PHOTOSYNTHESIS. THE LIGHT REACTIONS. The Light Reactions. Begin when photons strike the photosynthetic membrane. The process can be divided into three parts. Photoexcitation: absorption of a photon by an electron of chlorophyll
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PHOTOSYNTHESIS THE LIGHT REACTIONS
The Light Reactions • Begin when photons strike the photosynthetic membrane. The process can be divided into three parts. • Photoexcitation: absorption of a photon by an electron of chlorophyll • Electron transport: transfer of the excited electron through a series of membrane-bound electron carriers, resulting in the pumping of proton through the photosynthetic membrane, which creates a H+ reservoir and eventually reduces an electron acceptor • Chemiosmosis: the movement of protons through ATPase complexes to drive the phosphorylation of ADP to ATP
Photoexcitation • We recall that in an atom electrons want to occupy the lowest energy level, or its ground state. • When it gains energy and rises to a higher energy level, excitation takes place. • When it returns to its original level, heat and light (photon) are emitted. fluorescence
Photoexcitation of Chlorophyll • Chlorophyll in isolation displays these properties, but when it is associated with the photosynthetic membrane, the excited electron is immediately captured by the primary electron acceptor.
Primary Electron Acceptor • Excited electron is captured by the primary electron acceptor. • Redox reaction: chlorophyll is _______________ and primary acceptor is __________________.
Photosystems • Light is absorbed by chlorophyll or accessory pigment molecules that are associated with proteins in clusters.
Photosystems: Antenna Complex and Reaction Centres • Is composed of a number of chlorophyll molecules and accessory pigments set in a protein matrix in the thylakoid membrane. • The photon energy of the ant. pigment molecules transfer from pigment to pigment (resonance) until it reaches a chlorophyll a molecule in an area called the reaction centre. • The excited electron of the chlorophyll a is captured by the primary electron acceptor.
Photosystem I and II • Both are embedded in the thylakoid membranes. • Both contain the exact same chlorphylla molecules structurally, but PH. I has chlorophyll P700 and PH. II has chlorophyll P680. Why? • They differ in the wavelengths they best absorb, 700 nm and 680 nm respectively. • It’s caused by the different proteins associated with chlorophyll a in each photosystem.
Noncyclic Electron Flow & Chemiosmosis • The process in which photon-energized electrons flow from water to NADP+ through electron transport chains in thylakoid membranes, producing NADPH by reduction and ATP by chemiosmosis.
Overall... • Photon strikes photosystem II • Electron of chlorophyll P680 is excited • Electron captured by primary electron acceptor: pheophytin • Series of redox reactions • Electron transferred to plastoquinone, PQ. • Z protein, associated with PSII • Splits water into oxygen, H+, and e- • two of these e- is used to replace the missing electrons in _________________________. • H+ remaining in the thylakoid lumen • Oxygen leaves the cell
initial e-, now in _____, goes through an electron transport chain similar to that in ________________________. • THIS PROCESS OCCURS TWICE: ____ e- pass through the ETC.
The e- that leave PSII pass through the Q cycle • This causes protons to be transported from the stroma INTO the thylakoid lumen. • 4 H+ for each pair of electrons • Difference from cellular respiration? ________________________________________________ • CREATES A H+ GRADIENT FOR CHEMIOSMOSIS.
The two e- move through plastocyanin, Pc and other components of the ETC until they reach PSI. • PSI also continually undergoes the same electron excitation process (struck by photons) as PSII (therefore, looses 2 electrons) • The two e- originating from PSII replace the displaced e- in PSI. • Electrons from PSI pass through another ETC containing an iron-containing protein called ferredoxin (Fd). • Move to the NADP reductase that uses the two electrons and H+ ions from stroma: • NADP+ + 2e- + H+ NADPH
What about the electrochemical gradient produced by the Z protein? • Remember: protons are in the thylakoid lumen • H+ moves through the ATPase from lumen to the stroma ATP is formed! • Ratio: four H+ per ATP. • PHOTOPHOSPHORYLATION: light-dependant formation of ATP by chemiosmosis in photosynthesis.
Cyclic Electron Flow • In some cases, excited electrons take a cyclic pathway • Uses PSI only • Electron is passed to Fd Q cycle cytochrome chain (b6-f complex) back to chlorophyll P700. • Generates an H+ gradient for chemiosmotic ATP synthesis • Does not release electrons to generate NADPH
Phew! Light Reactions are Complex! • The overall goal : • Energy of light is transferred to ATP and NADPH. • Both of these substances play a critical role in carbon fixation, the next step.
Seatwork/Homework • Page 166 #1, 2, 3, 4, 5, 6, 7.