C02 assimilation

Schindler, C. and H. K. Lichtenthaler. 1996. Photosynthetic C02-assimilation, chlorophyll fluorescence and zeaxanthin accumulation in field grown maple trees in the course of a sunny and a cloudy day. J. Plant. Physiol. 148 399-412. 

This enzyme, similar to all C02 assimilating enzymes, contains biotin for a cofactor. Oxaloacetate is released from the mitochondria into the cytoplasm to enter gluconeogenesis. In the cytoplasm, oxaloacetate converts to phosphoenolpyruvate via a reaction catalyzed by phosphoenolpyruvate carboxylase ... 

Notice that phosphoglycerate kinase is named for the reverse reaction. Like all enzymes, it catalyzes the reaction in both directions. This enzyme acts in the direction suggested by its name during gluconeogenesis (see Fig. 14-16) and during photosynthetic C02 assimilation (see Fig. 20-4). 

FIGURE 20-4 The three stages of C02 assimilation in photosynthetic organisms. Stoichiometries of three key intermediates (numbers in parentheses) reveal the fate of carbon atoms entering and leaving the cycle. As shown here, three C02 are fixed for the net synthesis of one molecule of glyceraldehyde 3-phosphate. This cycle is the photosynthetic carbon reduction cycle, or the Calvin cycle. 

FIGURE 20-10 Third stage of C02 assimilation. This schematic diagram shows the interconversions of triose phosphates and pentose phosphates. Black dots represent the number of carbons in each compound. The starting materials are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Reactions catalyzed by transaldolase ( and ) and transketolase ((3) and ) produce pentose phosphates that are converted to ribulose 1,5-bisphosphate—ribose... 

One molecule of glyceraldehyde 3-phosphate is the net product of the carbon assimilation pathway. The other live triose phosphate molecules (15 carbons) are rearranged in steps to (S) of Figure 20-10 to form three molecules of ribulose 1,5-bisphosphate (15 carbons). The last step in this conversion requires one ATP per ribulose 1,5-bisphosphate, or a total of three ATP. Thus, in summary, for every molecule of triose phosphate produced by photosynthetic C02 assimilation, six NADPH and nine ATP are required. 

FIGURE 20-14 Stoichiometry of C02 assimilation in the Calvin cycle. For every three C02 molecules fixed, one molecule of triose phosphate (glyceraldehyde 3-phosphate) is produced and nine ATP and six NADPH are consumed. 

The chloroplast stroma contains all the enzymes necessary to convert the triose phosphates produced by C02 assimilation (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate) to starch, which is temporarily stored in the chloroplast as insoluble granules. Aldolase condenses the trioses to fructose 1,6-bisphos-phate fructose 1,6-bisphosphatase produces fructose 6-phosphate phosphohexose isomerase yields glucose 6-phosphate and phosphoglucomutase produces glucose 1-phosphate, the starting material for starch synthesis (see Section 20.3). 

Photosynthesis in vascular plants takes place in chloroplasts. In the C02-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce C02 to triose phosphates. These reactions occur in three stages the fixation reaction itself, catalyzed by rubisco reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate and regeneration of ribulose 1,5-bisphosphate from triose phosphates. 

An antiporter in the inner chloroplast membrane exchanges P, in the cytosol for 3-phosphoglycerate or dihydroxyacetone phosphate produced by C02 assimilation in the stroma. Oxidation of dihydroxyacetone phosphate in the cytosol generates ATP and NADH, thus moving ATP and reducing equivalents from the chloroplast to the cytosol. 

Once C02 is fixed into 3-phosphoglycerate in the bundle-sheath cells, the other reactions of the Calvin cycle take place exactly as described earlier. Thus in C4 plants, mesophyll cells carry out C02 assimilation by the C4 pathway and bundle-sheath cells synthesize starch and sucrose by the C3 pathway. 

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. 

Wolosiuk, R., Ballicora, M., Hagelin, K. (1993) The reductive pentose phosphate cycle for photosynthetic C02 assimilation enzyme modulation. FAS EB J. 7, 622-637. 

Identification of Key Intermediates in C02 Assimilation Calvin and his colleagues used the unicellular green alga Chlorella to study the carbon-assimilation reactions of photosynthesis. They incubated 14C02 with illuminated suspensions of algae and followed the time course of appearance... 

Pathway of C02 Assimilation in Maize If a maize (corn) plant is illuminated in the presence of 14C02, after about 1 second, more than 90% of all the radioactivity incorporated in the leaves is found at C-4 of malate, aspartate, and oxaloacetate. Only after 60 seconds does 14C appear at C-l of 3-phosphoglycerate. Explain. 

C02 assimilation. The amount of C02 available for photosynthesis decreases with decreasing C02 partial pressure at higher elevations, but this effect is offset by the increase in diffusion speed at lower air pressure (Gale 1972, 1973). The lower temperature at higher altitudes, however, decreases diffusion speed, and therefore the temperature lapse rate of the particular mountain determines whether C02 availability decreases (dry-moist lapse rate) or stays relatively constant (very wet lapse rate) (Smith and Donahue 1991). The lower air pressure at altitude does not just decrease C02 partial pressure but also 02 partial pressure, which results in lower photorespiration rates and more efficient photosynthesis. When all these effects are modeled, photosynthetic rates generally decrease with altitude, unless the temperature lapse rate is very low (which could occur in extremely wet mountain ranges), but the photosynthetic limitation is much less than expected based on just the partial pressure decrease (Terashima et al. 1995 Smith and lohnson 2007). 

A decrease in C02 assimilation by the ocean and land due to climate warming for Scenario A2 will lead to an additional increase in global warming by 1°C by the year 2100. 

An increase of C02 concentration in the atmosphere does not determine substantial fertilization of marine bioproductivity—but does lead to pH decrease. As temperature grows, C02 assimilation by the ocean decreases, but C02 emissions due to upwellings are reduced and the transport of excess carbon to deep layers of the ocean diminishes. The anthropogenically induced input of nutrients to the oceans through river run-off and deposition of atmospheric aerosols (especially nitrate and iron as elements of atmospheric aerosols) can affect bioproductivity. 

Table 3.11. The dynamics of the ratio of integral rates of (Hf) C02 assimilation by vegetation cover from the atmosphere with the natural distribution of soil-plant formations (Figure 3.8) and with its transformation according to the scenario in the second column.

Some ideas about the role the World Ocean plays in C02 assimilation from the atmosphere can be obtained from the data in Figures 3.10 and 3.11. Figure 3.12 gives... 

Figure 3.12. Longitude-averaged rates of atmospheric C02 assimilation by both land and ocean ecosystems with two scenarios of anthropogenic emissions of carbon 6.26GtC/yr (dashed curve, 2000) and 10.6GtC/yr (solid curve, predicted for 2020). Notation Ha = A//32 I Uh I IIa Hi Hi "<) (GtC yr-1).

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