BioPL 3420Spring 2016Lecture Topic 17Answer to Assigned Study QuestionPlant Cell WallsPlant Physiology LecturesBioPL 3420Spring 2016Lecture Topic 17 Answer to Assigned Study QuestionPlant Cell WallsStudy Question. The figure below is from a previous edition of our text. In lecture, Imentioned that understanding this figure was important to understanding how the rate ofcell expansion is regulated. Some brief information may be helpful to you. First, thefigure assumes that the apoplast surrounding this cell has a water potential of 0 (purewater, atmospheric pressure). The axes on the bottom of the graph show the turgorpressure (pressure potential) and water potential inside the cell. The graph assumes aconstant solute potential inside the cell of -0.7 MPa (solute potential + pressure potential= water potential, remember?) The solid line shows the rate of cell enlargement as afunction of turgor pressure. The dashed line shows the rate of water uptake by the cellas a function of the water potential difference between the apoplast and the cytoplasm.Remember that as a cell enlarges, the “new space” inside the cell is not a vacuum – it isfilled with water. Thus the intersection of the solid and dashed lines indicates the pointat which the rate of cell expansion (creating new volume inside the cell by loosening thewall) is equal to the rate at which water can enter the cell to fill that new space. Alsorecall that the expansion of the cell is initiated by a loosening of the wall. Consider thefollowing questions (and make sure to explain your answers):a) Why does the rate of cell expansion depend on the pressure potential (ψp) insidethe cell and the rate of water uptake depend on the difference in water potential(Δψw) between the inside and outside of the cell?b) What parameter(s) in this model is/are altered by the process of wall loosening?c) How does increasing the amount of wall loosening change the rate of cellenlargement and water uptake?d) What happens to the rate of cell expansion if the water potential outside the cellis less than 0 due to solutes in the apoplast?Answer:a) If the wall is loosened, the cell expands as a consequence of the turgor pressurepushing out on the wall and the wall stretching (irreversibly) as a consequence.In this model the pressure outside the cell was zero, so the difference in pressure(inside – outside) will just be the turgor pressure. Water is entering theexpanding cell by osmosis, which means that it is being driven by a difference inwater potential (which recall is a sum of pressure and solute potentials).Loosening the wall causes the cell volume to expand (the wall is not pushingback as much against the turgor pressure). This lowers (less positive) the turgorpressure and thus the water potential inside the cell relative to outside. It is thiswater potential difference that drives water uptake by osmosis.b) Wall loosening affects the parameter m in the equation .Loosening the wall makes m larger and the cell expands more rapidly for a giventurgor pressure.c) As m increases, the slope of the solid line (expansion rate of the cell) increases.As the slope of the GR line increases, the point of intersection between the solidand dashed line is at a lower turgor pressure, which causes a larger waterpotential difference between the inside and outside of the cell and thus a fasterrate of water uptake to match the faster rate of cell expansion.d) If the water potential outside the cell is negative (because of solutes in theapoplast), the corresponding turgor pressure inside the cell will be lower, and thedashed line will effectively shift to the left. This means that the point ofintersection between the solid and dashed lines will result in a slower growthrate, as expected if the driving force for water uptake by the cell is