Biology HMWRK 9 - Biology 5A Homework 9 January 27-28 2010...

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Biology 5A Homework 9 January 27-28, 2010 1. During the development of watermelon fruits, the fruit cells take up a large amount of sucrose that is delivered to the fruit through the stem. This homework problem involves an experiment investigating the sucrose cotransport system that transports sucrose across the plasma membrane of fruit cells. The cell wall of developing fruit cells is enzymically digested away, and the resulting protoplasts (live plant cells with their cell wall removed) are allowed to settle down on a Teflon film that divides the upper and lower portions of a chamber filled with liquid medium. The Teflon film contains tiny (5 micrometer diameter) pores. The protoplasts settle over and adhere to the pores, and are then ruptured, leaving plasma membrane fragments spanning across the pores and acting as a semipermeable barrier between the upper and lower portions of the chamber (see one such membrane fragment, highly magnified, in the accompanying figure). The upper chamber corresponds to what was the inside of the cells, and the lower chamber corresponds to what was the outside of the cells. A voltmeter is connected between the solutions in the upper and lower chambers, and another electrode inserted into the upper chamber measures the pH there. After the system is set up, 2 mM ATP is added to the upper chamber at time T1 (see accompanying graph), which causes the reading on the voltmeter to gradually increase (dotted line in graph). At a later time T2, a valve is opened, allowing sucrose to enter the lower chamber and causing the voltage to decrease somewhat. Which of the solid lines in the graph shows the most likely time course of changes of the pH in the upper chamber during this experiment? A. curve A B. curve B C. curve C D. curve D E. curve E
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