THE THERMODYNAMIC EFFICIENCY (QUANTUM DEMAND) AND DYNAMICS OF PHOTOSYNTHETIC GROWTH

Abstract

The commonly quoted values of maximum photosynthetic efficiency have been those obtained by determining the oxygen yield from suspensions of resting algal cells in which growth was disregarded. The unpredictability of the metabolism of resting cells severely vitiates the reliability of measurements made on their energy metabolism. Also the validity of the measurements with resting cells is made doubtful by anomalous values for the photosynthetic quotient (–δCO₂/δO₂). The measurements on resting cells fall into two categories: one in which the cells were suspended in acid media (pH 5) with a CO₂ partial pressure (p_CO2) ^of 5% atmospheric, and one in which the cells were suspended in alkaline media (about pH 9) with a p_co2 of 0.25% atmospheric. In acid media with 5% CO₂, the most probable value of the minimum quantum demand is 5 to 6 hv/O₂. With pH 9 media, equilibrated with 0.25% CO₂, the minimum quantum demand found is about 10 hv/O₂. This low efficiency seems to be caused by a sub-optimal CO₂ partial presure, since it has been observed that the value at alkaline pH agrees with that at acid pH provided the p_CO2 is maintained at 2 % atmospheric. This p_CQ2 effect has been neglected by many workers. To avoid the controversial methods using resting cells, it is essential to determine the photosynthetic efficiency of cells in a steady state of growth. The environmental conditions during growth of the cells have a strong influence on the efficiency of photosynthesis; for instance, the efficiency appears to be strongly dependent on the temperature during growth. Under light-limited conditions when the photosynthetic efficiency of growth is optimized the minimum quantum demand of algal cells is found to be 5 to 6 hv/O₂. The minimum quantum demand of CO₂ fixation varies from 1.1 to 1.4 times the value for O₂ depending on the nature of the nitrogen source for growth. Significant doubt must be attached to measurements of the maximum photosynthetic efficiency with isolated chloroplasts, on the grounds that in vitro conditions may impair their efficiency and that the efficiency may be affected by the growth conditions of the parent plant. Thus, a unified view of the experimental data indicates that the most probable value of the minimum quantum demand is 5 to 6 hv/O₂. The preference for the apparently sub-optimal value of about 10 hv/O₂ found with alkaline media and a p_co2 of 0.25 %, which is the prevailing view, is necessitated by the requirement of the Z-scheme paradigm of the mechanism of the electron transfer. Thus it appears that hypothesis rather than a unified view of the experimental data on the efficiency is dictating the view of the mechanism involved. A cell of Chlorella (strain 211/8k) fully charged with reducing equivalents and energy can continue to assimilate CO₂ and grow at the maximum rate (doubling time 3 h) for 9 s. It is calculated that exposure to 20 W m⁻²(daylight PAR) for 0.5 s is sufficient fully to charge such a cell with energy and reducing equivalents. This calculation predicts that, in the steady state of growth, cycles of exposure of each cell to 20 W m⁻²for 0.5 s followed by 9 s in the dark will support growth at the maximum rate. The theoretical expressions used to express the maximum thermodynamic efficiency of conversion of radiation to chemical work (η_T) are shown to be inconsistent. The correct value is taken to be given by Spanner's equation η_T= 1 −(T/T_r), where T is the ambient temperature and T_r is the radiation temperature. Hence, the maximum value of η_T for conversion of the PAR in sunlight to chemical work varies from 0.93 for unscattered sunlight to 0.70 if it is isotropically scattered. It is deduced that under the usual ambient conditions the value of η_T for photosynthesis will decrease by 0.043 for each log decrease in the irradiance. Contents Summary 3 I. Introduction 4 II. The stoichiometry of photosynthesis 6 III. Thermodynamic limits to photosynthetic efficiency 7 IV. The theoretical quantum demands for production of NADPH and ATP 8 V. The dynamics and energetics of photosynthetic growth 10 VI. The physiology of cells at or near zero growth rate 12 VII. Manometric measurements of quantum demands of resting cells 13 VIII. Non-manometric measurements of quantum demands of resting cells 16 IX. Quantum demands of vascular plants and isolated chloroplasts 18 X. The quantum demands of growing cells 19 XI. The influence of wavelength of radiation on photosynthetic efficiency 22 XII. Conclusion 23 XIII. Appendices 24 References 34