Ap Water Potential Sample Questions

Understanding and Mastering AP Water Potential: Sample Questions and Comprehensive Explanations

Water potential is a crucial concept in AP Biology, impacting plant physiology and ecological interactions. Understanding water potential requires grasping the interplay of solute potential (ψS), pressure potential (ψP), and the overall water potential (ψ). This article provides a comprehensive overview of water potential, including sample questions with detailed explanations to help you master this critical topic. We'll cover various scenarios and complexities to ensure you're well-prepared for the AP exam.

Introduction to Water Potential

Water potential (ψ) is the measure of the tendency of water to move from one area to another. It's expressed in units of pressure, typically megapascals (MPa). Water always moves from an area of higher water potential to an area of lower water potential. This movement is driven by two major components:

• Solute potential (ψS): This component reflects the effect of dissolved solutes on water potential. The more solutes present, the lower the solute potential. Pure water has a solute potential of 0 MPa. Adding solutes lowers this value, making it negative.

• Pressure potential (ψP): This component accounts for the physical pressure on the water. In plant cells, turgor pressure (the pressure exerted by the cell contents against the cell wall) contributes positively to pressure potential. In flaccid cells, pressure potential is 0 MPa. Negative pressure potential can occur in the xylem of plants (tension).

The total water potential is calculated as: ψ = ψS + ψP

Understanding this equation and its components is fundamental to solving water potential problems.

Sample Questions and Detailed Explanations

Let's delve into several sample questions, ranging in complexity, to illustrate the application of water potential concepts.

Question 1:

A plant cell has a solute potential (ψS) of -0.6 MPa and a pressure potential (ψP) of 0.4 MPa. What is the overall water potential (ψ) of the cell? Will water move into or out of this cell if placed in a solution with a water potential of -0.8 MPa?

Explanation:

First, calculate the overall water potential of the cell:

ψ = ψS + ψP = -0.6 MPa + 0.4 MPa = -0.2 MPa

Next, compare the water potential of the cell (-0.2 MPa) to the water potential of the surrounding solution (-0.8 MPa). Water moves from a higher water potential to a lower water potential. Since the cell has a higher water potential than the solution, water will move out of the cell. This will lead to plasmolysis (the shrinking of the cytoplasm away from the cell wall).

Question 2:

Two plant cells are placed in separate solutions. Cell A has a ψ of -0.5 MPa and is placed in a solution with a ψ of -0.8 MPa. Cell B has a ψ of -0.2 MPa and is placed in a solution with a ψ of -0.1 MPa. Describe the net movement of water in each scenario.

Explanation:

• Cell A: The solution (-0.8 MPa) has a lower water potential than Cell A (-0.5 MPa). Therefore, water will move out of Cell A into the solution.

• Cell B: The solution (-0.1 MPa) has a higher water potential than Cell B (-0.2 MPa). Therefore, water will move into Cell B from the solution.

Question 3:

A researcher measures the water potential of a leaf cell to be -0.7 MPa. The pressure potential of the cell is 0.2 MPa. What is the solute potential of the leaf cell? What would happen to the cell if it were placed in a solution with a water potential of -0.9 MPa?

Explanation:

We can rearrange the water potential equation to solve for solute potential:

ψS = ψ - ψP = -0.7 MPa - 0.2 MPa = -0.9 MPa

If placed in a solution with a water potential of -0.9 MPa, there would be no net movement of water. The water potential of the cell and the solution are equal. This is an example of isotonic conditions.

Question 4:

Explain the relationship between water potential, solute concentration, and the movement of water across a selectively permeable membrane.

Explanation:

Water potential is directly related to solute concentration. Higher solute concentration leads to lower water potential. Water moves across a selectively permeable membrane from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration) until equilibrium is reached. This movement is driven by osmosis.

Question 5:

A wilted plant is placed in a beaker of pure water. Describe the changes in water potential within the plant cells and the movement of water, explaining why the plant revives.

Explanation:

A wilted plant has low turgor pressure, resulting in a low pressure potential (ψP) and therefore a low overall water potential (ψ). When placed in pure water (ψ = 0 MPa), the water potential of the pure water is significantly higher than that of the plant cells. Water moves from the higher water potential (pure water) into the plant cells via osmosis. This increases the pressure potential (ψP) within the plant cells, restoring turgor pressure and causing the plant to revive.

Question 6 (More Advanced):

Two solutions are separated by a selectively permeable membrane. Solution A contains 0.1M sucrose, and solution B contains 0.2M sucrose. Both solutions are under atmospheric pressure. Calculate the solute potential of each solution, assuming the pressure potential is 0 MPa in both. (Use the equation ψS = -iCRT, where i is the ionization constant (1 for sucrose), C is the molar concentration, R is the pressure constant (0.00831 L MPa/mol K), and T is the temperature in Kelvin (assume 293K)). Which way will water move?

Explanation:

First, calculate the solute potential for each solution:

• Solution A: ψS = - (1)(0.1 mol/L)(0.00831 L MPa/mol K)(293 K) ≈ -0.24 MPa
• Solution B: ψS = - (1)(0.2 mol/L)(0.00831 L MPa/mol K)(293 K) ≈ -0.49 MPa

Since pressure potential is 0 in both, the water potential (ψ) is equal to the solute potential (ψS). Water will move from Solution A (-0.24 MPa) to Solution B (-0.49 MPa), as Solution A has a higher water potential.

Further Considerations & Applications

This detailed explanation of water potential and the provided sample questions should equip you to tackle various problem types. Remember that understanding the fundamental concepts of solute potential and pressure potential, and their relationship to overall water potential, is key to success. Practice applying the equation ψ = ψS + ψP and remembering that water moves from areas of higher to lower water potential.

Furthermore, consider these additional application points:

• Plant adaptations: How do different plant adaptations (e.g., root systems, leaf structures) affect water potential and water uptake?
• Environmental factors: How do factors like salinity, temperature, and humidity affect water potential and plant growth?
• Animal cells: While primarily focused on plants, understanding water potential is also relevant to animal cells and their osmotic balance.

By thoroughly understanding these concepts and practicing numerous problems, you will significantly improve your understanding and performance when it comes to AP water potential questions. Remember to consult your textbook and class notes for further clarification and practice problems. Good luck!