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I came across the definition of photorespiration as a process that forms CO2 from O2 in the presence of light. However, the CO2 is released only when the plant cell undergoes the conversion of glycolate to PGA, i.e in the glycolate pathway.

Most textbooks seem to use the term photorespiration and glycolate pathway interchangeably, but then they seem to say that the glycolate pathway evolved to regain the energy lost in the formation of glycolate, whereas photorespiration is wasteful. How is this possible?

Shouldn't photorespiration be a conservation pathway as well? It is trying to regain lost energy from glycolate, isn't it? or Have I got the two terms completely confused?

Please Explain

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  • $\begingroup$ I don't even think that photorespiration is wasteful. It seems to me like plants generate CO2 for photosynthesis when amount of CO2 in environment is less. $\endgroup$ – another 'Homo sapien' Feb 29 '16 at 13:32
  • $\begingroup$ Can you actually describe what process according to you is photorespiration? $\endgroup$ – Chitinous Exoskeleton Feb 29 '16 at 13:37
  • $\begingroup$ As per what I have studied, I can define photorespiration as the process in which RuBisCO uses O2 and produces CO2 when amount of Oxygen in environment is higher than that of CO2. But, though it uses more ATP and also produces ammonia, it seems to me like plant is generating CO2 for its own benifit i.e. photosynthesis. So it does not seem totally wasteful. $\endgroup$ – another 'Homo sapien' Feb 29 '16 at 13:45
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    $\begingroup$ That would actually make some sense as the feedback for that would be less photorespiration and more photsynthesis due to increase in CO2 concentration. So in those terms glycolate pathway is just a pathway for photorespiration, right? $\endgroup$ – Chitinous Exoskeleton Feb 29 '16 at 15:44
  • $\begingroup$ From plantsinaction.science.uq.edu.au/book/export/html/64 "Wang and Waygood (1962) had described the ‘glycolate pathway’, namely a series of reactions in which glycolate is oxidised to glyoxylate and aminated, first to form glycine and subsequently the three-carbon amino acid serine." So, it says that glycolate pathway is just a part of photorespiration in which glycolate is metabolized. $\endgroup$ – another 'Homo sapien' Mar 1 '16 at 8:52
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The simple answer to the question posed in the title of the question is as follows. The glycolate pathway is the series of reactions converting phosphogycolate to reusable carbon intermediates + CO2. Photorespiration is the combination of two processes:

  1. The oxidation of the enodiolate intermediate on Rubisco to phosphoglycoate + 3-phosphoglycerate.

  2. The processing of the phosphoglycolate produced by the glycolate pathway.

You cannot use the two interchangeably as there is no respiration (use of oxygen) in the glycolate pathway. The glycolate pathway is just the latter part of photorespiration. I'm not sure what the text books are that confuse the two, but Stryer presents this quite clearly in Figure 20.6 and 20.7 at http://www.ncbi.nlm.nih.gov/books/NBK22344/#A2795.

I am not familiar with the term "conservation pathway" in your final question. I would call the glycolate pathway a "salvage pathway" (although this term is more commonly used for recycling the skeletons of the products of macromolecular, esp. DNA, degradation). It does not salvage all of the carbon skeleton of phosphoglycolate as some carbon is lost to the atmosphere as CO2.

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