1. Concept 10.1: Photosynthesis converts light energy to the chemical energy of food Carried out by photoautotrophs, using the energy of sunlight to make organic molecules from H 2 O and CO 2 Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O

2. Photosynthesis as a Redox Process Photosynthesis is a redox process in which H 2 O is oxidized and C O 2 is reduced Chloroplasts split H 2 O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Light Fig. 10-5-1 H2OH2O Chloroplast Light Reactions NADP + P ADP i +

5. Light Fig. 10-5-2 H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2

6. Light Fig. 10-5-3 H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2

7. Light Fig. 10-5-4 H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 [CH 2 O] (sugar)

8. Chloroplasts: The Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis CO 2 enters and O 2 exits the leaf through microscopic pores called stomata Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9. Chloroplasts – found mainly in cells of the mesophyll – Contain chlorophyll=green pigment – The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense fluid Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

10. Fig. 10-3b 1 µm Thylakoid space Chloroplast Granum Intermembrane space Inner membrane Outer membrane Stroma Thylakoid

11. Photosynthetic Pigments: The Light Receptors In each chloroplast are functional groups called photosynthetic units – 1. Contain about 300 molecules of pigment – 2. contain one specialized chlorophyll a molecule called the reaction center – Other molecules function like antennae – Reaction center can trap energy and pass it along an enzyme-linked series of reactions to convert energy to a usable form Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

12. Fig. 10-10 Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center in chlorophyll a CH 3 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown CHO in chlorophyll b

13. Chlorophyll a is the main photosynthetic pigment; violet- blue and red light work best for photosynthesis – Reflects green, yellow, orange Chlorophyll b (other reds and blues), broadens the spectrum used for photosynthesis and provides protection Carotenoids (yellow/orange) absorb excessive light that would damage chlorophyll **All pigments absorb wavdifferent elengths!** Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

14. Light also behaves as though it consists of discrete particles, called photons Light is a form of electromagnetic energy. (Visible light 380-750 nm)

15. Excitation of Chlorophyll by Light Electrons within chlorophyll are excited to higher energy levels by photons What happens to excited electron? – Energy is dissipated as heat – Energy reemitted as light = fluorescence (in test tube) – Energy may cause a chemical reaction (in chloroplast) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16. Fig. 10-11 (a) Excitation of isolated chlorophyll molecule Heat Excited state (b) Fluorescence Photon Ground state Photon (fluorescence) Energy of electron e–e– Chlorophyll molecule

17. Concept 10.2: The Light Reactions Light energy drives the transfer of e- and H+ from water to NADP+  NADPH H 2 0 is split and O 2 is given off Light reactions also carry out photophosphorylation converting ADP to ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

18. Ground state e- + photon  excited e- – Unstable – Drops back – Energy can be trapped by ETC Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

19. Photosystem I = P700 (absorbs 700 nm) Photosystem II= P680 (absorbs 680 nm) Have identical chlorophyll a but different associated proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

20. Fig. 10-12 THYLAKOID SPACE (INTERIOR OF THYLAKOID) STROMA e–e– Pigment molecules Photon Transfer of energy Special pair of chlorophyll a molecules Thylakoid membrane Photosystem Primary electron acceptor Reaction-center complex Light-harvesting complexes

21. Linear Electron Flow (non-cyclic photophosphorylation) Occurs in green plants and a few bacteria 1. Photon strikes P680 reaction center (Photosystem II). 2. Electron passed down ETC to P700. 3. Electrons in P680 replaced by splitting H 2 O 4./5. Some energy in chain used to make ATP by chemiosmosis across the thylakoid membrane **This is the step where O 2 is released!** Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

22. Pigment molecules Light P680 e–e– 2 1 Fig. 10-13-1 Photosystem II (PS II) Primary acceptor P680 + (P680 that is missing an electron) is a very strong oxidizing agent

23. Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 Fig. 10-13-2 Photosystem II (PS II)

24. Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Fig. 10-13-3 Photosystem II (PS II)

25. 6. Photon strikes P700 reaction center. P700 + (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain 7. Electrons passed through a second ETC. 8. As e- pass down ETC, NADP is reduced  NADPH (2 e- required for reduction) **NADPH and ATP go into the Calvin Cycle.** Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

26. Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fig. 10-13-4 Photosystem II (PS II)

27. Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fd Electron transport chain NADP + reductase NADP + + H + NADPH 8 7 e–e– e–e– 6 Fig. 10-13-5 Photosystem II (PS II)

28. Fig. 10-14 Mill makes ATP e–e– NADPH Photon e–e– e–e– e–e– e–e– e–e– ATP Photosystem IIPhotosystem I e–e–

29. Cyclic Electron Flow (cyclic photophosphorylation) – Less efficient – Only uses P700 (PS I) 1. Photon strikes P700 reaction center. 2. Energy passed down ETC back to P700. 3. Generates ATP. 4. No production of NADPH or release of O2. **Like oxidative phosphorylation in the mitochondria!** Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

