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Photosynthesis – the Details

Photosynthesis – the Details. Chapter 3.3. 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

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Photosynthesis – the Details

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  1. Photosynthesis – the Details Chapter 3.3

  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. Light Reactions • Photoexcitation • the absorption of a photon by an electron of chlorophyll • Electron transport • pumping protons through the photosynthetic membrane creates a proton reservoir; reduces electron acceptor • Chemiosmosis • the movement of protons through ATPase complexes to drive phosphorylation of ADP to ATP

  4. 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

  5. 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

  6. 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.

  7. Photosystems I & 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

  8. 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

  9. H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ to theCalvin Cycle ADP + Pi H+ ETC of Photosynthesis 3 1 4 ATP

  10. H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ to theCalvin Cycle ADP + Pi H+ ETC of Photosynthesis 3 1 2 4 ATP

  11. to theCalvin Cycle ETC of Photosynthesis electron carrier 6 5 $$ in the bank…reducing power

  12. ETC of Photosynthesis split H2O

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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

  18. 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.

  19. Cyclic flow • Photosystem I only • Electron excited and trapped by primary electron acceptor • Electron passed to Fd (ferrodoxin) • 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

  20. Noncyclic Photophosphorylation • Light reactions elevate electrons in 2 steps (PS II & PS I) • PS II generates energy as ATP • PS I generates reducing power as NADPH

  21. Cyclic photophosphorylation • If PS I can’t pass electron to NADP…it cycles back to PS II & makes more ATP, but noNADPH • coordinates light reactions to Calvin cycle • Calvin cycle uses more ATP than NADPH X

  22. Photophosphorylation cyclic photophosphorylation noncyclic photophosphorylation

  23. The Calvin Cycle • Occurs in the stroma of chloroplasts. • Cyclical reactions similar to the Krebs Cycle. • Divided into three phases: • Carbon Fixation • Reduction Reactions • Regeneration of RuBP

  24. The Calvin Cycle Phase 1: Carbon Fixation • 3 CO2 are added to RuBP to form 3 unstable 6-carbon intermediates. • The intermediates split into six 3-carbon molecules called PGA. • These reactions are catalyzed by rubisco.

  25. The Calvin Cycle Phase 2: Reduction Reactions • 6 PGAs are phosphorylated by 6 ATPs to form 6 molecules of 1, 3-BPG. • 6 NADPH molecules reduce the six 1,3-BPG to 6 G3P or PGAL. • One molecule of G3P exits the cycle as a final product.

  26. The Calvin Cycle Phase 3: Regeneration of RuBP • 3 ATP are used to rearrange the remaining 5 G3P into 3 molecules of RuBP. • The cycle continues with the RuBP fixing more CO2.

  27. To Produce One G3P… 3 RuBP + 3 CO2 + 9 ATP + 6 NADPH + 5 H2O  9 ADP + 8 Pi + 6 NADP+ + G3P + 3 RuBP

  28. 1C 3C 3C CO2 C 5C C 3 ATP C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C 3 ADP 3C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C 6C = = C C C 6 ATP 6 NADPH H H H H H H | | | | | | – – C C C 6 NADP 6 ADP Calvin cycle C C C 1. Carbon fixation 3. Regenerationof RuBP RuBP Rubisco ribulose bisphosphate starch,sucrose,cellulose& more ribulose bisphosphate carboxylase used to makeglucose glyceraldehyde-3-P PGA G3P phosphoglycerate 2. Reduction

  29. Calvin Cycle Stroma of chloroplast Carbon dioxide (CO2) Rubisco Ribulose 1,5-bisphosphate (RuBP) (5C) 3-phosphoglycerate (3C) (PGA) 6 ATP Carbon fixation 3 ADP 6 ADP Reforming RuBP 1,3-bisphosphoglycerate (3C) 3 ATP 6 NADPH 2Pi Reverse of glycolysis 6 NADP+ 6Pi Glyceraldehyde 3-phosphate (3C) Glyceraldehyde 3-phosphate (3C) (G3P) Glyceraldehyde 3-phosphate (3C) (G3P) Glucose and other sugars

  30. To G-3-P and Beyond! • Glyceraldehyde-3-P • end product of Calvin cycle • energy rich 3 carbon sugar • G-3-P= important intermediate • G-3-P  glucose   carbohydrates   lipids   amino acids   nucleic acids

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