23/09/2021
The C-4 cycle requires compartmentalized photosynthetic reactions into two distinct cell types: bundle sheath cells (BSCs) and mesophyll cells (MCs). The C4 photosynthesis is classified into three biochemical subtypes, according to their different decarboxylation mechanisms. The first enzyme of the C4 cycle, PEPC, is common to all three subtypes. The product of PEPC, oxaloacetate (OAA), can be either reduced to malate by malate dehydrogenase (MDH) or converted to aspartate (Asp) by aspartate aminotransferase. Malate is then transported to BSCs in NADP-malic enzyme (ME) subtype plants while Asp is transported in NAD-ME and PEPCK types. In the NADP-ME type, malate is used to generate pyruvate and CO2, with the formation of NADPH catalysed by NADP-ME. In NAD-ME type and PEPCK type, aspartate is converted back to OAA in mitochondria or bundle sheath cytoplasm, respectively. In the NAD-ME type, OAA is reduced to malate by MDH, and the NAD-ME then catalyses splitting of malate to release CO2 and pyruvate in mitochondria. In the PEP-carboxykinase (PEPCK) type, most of the OAA is converted to PEP and CO2 in BSC cytosol by PEPCK while rest is converted in to Malate which is then decarboxylated in to Pyruvate and CO2 in BSC mitochondria.
Decarboxylation in BSCs, together with the CO2 diffusion barrier, elevates the CO2 concentration around RuBisCo, 10 folds above the air level. (A PART OF MY CLASS)