Photosystem Rubisco

Overall, the C4 cycle consumes two ATP equivalents to deliver a CO2 to Rubisco. During active photosynthesis, this is not a problem—plenty of ATP exists from the action of Photosystems I and II. Why, then, don t C4 plants take over the world Probably because the increased energy demands make these plants less efficient under conditions where sunlight is limited. Consistent with this idea, C4 plants are mostly confined to tropical climates, while the C3 plants predominate in more temperate regions. 

Although the bundle sheath chloroplasts contain all the enzymes of the RPP cycle, there is now evidence that some of the 3-PGA formed by the activity of rubisco is exported to the mesophyll cells [9]. Bundle sheath chloroplasts of maize are deficient in photosystem II activity and apparently cannot produce sufficient NADPH to reduce all of the 3-PGA formed to triose phosphate. Responsibility for this step is thus shared with the mesophyll chloroplasts which recycle triose phosphate to the bundle sheath as DHAP. This transport of 3-PGA from bundle sheath to mesophyll permits the synthesis of sucrose in the mesophyll cell cytoplasm. The evidence suggests that the mesophyll cells are the major site of sucrose synthesis [10-13]. Sucrose phosphate synthetase, one of the regulatory enzymes of sucrose synthesis and fructose 6-phosphate, 2-kinase (Fru-6-P,2K), the enzyme synthesizing fructose 2,6-bisphosphate — a potent regulator of cytoplasmic sucrose synthesis (see Section 5.4.1) — are both almost completely confined to the mesophyll cells. 

Have different chemical compositions, for example, it can be anticipated that photosynthetic organs of plants bear higher concentrations of both Mg (chlorophyll, rubisco protein which does attach CO molecules to ribulose-bis-phosphate) and Mn (photosystem II). The above photosynthetic organs are connected to... 

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