1. S SO 4 2 - S2-S2-

8. SO 4 2– + ATP + 8 e – + 8 H + = = S 2– + 4 H 2 O + AMP + PP i

12. APS sulfotransferase, using APS as a sulfuryl donor, transfers sulfate to an acceptor thiol compound, possibly glutathione. The resulting organic thiosulfonate is reduced to thiosulfide by a ferredoxin-dependent thiosulfonate reductase.

13. APS is first phosphorylated to PAPS by APS kinase. PAPS reductase then produces sulfite by using electrons donated from thioredoxin. Sulfite reductase then completes the reduction to sulfide by using electrons from ferredoxin.

17. major endproduct of the reductive sulfate assimilation, glutathione, is the major nonprotein thiol in plants (its milimolar concentrations far exceed the micromolar conc. of cysteine) involved in the storage and long distance transport of reduced sulfur, in signal transduction pathways, in scavenging active oxygen species, in detoxifying xenobiotics and serving as a substrate for phytochelatin synthesis

21. Two phytochelatin molecules [(gama-Glu-Cys) 2 Gly] coordinated with an ion of cadmium. By binding toxic heavy metal ions, phytochelatins remove them from the cellular machinery.

22. A model for the mechanism of CdS mineralization and sequestration in plant cells mediated by phytochelatins

26. (A) The phytoplankton-climate connection. Phytoplankton-produced DMSP (dimethylsulfoniopropionate) is broken down by bacteria to DMS (dimethyl sulfide) and acrylate. DMS volatilizes and is oxidized to DMSO and to sulfate, which nucleates water droplets, leading to cloud formation.

27. (B) Sulfate is returned to the sea dissolved in rain. Because cloud cover reduces the growth of phytoplankton and is accompanied by atmospheric cooling, phytoplankton have been proposed to serve as a homeostatic climate regulation mechanism. Dimethylsulfoniopropionate (DMSP) has many roles, e.g. osmoprotectant, cryoprotectant repellant against planctonic herbivores