Photosynthesis: Light-Dependent Reactions

• Photosynthesis is endergonic because it is anabolic and non-spontaneous.
• Photosynthesis occurs in three stages.
• Stage 1: Capturing light energy.
• Stage 2: Using the captured light energy to make ATP and NADPH.
• Stage 3: Using the free energy (ATP) and reducing power of NADPH to make organic compounds (carbon-based) like glucose from CO2, assembling G3P.
• Stages 1 and 2 are the light-dependent reactions.
• Light-dependent reactions are a series of reactions energized by light.
• Light-dependent reactions take place on the thylakoid membranes.

Light Reactions - Non-Cyclic Electron Flow

• Ground state chlorophyll absorbs photons, exciting electrons.
• Unstable excited electrons return to ground state.
• Electrons return to ground state by fluorescing red (if not in a photosystem) or transferring energy to chlorophyll a (if in a photosystem).
• The antenna complex is composed of chlorophyll and accessory pigments.
• The antenna complex absorbs photons, transferring energy to the reaction center.
• Reaction center chlorophyll a absorbs energy, exciting an electron.
• The excited electron is transferred to the primary acceptor via a redox reaction where chlorophyll is oxidized, and the acceptor is reduced.
• Photosystem II (P680): A photon excites P680’s electron, transferring it to the primary electron acceptor, leaving P680 positively charged.
• Water-Splitting Complex: oxidizes water, replacing P680's lost electrons, releasing H+ into the lumen and O₂ as waste.
• Electron Transport Chain (ETC): The primary acceptor transfers electrons to pQ (plastoquinone), an electron shuttle in the membrane.
• pQ donates electrons to the cytochrome complex, pumping H+ into the lumen.
• Electrons pass from cytochrome to pC (plastocyanin), shuttling them to Photosystem I.
• Photosystem I (P700): A photon excites P700’s electron, transferring it to the primary acceptor.
• Electrons lost from P700 are replaced by electrons from Photosystem II.
• Primary acceptor transfers electrons to Fd (ferredoxin), which reduces NADP+ reductase.
• NADP+ reductase forms NADPH from NADP+ and H+, consuming H+ from the stroma.
• A H+ gradient in the lumen is created by H+ pumping via the cytochrome complex and H+ release from water splitting.
• ATP synthase uses the proton gradient to generate ATP.
• Water splitting increases H+ concentration in the lumen.
• The cytochrome complex and pQ pump H+ into the lumen, increasing H+ concentration.
• NADP+ reductase gains a proton from the stroma.
• Without sunlight, the proton gradient is not maintained.
• The proton gradient faces the opposite direction of the one in cellular respiration because ATP synthase faces outwards since ATP is needed outside the thylakoid.

Light Reactions - Cyclic Electron Flow

• Photosystem I can function independently of photosystem II.
• Electrons are transferred to ferredoxin but are not given to NADP+ reductase, rather to pQ and then the cytochrome complex and then pC.
• This causes protons to be pumped and electrons to be cycled back to photosystem I.
• NADPH is not formed, and water is not oxidized in cyclic flow, but it is in normal flow.
• Cyclic electron flow is also known as cyclic photophosphorylation.
• Light-independent reactions (Calvin cycle) require more ATP than is produced by non-cyclic electron flow.