PhotosynthesisPhotosynthesis

The Light Dependent Reactions The Light Dependent Reactions

FormulaFormula

6 CO6 CO2 2 + 6 H+ 6 H22O + Light Energy O + Light Energy [[CHCH22O] + 6OO] + 6O22

Chlorophyll

There are 3 stages to Photosynthesis:There are 3 stages to Photosynthesis:

Stage 1: Capture of light Stage 1: Capture of light energyenergy

Stage 2: Energy is used to Stage 2: Energy is used to make ATP and make ATP and

reduced NADPreduced NADP++

Stage 3: Carbon FixationStage 3: Carbon Fixation

LIGHT REACTIONS-Take place

on the thylakoid

membrane

-Takes place in the stroma

The Light ReactionsThe Light Reactions Begin when photons strike a Begin when photons strike a

photosynthetic membrane.photosynthetic membrane.

Can be divided into three parts:Can be divided into three parts:

1.1. PHOTOEXCITATIONPHOTOEXCITATION

2.2. ELECTRON TRANSPORTELECTRON TRANSPORT

3.3. CHEMIOSMOSISCHEMIOSMOSIS

1. Photoexcitation …1. Photoexcitation … Is the absorption of a photon by an Is the absorption of a photon by an

electron of chlorophyll.electron of chlorophyll.

Before a photon of light strikes a Before a photon of light strikes a chlorophyll molecule, the chlorophyll chlorophyll molecule, the chlorophyll electrons are at the lowest possible electrons are at the lowest possible energy level – the energy level – the ground stateground state

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

ExcitationExcitation

The excited electron is unstable and will The excited electron is unstable and will return back to its ground state.return back to its ground state.

But it has to release the energy it But it has to release the energy it absorbed from the photon. absorbed from the photon.

The energy will be released in the form of The energy will be released in the form of heat and light (photons).heat and light (photons).

This rapid loss of energy (in the form of This rapid loss of energy (in the form of light) is called light) is called FLUORESCENCE.FLUORESCENCE.

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

However, this only happens when the However, this only happens when the chlorophyll molecule is separated from chlorophyll molecule is separated from the photosynthetic membrane in which it the photosynthetic membrane in which it is normally embedded in.is normally embedded in.

Most chlorophyll molecules do not Most chlorophyll molecules do not fluoresce when associated with a fluoresce when associated with a photosynthetic membrane because the photosynthetic membrane because the excited electron is captured by a special excited electron is captured by a special primary electron acceptor primary electron acceptor molecule.molecule.

PhotosystemsPhotosystems In a functioning chloroplast, light is NOT In a functioning chloroplast, light is NOT

absorbed by absorbed by independentindependent pigment pigment molecules.molecules.

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

PHOTOSYSTEM

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

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

An electron in this chlorophyll An electron in this chlorophyll aa absorbs absorbs the energy, becomes excited, and jumps the energy, becomes excited, and jumps to a higher energy level.to a higher energy level.

But instead of transferring the energy to But instead of transferring the energy to another pigment, the excited electron is another pigment, the excited electron is transferred to the transferred to the primary electron primary electron acceptoracceptor..

This is a This is a redox redox reaction.reaction.

Chlorophyll is oxidized Chlorophyll is oxidized (it loses an electron)(it loses an electron)

The primary acceptor The primary acceptor is reduced (it gains an electron).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 The primary electron acceptor then passes the electron off into the ETC passes the electron off into the ETC chain embedded in the thylakoid chain embedded in the thylakoid membranemembrane

There are 2 types of photosystems in the There are 2 types of photosystems in the thylakoid membrane.thylakoid membrane.

Photosystem I (P700):Photosystem I (P700): which has a which has a chlorophyll chlorophyll aa in the reaction centre which in the reaction centre which absorbs wavelengths of 700nm.absorbs wavelengths of 700nm.

