Biochemistry-II - Photosynthesis - Quick Guides

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Study GuideBiochemistry-II–Photosynthesis1.Light Reactions of PhotosynthesisThelight reactionsare the first stage of photosynthesis. They convertlight energy into chemicalenergyin the form ofATP and NADPH, which are later used in the dark reactions (Calvin cycle) tomake carbohydrates.1.1Overview of the Light ReactionsPhotosynthesis begins when pigments in thelight-harvesting complexabsorb photons of visiblelight. When a pigment absorbs light, it becomesexcitedand transfers this energy to a specialchlorophyll molecule in thereaction center.There aretwo types of reaction centers:•Photosystem II (PSII)•Photosystem I (PSI)Together, PSI and PSII carry out the light reactions. Electron flow through these systems creates:•Aproton gradient(used to make ATP)•NADPH(used as reducing power)Unlike mitochondrial electron transport, thefinal electron acceptor is NADP⁺, not oxygen.

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Study Guide1.2Redox and Energy ConceptsThe movement of electrons during the light reactions follows changes infree energyandreductionpotential.The relationship between free energy change and reduction potential is:ΔG°′ = −nE°′This equation explains why excited chlorophyll molecules can donate electrons more easily thanunexcited ones.1.3Photosystem II (PSII)Reaction Center P680The reaction center chlorophyll of PSII is calledP680, named for its absorption of light at680 nm.•When P680 absorbs light, it becomes excited (P680*)•P680*is a strong electron donor•It transfers an electron toplastoquinone (PQ)

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Study GuideFigure 11.4Role of PlastoquinonePlastoquinone behaves similarly tocoenzyme Qin mitochondria:•It can acceptone or two electrons•For each electron accepted, it also takes up aproton (H⁺) from the stromaThis proton uptake:•Makes thestroma more basic•Contributes to theproton gradient1.5Electron Transport and Proton PumpingAfter plastoquinone:•Electrons move through thecytochrome b₆f complex•Protons are pumped into thethylakoid lumen•Electrons are transferred toplastocyanin•Finally, electrons are delivered toPhotosystem I

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Study GuideThe accumulation of protons in the thylakoid lumen drivesATP synthesisvia ATP synthase, similar tooxidative phosphorylation.1.6Replacement of Electrons in PSIIOnce P680 loses an electron, it becomesoxidizedand must be reduced before it can function again.This reduction comes fromwater.The reaction is:2 H₂O + [Manganese center]oxidized→O₂+ [Manganese center]reduced+ 4 H⁺This reaction occurs at themanganese center, a metalloprotein complex:•Water is split•Oxygen (O₂) is released•Protons are released into the thylakoid lumen•Electrons reduce oxidized P680This step is thesource of oxygenreleased during photosynthesis.1.7Photosystem I (PSI)Reaction Center P700The reaction center of PSI is calledP700, which absorbs light at700 nm.•Light excites P700 toP700*•P700* donates an electron to a series of carriers•The electron is ultimately transferred toferredoxin2 Fdreduced+ NADP⁺+ H⁺→ 2 Fdoxidized+ NADPHFormation of NADPHReduced ferredoxin transfers electrons toNADP⁺, formingNADPH:

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Study GuideNADPH provides thereducing powerrequired for carbon dioxide fixation during the Calvin cycle.Regeneration of P700After P700 donates an electron:•It becomes oxidized•It accepts an electron from thecytochrome b₆f complex•This electron originally came from PSIIOnce reduced, P700 is ready to absorb light again and repeat the cycle.Summary of the Light Reactions•Light energy excites chlorophyll in PSII and PSI•Electrons flow through an electron transport chain•Aproton gradientis formed across the thylakoid membrane•ATP is produced byphotophosphorylation•NADP⁺is reduced toNADPH•Water is split, releasingoxygenKey Takeaways•PSII generates ATP and releases oxygen•PSI produces NADPH•Both systems work together through electron flow•Light reactions convert solar energy into chemical energy2.Cyclic Electron FlowSometimes, the cell hasplenty of NADPH but not enough NADP⁺. When this happens, a problemcould occur during the light reactions. Normally,reduced ferredoxin passes its electrons to NADP⁺

