1. Prof. Dr. Samih Tamimi Bio 304101 Photosynthesis Biology 304101

2. Prof. Dr. Samih Tamimi Bio 304101 The Two Stages of Photosynthesis: A Preview Photosynthesis consists of two processes The Light reactions NEEDS LIGHT Light Dependent Reactions The Calvin cycle A.k.a- Dark Reactions or Light Independent Reactions DOES NOT NEED LIGHT

3. Prof. Dr. Samih Tamimi Bio 304101 The Light Reactions  Occur in the grana (& thylakoids)  Convert solar energy to chemical energy  Chlorophyll absorbs solar energy  Split water  release oxygen gas (a by-product)  produce ATP (using chemiosmosis)  Forms NADPH from NADP+ (an e- acceptor)  Temporarily stores high energy e-’s  “Electron shuttle bus”

4. Prof. Dr. Samih Tamimi Bio 304101 The Calvin Cycle  Occurs in the stroma  Forms SUGAR from carbon dioxide  Carbon fixation occurs (CO 2  fixed carbon)  using ATP for energy and NADPH for reducing power (adding e-s to fixed carbon)  Fixed carbon  carbohydrate

5. Prof. Dr. Samih Tamimi Bio 304101  An overview of photosynthesis H2OH2O CO 2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH 2 O] (sugar) NADPH NADP  ADP + P O2O2 ATP G3P

6. Prof. Dr. Samih Tamimi Bio 304101 Light Reactions (in detail)  The light reactions convert solar energy to the chemical energy of ATP and NADPH

7. Prof. Dr. Samih Tamimi Bio 304101 RECALL: Color we SEE = color most reflected by pigment; other colors (wavelengths) are absorbed  BLACK  all colors are reflected Light Reflected Light Chloroplast Absorbed light Granum Transmitted light

8. Prof. Dr. Samih Tamimi Bio 304101 Spectrophotometer  Machine that sends light through pigments  measures fraction of light transmitted and absorbed at each wavelength  Produces an absorption spectrum

9. Prof. Dr. Samih Tamimi Bio 304101  An absorption spectrum  graph plotting light absorption versus wavelength White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue light 1 2 3 4 0 100 0

10. Prof. Dr. Samih Tamimi Bio 304101  The absorption spectra of three types of pigments in chloroplasts

11. Prof. Dr. Samih Tamimi Bio 304101  The action spectrum of a pigment  Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis Rate of photosynthesis (measured by O 2 release) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b & carotenoids.

12. Prof. Dr. Samih Tamimi Bio 304101  Chlorophyll a  The main photosynthetic pigment (primary pigment)  Accessory Pigments  Absorb different wavelengths of light  pass energy to chlorophyll a

13. Prof. Dr. Samih Tamimi Bio 304101 Excitation of Chlorophyll by Light  When a pigment absorbs light  It goes from a ground state (stable) to an excited state (unstable)

14. Prof. Dr. Samih Tamimi Bio 304101 Photosystems  Reaction centers used in Light Reactions  Made of light harvesting proteins (complexes)  Funnel (move) energy of photons (light pieces) to the middle of reaction center  INSIDE thylakoid membrane  2 DIFFERENT CENTERS  Photosystem II  Photosystem I

15. Prof. Dr. Samih Tamimi Bio 304101 Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) e–e– When a reaction-center “special” chlorophyll a molecule absorbs energy An electron gets bumped up to a primary electron acceptor http://www.bio.miami. edu/~cmallery/255/2 55phts/photosynthesi s.swf http://www.bio.miami. edu/~cmallery/255/2 55phts/photosynthesi s.swf

16. Prof. Dr. Samih Tamimi Bio 304101 2 Different Photosystems  BOTH found in thylakoid membrane  2 types  photosystems II (PII)  Uses chlorophyll a called P680  1 ST photosystem in membrane  photosystems I (PI)  Uses chlorophyll a called P700  2 ND photosystem in membrane

17. Prof. Dr. Samih Tamimi Bio 304101 2 types of e- flow 1. Non-cyclic photophosphorylation 1. Cyclic photophosphorylation

