|Lecture Notes: 26 January||
The Light Reactions, cont.
So what is light energy used for? Look at the "Z" scheme for photosynthetic electron transport. (text Figure 19-49, p 733) [overhead V&V 22-16] Note the three major complexes are not directly connected. Like the mitochondrial complexes they are connected by carriers which diffuse between them. The cytochrome b6/cytochrome f complex is analogous to Complex III in mitochondria: same electron path and a (plastoquinone) Q cycle for proton pumping.
The initial removal of electrons from water to give oxygen uses a manganese complex.
Note the stoichiometry: one O2 : 2 H2O : 4 e- : 8 H+ pumped : 2 ATP : 2 NADPH
The overall proton pumping and ATP synthesis process is summarized in the overhead showing the distribution of the components of the Z scheme [text Figure 19-52, p 736; overhead Patterson 19-6].
Cyclic photophosphorylation involves PSI and cytochrome b5f and the Q pool (Figure 15.10; 14.11).
We have just looked at the so-called Light Reactions of photosynthesis: the process whereby the energy in sunlight is converted by plants into biological useful energy (ATP) and reducing equivalents (NADPH). Now we want to look at how plants use ATP and NADPH to capture ("fix") carbon dioxide and make glucose: the Calvin Cycle.
Most of the reactions of the Calvin Cycle are familiar. Thus on the Calvin Cycle pathway diagram reactions 1-5 (numbered in bold italics) are from Gluconeogenesis/Glycolysis, and reactions 6-8 (numbered in outline font) are from the Pentose Phosphate Shunt, while reactions 9 & 11 are variations of familiar enzyme catalyzed reactions. The only really new reaction is catalyzed by Ribulose-1,5-bis phosphate carboxylase.
Let's begin our look at the Calvin cycle with the first unique reaction of the cycle: the phosphorylation of Ribulose-5-P to Ru-1,5-bis P by Ru-5-P kinase (reaction 9 on the pathway diagram). The strategy here is to create a precursor which is symmetrically phosphorylated so that it can be cleaved into two 3-PGA's.
In the next reaction, catalyzed by Ribulose-1,5-bis phosphate carboxylase (RuBisCo), carbon dioxide is added to the keto-carbon, giving a six carbon sugar which can now be cleaved into the two three carbon PGA's.
Once 3-PGA is formed the reactions of glycolysis/gluconeogenesis interconvert it to Ga-3-P and F-6-P, intermediates of the pentose-P pathway, and DHAP. These intermediates are then interconverted to reform Ru-5-P. For every three carbon dioxides incorporated by RuBisCo, one extra 3-PGA is formed which can be used for the synthesis of glucose etc. The overall stoichiometry of the cycle is shown on the Calvin Cycle Flow Diagram. This diagram emphasizes recycling vs. incorporation. (packet)
Last modified 26 January 2009