Plant photorespiration

At ttie compensation point, photosynthesis is just too slow to outrun (mostly photo) respiration. C3 plants photorespire so much, that their compensation point is c. 0.005%. C4 plants do not photorespire, so they have near zero compensation points (Fig 13.14). 

Lorimer, G. H. Andrews, T. J. 1973. Plant photorespiration - an inevitable consequence of the existence of atmospheric oxygen. Nature, 243,359. 

Accordingly, of 799 genera in the Poaceae family of grasses, 407 possess the C4 assimilation mechanism (Watson Dallwitz, 1992). In C4 plants photorespiration is inhibited by a higher concentration of CO2 around the Rubisco carbon fixing enzyme, which also facilitates carbon assimilation with less water use (Downes, 1969). Therefore, higher temperatures and limited water availability allow C4 plants to outcompete C3 types, which is the observed pattern in the field. 

Osmond and Bjorkman (1975) found that light-dependent CO2 fixation in K. daigremontiana was inhibited by O2 in the same manner as C3 plants, whereas dark CO2 fixation was independent of 4%, 26%, or 36% O2. Further, O2 inhibition of photosynthetic CO2 fixation is accompanied by a high CO2 compensation point (Allaway et al., 1974a). Unlike C3 plant photorespiration, however, the O2 inhibition of photosynthetic CO2 fixation is not eliminated by high CO2 tension, and the postillumination CO2 burst is present at low O2 (Osmond and Bjorkman, 1975 see Fig. 5.13). 

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

DOUCE, R., NEUBURGER, M., Biochemical dissection of photorespiration, Curr. Opin. Plant Biol., 1999,2, 214-222. 

Photosynthesis is the major means of dry matter accumulation in plants. Net photosynthesis is a measure of the gain in assimilate by the plant, after photorespiration and dark respiration losses have been accounted for, and is proportional to quantitative yield production.1... 

Plant mitochondria supply the cell with ATP during periods of low illumination or darkness by mechanisms entirely analogous to those used by nonphotosynthetic organisms. In the light, the principal source of mitochondrial NADH is a reaction in which glycine, produced by a process known as photorespiration, is converted to serine (see Fig. 20-21) ... 

In the dark, plants also carry out mitochondrial respiration, the oxidation of substrates to C02 and the conversion of 02 to H20. And there is another process in plants that, like mitochondrial respiration, consumes 02 and produces C02 and, like photosynthesis, is driven by light. This process, photorespiration, is a costly side reaction of photosynthesis, a result of the lack of specificity of the enzyme rubisco. In this section we describe this side reaction and the strategies plants use to minimize its metabolic consequences. 

The combined activity of the rubisco oxygenase and the glycolate salvage pathway consumes 02 and produces C02—hence the name photorespiration. This pathway is perhaps better called the oxidative photosynthetic carbon cycle or C2 cycle, names that do not invite comparison with respiration in mitochondria. Unlike mitochondrial respiration, photorespiration does not conserve energy and may actually inhibit net biomass formation as much as 50%. This inefficiency has led to evolutionary adaptations in the carbon-assimilation processes, particularly in plants that have evolved in warm climates. 

In many plants that grow in the tropics (and in temperate-zone crop plants native to the tropics, such as maize, sugarcane, and sorghum) a mechanism has evolved to circumvent the problem of wasteful photorespiration. The step in which C02 is fixed into a three-carbon product, 3-phosphoglycerate, is preceded by several steps, one of which is temporary fixation of C02 into a four-carbon compound. Plants that use this process are referred to as C4 plants, and the assimilation process as C4 metabolism or the C4 pathway. Plants that use the carbon-assimilation method we have described thus far, in which the first step is reaction of C02 with ribulose 1,5-bisphosphate to form 3-phosphoglycerate, are called C3 plants. 

In C4 plants, the carbon-assimilation pathway minimizes photorespiration C02 is first fixed in mesophyll cells into a four-carbon compound, which passes into bundle-sheath cells and releases C02 in high concentrations. The released C02 is fixed by rubisco, and the remaining reactions of the Calvin cycle occur as in C3 plants. 

This wonderful book—up to date and authoritative—covers all aspects of plant biochemistry and molecular biology. The following chapters cover carbohydrate synthesis in greater depth Malkin, R. Niyogi, K., Chapter 12, Photosynthesis (pp. 568-629) Dennis, D.T. Blakeley, S.D., Chapter 13, Carbohydrate Metabolism (pp. 630-675) Siedow, J.N. Day, DA., Chapter 14, Respiration and Photorespiration (pp. 676-729). 

Another excellent textbook of plant biochemistry. Especially useful are Chapter 6, Photosynthetic C02 Assimilation by the Calvin Cycle Chapter 7, Photorespiration and Chapter 9, Polysaccharides. 

In most plants photosynthesis is also strongly inhibited by 02. This observation led to the discovery that 02 competes directly for C02 at the active site of rubisco in a process called photorespiration. Chloroplasts inhibited by oxygen produce glycolate in large amounts2823 as a result of the reaction of the intermediate enediolate ion formed in step b of Eq. 13-48 with 02... 

The C4 cycle for concentration of carbon dioxide. The C4 plants reduce their rate of photorespiration by using a C02 concentrating mechanism that enables them to avoid the competition from 02. All species of C4 plants have a characteristic internal leaf anatomy in which a single dense layer of dark green cells surrounds the vascular bundles in the leaves. This bundle sheath is surrounded by a loosely packed layer of... 

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