Carbon concentration mechanisms

The best known carbon concentrating mechanism (CCM), is that of freshwater cyanobacteria... 

Figure 9 A model of the cyanobacterial carbon concentrating mechanism from Synechococcus PCC 7942, a freshwater species. Solid circles represent transporters located on the plasma membrane and interior to the cell wall. Boxes represent the catalyzing enzymes CA, Carbonic Anhydrase RUBISCO, Ribulose 1-5 bisphosphate Carboxylase Oxygenase. The carboxysome is the site of carbon fixation (dark reactions) and the thylakoid is the site of the light reactions of photosynthesis (after Badger et aL, 2002).

The increase in 02 and concomitant decrease in C02 caused by the evolution of oxygenic autotrophs on the Earth, has resulted in an undersaturation of the main (andancient) carboxylating enzyme, ribulose-l,5-bisphosphate carboxylase, responsible for the first step in carbon fixation—the dark reaction of photosynthesis. To palliate this difficulty, a number of species of marine phytoplankton have evolved carbon concentrating mechanisms that all involve some forms of... 

Fielding AS, Turpin DH, Guy RD, Calvert SE, Crawford DW, Harrison PJ (1998) Influence of the carbon concentrating mechanism on carbon stable isotope discrimination by the marine diatom Thalassiosira pseudonana. Canadian J Botany 76 1098-1103... 

The ambient concentration of CO2 in seawater ( 10 pM) is low compared to the half saturation constant of RubisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) the enzyme that fixes the inorganic carbon into PG3 (3-phos-phoglycerate) in the first step of the Calvin cycle (the dark reaction of photosynthesis). Microalgae have thus evolved various carbon concentrating mechanisms (CCM) to augment the CO2 concentration at the site of fixation [55]. These mechanisms all involve interconversion between CO2 and HCOJ at some point. But at neutral pH the hydration/dehydration reaction of CO2/ HCOj" has a half life of 30 s, much too slow for a cellular process (for example, diffusion from one end of the cell to the other takes on the order of 10 ms), and requires catalysis. The enzyme CA is an extraordinary effective catalyst some CA catalyze the CO2/HCO3 reaction at a rate that nearly reaches the limit imposed by the diffusion of molecules [56]. 

Cadmium carbonic anhydrase (CDC A) is the first member of a new class of carbonic anhydrases, the class. CDCAl, which uses Cd as its metal cofactor when Zn is limiting, was isolated from the marine diatom T. weissflogii. The amino acid sequence of CDCAl contains a triple repeat with 85% identity between repeats [10]. CDCAl is a key enzyme in the carbon concentrating mechanism (CCM) through which T. weissflogii increases the concentration of CO2 at the site of fixation byRuBisCO [92]. 

Differences in alloy carbon concentration, heat treatment, and mechanical forming usually produce only small differences in corrosion rate in a pH range of 4—10. It is less certain how corrosion rates vary at high and low pH due to these factors. Cast irons containing graphite particles may experience a unique form of attack called graphitic corrosion (see Chap. 17, Graphitic Corrosion ). 

Figure 11.7 shows how the mechanical properties of normalised carbon steels change with carbon content. Both the yield strength and tensile strength increase linearly with carbon content. This is what we would expect the FejC acts as a strengthening phase, and the proportion of FojC in the steel is linear in carbon concentration (Fig. 11.6a). The ductility, on the other hand, falls rapidly as the carbon content goes up (Fig. 11.7) because the a-FejC interfaces in pearlite are good at nucleating cracks.

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

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... 

Blood carbonic acid and hydrogen carbonate concentrations are controlled by independent mechanisms. Carbonic acid concentration is controlled by respiration as we exhale, we deplete our system of C02, and hence deplete it of H2C03 too. This decrease in acid concentration raises the blood pH. Breathing faster and more deeply increases the amount of C02 exhaled and hence decreases the carbonic acid concentration in the blood, which in turn V ... 

Network structure and reaction mechanisms in high pressure vulcanisation (HPV) and peroxide vulcanisation of BR was studied by 13C solid-state NMR [43]. Different samples of polybutadiene (51% trans, 38% cis, and 11 % vinyl) were peroxide cured with dicumyl peroxide on a silica carrier and by the HPV conditions of 250 °C and 293 MPa. The 13C NMR spectra from peroxide and HPV cures were compared to a control samples heated to 250 °C for 6 minutes under atmospheric pressure. Although no new isolated strong peaks were detected in either the peroxide or HPV vulcanisations, small increases in both spectra were observed at 29.5, 36.0, 46.5, and 48.0 ppm. These peaks compare favourably with calculated shifts from structures that arise from main chain radical addition to the pendent vinyl groups. These assignments are further reinforced by the observation that the vinyl carbon concentration is substantially reduced during vulcanisation in both peroxide and HPV curing. Two peaks at 39.5 and 42.5 ppm appear only in the peroxide spectrum. Cis-trans isomerisation was absent in both cures. 

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