Photosynthesis The Light Dependent Reactions

Formula 6 CO H 2 O + Light Energy [CH 2 O] + 6O 2 Chlorophyll

 There are 3 stages to Photosynthesis: Stage 1: Capture of light energy energy Stage 2: Energy is used to make ATP and make ATP and reduced NADP + reduced NADP + Stage 3: Carbon Fixation LIGHT REACTIONS -Take place on the thylakoid membrane -Takes place in the stroma

The Light Reactions  Begin when photons strike a photosynthetic membrane.  Can be divided into three parts: 1.PHOTOEXCITATION 2.ELECTRON TRANSPORT 3.CHEMIOSMOSIS

1. Photoexcitation …  Is the absorption of a photon by an electron of chlorophyll.  Before a photon of light strikes a chlorophyll molecule, the chlorophyll electrons are at the lowest possible energy level – the ground state

 When the photon is absorbed by a chlorophyll electron, the electron gains energy and jumps to a higher energy level. This process is called EXCITATION. Excitation

 The excited electron is unstable and will return back to its ground state.  But it has to release the energy it absorbed from the photon.  The energy will be released in the form of heat and light (photons).  This rapid loss of energy (in the form of light) is called FLUORESCENCE.

 Like other pigments, chlorophyll emits a photon of light when one of its electrons return to its ground states.

 However, this only happens when the chlorophyll molecule is separated from the photosynthetic membrane in which it is normally embedded in.  Most chlorophyll molecules do not fluoresce when associated with a photosynthetic membrane because the excited electron is captured by a special primary electron acceptor molecule.

Photosystems  In a functioning chloroplast, light is NOT absorbed by independent pigment molecules.  Light is absorbed by chlorophyll or accessory pigment molecules that are associated with proteins in clusters called photosystems.

PHOTOSYSTEM

 A photosystem consists of several pigment molecules (chlorophylls and acessory pigments) and a chlorophyll a molecule embedded in the thylakoid membrane.

 The pigments absorbs photons and transfers the energy from pigment to pigment until it reaches a chlorophyll a molecule.

 An electron in this chlorophyll a absorbs the energy, becomes excited, and jumps to a higher energy level.  But instead of transferring the energy to another pigment, the excited electron is transferred to the primary electron acceptor.

 This is a redox reaction.  Chlorophyll is oxidized (it loses an electron)  The primary acceptor is reduced (it gains an electron). * Independent chlorophyll molecules fluoresce because there is not primary electron acceptor to receive the excited electron.

 The primary electron acceptor then passes the electron off into the ETC chain embedded in the thylakoid membrane

 There are 2 types of photosystems in the thylakoid membrane.  Photosystem I (P700): which has a chlorophyll a in the reaction centre which absorbs wavelengths of 700nm.  Photosystem II (P680): which has a chlorophyll a in the reaction centre which absorbs wavelengths of 680nm.

2. Electron Transport…  Is the transfer of the excited electron through a series of membrane-bound electron carriers, resulting in the pumping of a proton through the photosynthetic membrane, creating a H + reservoir and reducing an electron acceptor.  This a noncyclic electron flow.

Photosystem II Photosystem I

 Energized electrons from photosystem I are passed down an electron transport chain containing the protein ferredoxin (Fd) and added to NADP+ to form NADPH. electron transport chainelectron transport chain

 Meanwhile, energized electrons from photosystem II are captured by a primary electron acceptor called plastoquinone (Q) and are passed through another electron transport chain.

 Their energy is used to pump hydrogen ions (H+) from the stroma into the thylakoid compartment, creating a concentration gradient. hydrogen ionsstroma thylakoidhydrogen ionsstroma thylakoid  Electrons leaving this electron transport chain enter photosystem I, replenishing its lost electrons.

 Photosystem II replenishes its electrons by splitting water with a Z protein associated with the thylakoid membrane.  Hydrogen ions and oxygen are released into the thylakoid compartment. This is where the oxygen gas generated by photosynthesis comes from. photosynthesis

 The electrons are used to replenish photosystem II.  The protons drive Chemiosmosis.  The oxygen is released into the atmosphere.

Noncyclic Electron Flow  The process is non-cyclic because once an electron is lost by a reaction centre chlorophyll within a photosystem, it does not return to that system.  The electron ends up in NADPH.

 NOTE: 2 electrons are required to reduce NADP+ to NADPH.  (A pair of electrons will move through the ETC chain together)

Noncyclic Electron Flow

Cyclic Electron Flow  Occasionally, excited electrons can take a cyclic pathway called cyclic electron flow that only uses photosystem I (P700).  In this pathway, the electron released from photosystem I is passed to ferredoxin, and the goes to the Q cycle and back to P700.

Cyclic Electron Flow

 The cyclic pathway generates a proton gradient for chemiosmotic ATP synthesis, but does not release electrons to generate NADPH.  NADPH is required for carbon fixation.

3. Chemiosmosis …  Is the movement of protons through ATPase complexes to drive the phosphorylation of ADP to ATP.  The protons that accumulate in the thylakoid space contribute to an electrochemical gradient that drives this process.  Since light is required to create the proton gradient, the process is called photophosphorylation.

Chemiosmosis

Goal of Light Dependent Reactions  To transfer the energy of light to ATP and NADPH.  Both of these substance will play a critical role in the next stage of photosynthesis: CARBON FIXATION.