Photosynthesis powers life on Earth. Light reactions, occurring in thylakoid membranes, kick off this process. Two photosystems work together, capturing light energy and converting it into chemical energy.
Electrons flow through these systems, driving ATP production. Meanwhile, NADPH is formed, storing energy for later use. Together, ATP and NADPH fuel the Calvin cycle, turning CO2 into sugar.
Photosystems and Light Harvesting
Structure and Components of Photosystems
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- Photosystem I contains P700 reaction center absorbs light at 700 nm wavelength
- Photosystem II houses P680 reaction center captures light at 680 nm wavelength
- Chlorophyll serves as primary pigment in photosystems absorbs red and blue light reflects green
- Light-harvesting complex surrounds reaction centers consists of proteins and accessory pigments (carotenoids, phycobilins)
- Reaction center acts as the core of photosystems where light energy converts to chemical energy
- Z-scheme illustrates electron flow between photosystems represents overall process of light reactions
Function and Interaction of Photosystems
- Photosystem II initiates light reactions by splitting water molecules
- Electrons from PSII travel through electron transport chain to Photosystem I
- PSI further energizes electrons using additional light energy
- Light-harvesting complex funnels light energy to reaction centers increases efficiency of light capture
- Chlorophyll molecules in reaction centers become excited by absorbed light energy
- Z-scheme demonstrates how electrons move from water to NADP+ through both photosystems
Electron Transport and Energy Production
Electron Flow and ATP Synthesis
- Electron transport chain consists of protein complexes (cytochrome b6f complex, plastoquinone, plastocyanin)
- Electrons flow through ETC from PSII to PSI generating proton gradient
- ATP synthase utilizes proton gradient to produce ATP through rotational catalysis
- Chemiosmosis drives ATP production by harnessing energy from proton concentration difference
NADPH Production and Energy Storage
- NADP+ receives energized electrons from PSI forming NADPH
- NADPH serves as reducing agent for carbon fixation in Calvin cycle
- Ferredoxin-NADP+ reductase catalyzes final step of NADPH production
- ATP and NADPH produced during light reactions power dark reactions of photosynthesis
Thylakoid Membrane and Photolysis
Thylakoid Membrane Structure and Function
- Thylakoid membrane forms flattened sacs within chloroplasts
- Membrane contains integral proteins including photosystems ATP synthase and electron carriers
- Grana consist of stacked thylakoids increase surface area for light absorption
- Stroma lamellae connect grana stacks allow for efficient energy transfer
Photolysis and Oxygen Evolution
- Photolysis occurs in oxygen-evolving complex of PSII
- Process splits water molecules into protons electrons and oxygen
- Manganese cluster in OEC catalyzes water-splitting reaction
- Oxygen released as byproduct of photolysis essential for aerobic life
- Protons from photolysis contribute to proton gradient used in ATP synthesis
- Electrons from water splitting replace those lost by P680 in PSII reaction center