Assignment 2 January 26, 1999

Provide answers to the following questions/problems based on the assigned readings, lectures and web notes (including direct links). Due by the start of class, February 2, 1999. (10 points)

  1. Describe mechanisms of exporting reducing equivalents from chloroplasts to other subcellular compartments. (3 points)

  2. Reducing equivalents are exported from chloroplasts via the malate/oxaloacetate cycle and triose phosphate/3-phosphoglycerate shuttle as described in Figs. 7.10 & 7.11 of the Heldt text.

  3. Discuss the prospects of improving the efficiency of the dark reactions of photosynthesis or CO2 fixation. Exclude converting C3 to C4 plants which is not very feasible given current knowledge and technology. (3 points)
  4. The efficiency of CO2 fixation has improved since the beginning of the evolution of RUBISCO, but not much if at all in the evolution of higher plants. Numerous attempts to improve the efficiency of CO2 fixation of higher plant RUBISCOs including mutagenesis and protein engineering have failed. The protein engineering efforts are complicated by the fact that the carboxylase and oxygenase activities use the same active site and the molecular similarities of CO2 and O2. The recent finding of a much more efficient RUBISCO in thermophilic red algae provides renewed hope for of improving the efficiency of the dark reactions of photosynthesis or CO2 fixation. This could be done either by replacing native RUBISCO with thermophilic red algae RUBISCO or studying the mechanism of the increased efficiency of the thermophilic red algae RUBISCO to provide a basis of "enlightened" re-engineering of higher plant RUBISCO. This is still not expected to be easy.

  5. Calculate relative carboxylase to oxygenase activity of C3 plants at 2.5 times the preindustrial atmospheric CO2 levels (500 ppm). (1 point)
  6. V(Ru-O)  =   Vmax(Ru-O) x km(CO2, Ru-C) x [O2]
    V(Ru-C)       Vmax(Ru-C)     km(O2, Ru-O)     [CO2]

    This = 0.3/0.7 at current atmospheric CO2 levels = 0.43

    0.43  =  X  x  10  x 270 ;    so X = 0.43
                        535     5

    Current atmospheric CO2 is 0.035% = 350 ppm which translates into [CO2] in leaf mesophyll cells of ~ 5 m M. 500 ppm CO2 = 1.43 x current levels resulting in a mesophyll CO2 level of ~ 7.15 mM

    at 500 ppm CO2   V(Ru-O)   =   0.43 x 10    x   270   =   0.43 x 2700  =  0.3
                                V(RuC)                 535        7.15                  3825

      0.3      =    0.23 of RuBP oxygenated; 77% carboxylated
    1 + 0.3

  7. What are the 2 most abundant polymers in plants (and therefore in nature)? What are their functions? (1 point)
  8. Cellulose and lignin. They are both structural components of plant cell walls. Cellulose is the major structural component of both primary and secondary cell walls. Lignin is a very important structural component of very strong secondary cell walls.

  9. Give two reasons why glucose does not accumulate to high levels in actively photosynthesizing leaves. (2 points)
  10. As a reducing sugar, glucose is reactive and can damage cellular components, particularly enzymes, by forming Schiff bases. Also high accumulation of glucose would excessively increase the osmolarity making cellular solutions too hypertonic for normal functioning.