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Learn about the Difference between Photorespiration and Dark Respiration.
1. In practically all aerobic tissues, dark respiration is a common feature. On the other hand, photorespiration is demonstrated only by those plants in which carbon fixation occurs via the C3 cycle.
2. In dark respiration, O2 uptake is restricted to only one kind of organelle, the mitochondrion, whereas O2 uptake in C2 cycle takes place in three organelles, viz., chloroplast (RuBP oxygenase), peroxisome (glycolate oxidase) and probably in the mitochondrion (cytochrome oxidase).
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3. The rate of dark respiration is neither markedly influenced by CO2 and O2 concentrations, nor is there any competition between the two gases. On the other hand, the operation of the C2 cycle is largely controlled by the levels of these two gases, with competition evident between CO2 and O2.
4. There is no obligatory connection between reactions of carbon reduction and those of dark respiration. In fact, the metabolic events associated with these two pathways are separated both with regard to space and time. On the contrary, C2 oxidation cycle is intimately connected with C3 carbon reduction cycle since the substrate RuBP for the C2 cycle is derived directly from the C3 cycle.
5. In dark respiration, there are several reactions that produce CO2, but O2 is consumed in only one reaction, viz., cytochrome oxidase. On the other hand, photo respiratory cycle contains only one reaction, viz., glycine decarboxylase that liberates CO2, but three reactions, viz., RuBP oxygenase, glycolate oxidase and possibly cytochrome oxidase, that utilize O2.
6. Ammonia is not released in dark respiration. In contrast, the C2 cycle releases ammonia, which is, however, used in GS reaction.
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7. In dark respiration, about 40 per cent of the energy available from glucose oxidation is conserved in the form of ATP. In contrast, ATP formation through C2 cycle by re-oxidation of NADH produced in glycine decarboxylase reaction is not an obligatory function, rather an input of energy is necessary to drive the C2 cycle.
