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Study Questions

1. What is meant by the term "active transport"? What is the difference between "primary" and "secondary" active transport?

2. What is the meaning of the Nernst equation? If the current carried by K⁺ ions diffusing through a K⁺ channel is plotted against the membrane potential, at what potential will the direction of current reverse?

3. Both membrane channels and carriers show changes in protein conformation. What is the role of such conformation changes (a) in channels, and (b) in carriers?

4. How can the operation of the plasma membrane H⁺-ATPase affect (a) the diffusion of K⁺ through membrane channels, and (b) the accumulation of sugars from the apoplast into the cell?

5. Detection of external signals by plant cells often results in a transient rise in the concentration of Ca²⁺ ions in the cytoplasm. What transport processes can cause such an increase in cytoplasmic Ca²⁺, and what transport processes can remove the excess Ca²⁺ and restore the level of Ca²⁺ to its resting state?

6. In the transport of an ion from the soil solution to the xylem, what is the minimum number of times it must cross a cell membrane? What transport mechanisms are likely to be involved at each membrane?