30. Fig. 10-15 ATP Photosystem II Photosystem I Primary acceptor Pq Cytochrome complex Fd Pc Primary acceptor Fd NADP + reductase NADPH NADP + + H +

31. Some organisms such as purple sulfur bacteria have PS I but not PS II Cyclic electron flow is thought to have evolved before linear electron flow Cyclic electron flow may protect cells from light-induced damage Generates more ATP because the Calvin Cycle uses more ATP than produced by linear electron flow. In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H 2 O to NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

32. Fig. 10-16 Key Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain H+H+ Diffusion Matrix Higher [H + ] Lower [H + ] Stroma ATP synthase ADP + P i H+H+ ATP Thylakoid space Thylakoid membrane

33. Fig. 10-17 Light Fd Cytochrome complex ADP + i H+H+ ATP P synthase To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane THYLAKOID SPACE (high H + concentration) STROMA (low H + concentration) Photosystem II Photosystem I 4 H + Pq Pc Light NADP + reductase NADP + + H + NADPH +2 H + H2OH2O O2O2 e–e– e–e– 1/21/2 1 2 3

34. Concept 10.3: The Calvin Cycle Does not require light directly (occurs in daylight for most plants!) Begins with carbon fixation = Carbon from CO 2 is incorporated into organic molecules 3 CO 2 per cycle! Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

35. The Calvin Cycle then reduces the carbon into carbohydrate by the addition of e- from NADPH Produces one 3C sugar (G3P glyceraldehyde- 3-phospate) per cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

36. Process (for each CO 2 ) 1. CO 2 attached to a five C sugar (RuBP) – catalyzed by enzyme rubisco (most abundant protein on Earth!) 2. Forms unstable 6-C intermediate which splits into 2 3-C sugars. 3. 3-C sugars phosphorylated by ATP.

37. Fig. 10-18-1 Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P

38. Process continued… 4. 3-C sugars reduced by NADPH 5. G3P produced *6 G3P are produced for each Calvin Cycle -1 exits cycle - 5 remain in cycle  regenerate RuBP 6. Rest of Cycle = regenerate RuBP (ATP needed!) It takes 2 cycles to produce one glucose!

39. Fig. 10-18-2 Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P ATP 6 6 ADP P P 6 1,3-Bisphosphoglycerate 6 P P 6 6 6 NADP + NADPH i Phase 2: Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar) Glucose and other organic compounds Calvin Cycle

40. Fig. 10-18-3 Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P ATP 6 6 ADP P P 6 1,3-Bisphosphoglycerate 6 P P 6 6 6 NADP + NADPH i Phase 2: Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar) Glucose and other organic compounds Calvin Cycle 3 3 ADP ATP 5 P Phase 3: Regeneration of the CO 2 acceptor (RuBP) G3P

41. Fig. 10-21 Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain CO 2 NADP + ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2OH2O O2O2

42. Concept 10.4: Special Cases of Photosynthesis Most plants = C3 plants CO 2 fixed by rubisco and first product =3C sugar In dry, hot weather, plants close stomata to reduce transpiration  Reduces CO 2 intake Rubisco binds to O 2 when [CO 2 ] is low – Photorespiration: consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar (decreases photosynthesis) – In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

43. C 4 Plants C 4 plants form a four C compound as first product into four-carbon compounds – Ex: sugar cane, corn, and some grasses 2 types of photosynthetic cells Bundle-sheath cells, tightly packed around veins Mesophyll cells located outside bundle sheath CO 2 is fixed in the mesophyll by the enzyme PEP carboxylase  4C compound 4C compound then enters the bundle sheath where CO 2 is released and is used by rubisco in the Calvin cycle. ** C4 pathway minimizes photorespiration and enhances sugar production!** Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

44. Fig. 10-19 C 4 leaf anatomy Mesophyll cell Photosynthetic cells of C 4 plant leaf Bundle- sheath cell Vein (vascular tissue) Stoma The C 4 pathway Mesophyll cell CO 2 PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP ATP Pyruvate (3C) CO 2 Bundle- sheath cell Calvin Cycle Sugar Vascular tissue

45. CAM Plants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants-no special cells, open their stomata at night, incorporating CO 2 into organic acids Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle Ex. Cactus and pineapple Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

46. Fig. 10-20 CO 2 Sugarcane Mesophyll cell CO 2 C4C4 Bundle- sheath cell Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Pineapple Night Day CAM Sugar Calvin Cycle Calvin Cycle Organic acid (a) Spatial separation of steps (b) Temporal separation of steps CO 2 1 2