Figure 10.15 Photosystem II Photosystem I 4 Photons 4 Photons Electron tra nsport chain produced via proton-motive force Energy of electron Higher Lower Figure 10.15 The Z-Scheme Model Links Photosystems II and I.
How does ATP get produced by the photosystems
The Enhancement Effect • The Z scheme explains the enhancement effect • Photosynthesis is more efficient when • Both 680-nm and 700-nm wavelengths are available • Allowing both photosystems to run at maximum rates • Photosystem I occasionally transfers electrons to photosystem II’s electron transport chain to • Increase ATP production • Instead of using the electrons to reduce NADP
The Enhancement Effect • This cyclic photophosphorylation • Coexists with the Z scheme • Produces additional ATP
Figure 10.16 Chloroplast stroma Thylakoid lumen Antenna complex Photosystem II Photosystem I Cytochrome complex ATP synthase Proton- motive force NADP reductase Photon Photon Most abundant in membranes of grana Most abundant in membranes exterior to grana Figure 10.16 Electrons Are Passed from Water to NADP + in a Linear Pathway.
Figure 10.17 Energy of electron Higher Lower 2 Photons produced via proton-motive force Photosystem I Figure 10.17 Cyclic Electron Flow Leads to ATP Production.
The Location of Photosystem I and Photosystem II • Photosystem II is much more abundant in the interior, stacked membranes of grana • Photosystem I and ATP synthase are much more common in the exterior, unstacked membranes • The stroma • Is the site of ATP production • Where the proton gradient established by PS II drives protons
The Calvin Cycle and Carbon Fixation Two separate but linked processes in photosynthesis: 1.The energy transformation of the light- dependent reactions 2.The carbon dioxide reduction of the Calvin cycle In the presence of light ATP and NADPH are produced by photosystems I and II
The reactions that produce sugar from carbon dioxide in the Calvin cycle are light- independent • Require the ATP and NADPH • Produced by the light-dependent reactions The Calvin Cycle and Carbon Fixation
The Calvin Cycle The Calvin cycle has three phases: 1. Fixation : CO 2 reacts with ribulose bisphosphate (RuBP) Produces two 3-phosphoglycerate molecules Attachment of CO 2 to an organic compound is carbon fixation 2. Reduction : The 3-phosphoglycerate molecules are: Phosphorylated by ATP Reduced by NADPH Producing glyceraldehyde 3-phosphate (G3P) 3. Regeneration : The remaining G3P is used in reactions that regenerate RuBP
The Calvin Cycle • This cycle of reactions occurs in the chloroplast’s stroma • One turn of the Calvin cycle fixes one molecule of CO 2 • 3 turns of the Calvin cycle are required • To produce 1 molecule of G3P • The discovery of the Calvin cycle clarified • How the ATP and NADPH produced by light- capturing reactions • Allow cells to reduce CO 2 to carbohydrate
Figure 10.19 (a) The Calvin cycle has three phases.
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