Where It Starts – Photosynthesis

1. Where It Starts – Photosynthesis Chapter 6

2. Sunlight as an Energy Source • Photosynthesis runs on a fraction of the electromagnetic spectrum, or the full range of energy radiating from the sun

3. Visible Light • Wavelengths humans perceive as different colors • Violet (380 nm) to red (750 nm) • Longer wavelengths, lower energy

4. Electromagnetic Spectrum Shortest Gamma rays wavelength X-rays UV radiation Visible light Infrared radiation Microwaves Longest Radio waves wavelength

5. Photons • Packets of light energy • Each type of photon has fixed amount of energy • Photons having most energy travel as shortest wavelength (blue-green light)

6. Pigments • Light-absorbing molecules • Absorb some wavelengths and transmit others • Color you see are the wavelengths not absorbed

7. Pigment Structure • Light-catching part of molecule often has alternating single and double bonds • These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light

8. Variety of Pigments Chlorophylls a and b Carotenoids Xanthophylls Phycobilins Anthocyanins

9. Chlorophylls Main pigments in most photoautotrophs chlorophyll a Wavelength absorption (%) chlorophyll b Wavelength (nanometers)

10. Carotenoids • Found in all photoautotrophs • Absorb blue-violet and blue-green that chlorophylls miss • Reflect red, yellow, orange wavelengths • Two types • Carotenes - pure hydrocarbons • Xanthophylls - contain oxygen

11. Yellow, brown, purple, or blue accessory pigments Xanthophylls

12. Phycobilins & Anthocyanins Red to purple pigments • Phycobilins • Found in red algae and cyanobacteria • Anthocyanins • Give many flowers their colors

13. T.E. Englemann’s Experiment Background • Certain bacterial cells will move toward places where oxygen concentration is high • Photosynthesis produces oxygen

14. T.E. Englemann’s Experiment Hypothesis • Movement of bacteria can be used to determine optimal light wavelengths for photosynthesis

15. T.E. Englemann’s Experiment Method • Algal strand placed on microscope slide and illuminated by light of varying wavelengths • Oxygen-requiring bacteria placed on same slide

16. T.E. Englemann’s Experiment Results Bacteria congregated where red and violet wavelengths illuminated alga Conclusion Bacteria moved to where algal cells released more oxygen – areas illuminated by the most effective light for photosynthesis

17. T.E. Englemann’s Experiment

18. Light-Dependent Reactions • Pigments absorb light energy, give up e- which enter electron transfer chains • Water molecules are split, ATP and NADH are formed, and oxygen is released • Pigments that gave up electrons get replacements

19. Light-Independent Reactions • Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma • Calvin-Benson cycle

20. Photosynthesis Equation

21. Chloroplasts Organelles of photosynthesis

22. Inside the Chloroplast • Two outer membranes enclose a semifluid interior, the stroma • Thylakoid membrane inside the stroma

23. Inside the Chloroplast • Photosystems are embedded in thylakoids, containing 200 to 300 pigments and other molecules that trap sun’s energy • Two types of photosystems: I and II

24. Carbon and Energy Sources • Photoautotrophs • Carbon source is carbon dioxide • Energy source is sunlight • Heterotrophs • Get carbon and energy by eating autotrophs or one another

25. Photoautotrophs • Capture sunlight energy and use it to carry out photosynthesis • Plants • Some bacteria • Many protistans

26. Photosynthesis Energy-storing pathway Releases oxygen Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide Linked Processes

27. Two Stages of Photosynthesis

28. Excitation of Electrons • Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment • Amount of energy needed varies among pigment molecules

29. Pigments in Photosynthesis • Bacteria • Pigments in plasma membranes • Plants • Pigments embedded in thylakoid membrane system • Pigments and proteins organized into photosystems • Photosystems located next to electron transfer chains

30. Photosystem Function: Harvester Pigments • Most pigments in photosystem are harvester pigments • When excited by light energy, these pigments transfer energy to adjacent pigment molecules • Each transfer involves energy loss

31. Photosystem Function: Reaction Center • Energy is reduced to level that can be captured by molecule of chlorophyll a • This molecule (P700 or P680) is the reaction center of a photosystem • Reaction center accepts energy and donates electron to acceptor molecule

32. Electron Transfer Chains • Adjacent to photosystem • Acceptor molecule donates electrons from reaction center • As electrons flow through chain, energy they release is used to produce ATP and, in some cases, NADPH

33. Cyclic Electron Flow • Electrons • are donated by P700 in photosystem I to acceptor molecule • flow through electron transfer chain and back to P700 • Electron flow drives ATP formation • No NADPH is formed

34. Noncyclic Electron Flow • Two-step pathway for light absorption and electron excitation • Uses two photosystems: type I and type II • Produces ATP and NADPH • Involves photolysis - splitting of water

35. ATP and NADPH Formation

36. ATP Formation • When water is split during photolysis, hydrogen ions are released into thylakoid compartment • More hydrogen ions are pumped into the thylakoid compartment when the electron transfer chain operates

37. ATP Formation • Electrical and H+concentration gradient exists between thylakoid compartment and stroma • H+ flows down gradients into stroma through ATP synthesis • Flow of ions drives formation of ATP

38. Energy Transfers

39. Energy Transfers

40. Overall reactants Carbon dioxide ATP NADPH Overall products Glucose ADP NADP+ Calvin-Benson Cycle Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

41. Calvin-Benson Cycle

42. Building Glucose • PGA accepts • phosphate from ATP • hydrogen and electrons from NADPH • PGAL (phosphoglyceraldehyde) forms • When 12 PGAL have formed • 10 are used to regenerate RuBP • 2 combine to form phosphorylated glucose

43. Using the Products of Photosynthesis • Phosphorylated glucose is the building block for: • Sucrose • The most easily transported plant carbohydrate • Starch • The most common storage form

44. The C3 Pathway • In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA • Because the first intermediate has three carbons, the pathway is called the C3 pathway

45. Photorespiration in C3 Plants • On hot, dry days stomata close • Inside leaf • Oxygen levels rise • Carbon dioxide levels drop • Rubisco attaches RuBP to oxygen instead of carbon dioxide • Only one PGAL forms instead of two

46. C4 Plants • Carbon dioxide is fixed twice • In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate • Oxaloacetate is transferred to bundle-sheath cells • Carbon dioxide is released and fixed again in Calvin-Benson cycle

47. C4 Plants

48. CAM Plants • Carbon is fixed twice (in same cells) • Night • Carbon dioxideis fixed to form organic acids • Day • Carbon dioxide is released and fixed in Calvin-Benson cycle

49. Summary of Photosynthesis