Chapter 19

To boldly go. (a) It can be argued that, if life were to exist elsewhere in the universe, it would require some process like photosynthesis. Why is this argument reasonable? (b) If the Enterprise were to land on a distant plant and find no measurable oxygen in the atmosphere, could the crew conclude that photosynthesis is not taking place?

Weed killer \(1 .\) Dichlorophenyldimethylurea (DCMU), a herbicide, interferes with photophosphorylation and \(\mathrm{O}_{2}\) evolution. However, it does not block \(\mathrm{O}_{2}\) evolution in the presence of an artificial electron acceptor such as ferricyanide. Propose a site for the inhibitory action of DCMU.

Hill reaction. In \(1939,\) Robert Hill discovered that chloroplasts evolve \(\mathrm{O}_{2}\) when they are illuminated in the presence of an artificial electron acceptor such as ferricyanide \(\left[\mathrm{Fe}^{3+}(\mathrm{CN})_{6}\right]^{3-}\) Ferricyanide is reduced to ferrocyanide \(\left[\mathrm{Fe}^{2+}(\mathrm{CN})_{6}\right]^{4-}\) in this process. No NADPH or reduced plastocyanin is produced. Propose a mechanism for the Hill reaction.