Photosynthesis The Light Reactions

6 CO2 + 6 H2O + Light Energy [CH2O] + 6O2 Formula 6 CO2 + 6 H2O + Light Energy [CH2O] + 6O2 Chlorophyll

Take place on the thylakoid membrane There are 3 stages to Photosynthesis: Stage 1: Capture of light energy Stage 2: Energy is used to make ATP and 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: PHOTOEXCITATION ELECTRON TRANSPORT 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

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 an antenna complex and a reaction centre. The antenna complex is a web of chlorophyll and accessory pigment molecules embedded in the thylakoid membrane.

An antenna pigment absorbs a photon and transfers the energy from pigment to pigment until it reaches a chlorophyll a molecule. This area is called the reaction center. The reaction centre is always 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 (red arrow)

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.

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 (red light) Photosystem II (P680): which has a chlorophyll a in the reaction centre which absorbs wavelengths of 680nm (red light) Their absorbtion spectra are slightly different because of the effects of the proteins that they are associated with in the reaction centre

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 I Photosystem II

Energized electrons from photosystem II are captured by a primary electron acceptor called plastoquinone (Q) and are passed through an electron transport chain.

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

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

Z Protein 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.

Components of Water The electrons from the water are used to replenish photosystem II. The protons from the water drive Chemiosmosis. The oxygen from water 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. The entire process happens twice for each NADP+ reduced to NADPH

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, then back to the b6-f complex 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 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.