Re: O2 in C3 plants--photorespiration and photosynthesis

You're right. RuBP carboxylase (Rubisco) is not the most effeicient 
enzyme in the world and this is because one of its substrates is the 
oxygen that is produced in the light reactions. Luckily, the rate of 
reaction of RuBPwith CO2 is about 80 times faster than with O2 with a 
typical plantRubisco.  Because there is more oxygen than CO2 in the cell, 
carboxylation of RuBP is about three times faster than oxygenation.  However,
the solubility of CO2 is dependent on temperature.  It decreases as the
temperature rises.  Therefore, photosynthesis is less efficient at high
temperatures.  At temperatures higher than 30 C, the C4 pathway is more 
energetically efficient than C3 despite the ATP that must be expended
in order to concentrate CO2.
Originally, Rubisco may have been more efficient; that is the earth's
atmosphere was considerably more rich in CO2 than it is now when Rubisco
was first synthesized in primative organisms.  Indeed, the orginal
Rubisco, (Form II, still found in anaerobic proteobacteria and
aerobic dinoflagellates!) has a higher affinity for O2 than the more
modern and common Form I.
One thing that may help is the fact that oxygen is evolved in the
thylakoid lumen and Rubisco is in the stroma.  So, even with diffusion, 
the enzyme might not be exposed to super-high concentrations of O2.