## 10 Photorespiration  
As described above, according to the recent study published by [PERSON] et al., the catalytic efficiencies of the carboxylation and oxygenation reactions are strictly related. Photosynthetic organisms (such as plants and bacteria) with similar cellular physiology have adapted to share similar S\({}_{\text{C/O}}\) selectivity. This observation can indirectly indicate that efforts aiming at improving S\({}_{\text{C/O}}\) value by directed mutagenesis would probably be unproductive.  
However, a recent study from the Farquhar Group [12] opened a new research scenario, having evidenced the existence of biochemical links between the photosynthesis and photorespiration pathways. The study shed a new light on the photorespiration process and some of its beneficial features, giving a possible explanation as to why photosynthetic organisms are not directly related.  
Figure 18: (**a**) Histograms of Rubisco kinetic parameters S\({}_{\text{C/O}}\) and \(k_{cat,C}\) experimentally determined for several hundred species at 25 \({}^{\circ}\)C and pH = 8. The values of kinetic parameters are reported along the \(X\)-axis (log scale) and the number of data points is reported along the \(Y\)-axis. The parameter distributions for Form I Rubisco are represented in blue while the parameter distributions for Form II Rubisco are represented in yellow. Dashed lines give the median value. N is the number of Form I Rubisco for which S\({}_{\text{C/O}}\) and \(k_{cat,C}\) parameters have been considered and \(\sigma^{*}\) is the multiplicative standard deviation of Form I values; (**b**) diagram showing \(k_{cat,C}/\)K\({}_{\text{C}}\) and \(k_{cat,O}/\)K\({}_{\text{O}}\) are strongly correlated on a log–log scale (R = 0.94; \(p<10^{-10}\)). Light green point refers to Rubisco from C3 plants; mustard points refer to Rubisco from C4 plants; deep green points refer to Rubisco from cyanobacteria. (Figure 18 from ref. [51], adapted and used by permission of American Chemical Society (copyright © 2019)).  
organisms have evolved by \"tolerating\" the oxygenation reaction instead of \"eliminating\" this side reaction.  
Over the last four decades, according to the [PERSON] et al., (FvCB model) photosynthesis model, [11] the RuBP-oxygenation reaction leading to 2-phosphoglycolate production has been considered a deleterious process, as the toxic intermediate 2-phosphoglycolate must be rapidly metabolized by the photorespiratory metabolism causing the following two drawbacks: (i) reduction of RuBP available for the carboxylation reaction; (ii) recycle of glycolate back to RuBP in a process that co-produces CO2 and NH3. CO2 is usually released from leaf while NH3 recycling is energetically costly. Overall, approximately 2 mol of 2-phosphoglycolate produced undo one carboxylation reaction, causing a decrease of carbon fixation efficiency into hexose sugars by up to 50%. However, the recent study points out that photorespiratory metabolism increases de novo nitrogen uptake (as NO3-) and reduction (to NH4+) by terrestrial plants, increasing availability of the precious element that enters the 2-oxo-glutarate/glutamic acid cycle harnessing, thus, glyoxylate conversion into glycine (Figure 19). Therefore, the negative effect due to reduced CO2-fixation into hexose sugars is counterbalanced by an increased synthesis of glycine that is partially converted into serine. Overall, photorespiration, despite reducing the Rubisco carboxylation efficiency, seems to improve the overall photosynthetic performance.