Photosynthesis: Energy from Sunlight

1. Photosynthesis: Energy from Sunlight

2. 10 Photosynthesis: Energy from Sunlight • 10.1 What Is Photosynthesis? • 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • 10.3 How Is Chemical Energy Used to Synthesize Carbohydrates? • 10.4 How Have Plants Adapted Photosynthesis to Environmental Conditions? • 10.5 How Does Photosynthesis Interact with Other Pathways?

3. 10 Photosynthesis: Energy from Sunlight To predict how plants will respond to rising CO2 levels, biologists have performed large-scale experiments. Photosynthesis rates increase as atmospheric CO2 concentration increases. Opening Question: What possible effects will increased atmospheric CO2 have on global food production?

4. 10.1 What Is Photosynthesis? • Photosynthesis: “synthesis from light” • Energy from sunlight is captured and used to convert CO2 to more complex carbon compounds.

5. Figure 10.1 The Ingredients for Photosynthesis

6. 10.1 What Is Photosynthesis? • Using stable 18O isotopes, Ruben and Kamen determined that water is the source of O2 released during photosynthesis:

7. Figure 10.2 The Source of the Oxygen Produced by Photosynthesis

8. Working with Data 10.1: Water Is the Source of the Oxygen Produced by Photosynthesis • The stable isotope 18O was used to confirm the hypothesis that O2 generated during photosynthesis came from water. • Algal cells were exposed to water, and CO2 generated from K2CO3 and KHCO3–.

9. Working with Data 10.1: Water Is the Source of the Oxygen Produced by Photosynthesis • Experiment 1: water contained more 18O than 16O. • Experiment 2: the CO2 contained more 18O than 16O. • A mass spectrometer measured the isotopic content of reactants and the O2 produced.

10. Working with Data 10.1: Water Is the Source of the Oxygen Produced by Photosynthesis • Question 1: • In Experiment 1, was the isotopic ratio of O2 similar to that of H2O or to that of CO2? • What about in Experiment 2?

11. Working with Data 10.1, Table 1

12. Working with Data 10.1: Water Is the Source of the Oxygen Produced by Photosynthesis • Question 2: • What can you conclude from these data?

13. 10.1 What Is Photosynthesis? • Photosynthesis is an oxidation–reduction process. • Oxygen atoms in H2O are in a reduced state; they are oxidized to O2. • Carbon atoms are in the oxidized state in CO2; they are reduced to a carbohydrate.

14. 10.1 What Is Photosynthesis? • Water is the donor of protons and electrons in oxygenic photosynthesis. • In anoxygenic photosynthesis, other molecules donate the protons and electrons. • Example: purple sulfur bacteria use H2S.

15. 10.1 What Is Photosynthesis? • Two pathways occur in different parts of the chloroplast: • Light reactions: Convert light energy to chemical energy as ATP and NADPH. • Light-independent reactions: Use ATP and NADPH (from the light reactions) plus CO2 to produce carbohydrates.

16. Figure 10.3 An Overview of Photosynthesis

17. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Light is a form of energy—electromagnetic radiation. • It is propagated as waves—the amount of energy is inversely proportional to its wavelength. • Light also behaves as particles, called photons.

18. Figure 10.4 The Electromagnetic Spectrum

19. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Certain molecules absorb photons of specific wavelengths. • When a photon hits a molecule, it can: • Bounce off—scattered or reflected • Pass through—transmitted • Be absorbed, adding energy to the molecule (excited state)

20. In-Text Art, Ch. 10, p. 189

21. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • The absorbed energy boosts an electron in the molecule into a shell farther from the nucleus. • This electron is held less firmly— making the molecule more unstable and reactive.

22. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Molecules that absorb specific wavelengths in the visible range are called pigments. • Other wavelengths are scattered or transmitted, which imparts the colors that we see. • Chlorophyll absorbs blue and red light and scatters green.

23. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Absorption spectrum: plot of wavelengths absorbed by a pigment. • Action spectrum: plot of photosynthesis against wavelengths of light to which it is exposed. • The rate of photosynthesis can be measured by the amount of O2 released.

24. Figure 10.5 Absorption and Action Spectra

25. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • The major pigment in photosynthesis is chlorophyll a. • It has a hydrocarbon “tail” that anchors it in a protein complex in the thylakoid membrane called a photosystem.

