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C4 Photosynthesis

C4 Photosynthesis. Undergraduate Level Notes. Overview. Mesophyll Cell. CCM works on the basis of spatial separation of CO 2 drawdown and actual fixation by RuBisCO . Facilitated by Kranz anatomy, shown left. CO 2 fixed into 4C acids in mesophyll cells.

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C4 Photosynthesis

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  1. C4 Photosynthesis Undergraduate Level Notes

  2. Overview Mesophyll Cell • CCM works on the basis of spatial separation of CO2 drawdown and actual fixation by RuBisCO. • Facilitated by Kranz anatomy, shown left. • CO2 fixedinto 4C acids in mesophyll cells. • 4C acids transported to bundle sheath cells where CO2 is released to allow carbon fixation by RuBisCO. • Has the effect of concentrating CO2 around RuBisCO, increasing carboxylation efficiency. • These processes are spatially separated by segregating the necessary enzymes (PEPC and RuBisCO respectively) into the two cell types (mesophyll and bundle sheath). Bundle Sheath Cell Vascular Tissue

  3. In the Mesophyll... CO2 Carbonic anhydrases catalyse the interconversion between CO2 from the air and HCO3-. Mesophyll Cell PEP CO2 HCO3- The cytosolic enzyme PEPC catalyses the formation of oxaloacetate (OA) from HCO3- and PEP from the chloroplast. Malate OA OA is usually (see “Variations on the Pathway” below) converted to malate (a 4C acid) in the mesophyll chloroplast, by the action of NADP-malate dehydrogenase. The 4C acid diffuses from the mesophyll to the bundle sheath cells. Bundle Sheath Cell Malate

  4. Mesophyll Cell Pi + ATP In the Bundle Sheath... Pyruvate PEP The most important step of the pathway in the BS cells is decarboxylation of the 4C acid to yield CO2 for final fixation by RuBisCO in the CBB cycle, which occurs as normal in the bundle sheath cells. Bundle Sheath Cell Malate CO2 The enzyme catalysing the decarboxylation and the location within the cell of this step is again quite variable (see below), but most common is NADP-malic enzyme in the chloroplast. RuBP CBB Cycle The 3C compound, normally pyruvate,yielded by the decarboxylation diffuses back to the mesophyll cells. In the chloroplast, pyruvate is converted into PEP by the action of pyruvate phosphate dikinase. Sugars

  5. Variations on the Pathway • Variation on the pathway outlined above occurs at three stages: • OA can either be converted to malate (as above) by NADP-malatedehydrogenase, or alternatively to aspartate, by aspartateaminotransferase. • Decarboxylation in the BS can be catalysed either by NADP-malic enzyme, as above, NAD-malic enzyme, or PEPCK (PEP carboxykinase).

  6. Variations on the Pathway • The pyruvate precursor to PEP may be produced in the BS chloroplast then transported to the mesophyll chloroplast (as above), or the product of decarboxylation in the BS may be alanine, which is transported back to the mesophyll before being converted into pyruvate. • These variations also entail differences in the compartmentalisation of each of the processes, but the details of this need not concern us at this level.

  7. Summary • Most C4 plants have a funtional differentiation of photosynthetic cells into two tissue types: mesophyll and bundle sheath (Kranz anatomy) • CO2, having been converted to HCO3-, is “fixed” as a 4C acid by PEPC, in the mesophyll. • Decarboxylation and fixation by RuBisCO in the CBB cycle occurs in the BS – the remaining 3C acid regenerates PEP in the mesophyll. • The enzymes, compartments and processes involved in each of these steps are highly variable.

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