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Study Guideto form NADPH. However, if most of the NADP⁺has already been converted into NADPH, there isnowhere for the electrons to go.If electrons had no alternative path, they wouldaccumulate in the photosystems. All the electroncarriers would remain in a reduced state, andphotosynthesis would come to a halt.Fortunately, this does not happen because plants have abackup pathwaycalledcyclic electronflow.2.1How Cyclic Electron Flow WorksIn cyclic electron flow:•Electrons fromreduced ferredoxinare redirected•Instead of reducing NADP⁺, the electrons are passed back to thecytochrome b₆f complex•From cytochrome b₆f, the electrons return toPhotosystem IBecause the electrons cycle back rather than leaving the system,no NADPH is producedin thispathway.2.2Importance of Cyclic Electron FlowEven though NADPH is not made, cyclic electron flow is still very important because:•Electron movement through cytochrome b₆fpumps protons into the thylakoid lumen•These protons increase theproton gradient•The proton gradient drivesATP synthesisby ATP synthaseThe ATP produced supplements the ATP made during normal (non-cyclic) electron flow involvingPhotosystem II.Key Takeaways•Cyclic electron flow occurs whenNADP⁺is limited•Electrons are recycled back to Photosystem I•ATP is produced, butNADPH and O₂are not•This pathway prevents electron backup and keeps photosynthesis running efficiently

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Study Guide3.Z-Scheme of PhotosynthesisFigure 1TheZ-schemeis a way to visualize themovement of electrons and energy changesduring thelight reactions of photosynthesis. It shows how electrons gain energy from light and then lose energyas they move through the electron transport chain.3.1Understanding the Vertical AxisIn the Z-scheme diagram, thevertical axis represents reduction potential.•Molecules placedhigher on the diagramhave amore negative reduction potential•This means they arebetter electron donors•Molecules lower on the diagram arebetter electron acceptorsThis arrangement helps explain why electrons move in a specific direction during photosynthesis.

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Study Guide3.2Electron Flow from Water to Photosystem IIThe process begins on theleft sideof the Z-scheme:•Water molecules are oxidized•Electrons are removed from water and passed to theoxidized form of P680•This reaction releases:oOxygen (O₂)oProtons (H⁺)into the thylakoid lumenThis step replaces the electrons lost by Photosystem II.3.3Excitation and Electron Transfer in Photosystem II•WhenP680 absorbs a photon, it becomes excited toP680*•P680* moves to a much higher energy level in the Z-scheme•In this excited state, P680* is astrong reducing agent•It donates an electron to thequinone–cytochrome b₆f electron transport chainAs electrons pass through this chain:•Protons are pumpedinto the thylakoid lumen•This contributes to theproton gradientneeded for ATP synthesis3.4Transfer of Electrons to Photosystem I•Electrons from the cytochrome b₆f complex are transferred toPhotosystem I•This reduces the PSI reaction center chlorophyll,P700WhenP700 absorbs light, it becomes excited toP700*, reaching a high energy level in the Z-scheme.3.5Formation of NADPH or Cyclic Electron FlowFromP700*, electrons can followtwo possible paths:

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Study Guide1. Non-cyclic Electron Flow•Electrons are transferred toNADP⁺•NADPHis formed•NADPH provides reducing power for carbon fixation in the Calvin cycle2. Cyclic Electron Flow•Electrons return to thecytochrome b₆f complex•This pathway producesATP only•No NADPH is formedSummary of the Z-Scheme•Electrons start fromwaterand end atNADP⁺•Light energy boosts electrons twice: once inPSIIand once inPSI•Electron flow creates aproton gradient•The proton gradient drivesATP synthesis•NADPH is produced for use in the dark reactionsKey Takeaways•The Z-scheme explains energy changes during the light reactions•PSII and PSI work together to move electrons uphill and downhill energetically•Light energy is converted intoATP and NADPH•Cyclic electron flow provides flexibility when ATP demand is high4.ATP Synthesis in PhotosynthesisATP synthesis during the light reactions of photosynthesis is carried out by a large protein complexcalledATP synthase. This complex is also known as thecoupling factorbecause it couples protonmovement to ATP formation.

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Study GuideFigure 14.1Structure of ATP SynthaseATP synthase has a distinctivemushroom-like shape:•Thehead (cap)of the enzyme faces thestroma•Thestalkpasses through thethylakoid membraneThis structure allows ATP synthase to use the energy stored in a proton gradient to convertADP andinorganic phosphate (Pi) into ATP.4.2Creation of the Proton GradientDuring the light reactions:•Photosystem II (PSII)andPhotosystem I (PSI)move protons into thethylakoid lumen•Water splitting at PSII adds additional protons to the lumen•Electron transport through the cytochrome b₆f complex also contributes to proton pumping

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