THE LIGHT REACTIONS

 Begin when photons strike the photosynthetic membrane. The process can be divided into three parts. 1) Photoexcitation: absorption of a photon by an electron of chlorophyll 2) 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 3) Chemiosmosis: the movement of protons through ATPase complexes to drive the phosphorylation of ADP to ATP

 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

 Chlorophyll in isolation, when bombarded with white light, emits flourescence: due to the when electrons fall from higher energy to lower energy.  but when it is associated with the photosynthetic membrane, the excited electron is immediately captured by the primary electron acceptor.

 Excited electron is captured by the primary electron acceptor.  Redox reaction: chlorophyll is _______________ and primary acceptor is __________________.

 Light is absorbed by chlorophyll or accessory pigment molecules that are associated with proteins in clusters.

 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.

 Both are embedded in the thylakoid membranes.  Both contain the exact same chlorphyll a 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.

 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.

 Photon strikes photosystem II ◦ Electron of chlorophyll P680 are 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 2e- 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, loses 2 electrons) ◦ The 2e- 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.

 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

 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.

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