Overview: The Process That Feeds the Biosphere Photosynthesis

1. Overview: The Process That Feeds the Biosphere • Photosynthesis • Is the process that converts solar energy into chemical energy

2. Plants and other autotrophs • Are the producers of the biosphere • Plants are photoautotrophs • They use the energy of sunlight to make organic molecules from water and carbon dioxide

3. These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). (a) Plants (c) Unicellular protist 10 m (e) Pruple sulfur bacteria 1.5 m Figure 10.2 (d) Cyanobacteria (b) Multicellular algae 40 m • Photosynthesis • Occurs in plants, algae, certain other protists, and some prokaryotes

4. Heterotrophs • Obtain their organic material from other organisms • Are the consumers of the biosphere

5. Concept 10.1: Photosynthesis converts light energy to the chemical energy of food

6. Leaf cross section Vein Mesophyll CO2 O2 Stomata Figure 10.3 Chloroplasts: The Sites of Photosynthesis in Plants • The leaves of plants • Are the major sites of photosynthesis

7. Mesophyll Chloroplast 5 µm Outer membrane Thylakoid Intermembrane space Thylakoid space Granum Stroma Inner membrane 1 µm • Chloroplasts • Are the organelles in which photosynthesis occurs • Contain thylakoids and grana

8. Tracking Atoms Through Photosynthesis: Scientific Inquiry • Photosynthesis is summarized as 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O

9. Photosynthesis as a Redox Process • Photosynthesis is a redox process • Water is oxidized, carbon dioxide is reduced • This is the reverse flow of electrons as in cellular respiration

10. The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of two processes • The light reactions • The Calvin cycle

11. The light reactions • Occur in the grana • Split water, release oxygen, produce ATP, and form NADPH • The ATP is produced by photophosphorylation – generating ATP using chemiosmosis to power the addition of phosphate to ADP • The third way of generating ATP we’ve seen

12. The Calvin cycle • Occurs in the stroma • Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

13. H2O CO2 Light NADP  ADP + P LIGHT REACTIONS CALVIN CYCLE ATP NADPH Chloroplast [CH2O] (sugar) O2 Figure 10.5 • An overview of photosynthesis

14. Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

15. The absorption spectra of chloroplast pigments • Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

16. Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Chlorophyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Figure 10.9 • The absorption spectra of three types of pigments in chloroplasts

17. CH3 in chlorophyll a in chlorophyll b CHO CH2 CH3 CH H C C C Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center C C CH3 C C H3C CH2 C N C N H C C Mg H N C C N H3C C C CH3 C C C C C H H CH2 H C C O CH2 O O C O O CH3 CH2 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Figure 10.10 • Chlorophyll a • Is the main photosynthetic pigment • Chlorophyll b • Is an accessory pigment

18. Other accessory pigments • Absorb different wavelengths of light and pass the energy to chlorophyll a

19. Excited state e– Heat Energy of election Photon (fluorescence) Ground state Chlorophyll molecule Photon Figure 10.11 A Excitation of Chlorophyll by Light • When a pigment absorbs light • It goes from a ground state to an excited state, which is unstable

20. Thylakoid Photosystem Photon STROMA Light-harvesting complexes Reaction center Primary election acceptor e– Thylakoid membrane Special chlorophyll a molecules Transfer of energy Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes • A photosystem • Is composed of a reaction center surrounded by a number of light-harvesting complexes

21. The light-harvesting complexes • Consist of pigment molecules bound to particular proteins • Act as an antenna for the reaction center • Funnel the energy of photons of light to the reaction center

22. Reaction center – • Is a protein complex, including two special chlorophyll molecules and the primary electron acceptor

23. First step of light reactions • When a reaction-center chlorophyll molecule absorbs energy • One of its electrons gets bumped up to a primary electron acceptor • This is a redox rxn

24. The thylakoid membrane • Is populated by two types of photosystems, I and II

25. Noncyclic Electron Flow • Noncyclic electron flow • Is the primary pathway of energy transformation in the light reactions • Produces ATP and NADPH which will provide chemical energy and reducing power to the sugar making Calvin cycle

26. H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH Electron Transport chain O2 [CH2O] (sugar) Primary acceptor 7 Primary acceptor 4 Fd Electron transport chain Pq 2 e 8 e– e H2O NADP+ + 2 H+ Cytochrome complex 2 H+ NADP+ reductase + 3 NADPH O2 PC e– + H+ P700 e– 5 Light P680 Light 1 6 ATP Photosystem-I (PS I) Photosystem II (PS II) Figure 10.13 • Produces NADPH, ATP, and oxygen

27. Cyclic Electron Flow • Under certain conditions • Photoexcited electrons take an alternative path • This is because the Calvin cycle needs more ATP than NADPH.

28. Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II Figure 10.15 • In cyclic electron flow • Only photosystem I is used • Only ATP is produced

29. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria • Chloroplasts and mitochondria • Generate ATP by the same basic mechanism: chemiosmosis • But use different sources of energy to accomplish this

30. In both organelles • Redox reactions of electron transport chains generate a H+ gradient across a membrane • ATP synthase • Uses this proton-motive force to make ATP

31. H2O CO2 LIGHT NADP+ ADP CALVIN CYCLE LIGHT REACTOR ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome complex Photosystem II Photosystem I NADP+ reductase Light 2 H+ 3 NADP+ + 2H+ Fd NADPH + H+ Pq Pc 2 H2O 1⁄2 O2 THYLAKOID SPACE (High H+ concentration) 1 2 H+ +2 H+ To Calvin cycle ATP synthase Thylakoid membrane STROMA (Low H+ concentration) ADP ATP P H+ Figure 10.17 • The light reactions and chemiosmosis: the organization of the thylakoid membrane

32. Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar • The Calvin cycle • Is similar to the citric acid cycle • Occurs in the stroma

33. The Calvin cycle has three phases • Carbon fixation • Reduction • Regeneration of the CO2 acceptor

34. H2O Input CO2 Light 3 (Entering one at a time) NADP+ CO2 ADP CALVINCYCLE LIGHTREACTION ATP NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-livedintermediate P 6 3 P P Ribulose bisphosphate(RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 ADP 6 P P 3 ATP 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 Glucose andother organiccompounds G3P(a sugar)Output Figure 10.18 • The Calvin cycle Phase 1: Carbon fixation Phase 3:Regeneration ofthe CO2 acceptor(RuBP) Phase 2:Reduction

35. Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

36. On hot, dry days, plants close their stomata • Conserving water but limiting access to CO2 • Causing oxygen to build up

37. Photorespiration: An Evolutionary Relic? • In photorespiration • O2 substitutes for CO2 in the active site of the enzyme rubisco • The photosynthetic rate is reduced • No sugar is made, ATP is used, not made • Hold over from earlier time

38. C4 Plants • C4 plants minimize the cost of photorespiration • By incorporating CO2 into four carbon compounds in mesophyll cells • These four carbon compounds • Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

39. Mesophyll cell Mesophyll cell Photosynthetic cells of C4 plant leaf PEP carboxylase Bundle- sheath cell CO2 CO2 PEP (3 C) Oxaloacetate (4 C) ADP Vein (vascular tissue) Malate (4 C) ATP C4 leaf anatomy Pyruate (3 C) Bundle- Sheath cell CO2 Stoma CALVIN CYCLE Sugar Vascular tissue Figure 10.19 • C4 leaf anatomy and the C4 pathway

40. CAM Plants • CAM plants • Open their stomata at night, incorporating CO2 into organic acids • During the day, the stomata close • And the CO2 is released from the organic acids for use in the Calvin cycle

41. 2 1 Pineapple Sugarcane C4 CAM CO2 CO2 Mesophyll Cell Night CO2 incorporated into four-carbon organic acids (carbon fixation) Organic acid Organic acid Bundle- sheath cell (b) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times. Day (a) Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. Organic acids release CO2 to Calvin cycle CALVINCYCLE CALVINCYCLE Sugar Sugar Figure 10.20 • The CAM pathway is similar to the C4 pathway

42. Light reaction Calvin cycle H2O CO2 Light NADP+ ADP + P1 RuBP 3-Phosphoglycerate Photosystem II Electron transport chain Photosystem I ATP G3P Starch (storage) NADPH Amino acids Fatty acids Chloroplast O2 Sucrose (export) Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H2O and release O2 to the atmosphere Figure 10.21 The Importance of Photosynthesis: A Review • A review of photosynthesis

43. Organic compounds produced by photosynthesis • Provide the energy and building material for ecosystems