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2.3.3 - C4 plants and unicellular algae avoid photorespiration

At around the same time as the nature of photorespiration was becoming clearer Hatch and Slack (1966) demonstrated that in tropical grasses (initially sugar cane) the first-formed products of photosynthetic CO2 fixation were the four-carbon acids oxalacetate, malate and aspartate, rather than the 3-PGA formed in the PCR cycle. Furthermore, the carboxylation reaction involved PEP carboxylase and carbon was subsequently transferred to PCR cycle intermediates. As noted earlier (Section 2.2) C4 plants show no apparent CO2 release in light. The explanation lies in their anatomy and multiple carboxylation reactions rather than in the absence of the pathway of photorespiration. Bundle sheath cells are equipped with a CO2-concentrating mechanism that favours carboxylation over oxygenation reactions due to increased partial pressure of CO2, while photorespiratory release of CO2 is further prevented through the activity of PEP carboxylase which refixes any respired CO2 formed from the oxygenase function of Rubisco.

Unicellular green algae also posed a problem for the simple extrapolation of early models for photorespiratory metabolism in C3 leaves. Although organisms such as Chlorella had been used to establish the PCR cycle, and indeed provided much early evidence for effects of O2 on photosynthesis and formation of glycolate in light, they also appeared to lack CO2 evolution in light (Lloyd et al. 1977). In this case the explanation lies in a CO2-concentrating mechanism which effectively increases the internal pool of inorganic carbon (CO2 and HCO3) thereby favouring the carboxylase function of Rubisco over its oxygenase function.