Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Photorespiration Limits the Productivity of C3 Plants

In both C₃ and C₄ plants, Respiration and phosphorylation processes take place in the Cell/35.html">Mitochondria of green leaf Cells during the night, accompanied by The breakdown of substrates produced by Photosynthesis during the preceding light periods. This raises the question: do leaf cells also respire in the light, during active photosynthesis, or is mitochondrial respiration switched off for this time? Careful measurements of oxygen evolution and CO₂ uptake rates have shown that C₃ plants do indeed respire in the light and consume a certain amount of oxygen while photosynthesis—which evolves oxygen—is actively ongoing. However, this respiration cannot be entirely attributed to mitochondria. It turned out to be only partially inhibited by cyanide, an inhibitor of mitochondrial cytochrome oxidase. The cyanide-insensitive respiration observed in C₃ plants in the light is known as Photorespiration.

Photorespiration appears to be a wasteful process. First, part of the reducing power generated in the light reactions is diverted in this way to reduce oxygen instead of being utilized for Biosynthesis. Second, unlike mitochondrial respiration, photorespiration is not coupled to Oxidative Phosphorylation. Consequently, a significant portion of the solar energy captured in the light reactions is squandered during photorespiration. A third circumstance is also important to us: photorespiration is particularly active in C₃ plants, whereas it is virtually absent in plants of tropical origin.

The primary substrate oxidized during photorespiration in C₃ plants is glycolic acid (Fig. 23-28). In the Peroxisomes of leaf cells, glycolate is oxidized to glyoxylate, which is then converted into Glycine and other products. The reaction leading to The formation of glycolate in plant cells is highly unusual. Glycolate is formed As a result of the oxidative Cleavage of ribulose 1,5-bisphosphate. This reaction is catalyzed by ribulose bisphosphate carboxylase—the very same enzyme that catalyzes CO₂ fixation leading to the formation of phosphoglycerate. How is this possible?

The reason is that ribulose bisphosphate carboxylase is capable of catalyzing the reaction of ribulose bisphosphate with either CO₂ or O₂. When the CO₂ concentration is low and the O₂ concentration is relatively high, the O₂ molecule not only competes with CO₂ but can actually replace it. This unusual reaction results in ribulose bisphosphate undergoing oxygenation rather than carboxylation in C₃ plants. Oxygenation yields phosphoglycolate and 3-phosphoglycerate (Fig. 23-29), instead of the two molecules of 3-phosphoglycerate produced by carboxylation. Phosphoglycolate is then hydrolyzed to form free glycolate, which serves as the substrate for photorespiration.

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Fig. 23-28. Hydrolysis of phosphoglycolate yields glycolate, the substrate for photorespiration. Glycolate is oxidized to glyoxylate, CO₂, and other products.

Fig. 23-29. Oxygenation of ribulose 1,5-bisphosphate. In this reaction, the normal substrate, CO₂, is replaced by oxygen; consequently, phosphoglycolate is formed instead of a second molecule of 3-phosphoglycerate.

In contrast, in C₄ plants, the CO₂/O₂ ratio in bundle-sheath cells always remains relatively high thanks to the preceding C₄ pathway; i.e., conditions here favor the carboxylation of ribulose 1,5-bisphosphate. Furthermore, the closure of Stomata in the leaves of C₄ plants not only prevents Water loss but also restricts the influx of atmospheric oxygen into the leaves.