ψs = -0.5 MPa is placed into a solution of sucrose with Ψp = 0 and Ψs = -0.9 MPa. Will water move into or out of the cell?
6. In problem 5, a cell biologist could inject protein into the cell to raise the Ψs. What value of Ψs is needed to prevent an inflow or outflow of water from the cell?
7. Soil has a Ψ = -0.3 MPa and air has a Ψ value of -100 MPa, and the Ψs of leaves and roots is about the same. From this information, is the Ψp higher in root xylem or in leaf xylem?
8. In phloem, on a sunny day, the concentration of sucrose is higher in leaf sieve cells than in surrounding tissues. This causes water to migrate (by osmosis) into the sieve cells. Under these conditions does sap flow toward the leaves or away from the leaves in the phloem?
9. Given the answer to problem 8, where is Ψp higher, in stems or in leaves?
10. At night, sucrose is higher in the roots than in the leaves. Given this, what would you predict about the movement of sap at night? Why?