Respiration in the light and photorespiration

Photorespiration is a mechanism of CO2 recycling. CO2 is depleted near the centres of carboxylation, which results in oxygenation at Rubisco level. Photorespiration provides a feedback releasing a part of assimilated CO2. This results in oscillatory phenomena during photosynthetic CO2 fixation [1].

In the evolution of biosphere, photorespiration was important for establishing O2 and CO2 concentration in the biosphere by imposing limits of concentrations of both gases. I propose that the oscillations of CO2 in the biosphere are of the same origin as in the leaf [2].

Respiration in the light is possible due to a continuous build-up of ADP in the intermembrane space of mitochondria due to adenylate kinase activity. This provides a mechanism of equilibration of adenylates, maintenance of pH gradients and metabolite concentrations in subcellular compartments [3]. Operation of partial tricarboxylic acid cycle is possible due to fine mechanisms of switching NADP-isocitrate dehydrogenase from the forward to reverse reaction depending on redox state in mitochondria [4].

 Hypoxic metabolism in plants

Lactic and alcoholic fermentation are not the only strategies for adaptation to low oxygen. An alternative pathway involves participation of plant hemoglobin (Hb) in the maintenance of redox level in hypoxic plant cell. This is possible due to the operation of Hb/NO cycle in which nitric oxide (NO) is formed from nitrite by anaerobic mitochondria and then oxygenated by Hb to nitrate [5]. Anaerobic mitochondria can generate ATP using nitrite as electron acceptor [6].

Glyoxylate cycle in plants and animals

The glyoxylate cycle makes possible the conversion of fatty acids to carbohydrates. It is also an important supplier of organic acids for acidification in germinating maize seeds. We discovered that in mammals, the glyoxylate cycle is induced upon starvation and during diabetes [7].

Theoretical biology

My vision of biological organization is based on the principles of the quantum measurement theory, which can be considered as a mirrored image of theoretical biology [8]. I define life as a self-organizing and self-generating activity of open non-equilibrium systems determined by their internal semiotic structure. Life by its existence (in self-reflecting loops) establishes basic physical parameters of the Universe [9,10]. 

References

1. Roussel MR, Ivlev AA, Igamberdiev AU (2007) Oscillations of the internal CO2 concentration in tobacco leaves transferred to low CO2. Journal of Plant Physiology 164: 1188-1196

2. Igamberdiev AU, Lea PJ (2006) Land plants equilibrate O2 and CO2 concentrations in the atmosphere. Photosynthesis Research 87: 177-194

3. Igamberdiev AU, Kleczkowski LA (2003) Membrane potential, adenylate levels and Mg2+ are interconnected via adenylate kinase equilibrium in plant cells. Biochimica et Biophysica Acta – Bioenergetics 1607: 111-119

4. Igamberdiev AU, Gardeström P (2003) Regulation of NAD- and NADP-dependent isocitrate dehydrogenases by reduction levels of pyridine nucleotides in mitochondria and cytosol of pea leaves. Biochimica et Biophysica Acta – Bioenergetics 1606: 117-125

5. Igamberdiev AU, Baron K, Manac’h N, Stoimenova M, Hill RD (2005) The haemoglobin/nitric oxide cycle: involvement in flooding stress and effects on hormone signalling. Annals of Botany 96: 557-564

6. Stoimenova M, Igamberdiev AU, Gupta KJ, Hill RD (2007) Nitrite-driven anaerobic ATP synthesis in barley and rice root mitochondria. Planta 226: 465-474

7. Popov VN, Volvenkin SV, Eprintsev AT, Igamberdiev AU (1998) Glyoxylate cycle enzymes are present in liver peroxisomes of alloxan-treated rats. FEBS Letters 440: 55-58

8. Igamberdiev AU (1993) Quantum mechanical properties of biosystems - a framework for complexity, structural stability, and transformations. BioSystems 31: 65-73

9. Igamberdiev AU (2004) Quantum computation, non-demolition measurements, and reflective control in living systems. BioSystems 77: 47-56

10. Igamberdiev AU (2007) Quantum limits of computation and emergence of life. BioSystems 90: 340-349