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 (p)• What contributes more to PMF, or pH?

• In the light pH=3

• is negligible due to counter ion movement in and out of the lumen

• G for export of one mole H+ across thylakoid membrane = -17 kJ/mole

• Go’ 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 (pH ~3.5).

• Photophosphorylation produces 3 ATPs per O2 produced

Non-cyclic photosynthetic ETC

cyclic photosynthetic ETC

•NADPH and ATP produced

•Involves both PSI and PSII

•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

• 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)

Arrangement of photosystems in thylakoid

membrane