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Chapter 38
What would Ψw, Ψs, and Ψp of the cell in (a) be at equilibrium if it had been placed in a
solution with a Ψs of -0.05? (Figure 38.4)
Answer: Before equilibrium, the solute potential of the solution is -0.5 MPa, and that of
the cell is -0.2 MPa. Since the solution contains more solute than does the cell, water will
leave the cell to the point that the cell is plasmolyzed. Initial turgor pressure (Ψp ) of the
cell = 0.05 MPa, while that of the solution is 0 MPa. At equilibrium, both the solution
and the cell will have the same Ψw. Ψcell = -0.2 MPa + 0.5 MPa = 0.3 MPa before
equilibrium is reached. At equilibrium, Ψcell = Ψsolution =-0.5MPa, thus Ψw cell = -0.5 MPa.
At equilibrium, the plasmolyzed cell ΨP = 0 MPa. Finally, using the relationship ΨW(cell) =
Ψp + Ψs and ΨW(cell) = -0.5 MPa, ΨP(cel) = 0 MPa, then Ψs(cell) = -0.5 MPa.
Which route would be the fastest for water movement? Would this always be the best
way to move nutrients into the plant? (Figure 38.8)
Answer: The fastest route for water movement through cells has the least hindrance, and
thus is the symplast route. The route that exerts the most control over what substances
enter and leave the cell is the transmembrane route, which is then the best route for
moving nutrients into the plant.
If a mutation increased the radius of a xylem vessel threefold, how would the movement
of water through the plant be affected? (Page 765)
Answer: If a mutation increases the radius, r, of a xylem vessel threefold, then the
movement of water through the vessel would increase 81-fold (r 4 = 34 = 81). A plant with
larger diameter vessels can move much more water up its stems.