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Photosynthesis

Uni of Notts, Genes, Molecules & Cells, first year

TermDefinition
Photosynthesis experiment: Joseph Preistly (1771) A candle in a jar may have its air restored by a plant
2 main stages of photosynthesis Stage 1: Water is oxidised using sunlight to yield high energy electrons Stage 2: Those electrons are used to reduce carbon dioxide to produce useful organic molecules
Light Harvesting Complexes (LHCs) Membrane bound thylakoid pigment-protein complexes used in photosystems to maximise energy gathering efficiency using multiple pigments for multiple electromagnetic wavelengths
How EM radiation excites electrons If photons carry the correct energy (hv) to equal the difference in energy levels between the grounded orbital (HOMO) & the unoccupied excited orbital (LUMO) then electrons will be excited
How electrons return to grounded state Energy transfer (resonance to nearby pigments until a RC), photooxidation (being transferred to an unexcited molecule, redox potential), or quenching (emitting energy as heat or light)
Roles of photosystems in photosynthesis PSII oxidises water to liberate electrons & protons (for a proton gradient) to send to PSI to reduce NADP+ to NADPH so that its biosynthetic reducing power can be used on carbon dioxide
Role of plastoquinone (PQ) Similar to ubiquinone, lipid soluble ketone in thylakoid membranes. Electrons & protons from PSII reduce it to 2' alcohol plastoquinol (PQH2). It reduces a cytochrome complex & releases the protons to form the gradient
Oxygen Evolving Complex (OEC) Metallic Mn4CaO5 complex with protein ligands. Is oxidised once for every photon absorbed, reaching an oxidation state of +4 for 4 photons. It then reacts with 2H2O to oxidise it & evolve oxygen
Processes in PSII PQH2 docks at cytochrome b6f, electrons flow through a Q cycle to be accepted at plastocyanin. Moves to the P700 RC using internal carriers to ferrodoxin to reduce NADP+ or cyclic photophosphorylation
Plastocyanin (Pc) & ferredoxin Plastocyanin: Small mobile water soluble Cu based protein moving freely through the lumen to carry one electron at a time Ferredoxin: Small Fe-S based stroma protein that decides the fate of the electron
Q cycle PQH2 releases 2 electrons & protons, high-potential path goes Fe-S Reiske protein complex to cytochrome f to Pc to PSI while the other low potential path goes through cytochrome bI & bh to reduce a semiquinone to PQH2. This happens twice
How NADP+ is reduced Ferredoxin-NADP+ reductase is a flavoprotein (contains prosthetic FAD) which accepts a proton & 2 electrons from 2 ferredoxins to synthesise a hydride ion to react with NADP+
How photon absorption translates to ATP yield Light Dependent Reaction causes buildup of protons on thylakoid lumen so chemiosmosis can happen. For every noncyclic obtained photon, 1 ATP is produced. At saturating light intensity, photon gradient is pH 3.5
Adaptations to overcome low efficiency of Rubisco Rubisco catalyses 3 reactions/second which is too slow to be compatible with plant life. Leaves have adapted to have so many that 50% of their proteins are Rubisco
How Rubisco is inhibited in the chloroplast 2-carboxyarabinitol resembles a Calvin cycle intermediate & binds to Ruisco at night time to inactivate it, it also binds too strongly to sugar phosphate. Rubisco activase uses ATP & light to eject inhibitors
Created by: Beech47
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