You're viewing 3 free preview questions for Biology SL — subscribe to unlock 45 more, plus the Full Question Bank, Mock Papers & Worksheets.Upgrade

Water Potential · Paper 2 — Free Biology SL Practice Questions

1MasterySAQ-SConcept of water potential in plants5 marksPaper 2~8 min
A student places a plant cell from a freshwater aquatic plant (Elodea canadensis) into a concentrated salt solution. The cell initially has a solute potential (Ψs\Psi_s) of −0.8 MPa-0.8\,\text{MPa} and a pressure potential (Ψp\Psi_p) of +0.3 MPa+0.3\,\text{MPa}. The external salt solution has a water potential (Ψw\Psi_w) of −1.4 MPa-1.4\,\text{MPa}. After 10 minutes, the cell has undergone plasmolysis.
(a)
State the initial water potential (Ψw\Psi_w) of the cell. [1 mark]
(b)
Describe the direction of net water movement and the change in pressure potential (Ψp\Psi_p) as plasmolysis occurs. [2 marks]
(c)
Predict whether net water movement into or out of the cell will continue after plasmolysis complete, given that the cell's solute potential (Ψs\Psi_s) at the point of incipient plasmolysis −1.4 MPa-1.4\,\text{MPa}. Justify your answer using water potential values. [2 marks]
diagram

Solutions

2MasterySAQ-SConcept of water potential in plants5 marksPaper 2~8 min
A student uses a pressure bomb to measure water potential in a branch of a maple tree (Acer saccharum). The branch is cut and placed in the pressure chamber with the cut end exposed. Pressure is applied until xylem sap just reappears at the cut surface. The pressure required is 0.8 MPa0.8\,\text{MPa}. In a pressure bomb, the applied pressure equals the magnitude of the xylem water potential, and the xylem water potential approximates the leaf cell water potential.
(a)
State the water potential of the leaf cells, including its sign. [1 mark]
(b)
The solute potential (Ψs\Psi_s) of the leaf cells is −1.5 MPa-1.5\,\text{MPa}. Calculate the pressure potential (Ψp\Psi_p) of the leaf cells. [2 marks]
(c)
Root cells have a water potential of −0.3 MPa-0.3\,\text{MPa}. Using the values from (a) and (b), explain how water moves from roots to leaves through the xylem, and deduce whether the pressure potential calculated in (b) is consistent with water movement occurring by the cohesion-tension mechanism. [2 marks]
diagram

Solutions

3ChallengeSAQ-LConcept of water potential in plants7 marksPaper 2~11 min

Data

- Water potential of soil solution: Ψsoil=−1.2 MPa\Psi_{soil} = -1.2\,\text{MPa} - Balancing pressure recorded when sap just begins to exude: Pbalance=+0.8 MPaP_{balance} = +0.8\,\text{MPa} - Standard formula: Ψw=Ψs+Ψp\Psi_w = \Psi_s + \Psi_p
A plant physiologist investigates the water relations of the halophyte Salicornia europaea, which grows in coastal salt marshes. Cells in the stem accumulate high concentrations of sodium chloride (NaCl) and the organic solute glycine betaine to maintain turgor despite the high salinity of the surrounding soil water. A 5.0 cm5.0\,\text{cm} segment of S. europaea stem is placed in a sealed pressure chamber (Scholander bomb) containing soil solution. The stem segment is allowed to equilibrate fully with the soil solution before measurements are taken.
(a)
State the water potential (Ψw\Psi_w) of the stem cells after equilibration with the soil solution. [1 mark]
(b)
The balancing pressure recorded equals the pressure potential (Ψp\Psi_p) of the stem cells. Calculate the solute potential (Ψs\Psi_s) of the stem cells. [2 marks]
(c)
Explain how the accumulation of NaCl and glycine betaine in the vacuoles of S. europaea stem cells enables the plant to maintain a positive turgor pressure (Ψp\Psi_p) when growing in soil where Ψsoil=−1.2 MPa\Psi_{soil} = -1.2\,\text{MPa}. Refer to the values calculated in (a) and (b) in your answer. [2 marks]
(d)
Evaluate the use of the Scholander pressure chamber technique for measuring water potential in halophyte stem cells. In your answer, discuss one strength and one limitation, with specific reference to the assumption that the stem segment equilibrates fully with the surrounding solution before measurement. [2 marks]
diagram

Solutions

Questions 4-48

Unlock these questions with Premium

Sign Up →

That's every free question here — 3 of 48.

+45 more questions, plus Mock Papers & Worksheets, are waiting in Premium.

Upgrade