text: Experimentation with respiratory oxidase inhibitors (for instance, cyanide) on unicellular algae has revealed interactive pathways to be present between chloroplasts and mitochondria. Metabolic pathways responsible for photosynthesis are present in chloroplasts, whereas respiratory metabolic pathways are present in mitochondria. In these pathways, metabolic carriers (like phosphate) exchange NAD(P)H molecules between photosynthetic and respiratory ETCs. Evidence using mass spectrometry on algae and photosynthetic mutants of Chlamydomonas, discovered that oxygen molecules were also being exchanged between photosynthetic and chlororespiratory ETCs.  The mutant Chlamydomonas alga species, lacks photosystems one and two (PS I and PS II), so when the alga underwent flash-induced PS I activity, it resulted in no effect on mitochondrial pathways of respiration. Instead, this flash-induced PS I activity caused an exchange between photosynthetic and chlororespiratory ETCs, which was observed using polarography. This flash of PS I activity is triggered by an over-reduction of the PQ pool and/or lack of the pyridine nucleotide in the thylakoid membrane. A reduction in such molecules then stimulates NADPH and PTOX molecules to trigger chlororespiratory pathways.
  Furthermore, in the absence of light (and thus photosynthesis), chlororespiration plays an integral role in enabling metabolic pathways to compensate for chemical energy synthesis. This is achieved through the oxidation of stromal compounds, which increases the PQ pool and allows for the chlororespiratory ETC to take place.