18. Prof. Dr. Samih Tamimi Bio 304101 Noncyclic Electron Flow Steps 1. PII  excited e- to primary e- acceptor 2. Photolysis- water splits by enzyme  e-s are replaced from lost chl a P680 H 2 0  2 H+ + 2e- + ½ O 2 (2 O’s combine and O 2 is released) 3. Electron Transport Chain  proteins in thylakoid membrane pass e-s (become reduced)  Flow of e-’s is exergonic  releases energy to make ATP  Proteins used = Cytochromes, PC, and PQ complexes

19. Prof. Dr. Samih Tamimi Bio 304101 4. Chemiosomosis – the process that forms ATP during light reactions  Protons (H+) are pumped ACTIVELY into thylakoid space (lumen) from stroma durin electron transport  Protons (H+) from split water build up in thylakoid space (lumen)  MORE acidic  H+’s then DIFFUSE down ATP synthase channels in stroma

20. Prof. Dr. Samih Tamimi Bio 304101  Non cyclic light reactions & Chemiosmosis

21. Prof. Dr. Samih Tamimi Bio 304101 Cyclic Electron Flow  Under certain conditions  Photoexcited electrons take an alternative path (shorter pathway)  Why use this pathway?  Sugar production (Calvin Cycle) uses a lot more ATP than NADPH  Sometimes, autotrophs run low on ATP  needs to replenish ATP levels and uses cycle e- flow

22. Prof. Dr. Samih Tamimi Bio 304101 Why is it “cyclic”?  This process is cyclic since electrons return to the reaction center.  An electron donor (i.e.- water) is NOT required and oxygen is NOT produced.

23. Prof. Dr. Samih Tamimi Bio 304101 Cyclic Electron Flow Steps 1. Photon hits PS1 2. E-s enter PSI P700  a primary e- acceptor 3. E-s travel BACK to P700 through FD cytochrome complex and PC ATP is produced using ATP synthase and H+ diffusion…NO NADPH!!!

24. Prof. Dr. Samih Tamimi Bio 304101  In cyclic electron flow  Only photosystem I is used  Only ATP is produced NO NADPH

25. Prof. Dr. Samih Tamimi Bio 304101 NON CYCLIC CYCLIC

26. Prof. Dr. Samih Tamimi Bio 304101 The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar  The Calvin cycle  Occurs in the stroma

27. Prof. Dr. Samih Tamimi Bio 304101 Recall…

28. Prof. Dr. Samih Tamimi Bio 304101 ?H?H A L G E + F C D M B I + J K

29. Prof. Dr. Samih Tamimi Bio 304101 ?H?H A L G E + F C D M B I + J K

30. Prof. Dr. Samih Tamimi Bio 304101 The Calvin cycle has three phases 1. Carbon fixation 2. Reduction 3. Regeneration of the CO 2 acceptor (RuBP)

31. Prof. Dr. Samih Tamimi Bio 304101 The Calvin Cycle Steps CARBON FIXATION 1. CO2 enters cycle and attached to a 5- carbon sugar called ribulose bisphosphate (RuBP) forming 6-C molecule (unstable)  Enzyme RUBISCO catalyzes reaction 2. Unstable 6-C molecule immediately breaks down to two 3-C molecules called 3-phosphoglycerate (3-PGA)

32. Prof. Dr. Samih Tamimi Bio 304101 REDUCTION 3. Each 3-phosphoglycerate (3-PGA) gets an additional phosphate from ATP (from LIGHT RXN)  becomes 1,3 bis phosphoglycerate 4. NADPH reduces 1,3 bisphosphoglycerate to Glyceraldehyde-3-phosphate (G3P) G3P = a sugar that stores potential energy Every 3 CO 2  yields 6 G3P’s BUT only 1 can be counted in net gain for carbohydrate

33. Prof. Dr. Samih Tamimi Bio 304101 REGENERATION OF CO2 ACCEPTOR (RuBP) 5. The C- skeletons of 5 G3P molecules are rearranged into 3 RuBP molecules ATP is used !!!!

34. Prof. Dr. Samih Tamimi Bio 304101 The Calvin cycle Phase 1: Carbon fixation Phase 2: Reduction Phase 3: Regeneration of the CO 2 acceptor (RuBP) output NOTE: MORE ATP is needed than NADPH!!

35. Prof. Dr. Samih Tamimi Bio 304101 Calvin Cycle Overview  For 1 G3P molecule made  9 ATP molecules are used  6 NADPH molecules are used  G3P (starting material to make other organic molecules (glucose, starch, etc.)