Photosystem II (P680):Photosystem II (P680): which has a which has a chlorophyll chlorophyll aa in the reaction centre which in the reaction centre which absorbs wavelengths of 680nm.absorbs wavelengths of 680nm.

2. Electron Transport…2. Electron Transport… Is the transfer of the excited electron Is the transfer of the excited electron

through a series of membrane-bound through a series of membrane-bound electron carriers, resulting in the pumping electron carriers, resulting in the pumping of a proton through the photosynthetic of a proton through the photosynthetic membrane, creating a Hmembrane, creating a H++ reservoir and reservoir and reducing an electron acceptor.reducing an electron acceptor.

This a This a noncyclic electron flownoncyclic electron flow..

Photosystem II

Photosystem I

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

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

Their energy is used to pump hydrogen Their energy is used to pump hydrogen ions (H+) from the stroma into the ions (H+) from the stroma into the thylakoid compartment, creating a thylakoid compartment, creating a concentration gradient. concentration gradient.

Electrons leaving this electron transport Electrons leaving this electron transport chain enter photosystem I, replenishing chain enter photosystem I, replenishing its lost electrons. its lost electrons.

Photosystem II replenishes its electrons Photosystem II replenishes its electrons by splitting water with a Z protein by splitting water with a Z protein associated with the thylakoid membrane.associated with the thylakoid membrane.

Hydrogen ions and oxygen are released Hydrogen ions and oxygen are released into the thylakoid compartment. This is into the thylakoid compartment. This is where the oxygen gas generated by where the oxygen gas generated by photosynthesis comes from.photosynthesis comes from.

The electrons are used to replenish The electrons are used to replenish photosystem II.photosystem II.

The protons drive Chemiosmosis.The protons drive Chemiosmosis.

The oxygen is released into the The oxygen is released into the atmosphere.atmosphere.

Noncyclic Electron FlowNoncyclic Electron Flow The process is non-cyclic because once The process is non-cyclic because once

an electron is lost by a reaction centre an electron is lost by a reaction centre chlorophyll within a photosystem, it does chlorophyll within a photosystem, it does not return to that system.not return to that system.

The electron ends up in NADPH.The electron ends up in NADPH.

NOTE: 2 electrons are required to reduce NOTE: 2 electrons are required to reduce NADP+ to NADPH.NADP+ to NADPH.

(A pair of electrons will move through the (A pair of electrons will move through the ETC chain together)ETC chain together)

Noncyclic Electron FlowNoncyclic Electron Flow

Cyclic Electron FlowCyclic Electron Flow Occasionally, excited electrons can take Occasionally, excited electrons can take

a cyclic pathway called a cyclic pathway called cyclic electron cyclic electron flow flow that only uses photosystem I (P700).that only uses photosystem I (P700).

In this pathway, the electron released In this pathway, the electron released from photosystem I is passed to from photosystem I is passed to ferredoxin, and the goes to the Q cycle ferredoxin, and the goes to the Q cycle and back to P700.and back to P700.

Cyclic Electron FlowCyclic Electron Flow

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

NADPH is required for carbon fixation.NADPH is required for carbon fixation.

3. Chemiosmosis …3. Chemiosmosis … Is the movement of protons through Is the movement of protons through

ATPase complexes to drive the ATPase complexes to drive the phosphorylation of ADP to ATP.phosphorylation of ADP to ATP.

The protons that accumulate in the The protons that accumulate in the thylakoid space contribute to an thylakoid space contribute to an electrochemical gradient that drives this electrochemical gradient that drives this process.process.

Since light is required to create the Since light is required to create the proton gradient, the process is called proton gradient, the process is called photophosphorylation.photophosphorylation.

Chemiosmosis

Goal of Light Dependent Goal of Light Dependent ReactionsReactions

To transfer the energy of light to ATP and To transfer the energy of light to ATP and NADPH.NADPH.

Both of these substance will play a Both of these substance will play a critical role in the next stage of critical role in the next stage of photosynthesis: CARBON FIXATION.photosynthesis: CARBON FIXATION.