The complexities of respiration in photosynthetic cells

Abstract

(3). The loosely held view that mitochondrial respiration is restricted by a high cytosolic ATP/ADP ratio in the light has been challenged by two recent reports. At the University of Adelaide, Dry and Wiskich 3 studied the respiration of pea leaf mitochondria in the presence of a creatine/ creatine kinase system and varying levels of ATP, and observed that ATP/ADP ratios of less than 20 exerted little control over respiration as long as the absolute concentration of ADP available for uptake into the mitochondrion (via the adenine nucleotide translocator) was not unusually low. Since literature values for the ratio of ATP to ADP outside the chloroplast in vivo are between 4 and 8, Dry and Wiskich concluded that any light/dark-mediated regulation of mitochondrial respiration cannot be based on a mechanism of changing external ATP/ADP ratios. Supportive evidence has come from the University of G6ttingen where Stitt, Lilley and Heldt 4 measured the levels of ATP, ADP and AMP in the chloroplasts, cytosol and mitochondria of wheat leaf protoplasts. Utilizing an ingenious arrangement of syringe, nylon net and membrane filters, they ruptured the protoplasts, separated the released subceilular fractions and quenched these in HCIO4 in about 0.1 s, thereby ensuring that measured levels best reflected those present in vivo. From their experiments, a cytosolic ATP/ ADP ratio of between 6 and 7 was obtained with illuminated protoplasts. Surprisingly an even higher value (about 9) was obtained with darkened protoplasts. As Stitt et al. point out, these values do not support the belief that mitochondrial oxidative phosphorylation is suppressed in the light by an increased cytosolic ATP/ADP ratio. If mitochondrial respiration is suppressed in the light, some other mechanism must be responsible, but the nature of this mechanism is at present unknown. The quantitative significance of the third O2-uptake process (ferredoxin-linked O2reduction) has also been questioned. Two groups 5' recently concluded that it is indeed a significant process in the light, but since it is then linked to the water-splitting system of photosynthesis, and not directly to any CO2-evolving process, it cannot be called respiration. However, a similar O2-uptake by chloroplasts may also occur in darkness and in this case can be looked upon as a form of respiration. At Brandeis University, Kow, Erbes and Gibbs 7 demonstrated that spinach chloroplast particles can transfer electrons from NADPH (produced during the oxidation of glyceraldehyde-3-phosphate to glycerate3-phosphate) to O,, via ferredoxin; they termed this chloroplast respiration. It does not appear that the energy of the NADPH is conserved. Kow et al. consider that this chloroplast respiration might be linked to the degradation of starch (which is synthesized within the chloroplast during photosynthesis) in that it could regenerate NADPH + for glyceraldehyde-3-phosphate