152-Disc_McCulloh4_Assign-2023fa

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University of Wisconsin, Madison *

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152

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Biology

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Dec 6, 2023

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docx

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Intro Bio 152 – Discussion assignment for week of October 2 nd Exam preparation 1. The exam is a little over a week away. Review all the course material thus far, and think of a review question that you would like to go over during your discussion this week. Then, write that question below. You will be sharing it with your classmates. It should be based on lecture material and should address specific thing(s) you would like to examine. Write a question with the goal of facilitating your exam preparation. Consider class material up to and including CAM photosynthesis (i.e., don’t include water relations). Some examples of not-so-great questions (if your question looks like one of these, you should revise it):  “I want to go over the different types of photosynthesis.”  “Will the plant groups be on the exam?”  “Primary vs. secondary growth” Your question: When are photorespiration and the Calvin Cycle used in place of regular photosynthesis? Water relations 2. You have a small, flaccid , living plant cell. You put this cell into a large open beaker of pure water and allow the cell to reach equilibrium. Remember, a flaccid cell has a Ψp of 0. a. In reaching equilibrium, would water move into the cell, move out of the cell, or can you not answer this question without more information? Explain your answer. Water would move into the cell because it is flaccid and therefore has empty space that would be filled b. After it has reached equilibrium, you leave this same cell in the beaker. You then add enough sugar to the water in the beaker to make its Ψ S = -0.3 MPa. Would water move into or out of the cell and what would be the water potential of the cell when it reached equilibrium in the new sugar solution? What would happen to the pressure of the cell? Water would move out of the cell, making room for some of the dissolved ions until it reaches a new equilibrium with the beaker water. The new water potential once equilibrium is reached would be –0.15. The pressure of the cell would become more negative with the addition of the solute. 3. The following question is about a different cell, not the one in the question above. You drop a root hair cell (Ψ S = -1.4 and Ψ P = 0.7) into an open beaker containing a solution where Ψ S = -0.4. When the cell reaches equilibrium with the solution in the beaker, the cell’s Ψ S is -1.3. What will be that cell’s resulting Ψ P and overall Ψ? What will be the effect on the cell? -0.4 - (-1.3) = 0.9 Pressure potential will be 0.9, water potential will be -0.4 4. Remember the Top Hat question from the Friday lecture (September 29)? Let’s try this again… Page 1 of 4

a. What is the correct answer? Why? Explain why that is the answer. C. -0.25MPa The average between –0.1 and –0.4 is –0.25 so this answer makes sense because the two cells will be reaching equilibrium b. Now, try this question again, but this time assume Cell A is much much much larger than Cell B. Which choice (A- D) is correct? Explain why that is the correct answer. I’d assume that the water potential would be closer to that of cell A’s since it has so much more volume, but it could also be impossible to tell. Transpiration 5. What is relative humidity? Don’t Google this. Define it using terms that were used in lecture on Friday (9/29). You’ll probably write an equation below. Great! Be sure also to explain what that equation means. (Partial pressure of water vapor / saturated vapor pressure) x 100 Relative humidity is the amount of moisture in the air relative to if it were fully saturated. 6. The figure below is very similar to one that Dr. McCulloh showed during the Friday lecture (9/29). On this figure, the dashed line curving upward shows the vapor pressure and temperature at 100% relative humidity (RH), as those variables increase. The leaf and air temperature start at the same temperature (T1). The air in the leaf is saturated with water vapor (i.e., at 100% RH), so the symbol indicating the leaf vapor pressure is on the dashed line. The relative humidity of the air is 50%, so the symbol indicating the air’s vapor pressure is halfway between the line and the x-axis. The difference between the leaf symbol and the air symbol is the vapor pressure difference. Page 2 of 4

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