26. Figure 10.6 The Molecular Structure of Chlorophyll a

27. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Chlorophyll a and accessory pigments (chlorophylls b and c, carotenoids, phycobilins) are arranged in light-harvesting complexes, or antenna systems. • Several complexes surround a reaction center in the photosystem.

28. Figure 10.7 Energy Transfer and Electron Transport (Part 2)

29. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Light energy is captured in the light harvesting complexes and transferred to the reaction centers. • Accessory pigments absorb light in other wavelengths, increasing the range of light that can be used. • Types of accessory pigments characterize different groups.

30. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • When a pigment molecule absorbs a photon, the excited state is unstable and the energy is quickly released. • The energy is absorbed by other pigment molecules and passed to chlorophyll a in a reaction center.

31. Figure 10.8 Noncyclic Electron Transport Uses Two Photosystems

32. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • The excited chlorophyll a molecule (Chl*) gives up an electron to an acceptor. • Chl* + acceptor → Chl+ + acceptor – • A redox reaction: The chlorophyll gets oxidized; the acceptor molecule is reduced.

33. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • The electron acceptor is the first in a chain of carriers in the thylakoid membrane. • The final electron acceptor is NADP+, which gets reduced: • NADP+ + H+ + 2e– → NADPH

34. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Noncyclic electron transport uses two photosystems: • Photosystem I has P700 chlorophyll —absorbs best at 700 nm. • Photosystem II has P680 chlorophyll —absorbs best at 680 nm.

35. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Photosystem II: • When excited chlorophyll (Chl*) gives up its electron, it is unstable, and grabs another electron (it is a strong oxidizer). • The electron comes from water: • 2Chl+ + H2O → 2Chl + 2H+ + ½O2

36. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • The energetic electrons are passed through a series of membrane-bound carriers to a final acceptor at a lower energy level. • A proton gradient is generated and is used by ATP synthase to make ATP.

37. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Photosystem I: • An excited electron from the Chl* reduces an acceptor. • The oxidized Chl+ takes an electron from the last carrier in photosystem II. • The energetic electron is passed through several carriers and reduces NADP+ to NADPH.

38. Figure 10.8 Noncyclic Electron Transport Uses Two Photosystems

39. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Cyclic electron transport: • Uses photosystem I and electron transport to produce ATP instead of NADPH. • Cyclic: the electron from the excited chlorophyll passes back to the same chlorophyll.

40. Figure 10.9 Cyclic Electron Transport Traps Light Energy as ATP

41. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • ATP is formed by photophosphorylation, a chemiosmotic mechanism. • H+ is transported across the thylakoid membrane into the lumen, creating an electrochemical gradient.

42. Figure 10.10 Photophosphorylation

43. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • Water oxidation creates more H+ in the thylakoid lumen and NADP+ reduction removes H+ in the stroma. • Both reactions contribute to the H+ gradient.

44. 10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? • High concentration of H+ in the lumen drives movement of H+ back into the stroma through protein channels. • The channels are ATP synthases that couple movement of protons with formation of ATP.

45. 10.3 How Is Chemical Energy Used to Synthesize Carbohydrates? • CO2 fixation: CO2 is reduced to carbohydrates. • Occurs in the stroma. • Energy in ATP and NADPH is used to reduce CO2.

46. 10.3 How Is Chemical Energy Used to Synthesize Carbohydrates? • Calvin and Benson used the 14C radioisotope to determine the sequence of reactions in CO2 fixation. • They exposed Chlorella to 14CO2, then extracted the organic compounds and separated them by paper chromatography.

47. Figure 10.11 Tracing the Pathway of CO2 (Part 1)

48. Figure 10.11 Tracing the Pathway of CO2 (Part 2)

49. 10.3 How Is Chemical Energy Used to Synthesize Carbohydrates? • The first compound to be formed is 3-phosphoglycerate, 3PG, a 3-carbon sugar phosphate.

50. 10.3 How Is Chemical Energy Used to Synthesize Carbohydrates? • The pathway of CO2 fixation is cyclic: the Calvin cycle. • CO2 first binds to 5-C RuBP; the 6-C compound immediately breaks down into two molecules of 3PG. • The enzyme rubisco (ribulose bisphoshate carboxylase/oxygenase) is the most abundant protein in the world.