Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Stoichiometry and Energetics of Metabolic Conversions
Stoichiometry of Cell Growth and Product Formation
Stoichiometry of Photosynthesis

Evaluating the yields of products from the photosynthetic reduction of СО₂ to CARBOHYDRATES is crucial both for food production processes and for determining the regeneration rates of raw Materials in biomass production. Current knowledge regarding The sequence of photosynthetic reactions and associated electron flows suggests that the theoretically maximum possible efficiency of Photosynthesis (expressed as the percentage ratio of the Free energy of glucose to the free energy of absorbed radiation at 700 nm) is 35%. Naturally, this figure represents a limiting value achievable only under ideal conditions. It does not account for factors such as the utilization of only a fraction of solar radiation in photosynthesis (corresponding to a reduction factor of approximately 0.43), light reflection and leaf shading (a reduction factor of 0.80 or less), and dark Respiration (a variable coefficient with a more or less typical value of 0.67). Multiplying all these coefficients by the theoretical photosynthetic efficiency yields a realistic (maximum) efficiency of about 8%.

Class="center">Table 5.13. Maximum mass yields of metabolic products Y^max_p/s (expressed in grams of metabolic product per gram of substrate), calculated using equation (5.72)^a

^a Reprinted with permission from: Eroshin V. K., Minkevich I. G., On the Upper Limit of Mass Yields of an Organic Product from an Organic Substrate, Biotech. Bioeng., 24, 2263 (1982).

The corresponding amount of biomass (in the form of starch) produced per unit area (m²) per unit time (e.g., per day) can be estimated as follows:

Solar radiation intensity depends on the season and geographical zone. For example, the average solar radiation intensity in the USA is 3930 kcal/(m²·day), reaching up to 6775 kcal/(m²·day) in the southwestern USA during the summer. The final term in equation (5.73) accounts for the loss of one Water molecule for each glucose molecule incorporated into the starch polymer chain.

Given experimental data characterizing the productivity of biomass formation and solar radiation intensity, equation (5.73) can be used to calculate the actual efficiency of photosynthesis in plants and Algae (Table 5.14). The values obtained in this manner range from 0.8% to 3.2%, which is significantly lower than the theoretical estimate. It is also worth noting that the efficiency in the group of so-called C₄ photosynthetic plants is considerably higher than in C₃ plants. Below, we briefly discuss the biochemical basis for this difference.

Table 5.14. Experimentally determined annual average and maximum values of photosynthetic efficiency and productivity in selected plants^a

^a Reprinted with permission from: Bassham J. A., Increasing Crop Production Through More-Controlled Photosynthesis: Science, 197, 630 (1977).

The phenomenon of Photorespiration (light-induced respiration)—namely, the uptake of oxygen and simultaneous release of СО₂ in the light—is a critical process affecting the overall growth efficiency of photosynthetic organisms. In this process, carbon fixation reactions within the plant yield an intermediate compound, ribulose-1,5-diphosphate (see Section 5.6.1), which subsequently breaks down as follows:

Phosphoglycolate is then hydrolyzed to form glycolate, which is subsequently converted into Glycine and other substances within plant Cell Organelles.

This oxidative process neither generates ATP nor releases electrons; consequently, it consumes photosynthetic energy. At the same time, the oxidation reaction (5.74) is inhibited by high concentrations of СО₂. The Cells of C₄ plants, in which The Calvin Cycle (Fig. 5.15) is uniquely modified, maintain a relatively high СО₂ concentration. In the mesophyll cells of C₄ plants, which are in direct contact with air, a series of chemical transformations takes place that can be summarized by the following equation:

Mesophyll cells:

Pyruvate + СО₂ + ATP + 2H₂O → malate + ATP + 2P_i      (5.75)

The malate thus formed diffuses into neighboring bundle sheath cells, where the Reactions of the Calvin cycle take place. Here, the СО₂ incorporated into malate is released According to the reaction:

Malate → pyruvate + СО₂      (5.76)

Ultimately, this metabolic cycle expends two high-energy phosphate bonds to maintain the high СО₂ concentration near ribulose diphosphate carboxylase, thereby minimizing photorespiration. Consequently, the photosynthesis of 1 mole of glucose requires an additional 12 moles of ATP. Thus, energy in C₄ plants is evidently expended on two competing processes: the СО₂ transport cycle pathway and enhanced photorespiration (in the absence of the СО₂ transport cycle). It is interesting to note that the activation energy of the photorespiration reaction (5.74) exceeds that of the carboxylation reaction (5.37), which is also catalyzed by ribulose diphosphate carboxylase. This fact may well explain why C₄ plants are more widespread in the tropics, whereas C₃ plants predominate in temperate zones.