Figp2.01a.png

Figure 2.1a (also shown in the first section of this chapter). Diagram of the chloroplast showing Rubisco’s carboxylation reaction of RuBP with CO₂ to produce two 3-PGA molecules, and the oxygenation reaction to produce one 3-PGA and one P-glycolate molecule.

Rubisco is a bifunctional enzyme capable of reacting either CO₂ or O₂ to RuBP in the active sites. Although Rubisco’s affinity for CO₂ is an order of magnitude higher than that for O₂, the high O₂ concentration (20%) relative to CO₂ (0.004%) in the Earth’s atmosphere leads to a ratio of 3:1 of carboxylation:oxygenation in C₃ plants exposed to air. The carboxylation reaction yields two molecules of 3-PGA while the oxygenation of RuBP yields one molecule of 3-PGA and one molecule of phosphoglycolate (P-glycolate), as shown in this figure.

The 3-carbon compound 3-PGA enters the Calvin cycle, but the 2-carbon compound 2-phosphoglycolate is a dead-end metabolite. Consequently, plants have evolved a series of metabolic reactions, termed photorespiration, aimed at salvaging some of the carbon stored in 2-phosphogylcolate and evolving the rest as CO₂. This process of CO₂ evolution is different from mitochondrial respiration which is described in Section 2.4. The historical evidence for photorespiration is presented below.