Thylakoid lumen

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The thylakoid lumen is the compartment in the chloroplast bounded by the thylakoid membrane. It plays a vital role for photophosphorylation during photosynthesis.

[edit] Chemiosmosis

During photosynthesis the lumen becomes acidic, as low as pH 4, compared to pH 8 in the stroma. This represents a 10,000 fold concentration gradient for protons across the thylakoid membrane. Thus, the chemiosmotic potential between the lumen and stroma is high enough to drive ATP synthesis using the ATP synthase. In this manner, the light-dependent reactions are coupled to the synthesis of ATP via the proton gradient.

[edit] Source of proton gradient

The protons in the lumen come from three primary sources.

• Photolysis by photosystem II oxidises water to oxygen, protons and electrons in the lumen.
• The transfer of electrons from photosystem II to plastoquinone during non-cyclic electron transport consumes two protons from the stroma. These are released in the lumen when the reduced plastoquinol is oxidized by the cytochrome b6/f protein complex on the lumen side of the thylakoid membrane. From the plastoquinone pool, electrons passs through the cytochrome b6f complex. This integral membrane assembly resembles cytochrome bc1.
• The reduction of plastoquinone by ferrodoxin during cyclic electron transport also transfers two protons from the stroma to the lumen.

The proton gradient is also caused by the consumption of protons in the stroma to make NADH from NAD+ at the NADP reductase.

[edit] Lumen specific proteins

The electron transport protein plastocyanin is present in the lumen and shuttles electrons from the cytochrome b6/f protein complex to photosystem I.