1. The Assimilation of Sulfur

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1. The Assimilation of Sulfur
2011-11-12 04:01:13
PMB 135 exam5

plant physiology and biochemistry exam 5
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  1. What is the effect of sulfur deficiency?
    • sulfur deficiency: sulfur is an essential macronutrient for plant growth and plants show yellowing when it is deficient
    • crop yields: reduces crop yields
  2. How do plants acquire sulfur?
    • from the soil as sulfate through the roots
    • sulfate is transported through plants
    • absorb sulfur dioxide from the atmosphere by absorption through the leaf surface
  3. What is the role of air pollution in sulfur nutrition?
    • accumulation of sulfur from air pollution
    • burning of fossil fuels
    • sulfate fertilization
  4. Roles of cysteine and methionine
    • sulfur-containing amino acids: sulfur is used to synthesize cys and met
    • animals and humans are unable to reduce sulfur - require from diet
    • sulfur proteins: cys is essential for met synthesis, critical components of proteins
    • - play critical roles in structural and catalytic functions of proteins
  5. cysteine is essential for forming thiol compounds
    • glutathione and phytochelatin
    • important for forming disulfide bonds
  6. disulfide bonds
    • formation: by oxidizing the thiol groups of two cys residues, oxidation by NADP+
    • form covalent linkages involved in establishing tertiary and quaternary protein structures
  7. structure and functions of glutathione
    • major end product - present in mM
    • structure: glutamate-cysteine-glycine

    • function:
    • protects cells from oxidative stress
    • glutathione reductase - for every one mole of oxidized glutathione (GSSG), one mole of NADPH is required to reduce GSSG -> GSH
    • redox control
    • transport and storage
    • response to environmental stresses
    • phytochelatin
  8. phytochelatins
    • glutathione peptides - form thiolate coordination complexes with heavy metals
    • low molecular weight phytochelatin-Cd complexes - detoxify toxic metals by binding the metal ions using the thiol groups as the ligand, transported into the vacuole by an ABC transporter
    • high molecular weight phytochelatin-Cd complexes - withing the vacuoule, includes the low molcular weight complex and Cd (brought in by antiporter) and sulfide
    • Cd sulfide crystallite - phytochelatins hold Cd in an ordered structure
  9. In what other ways is S important?
    • food flavor
    • pathogen defense
    • DMSP
  10. How is S important in food flavor?
    • allicin - sulfur compounds add flavors to food products, garlic, onions
    • alliinase enzyme in the vacuole - acts on alliin in the cytosol
    • crushing produces flavor - mixing the enzyme with the substrate produces allicin
  11. Which S compounds are involved in pathogen defense?
    • allicin - deters herbivores, antimicrobial agent
    • phtyalexins - defend plants against pathogen attacks, especially fungi like camalexin
    • glucosinolates - plant glycosides that break down to release volatile defense substances isothiocyanates and nitriles
  12. other roles for glucosinolates
    • antioxidant properties of some vegetables like broccoli
    • taste in condiments like mustard, horseradish
    • protects against cancer
  13. In what ways is DMSP important?
    • marine algae
    • DMSP roles:
    • 1. osmoprotectant - helps against osmotic stress
    • 2. cryoprotectant - protects from freezing damage
    • 3. climate-regulating mechanism
    • 4. repellent against plankton herbivores
  14. How is DMSP involved in climate regulation?
    • Phytoplankton produces DMSP: phytoplankton may regulate climate, DMSP is broken down to DMS by bacteria
    • DMS moves into atmosphere and is oxidized to sulfate, leads to cloud formation
    • clouds lead to cooling, sulfate is dissolved in rain, returns to ocean
    • cloud cover reduces growth of phytoplankton