Respiration is the exact reversal of the biochemical pathways of ..
Respiration is the reversal of the biochemical pathways …
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Initial and rapid fixation of CO2 within mesophyll cells results in the formation of a four-carbon compound which is then pumped to bundle sheath cells for decarboxylation and subsequent incorporation into the PCR cycle in that tissue. This neat division of labour hinges on specialised anatomy and has even resulted in evolution of distinct classes of chloroplasts in mesophyll compared with bundle sheath cells. Three biochemical variants of C4 photosynthesis (termed subtypes) are known to have evolved from C3 progenitor and in all cases with a recurring theme where the C4 cycle of mesophyll cells is complemented by a PCR cycle in bundle sheath cells, where Rubisco is exclusively localised. In effect, a biochemical ‘pump’ concentrates CO2 at Rubisco sites in bundle sheath cells thereby sustaining faster net rates of CO2 incorporation and virtually eliminating photorespiration. For this overall mechanism to have evolved, a complex combination of cell specialisation and differential gene expression was necessary. Figure 2.3a shows a low-magnification electron micrograph of a C4 leaf related to a generalised scheme for the C4 pathway.
All animals, , use aerobic respiration today, and early animals (, which are called metazoans today) may have also used aerobic respiration. Before the rise of eukaryotes, the dominant life forms, bacteria and archaea, had many chemical pathways to generate energy as they farmed that potential electron energy from a myriad of substances, such as , and photosynthesizers got their donor electrons from hydrogen sulfide, hydrogen, , , and other chemicals. If there is potential energy in electron bonds, bacteria and archaea will often find ways to harvest it. Many archaean and bacterial species thrive in harsh environments that would quickly kill any complex life, and those hardy organisms are called . In harsh environments, those organisms can go dormant for millennia and , waiting for appropriate conditions (usually related to available energy). In some environments, it can .
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From the previous section, the C4 pathway is obviously energetically more expensive than the C3 pathway in the absence of photorespiration. However, at higher temperatures the ratio of RuBP oxygenation to carboxylation is increased and the energy requirements of C3 photosynthesis can rise to more than five ATP and three NADPH per CO2 fixed in air (for these calculations see Hatch 1987).
By analogy with Calvin’s biochemical definition of the C3 pathway at Berkeley in the 1950s, the C4 pathway was also delineated with radioactively labelled CO2 (see Feature Essay 2.1). Significantly, and unlike C3 plants, 3-PGA is not the first compound to be labelled after a 14C pulse (Figure 2.3b). Specialised mesophyll cells carry out the initial steps of CO2 fixation utilising the enzyme phosphoenolpyruvate (PEP) carboxylase. The product of CO2 fixation, oxaloacetate, is a four-carbon organic acid, hence the designation ‘C4’ photosynthesis (or colloquially, C4 plant). A form of this four-carbon acid, either malate or aspartate depending on the C4 subtype, migrates to the bundle sheath cells which contain Rubisco and the PCR cycle. In the bundle sheath cells, CO2 is removed from the four-carbon acid by a specific decarboxylase and a three-carbon product returns to the mesophyll to be recycled to PEP for the carboxylation reaction. Thus, label first appears in the four-carbon acid after 14C feeding, followed by 3-PGA and, finally, in sucrose and starch (Figure 2.1b).
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A more dynamic approach to carbon fixation was needed to resolve this impasse. In particular, the biochemical fate of early products would have to be traced, and using a development of the open gas exchange system at Queens, Atkins et al. (1971) supplied 14CO2 in pulse–chase experiments to sunflower leaf tissue under conditions in which photorespiration was operating at high rates (21% O2) or in which it was absent (1% O2). A series of kinetic experiments showed that synthesis of 14C-glycine and 14C-serine was inhibited in low O2 and that the 14C precursor for their synthesis was derived from sugar bisphosphates of the PCR cycle, especially RuBP. Indeed RuBP was the obvious source of glycine carbon atoms and the kinetics of glycine turnover closely matched those of RuBP. As these authors concluded, ‘we can no longer view this (glycolate) pathway as an adjunct to the Calvin cycle but must incorporate it completely into the carbon fixation scheme for photosynthesis’ (Atkins et al. 1971).
The photosynthetic carbon reduction (PCR) cycle for CO2 fixation (Section 2.1) involves an initial carboxylation of ribulose-1,5-bisphosphate (RuBP) to form 3-PGA, but makes no provision for glycolate synthesis. However, Wang and Waygood (1962) had described the ‘glycolate pathway’, namely a series of reactions in which glycolate is oxidised to glyoxylate and aminated, first to form glycine and subsequently the three-carbon amino acid serine. The intracellular location of this pathway in leaves was established in a series of elegant studies by Tolbert and his colleagues who also established that leaf microbodies (peroxisomes) were responsible for glycolate oxidation and the synthesis of glycine. Kisaki and Tolbert (1969) suggested that the yield of CO2 from the condensation of two molecules of glycine to form serine could account for the CO2 evolved in photorespiration. This idea was incorporated in later formulations of the pathway.
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Of course by closing stomata in the light CAM plants minimize water loss when evaporative demand is highest (von Caemmerer and Griffiths 2009). The biomass production per unit water utilized in CAM was 6 times higher than for C3 plants and 2 times higher than for C4 plants when plants exhibiting all three photosynthetic pathways were grown together in a garden outdoors (Winter et al. 2005). The attribute of water-use efficiency undoubtedly contributes significantly to the success of CAM photosynthesis in nature, with CAM species outnumbering C4 species by about two to one. The paradoxes of CAM, a mode of photosynthesis that involves stomatal opening and CO2 uptake during the dark, continue to inform many aspects of plant biochemistry, physiology, ecology and evolution. This article draws heavily on two recent reviews (Borland et al. 2011; Winter et al. 2015).
Cellular Respiration & Photosynthesis Study Guide ..
The radiolabelling method of Ludwig and Krotkov had, for the first time, provided measurements of what could be regarded as a true estimate of light-driven respiration which was not complicated by transient effects (as the post-illumination burst had been), or by changes in CO2 concentration (as was the case for measurements in closed gas exchange systems or in CO2-free air) or by difficulties associated with detached organs. Ludwig and Canvin (1971) subsequently concluded that processes underlying photorespiration re-evolved 25% of the CO2 which was being fixed concurrently by photosynthesis. Such a rate of CO2 loss was not a trivial process so a biochemical basis for its operation had to be established, and particularly when photorespiration seemed to be quite different from known mechanisms of dark (mitochondrial) respiration.
attribute of all biochemical pathways
Crassulacean acid metabolism (CAM) is a water-conserving mode of photosynthesis that, like C4 photosynthesis, is a modification of the C3 photosynthetic pathway fitted with a CO2 concentrating mechanism (CCM) that can increase the [CO2] around ribulose bisphosphate carboxylase/oxygenase (Rubisco) by more than 10-fold and suppress photorespiration. The overall energy demand of the CAM pathway is only about 10% more than that of C3 photosynthesis, as costs of the CCM machinery are partially offset by reducing photorespiration.
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