Chapter 6

Which of the two photosystems operates at the most negative redox potential? Which generates the strongest reducing agent? Which must absorb four photons during each round of noncyclic photophosphorylation?

Which type of plant \(\left(\mathrm{C}_{3}, \mathrm{C}_{4}, \text { or } \mathrm{CAM}\right)\) might you expect to do best if exposed to continuous daylight under hot, dry conditions? Why?

Under what conditions would you expect a plant to be engaged most heavily in cyclic photophosphorylation?

Contrast the change in \(\mathrm{pH}\) of the medium that occurs when isoLated chloroplasts carry out photosynthesis compared to isolated mitochondria carrying out aerobic respiration.

Contrast the roles of PSI and PSII in generating the electrochemical gradient across the thylakoid membrane.

Would you agree that a \(\mathrm{C}_{3}\) plant has to expend more energy per molecule of \(\mathrm{CO}_{2}\) converted to carbohydrate than a \(\mathrm{C}_{4}\) plant? Why or why not?

In photosynthesis, the capture of light energy results in the release and subsequent transfer of electrons. From what molecules are the electrons originally derived? In what molecules do these electrons eventually reside?

If pheophytin and \(A_{0}\) (a chlorophyll \(a\) molecule) are the primary electron acceptors of PSII and PSI, respectively, what are the primary electron donors of each photosystem?

It has been suggested that low \(\mathrm{CO}_{2}\) levels play a key role in keeping \(\mathrm{O}_{2}\) levels stable at 21 percent. How is it possible that \(\mathrm{CO}_{2}\) levels could affect \(\mathrm{O}_{2}\) levels in the atmosphere?

Suppose \(\mathrm{CO}_{2}\) levels were to rise to 600 ppm, which in fact they probably were about 300 million years ago. What effect would you suppose this might have regarding competition between \(\mathrm{C}_{3}\) and \(\mathrm{C}_{4}\) plants?

Suppose you were to place a \(\mathrm{C}_{3}\) plant under hot, dry conditions and provide it with radioactively labeled \(^{18} \mathrm{O}_{2} .\) In what compounds would you find this label incorporated?