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Photosynthesis in plants

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.

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Photosynthesis in plants

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  1. 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

  2. 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: • Photoexcitation • Electron Transport • Chemiosmosis

  3. 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

  4. 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

  5. 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.

  6. 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

  7. 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

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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 http://vcell.ndsu.edu/animations/photosystemII/movie.htm http://vcell.ndsu.edu/animations/photosynthesis/movie.htm

  13. 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.

  14. http://highered.mcgraw-hill.com/sites/0070960526/student_view0/chapter5/animation_quiz_1.htmlhttp://highered.mcgraw-hill.com/sites/0070960526/student_view0/chapter5/animation_quiz_1.html

  15. 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 http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio12.swf::Cyclic%20and%20Noncyclic%20Photophosphorylation

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