Photophosphorylation Chapter 15 (part 2) Photophosphorylation

Photophosphorylation Light-Driven ATP Synthesis Electron transfer through the proteins of the Z scheme drives the generation of a proton gradient across the thylakoid membrane Protons pumped into the lumen of the thylakoids flow back out, driving the synthesis of ATP CF1-CFo ATP synthase is similar to the mitochondrial ATP synthase

Chloroplast CF1CFo ATPase Similar in structure to mitochondrial F1Fo ATPase CF1 domain (ATP synthesis) extends into the stroma. Many of the protein subunits are encoded by the chloroplast genome

Chloroplast Proton Motive Force (Dp) What contributes more to PMF, DY or DpH? In the light DpH=3 DY is negligible due to counter ion movement in and out of the lumen DG for export of one mole H+ across thylakoid membrane = -17 kJ/mole DGo’ for ATP formation = 30.5 kJ/mole Since 12 moles of protons gives –200 kJ of energy Experiment show that 3 ATPs are generated per mole of O2 produced

Energy Balance Sheet 8 photons (4 e-) generate 1 oxygen and 2 NADPH Photosynthetic ETC pumps between 8 and 12 protons across thylakoid membrane to generate proton gradient (DpH ~3.5). Photophosphorylation produces 3 ATPs per O2 produced

Non-cyclic photosynthetic ETC NADPH and ATP produced Involves both PSI and PSII cyclic photosynthetic ETC only ATP produced Involves only PSI

Cyclic Photosynthetic ETC Involves only PSI Reduced ferredoxin transfers e- to Cytobf complex which then re-reduces Plastocyanin and finally the oxidized P700 of PSI No NADPH produce.Only ATP Levels of NADP+ thought to regulated this process. Low NADP+ activates cyclic ETC Observed in vitro.

Arrangement of photosystems in thylakoid membrane

Arrangement of photosystems in thylakoid membrane PSII primarily present in granal lamellae Light harvesting antennae complexes (LHC) are also present in the granal lamellae. Under low light conditions LHCs are closely associated with PSII, Under high light condition the 2 disassociate. PSI and ATPase are in the stroma lamellae. Physical separation suggest that mobile electron carrier must be involved (i.e. PQ and Plastocyanin)