3.2 Electron transport and ATP synthesis

Photosynthesis is all about capturing light energy and turning it into chemical energy. The electron transport chain is the key player here, moving electrons through a series of proteins to create a proton gradient.

This gradient is then used by ATP synthase to make ATP, the energy currency of cells. It's like a molecular waterfall, with protons flowing downhill to power ATP production.

Photosystems and Electron Transport

Light-dependent reactions in photosynthesis

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• Photosystems are protein complexes involved in the light-dependent reactions of photosynthesis
• Photosystem II (PSII) and Photosystem I (PSI) work together to capture light energy and convert it into chemical energy
• Light energy is used to excite electrons, which are then transferred through an electron transport chain
• Electron transport chain consists of a series of redox reactions that generate a proton gradient across the thylakoid membrane

Components of the electron transport chain

• Plastoquinone (PQ) is a mobile electron carrier that accepts electrons from PSII and transfers them to the cytochrome b6f complex
• Cytochrome b6f complex is a proton pump that transfers electrons from PQ to plastocyanin while simultaneously pumping protons into the thylakoid lumen
• Plastocyanin (PC) is a mobile electron carrier that accepts electrons from the cytochrome b6f complex and transfers them to PSI
• Ferredoxin (Fd) is an electron acceptor that receives electrons from PSI and transfers them to NADP+ reductase
• NADP+ reductase catalyzes the reduction of NADP+ to NADPH using electrons from ferredoxin (provides reducing power for the Calvin cycle)

Electron flow and energy production

• Light energy excites electrons in PSII, which are then transferred to PSI via the electron transport chain (linear electron flow)
• As electrons flow through the electron transport chain, protons are pumped into the thylakoid lumen, creating a proton gradient
• Proton gradient is used by ATP synthase to generate ATP (chemiosmosis)
• In cyclic electron flow, electrons from PSI are transferred back to the cytochrome b6f complex via ferredoxin, generating ATP without producing NADPH (helps balance ATP and NADPH production)

ATP Synthesis

Proton gradient and chemiosmosis

• Proton gradient is established across the thylakoid membrane during the light-dependent reactions
• Protons accumulate in the thylakoid lumen as a result of water splitting in PSII and proton pumping by the cytochrome b6f complex
• ATP synthase uses the proton gradient to generate ATP through the process of chemiosmosis
• Protons flow down their concentration gradient through ATP synthase, driving the synthesis of ATP from ADP and inorganic phosphate (Pi)

Types of photophosphorylation

• Non-cyclic photophosphorylation involves linear electron flow from PSII to PSI, generating both ATP and NADPH
• Electrons from PSI reduce NADP+ to NADPH, while protons pumped into the thylakoid lumen drive ATP synthesis (produces ATP and NADPH in a ratio of 3:2)
• Cyclic photophosphorylation involves the recycling of electrons from PSI back to the cytochrome b6f complex via ferredoxin
• Cyclic electron flow generates ATP without producing NADPH (helps balance ATP and NADPH production to meet the demands of the Calvin cycle)
• Cyclic photophosphorylation is important under conditions of high ATP demand or low NADPH requirement (such as during photorespiration or nitrogen assimilation)