PHOTOSYNTHESISTHE 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

The Light Reactions

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

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.

Photoexcitation of Chlorophyll

Excited electron is captured by the primary electron acceptor.

Redox reaction: chlorophyll is _______________ and primary acceptor is __________________.

Primary Electron Acceptor

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.

Recap...

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.

Photosystem I and II

Photosystems & Light

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.

Noncyclic Electron Flow & 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: ___________________________

Overall (copy down what is on the board!)...

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.

Energy Changes in E- in noncyclic 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

Cyclic Electron Flow

Energy Changes in E- in cyclic electron flow

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.

Phew! Light Reactions are Complex!

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