1. 7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH  The second stage is the Calvin cycle, which occurs in the stroma of the chloroplast –It is a cyclic series of reactions that builds sugar molecules from CO 2 and the products of the light reactions –During the Calvin cycle, CO 2 is incorporated into organic compounds, a process called carbon fixation Copyright © 2009 Pearson Education, Inc.

2. 7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH  NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle –ATP from the light reactions provides chemical energy for the Calvin cycle –The Calvin cycle is often called the dark (or light- independent) reactions Copyright © 2009 Pearson Education, Inc.

3.  H2OH2O NADP + ADP P LIGHT REACTIONS (in thylakoids) Light Chloroplast

4. H2OH2O ADP P LIGHT REACTIONS (in thylakoids) Light Chloroplast NADPH ATP O2O2  NADP +

5. H2OH2O ADP P LIGHT REACTIONS (in thylakoids) Light Chloroplast NADPH ATP O2O2 CALVIN CYCLE (in stroma) Sugar CO 2  NADP +

6. THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY Copyright © 2009 Pearson Education, Inc.

7. 7.6 Visible radiation drives the light reactions  Sunlight contains energy called electromagnetic energy or radiation –Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths –Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves Copyright © 2009 Pearson Education, Inc.

8. 7.6 Visible radiation drives the light reactions  Light behaves as discrete packets of energy called photons –A photon is a fixed quantity of light energy, and the shorter the wavelength, the greater the energy Copyright © 2009 Pearson Education, Inc.

9. Wavelength (nm) 10 –5 nm Increasing energy Visible light 650 nm 10 –3 nm 1 nm10 3 nm10 6 nm 1 m 10 3 m 380 400 500 600700 750 Radio waves Micro- waves Infrared X-rays UV Gamma rays

10. 7.6 Visible radiation drives the light reactions  Pigments, molecules that absorb light, are built into the thylakoid membrane –Plant pigments absorb some wavelengths of light and transmit others –We see the color of the wavelengths that are transmitted; for example, chlorophyll transmits green Copyright © 2009 Pearson Education, Inc. Animation: Light and Pigments

11. Light Chloroplast Thylakoid Absorbed light Transmitted light Reflected light

12. 7.6 Visible radiation drives the light reactions  Chloroplasts contain several different pigments and all absorb light of different wavelengths –Chlorophyll a absorbs blue violet and red light and reflects green –Chlorophyll b absorbs blue and orange and reflects yellow-green –The carotenoids absorb mainly blue-green light and reflect yellow and orange Copyright © 2009 Pearson Education, Inc.

13. 7.7 Photosystems capture solar power  Pigments in chloroplasts are responsible for absorbing photons (capturing solar power), causing release of electrons –The electrons jump to a higher energy level—the excited state—where electrons are unstable –The electrons drop back down to their “ground state,” and, as they do, release their excess energy Copyright © 2009 Pearson Education, Inc.

14. Chlorophyll molecule Excited state Ground state Heat Photon (fluorescence) e–e–

16. Chlorophyll molecule Excited state Ground state Heat Photon (fluorescence) e–e–

17. 7.7 Photosystems capture solar power  The energy released could be lost as heat or light, but rather it is conserved as it is passed from one molecule to another molecule –All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems –Photosystems are light-harvesting complexes surrounding a reaction center complex Copyright © 2009 Pearson Education, Inc.

18. 7.7 Photosystems capture solar power  The energy is passed from molecule to molecule within the photosystem –Finally it reaches the reaction center where a primary electron acceptor accepts these electrons and consequently becomes reduced –This solar-powered transfer of an electron from the reaction center pigment to the primary electron acceptor is the first step of the light reactions Copyright © 2009 Pearson Education, Inc.

19. 7.7 Photosystems capture solar power  Two types of photosystems have been identified and are called photosystem I and photosystem II –Each type of photosystem has a characteristic reaction center –Photosystem II, which functions first, is called P680 because its pigment absorbs light with a wavelength of 680 nm –Photosystem I, which functions next, is called P700 because it absorbs light with a wavelength of 700 nm Copyright © 2009 Pearson Education, Inc.

20. Reaction center complex e–e– Primary electron acceptor Light-harvesting complexes Photon Photosystem Transfer of energy Pigment molecules Pair of Chlorophyll a molecules Thylakoid membrane

21. 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH  During the light reactions, light energy is transformed into the chemical energy of ATP and NADPH –To accomplish this, electrons removed from water pass from photosystem II to photosystem I and are accepted by NADP + –The bridge between photosystems II and I is an electron transport chain that provides energy for the synthesis of ATP Copyright © 2009 Pearson Education, Inc.

22. NADPH Photosystem II e–e– Mill makes ATP Photon Photosystem I ATP e–e– e–e– e–e– e–e– e–e– e–e– Photon

23. 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH  NADPH, ATP, and O 2 are the products of the light reactions Copyright © 2009 Pearson Education, Inc.

24. Stroma O2O2 H2OH2O 1212 H+H+ NADP + NADPH Photon Photosystem II Electron transport chain Provides energy for synthesis of by chemiosmosis + 2 Primary acceptor 1 Thylakoid mem- brane P680 2 4 3 Thylakoid space e–e– e–e– 5 Primary acceptor P700 6 Photon Photosystem I ATP H+H+ +

25. 7.9 Chemiosmosis powers ATP synthesis in the light reactions  Interestingly, chemiosmosis is the mechanism that not only is involved in oxidative phosphorylation in mitochondria but also generates ATP in chloroplasts –ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane Copyright © 2009 Pearson Education, Inc.

26. 7.9 Chemiosmosis powers ATP synthesis in the light reactions  ATP synthase couples the flow of H + to the phosphorylation of ADP –The chemiosmotic production of ATP in photosynthesis is called photophosphorylation Copyright © 2009 Pearson Education, Inc.

27. + O2O2 H2OH2O 1212 H+H+ NADP + H+H+ NADPH + 2 H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Photosystem II Photosystem I Electron transport chain ATP synthase Light Stroma (low H + concentration) Chloroplast Thylakoid membrane Thylakoid space (high H + concentration) ADP + PATP

28. + O2O2 H2OH2O 1212 H+H+ NADP + H+H+ NADPH + 2 H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Photosystem II Photosystem I Electron transport chain ATP synthase Light Stroma (low H + concentration) Thylakoid space (high H + concentration) ADP + P ATP