Carbon assimilation

Lawlor, D.W. Fock, H. (1978). Photosynthesis, respiration and carbon assimilation in water-stressed maize at two oxygen concentrations. Journal of Experimental Botany, 29, 579-93. 

AMORY, A.M, FORD, L., PAMMENTER, N.W., CRESSWELL, C.F., The use of 3-amino-1,2,4-triazole to investigate the short-term effects of oxygen toxicity on carbon assimilation by Pisum sativum seedlings, Plant Cell Env., 1992,15,655-663. 

The AE methods have been used to determine the effects of different algae as food sources in the bioaccumulation of radiolabelled essential (Co, Se, Zn) and nonessential trace metals (Ag, Am, Cd, Cr) in the mussel Mytilus edulis [94]. Assimilation of essential metals was correlated with carbon assimilation, but not nonessential metals. The distribution of the metal in the alga and the gut passage time in the mussel was found to be important. 

Under certain conditions in fresh and wastewaters, essentially all the carbon is mineralized, and little or none accumulates in the biomass. This is unexplained because mineralization generates energy, and the metabolic pathways leading to the formation of C02 are assumed to involve biochemical sequences that result in carbon assimilation. The following are some examples reported by several workers [40,47,57,87] ... 

Only 1.2% of the carbon of 2,4-D added to stream water was converted to organic particulate matter, the solids fraction in water containing the microbial cells. This lack of significant carbon assimilation may be a result of the inability of the microorganisms to obtain carbon and energy for biosynthetic purposes at these low concentrations, the immediate use of the carbon for respiration in order for the cells to maintain their viability (i. e., for maintenance energy), or the rapid decomposition and mineralization of the cells and their constituents. 

In the second phase, transaldolase (with TPP as cofactor) and transketolase catalyze the interconversion of three-, four-, five-, six-, and seven-carbon sugars, with the reversible conversion of six pentose phosphates to five hexose phosphates. In the carbon-assimilating reactions of photosynthesis, the same enzymes catalyze the reverse process, called the reductive pentose phosphate pathway conversion of five hexose phosphates to six pentose phosphates. 

In photosynthetic eukaryotic cells, both the light-de-pendent and the carbon-assimilation reactions take place in the chloroplasts (Fig. 19-38), membrane-bounded intracellular organelles that are variable in shape and generally a few micrometers in diameter. Like mitochondria, they are surrounded by two membranes, an outer membrane that is permeable to small molecules and ions, and an inner membrane that encloses the internal compartment. This compartment contains many flattened, membrane-surrounded vesicles or sacs, the thylakoids, usually arranged in stacks called grana (Fig. 19-38b). Embedded in the thylakoid membranes (commonly called lamellae) are the photosynthetic pigments and the enzyme complexes that carry out the light reactions and ATP synthesis. The stroma (the aqueous phase enclosed by the inner membrane) contains most of the enzymes required for the carbon-assimilation reactions. 

The carbon-assimilation reactions of photosynthesis reduce C02 with electrons from NADPH and energy from ATP. 

By regulating the partitioning of electrons between NADP+ reduction and cyclic photophosphorylation, a plant adjusts the ratio of ATP to NADPH produced in the light-dependent reactions to match its needs for these products in the carbon-assimilation reactions and other biosynthetic processes. As we shall see in Chapter 20, the carbon-assimilation reactions require ATP and NADPH in the ratio 3 2. 

ATP synthase 704 FiATPase 708 rotational catalysis 711 P/O ratio 712 P/2e ratio 712 acceptor control 716 mass-action ratio 716 light-dependent reactions 723 light reactions 723 carbon-assimilation reactions 723 carbon-fixation reaction 723 thylakoid 724 stroma 724 exciton transfer 725 chlorophylls 725... 

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 20-15 The Pi-triose phosphate antiport system of the inner chloroplast membrane. This transporter facilitates the exchange of cytosolic P for stromal dihydroxyacetone phosphate. The products of photosynthetic carbon assimilation are thus moved into the cytosol... 

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. 

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... 

In hepatocytes and adipocytes, cytosolic NADPH is largely generated by the pentose phosphate pathway (see Fig. 14-21) and by malic enzyme (Fig. 21-9a). The NADP-linked malic enzyme that operates in the carbon-assimilation pathway of C4 plants (see Fig. 20-23) is unrelated in function. The pyruvate produced in the reaction shown in Figure 21-9a reenters the mitochondrion. In hepatocytes and in the mammary gland of lactating animals, the NADPH required for fatty acid biosynthesis is supplied primarily by the pentose phosphate pathway (Fig. 21-9b). 

Naturally occurring stable isotopes of C, N, and S have been used extensively for over a decade as direct tracers of element cycling in marine and terrestrial food webs (34-39). Carbon and sulfur isotopes fractionate very little between food and consumer thus their measurement indicates which primary producers or detrital pools are sources of C and S for consumers. For example, a study of plants and animals in Texas sand dunes showed that insect species had 813C values either like those of C3 plants or like those of C4 plants (-27 and -13%o, respectively). Rodent species had intermediate values near -20%o that indicated mixed diets of both C3 and C4 plants (40). The 13C measurements, used in simple linear mixing models, proved to be quick and reliable indicators of which plant sources provided the carbon assimilated by higher trophic levels. 

Keeley, J.E. Busch, G. (1984). Carbon assimilation characteristics of the aquatic CAM plant Isoetes howellii. Plant Physiology 76, 525-30. 

Ammonium, the primary product of nitrogen fixation, is transported to the host cell cytoplasm where it is assimilated into amides and, in some cases, further converted into ureides before being transported to the shoot. Since the physiological environment within the nodule is apparently different from the other parts of the plant, nodule-specific or nodule-abundant forms of several enzymes of the nitrogen and carbon assimilation pathways have evolved, and are induced to improve the efficiency of nitrogen and carbon metabolism in nodules. 

Harding, L. W. J., B. W. Meeson, and T. R. J. Fisher. 1986. Phytoplankton production in two east coast estuaries Photosynthesis-light functions and patterns of carbon assimilation in Chesapeake and Delaware Bays. Estuarine, Coastal and Shelf Science 23 773-806. 

Of course, an accurate assessment of the fluxes of chemical elements in the atmosphere-vegetation-soil system is only possible with a detailed inventory of land covers. For instance, Fang et al. (2001) have undertaken such an inventory for seven time periods over the territory of China, including both planted and natural forests. It was shown that a maximum rate (0.035 PgCyr-1) of carbon assimilation from the atmosphere was observed between 1989 and 1993. Under this, different types of forest had various time periods for a maximum rate of carbon assimilation. This confirms... 

The assimilation of C02 by land ecosystems is determined by the difference between carbon assimilation due to photosynthesis and loss of carbon due to respiration. However, the subsequent climatic impact shows itself only in several decades in... 

Nadelhoffer et al. (1999) showed that the missing sink can be explained neither by the contribution of the ocean nor carbon assimilation by boreal forests of the Northern Hemisphere. It was shown that the existing uncertainty about the volume of carbon sink in forests cannot be reduced by more than 0.25 1015 gCyr-1. In... 

A change of CO2 concentration in the atmosphere affects through feedbacks the carbon fluxes at the boundaries of natural media. The efficiency of carbon assimilation from the atmosphere by the ocean decreases with growing atmospheric C02 concentration due to the decreasing buffer capacity of its carbonate system. 

---