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Photosynthesis involving Rubisco …
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.
Kurek I, Chang TK, Bertain SM et al. (2007) Enhanced thermostability of Arabidopsis Rubisco activase improves photosynthesis and growth rates under moderate heat stress. The Plant Cell 19: 3230–3241.
Types of Photosynthesis: C3, C4 and CAM
Whitney SM and Andrews TJ (2003) Photosynthesis and growth of tobacco with a substituted bacterial Rubisco mirror the properties of the introduced enzyme. Plant Physiology 133: 287–294.
Crafts‐Brandner SJ and Salvucci ME (2000) Rubisco activase constrains the photosynthetic potential of leaves at high temperature and CO2. Proceedings of the National Academy of Sciences of the USA 97: 13430–13435.
Photosynthesis - Song with Free Worksheets and Activities
RuBP is therefore consumed and produced during the light-independent reactions and therefore these reactions form a cycle which is named the Calvin cycle.The stroma - Contains many enzymes, including rubisco, which are important for the reactions of the Calvin cycle.The thylakoids - Have a large surface area for light absorption and the space within them allows rapid accumulation of protons.The action spectrum of photosynthesis is a graph showing the rate of photosynthesis for each wavelength of light.
In leaves of C3 plants, the PCR cycle operates in all mesophyll chloroplasts, but in C4 plants the PCR cycle is restricted to bundle sheath cells (Figure 2.3). Rubisco is pivotal in this cycle, and can be used as a marker for sites of photosynthetic carbon reduction. Rubisco was visualised by localising this photosynthetic enzyme with antibodies via indirect immunofluorescent labelling (Hattersley et al. 1977; Figure 2.6). In this pioneering method, ‘primary’ rabbit anti-Rubisco serum (from rabbits injected with purified Rubisco) is first applied to fixed transverse sections of leaves. Rabbit antibodies to Rubisco bind to the enzyme in situ. Then ‘secondary’ sheep anti-rabbit immunoglobulin tagged with a fluorochrome (fluorescein isothiocyanate) is applied to the preparation. This fluorochrome binds specifically to the rabbit antibodies and fluoresces bright yellow wherever Rubisco is located (blue light excitation using an epifluorescence light microscope).
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This song makes it easy to learn how photosynthesis works
Portis AR Jr and Parry MAJ (2007) Discoveries in Rubisco (ribulose 1,5‐bisphosphate carboxylase/oxygenase): a historical perspective. Photosynthesis Research 94: 121–143.
C4 photosynthesis: how some plants avoid photorespiration
Rubisco, formally known as ribulose‐1,5‐bisphosphate (RuBP) carboxylase/oxygenase, is the chloroplast enzyme that assimilates atmospheric carbon dioxide during photosynthesis by combining it with RuBP to form two molecules of phosphoglycerate. However, Rubisco allows oxygen to react with RuBP in addition to carbon dioxide, producing phosphoglycolate, and thereby also initiates the process known as photorespiration. Genetic modification to overcome the limitations of Rubisco would have great agronomic importance.
IB Biology Notes - 8.2 Photosynthesis
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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