Abstract. A mechanistic model of photosynthesis is developed based on the characteristics of ribulose 1,5‐bisphosphate (RuBP) carboxylase and the assimilation of CO2 as an ordered reaction with RuBP binding before CO2. An equation is derived which considers the effects of light (for regeneration of RuBP) and CO2. Taking values for the maximum turnover of RuBP carboxylase at substrate saturation, the maximum carboxylation efficiency (maximum increase in rate per increase in CO2 concentration) and the minimum quantum requirement for the C3 pathway, photosynthesis in the absence of photorespiration is simulated. In the model, at varying concentrations of CO2, the efficiency of light utilization approaches a maximum value as photon flux density decreases. Similarly, with a given maximum carboxyation capacity, at varying photon flux densities the carboxylation efficiency approaches a constant maximum value (equal to Vmax/KmCO2) as CO2 is decreased. However, a decrease in the state of activation of RuBP carboxylase under low light results in a lower carboxylation efficiency. Limits on the rate of photosynthesis, as the maximum capacity for regeneration of RuBP is reduced relative to carboxylation potential, or as the maximum capacity of the carboxylase varies, are considered.
Photosynthesis is the origin of oxygenic life on the planet, and its models are the core of all models of plant biology, agriculture, environmental quality and global climate change. A theory is presented here, based on single process biochemical reactions of Rubisco, recognizing that: In the light, Rubisco activase helps separate Rubisco from the stored ribulose-1,5-bisphosphate (RuBP), activates Rubisco with carbamylation and addition of Mg²(+), and then produces two products, in two steps: (Step 1) Reaction of Rubisco with RuBP produces a Rubisco-enediol complex, which is the carboxylase-oxygenase enzyme (Enco) and (Step 2) Enco captures CO₂ and/or O₂ and produces intermediate products leading to production and release of 3-phosphoglycerate (PGA) and Rubisco. PGA interactively controls (1) the carboxylation-oxygenation, (2) electron transport, and (3) triosephosphate pathway of the Calvin-Benson cycle that leads to the release of glucose and regeneration of RuBP. Initially, the total enzyme participates in the two steps of the reaction transitionally and its rate follows Michaelis-Menten kinetics. But, for a continuous steady state, Rubisco must be divided into two concurrently active segments for the two steps. This causes a deviation of the steady state from the transitional rate. Kinetic models are developed that integrate the transitional and the steady state reactions. They are tested and successfully validated with verifiable experimental data. The single-process theory is compared to the widely used two-process theory of Farquhar et al. (1980. Planta 149, 78-90), which assumes that the carboxylation rate is either Rubisco-limited at low CO₂ levels such as CO₂ compensation point, or RuBP regeneration-limited at high CO₂. Since the photosynthesis rate cannot increase beyond the two-process theory's Rubisco limit at the CO₂ compensation point, net photosynthesis cannot increase above zero in daylight, and since there is always respiration at night, it leads to progressively negative daily CO₂ fixation with no possibility of oxygenic life on the planet. The Rubisco-limited theory at low CO₂ also contradicts all experimental evidence for low substrate reactions, and for all known enzymes, Rubisco included.
Abstract. A model is developed for photosynthesis and photorespiration in C3 plants, using an equation for the multisubslrate ordered reaction of ribulose 1,5‐bisphosphalc carboxylase‐oxygenase (Farazdaghi & Edwards, 1988). The model examines net CO2 fixation with O2 inhibition, and mutual inhibition when equilibrium exists between carboxylation and oxygenation (at the CO2 compensation point). It is based on the stoichiometry of energy requirements and O2, and CO2 exchange in the cycles, the quantum efficiency for RuBP generation, the maximum capacity for RuBP generation, the carboxylation efficiency with respect to [CO2], and the oxygenation efficiency with respect to [O2]. With increasing concentrations of CO2 above the CO2 compensation point, decreasing quantum flux density, or decreasing O2, simulations show that the rate of photorespiration progressively decreases. The two components of O2 inhibition of photosynthesis change disproportionately with increasing CO2 concentration. According to the model, the energy utilized during photosynthesis at the CO2 compensation point is about half that under atmospheric conditions.
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