1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: The Process That Feeds the Biosphere Photosynthesis – Is the process that converts solar energy into chemical energy

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis – Occurs in plants, algae, certain other protists, and some prokaryotes 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 (b) Multicellular algae (c) Unicellular protist 10  m 40  m (d) Cyanobacteria 1.5  m (e) Pruple sulfur bacteria Figure 10.2

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heterotrophs – Obtain their organic material from other organisms – Are the consumers of the biosphere

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 10.1: Photosynthesis converts light energy to the chemical energy of food

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts: The Sites of Photosynthesis in Plants The leaves of plants – Are the major sites of photosynthesis Vein Leaf cross section Figure 10.3 Mesophyll CO 2 O2O2 Stomata

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts – Are the organelles in which photosynth esis occurs – Contain thylakoids and grana Chloroplast Mesophyll 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis is summarized as 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Two Stages of Photosynthesis: A Preview Photosynthesis consists of two processes – The light reactions – The Calvin cycle

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle – Occurs in the stroma – Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An overview of photosynthesis H2OH2O CO 2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH 2 O] (sugar) NADPH NADP  ADP + P O2O2 Figure 10.5 ATP

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The absorption spectra of chloroplast pigments – Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The absorption spectra of three types of pigments in chloroplasts Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Absorption of light by chloroplast pigments Chlorophyll a (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Wavelength of light (nm) Chlorophyll b Carotenoids Figure 10.9

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chlorophyll a – Is the main photosynthetic pigment Chlorophyll b – Is an accessory pigment C CH CH 2 C C C C C CN N C H3CH3C C C C C C C C C N C C C C N Mg H H3CH3C H C CH 2 CH 3 H C H H CH 2 H CH 3 C O O O O O CHO in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Figure 10.10

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other accessory pigments – Absorb different wavelengths of light and pass the energy to chlorophyll a

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Excitation of Chlorophyll by Light When a pigment absorbs light – It goes from a ground state to an excited state, which is unstable Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground state Photon e–e– Figure 10.11 A

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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) Figure 10.12 e–e–

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reaction center – – Is a protein complex, including two special chlorophyll molecules and the primary electron acceptor

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The thylakoid membrane – Is populated by two types of photosystems, I and II

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Produces NADPH, ATP, and oxygen Figure 10.13 Photosystem II (PS II) Photosystem-I (PS I) ATP NADPH NADP + ADP CALVIN CYCLE CO 2 H2OH2O O2O2 [CH 2 O] (sugar) LIGHT REACTIONS Light Primary acceptor Pq Cytochrome complex PC e P680 e–e– e–e– O2O2 + H2OH2O 2 H + Light ATP Primary acceptor Fd e e–e– NADP + reductase Electron Transport chain Electron transport chain P700 Light NADPH NADP + + 2 H + + H + 1 5 7 2 3 4 6 8

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cyclic Electron Flow Under certain conditions – Photoexcited electrons take an alternative path – This is because the Calvin cycle needs more ATP than NADPH.

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In cyclic electron flow – Only photosystem I is used – Only ATP is produced Primary acceptor Pq Fd Cytochrome complex Pc Primary acceptor Fd NADP + reductase NADPH ATP Figure 10.15 Photosystem II Photosystem I NADP +

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The light reactions and chemiosmosis: the organization of the thylakoid membrane LIGHT REACTOR NADP + ADP ATP NADPH CALVIN CYCLE [CH 2 O] (sugar) STROMA (Low H + concentration) Photosystem II LIGHT H2OH2O CO 2 Cytochrome complex O2O2 H2OH2O O2O2 1 1⁄21⁄2 2 Photosystem I Light THYLAKOID SPACE (High H + concentration) STROMA (Low H + concentration) Thylakoid membrane ATP synthase Pq Pc Fd NADP + reductase NADPH + H + NADP + + 2H + To Calvin cycle ADP P ATP 3 H+H+ 2 H + +2 H + 2 H + Figure 10.17

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar The Calvin cycle – Is similar to the citric acid cycle – Occurs in the stroma

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle has three phases – Carbon fixation – Reduction – Regeneration of the CO 2 acceptor

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle (G3P) Input (Entering one at a time) CO 2 3 Rubisco Short-lived intermediate 3 PP P Ribulose bisphosphate (RuBP) P 3-Phosphoglycerate P6 P 6 1,3-Bisphoglycerate 6 NADPH 6 NADPH + 6 P P 6 Glyceraldehyde-3-phosphate (G3P) 6 ATP 3 ATP 3 ADP CALVIN CYCLE P 5 P 1 G3P (a sugar) Output Light H2OH2O CO 2 LIGHT REACTION ATP NADPH NADP + ADP [CH 2 O] (sugar) CALVIN CYCLE Figure 10.18 O2O2 6 ADP Glucose and other organic compounds Phase 1: Carbon fixation Phase 2: Reduction Phase 3: Regeneration of the CO 2 acceptor (RuBP)

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings On hot, dry days, plants close their stomata – Conserving water but limiting access to CO 2 – Causing oxygen to build up

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photorespiration: An Evolutionary Relic? In photorespiration – O 2 substitutes for CO 2 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C 4 Plants C 4 plants minimize the cost of photorespiration – By incorporating CO 2 into four carbon compounds in mesophyll cells – These four carbon compounds Are exported to bundle sheath cells, where they release CO 2 used in the Calvin cycle

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C 4 leaf anatomy and the C 4 pathway CO 2 Mesophyll cell Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C 4 plant leaf Stoma Mesophyll cell C 4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Bundle- Sheath cell CO 2 Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure 10.19 CO 2

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CAM Plants CAM plants – Open their stomata at night, incorporating CO 2 into organic acids – During the day, the stomata close – And the CO 2 is released from the organic acids for use in the Calvin cycle

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The CAM pathway is similar to the C 4 pathway Spatial separation of steps. In C 4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. (b) Pineapple Sugarcane Bundle- sheath cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO 2 Organic acid CALVIN CYCLE Sugar C4C4 CAM CO 2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 Organic acids release CO 2 to Calvin cycle Figure 10.20

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Importance of Photosynthesis: A Review A review of photosynthesis Light reactions: Are carried out by molecules in the thylakoid membranes Convert light energy to the chemical energy of ATP and NADPH Split H 2 O and release O 2 to the atmosphere Calvin cycle reactions: Take place in the stroma Use ATP and NADPH to convert CO 2 to the sugar G3P Return ADP, inorganic phosphate, and NADP+ to the light reactions O2O2 CO 2 H2OH2O Light Light reaction Calvin cycle NADP + ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast Figure 10.21

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organic compounds produced by photosynthesis – Provide the energy and building material for ecosystems