BCH/PPA 503 -- Plant
Lecture Three

Models integrating electron transport, ATP/ADP, chloroplastic compounds, inhibitors
and the regulation of CO2 fixation





The energetic relationship of the light reactions and CO2 fixation reactions of photosynthesis can be illustrated as follows:

This image is from M.J. Farabee's web site.


Some plants have evolved strategies for suppressing photorespiration.  One such adaptation is C4 photosynthesis in which CO2 is 1st fixed into a 4-C rather than a 3-C acid, as is the case for C3 photosynthesis. C4 plants possess 2 types of photosynthetic cells each with a different type of chloroplast and a unique anatomy ('Kranz' anatomy).  A layer of cells surrounding the vascular bundle, the bundle-sheath, is common in plants, but they are the only chloroplast containing cells in C4 plants.  These bundle-sheath cells are thick walled and sometimes suberized sealing them off from gas exchange.  In C4 plants phosphoenolpyruvate (PEP) is carboxylated into C4 acids by PEP carboxylase (PEPC) in mesophyll cells which are transported via plasmodesmata to the bundle-sheath cells where they are decarboxylated.  A simplified scheme of the NADP-malic enzyme type of C4 photosynthesis from Furbank and Taylor [Plant Cell 7,797 (1995)].

In the C4 photosynthetic pathway CO2 (in the form of HCO3-) is covalently added to PEP by PEPC to form the C4 acid, oxaloacetate.  In the case of the NADP+-malate enzyme type C4 pathway, OAA is reduced by malate dehydrogenase using NADPH from the light reactions to form malate, a C4 acid that is transported to the bundle-sheath cells.  Malate is decarboxylated in the bundle-sheath cells by NADP+-malic enzyme where the CO2 released is fixed by the PCR cycle as in C3 plants.  The 3-C compound pyruvate diffuses back to the mesophyll where it is phosphorylated in an ATP-dependent reaction catalyzed by pyruvate orthophosphate dikinase to generate the carbon acceptor PEP.

The net reaction catalyzed by the C4 pathway is to transfer CO2 from the mesophyll to the bundle-sheath at the expense of 2 ATPs per CO2 transferred.  It is in effect an ATP-driven CO2 pump.  It achieves concentrations of inorganic C (CO2 + HCO3-) ~ 150 µM (~ a CO2 concentration of 70 µM).  This is ~ 20 x the CO2 concentration in mesophyll cells and is sufficient to saturate photosynthesis and essentially completely suppress photorespiration.  PEPC has an affinity for inorganic C ~ that of RUBISCO [Km(HCO3-) = 30 µM, ~ 6.4 µM CO2 at pH 7].  This CO2 concentrating activity of C4 plants provides 3 potential advantages over C3 plants:







Many xerophytic plants have photosynthetic adaptations that can greatly improve water-use efficiency and effectively suppress photorespiration.  Water-use efficiencies (g CO2 fixed per kg H2O transpired) of CAM plants range from 10 - 40 compared to 1 - 3 for C3 plants and 2 - 5 for C4 plants.  The main feature of photosynthetic metabolism in CAM plants is assimilation of CO2 into oxaloacetic acid at night by PEPC that is then reduced to malic acid by malate dehydrogenase, which is stored in vacuoles.  During the day the malate is released from the vacuole where it is decarboxylated to provide CO2 for fixation by the Calvin-Benson cycle behind closed stomata.  Thus, CAM plants separate CO2 assimilation from CO2 fixation in time rather than spatially as in C4 plants.


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    This page was last modified January 17, 2000.