C4 cycle

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... 

The overall effect is to transport C02 from the mesophyll cells into the bundle sheath cells along with two reducing equivalents, which appear as NADPH following the action of the malic enzyme. The C02, the NADPH, and additional NADPH generated in the chloroplasts of the bundle sheath cells are then used in the Calvin-Benson cycle reactions to synthesize 3-phospho-glycerate and other materials. Of the C02 used in the bundle sheath cells, it is estimated that 85% comes via the C4 cycle and only 15% enters by direct diffusion. The advantage to the cell is a higher C02 tension, less competition with 02, and a marked reduction in photorespiration. 

Figure 23-38 The C4 cycle for concentrating C02 in the C4 plants. From Haag and Renger with alterations.440...

After PEP carboxylase makes the oxaloacetate, it is transported to the bundle sheath cells. First, NADPH reduces it to malate, and it is then transported to the bundle sheath cells. In the bundle sheath cells, malic enzyme cleaves the malate to pyruvate and C02 for Rubisco. This generates NADPH as well, so the C4 cycle consumes no reducing equivalents. Pyruvate is transported from the bundle sheath back to the mesophyll cells where it is rephosphorylated to phosphoenolpyruvate, expending the equivalent of two ATP high-energy phosphates. ... 

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. 

The C4 cycle can be viewed as an ATP-dependent C02 pump that delivers C02 from the mesophyll cells to the bundle-sheath cells, thereby suppressing photorespiration (Hatch and Osmond, 1976). The development of the C4 syndrome has resulted in considerable modifications of inter- and intracellular transport processes. Perhaps the most striking development with regard to the formation of assimilates is that sucrose and starch formation are not only compartmented within cells, but in C4 plants also may be largely compartmented between mesophyll and bundle-sheath cells. This has been achieved together with a profound alteration of the Benson-Calvin cycle function, in that 3PGA reduction is shared between the bundle-sheath and mesophyll chloroplasts in all the C4 subtypes. Moreover, since C4 plants are polyphyletic in origin, several different metabolic and structural answers have arisen in response to the same problem of how to concentrate C02. C4 plants have three distinct mechanisms based on decarboxylation by NADP+-malic enzyme, by NAD+-malic enzyme, or by phosphoenolpy-ruvate (PEP) carboxykinase in the bundle-sheath (Hatch and Osmond, 1976). 

Fig. 2. The C4 cycle of COj fixation in photosynthesis. The pathway shown is that occurring in Type-1 C4 plants such as Zea mays. Abbreviations RuBP, ribulose 1,5-bisphosphate PGA, 3-phosphogly-cerate PEP, phosphoeno/pyruvate OAA, oxaloacetate. The partial triose-P/PGA shuttle is based primarily on evidence demonstrating concentration gradients that would support metabolite flux between the two cell types.

The second pathway is called the C4 cycle because COj is initially converted to the four-carbon dicarboxylic acids, malic or aspartic acids (Fig. 3.3). Phos-phoenolpyruvic acid (I) reacts with one molecule of CO2 to form oxaloacetic acid (II) in the mesophyll of the biomass, and then malic or aspartic acid (III) is formed. The Q acid is transported to the bundle sheath cells, where decarboxylation occurs to regenerate pyruvic acid (IV), which is returned to... 

See also Calvin Cycle Reactions, C4 Cycle, Basic Processes of Photosynthesis, Relationship of Gluconeogenesis to Glycolysis (from Chapter 16), Pentose Phosphate Pathway (from Chapter 14)... 

Phosphoglycolate is subsequently dephosphorylated and passed into organelles called the peroxisomes where it is further oxidized, the glyoxylate is amidated, and glycine is produced. This process is referred to as photorespiration and it occurs under conditions where the oxygen concentration is high. The C4 cycle, which occurs in so-called C4 plants, bypasses some of the inefficiency of photosynthesis arising from photorespiration. 

See also Glycolysis, Gluconeogenesis, The C4 Cycle (from Chapter 17), Glycolysis Reaction Summaries, PEPCK, Enolase, Pyruvate Kinase... 

See also The C4 Cycle, Reactive Oxygen (from Chapter 15), Oxidases and Oxygenases (from Chapter 15)... 

Photorespiration is an inefficient process occurring in plants under conditions where C02 levels are low. Certain plants called C4 plants have evolved an additional photosynthetic pathway that helps conserve C02 released by photorespiration. This pathway is called the C4 cycle because it incorporates C02 into an intermediate with four carbons (oxaloacetate). By contrast, the Calvin cycle is sometimes called the C3 cycle because the reaction of C02 with ribulose-l,5-bisphosphate yields two molecules of 3-phosphoglycerate, a three - carbon compound. 

The C4 cycle is found in important crop species, such as maize and sugarcane, and is important in tropical plants, which are exposed to intense sunlight and high temperatures. Photorespiration is most active under these conditions. 

C4 plants concentrate their Calvin cycle photosynthesis in specialized bundle sheath cells, which lie below a layer of mesophyll cells (Figure 17.25). The mesophyll cells, which are most directly exposed to external C02, contain the enzymes of the C4 cycle. See Figure 17.26 for the reactions of the C4 cycle. The key reaction, which is the capture of C02 into oxaloacetate, is catalyzed by the enzyme phosphoenolpyruvate carboxylase and occurs in the mesophyll cells. 

Note that the C4 cycle costs 2 additional ATPs for every C02 fixed. This price may seem steep, but it appears to be worth paying under circumstances when photorespiration would dominate. 

See also C4 Cycle, Bundle Sheath Cells, Calvin Cycle, Photo respiration, Rubisco... 

C4 plants suppress photorespiration by concentrating CO2 at the site of ribulose-l,5-bisphosphate carboxylase (Rubisco). During photosynthesis in C4 leaves an inorganic carbon pool develops which is up to 10 x that expected by simple equilibration with external CO2 (1). The effective concentration of CO2 in bundle sheath cells requires that the mesophyll-bundle sheath cell interface be resistant to CO2 diffusion. The more "leaky" this interface is to CO2 the more energy must be expended by "overcycling" of the C4 cycle relative to net C02 assimilation to maintain a high bundle sheath CO2 concentration. The work presented here quantitatively examines the permeability of the bundle sheath-mesophyll interface to CO2 and its implications for C4 photosynthesis. 

C4 cycle A cycle in some plants that minimizes the wasteful effects of photorespiration by using an enzyme other than rubisco... 

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