Photosynthesis The Light Dependent Reactions Formula Chlorophyll 6

Photosynthesis The Light Dependent Reactions

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

§ There are 3 stages to Photosynthesis: Stage 1: Photoexcitation Capture of light energy Stage 2: ETC & Chemiosmosis Energy is used to make ATP and reduced NADP+ Stage 3: The Calvin Cycle 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 (Stage 1) 2. ELECTRON TRANSPORT 2) 3. CHEMIOSMOSIS (Stage

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

Excitation § 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.

§ 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 accessory 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 (P 700): which has a chlorophyll a in the reaction centre which absorbs wavelengths of 700 nm. § Photosystem II (P 680): which has a chlorophyll a in the reaction centre which absorbs wavelengths of 680 nm.

2. Electron Transport… § Is the transfer of the excited electron through a series of membrane-bound proteins and electron carriers – an electron transport chain! § The proteins and electron carriers are placed in order of increasing electronegativity. § The movement of the electrons through the ETC are redox reactions.

§ As the pair of electrons move through the ETC, energy is released as they move from molecule to molecule. § The energy released is used to pump a proton across the thylakoid membrane (from the stroma of the chloroplast into the thylakoid lumen), creating a H+ reservoir and reducing an electron acceptor.

Photosystem II

Photosystem I § The electrons from photosystem I move from: § Photosystem I ferredoxin (Fd) NADP recductase

§ NADP reductase reduces the co-enzyme NADP+ (in the stroma) by giving it the 2 electrons from the ETC and a H+ (from the stroma) to produce the reduced form NADPH From the stroma From the ETC From the stroma § NADP+ + 2 é + H+ Oxidized form NADPH Reduced form

§ Note: photosystem I has lost its electrons and cannot be excited again until they are replenished

Photosystem II § The electrons from photosystem II move from: § Photosystem II plastoquinone (PQ) the b 6 –f complex plastocyanin photosystem I

§ So the electrons from photosystem II end up replacing the electrons lost from the chlorophyll a of photosystem I! § Now, that they the electrons in photosystem I have been replaced, it can be excited again.

§ This means that eventually these electrons in photosystem II will end up in NADPH § Note: Photosystem II has lost electrons and cannot be excited again until they are replenished

Photolysis § Photosystem II replenishes its electrons by splitting water with a Z protein associated with the thylakoid membrane. § This is known as “Photolysis” because light was required to break up the H 2 O § Hydrogen ions and oxygen are released into the thylakoid compartment. This is where the oxygen gas generated by photosynthesis comes from.

H 2 O ½ O 2 + + 2 H + 2é § Hydrogen ions and oxygen are released into the thylakoid compartment. § This is where the oxygen gas generated by photosynthesis comes from.

Oxygen Pollution! § (Oxygen will diffuse out of the thylakoid and chloroplast and be used for cellular respiration in the mitochondria OR will diffuse out of the cell and out of the plant into the atmosphere through the stomata)

H 2 O ½ O 2 + 2 H+ + 2é § The electrons are used to replenish photosystem II. § The protons (H+) will be used to drive Chemiosmosis.

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 (P 700). § In this pathway, the electron released from photosystem I is passed to ferredoxin, and the goes to the Q cycle and back to P 700.

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.

Think About This § What happens when plants do not receive enough water? § What happens when plants do not receive enough sunlight?