Photosynthesis in plants Light energy is used to transform carbon dioxide and water to energy rich food molecules composed of glucose monomers There are 2 stages in this process

Photosynthesis: The Details Photosynthesis is divided into 2 sequential processes: the light reactions (stages 1 & 2) and carbon fixation (stage 3) The Light Reactions: Noncyclic Electron Flow -Convert solar energy to chemical energy -The process is divided into 3 parts: 1.Photoexcitation 2.Electron Transport 3.Chemiosmosis

1. Photoexcitation Electrons in chlorophyll molecules are initially at ground state When a molecule absorbs a photon, one of the electrons is elevated to an orbital where it has more potential energy

Photoexcitation In the photosynthetic membrane, a nearby molecule referred to as a Primary Electron Acceptor traps a high energy electron that has absorbed a photon This a redox reaction In chloroplasts – independent pigments do not absorb light, instead clusters of chlorophyll molecules and accessory pigments associated with proteins called photosystems absorb light

The Light Reactions Photosystems are embedded in the thylakoid membrane. They contain chlorophyll and accessory pigments that are associated with proteins. A photosystem consists of an antenna complex and a reaction centre.

Photosystems Photosystems I and II Of the many chlorophyll a molecules only one can trigger the light reactions by donating its excited electron to a primary electron acceptor The other chlorophyll a, chlorophyll b and carotenoid molecules function collaboratively as a light-gathering antenna that absorbs photons and passes the energy from pigment to pigment until it reaches the one chlorophyll a molecule in an area called the reaction centre

Photosystem I and II Photosystem I contains a specialized chlorophyll a molecule known as P700 since it best absorbs light with an average wavelength of 700 nm Photosystem II contains a specialized chlorophyll a molecule known as P680 since it best absorbs light with an average wavelength of 680 nm P700 and P680 chlorophyll a molecules are identical – they simply absorb at slightly different wavelengths because of the effects of the proteins they are associated with in the reaction centre

The Light Reactions Photosystem II (P680) Two photons strike photosystem II and excite 2 electrons from chlorophyll P680. The excited electrons are captured by a primary electron acceptor and are then transferred to plastoquinone (PQ) and the ETC.

The Light Reactions Photosystem II (P680) In the ETC, the 2 electrons pass through a proton pump (Q cycle). The Q cycle transports 4 protons from the stroma into the thylakoid lumen to create a proton gradient.

The Light Reactions Photosystem II (P680) The electrochemical gradient drives the photophosphorylation of ADP to ATP. 1 ATP forms for every 4 protons that pass through ATPase from the thylakoid lumen into the stroma.

The Light Reactions Photosystem II (P680) A Z protein splits water into 2 protons, 2 electrons and 1 oxygen atom. –The electrons replace those lost from chlorophyll P680. –The protons remain in the thylakoid space to add to the proton gradient. –Oxygen leaves as a byproduct.

Noncyclic Electron Transport and Chemiosmosis Photon excites 2 electrons of chlorophyll P680 Through series of redox reactions, electron transferred to PQ and then to ETC Z protein splits water and replaces missing electrons in P680 Electrons flow down an ETC to P700 providing energy to make ATP  since light is required for the establishment of proton gradient, this process is called photophosphorylation Excited electrons are stored as high energy-electrons in NADPH photosynthesis/movie.htm s/photosystemII/movie.htm

The Light Reactions Photosystem I (P700) Two photons strike photosystem I and excite 2 electrons from chlorophyll P700 (replaced by electrons from P680). These electrons pass through another ETC. The enzyme NADP reductase uses the 2 electrons and a proton from the stroma to reduce 1 NADP + to 1 NADPH.

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Cyclic flow Photosystem I only Electron excited and trapped by primary electron acceptor Electron passed to Fd Passes through Q cycle, b6-f complex and back to chlorophyll P700 Generates proton gradient for ATP synthesis, does NOT release electrons to generate NADPH Without NADPH, carbon fixation cannot occur hill.com/olcweb/cgi/pluginpop.cgi?it=swf:: 535::535::/sites/dl/free/ / /bio12.swf::Cyclic%20and%20Noncycli c%20Photophosphorylation