Chapter 10

1. Chapter 10 Photosynthesis

2. 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) Purple 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

3. Figure 10.1 Concept 10.1: Photosynthesis converts light energy to the chemical energy of food Plants are photoautotrophs They use the energy of sunlight to make organic molecules from water and carbon dioxide • Plants and other autotrophs are the producers of the biosphere

4. Photosynthesis is vital to all life on earth • Heterotrophs • Obtain their organic material from other organisms • Are the consumers of the biosphere • Photosynthetically-produced carbohydrates provide fuel for all

5. Overview: The Process That Feeds the Biosphere Photosynthesis • Is the process that converts solar energy into chemical energy • 1st photosynthetic organisms, 3 to 3.5 billion years old • These were probably responsible for changing the earth's atmosphere (use CO2 and H2O to synthesize glucose and oxygen) chlorophyll 6 CO2 + 12 H2O --------------- C6H12O6 + 6 O2 + 6 H2 O light

6. Reactants: 12 H2O 6 CO2 6 H2O 6 O2 C6H12O6 Products: Figure 10.4 The Splitting of Water • Photosynthesis is a redox process • Water is oxidized, carbon dioxide is reduced • Chloroplasts split water into • Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

7. Oxygen comes from Splitting Water • Van Niel's Hypothesis  (Stanford U., 1930's) • previously, it had been believed that the O2 given off in light reactions came from the splitting of CO2 • Van Niel studied autotrophic bacteria "purple sulfur bacteria" which did NOT use H2O in their food-making proceses: • CO2 + 2H2S -------light--------> (CH2O)n + H2O + 2S So general equation is: • CO2 + 2H2A ------------------------> (CH2O)n + H2O + 2A • Which proves it is the " H2A" that is, in fact, split to release the gas • Later, "heavy" oxygen18(radioactive isotope) was traced through a plant to prove it.

8. 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

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

10. Photosynthetic Membranes: the Thylakoid • Thylakoid: the structural unit of photosynthesis is usually a form of a flattened sac, or vesicle. • These form the internal membranes of a CHLOROPLAST. • There can be up to 500,000 chloroplasts per square mm of leaf surface • The Structure of the CHLOROPLAST • similar in structure to the mitochondria: surrounded by 2 membranes that are separates by an intermembrane space • 3rd layer inside: grana (stacks of thylakoids), surrounded by a dense solution: the stroma

11. 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 Pigments • Chlorophyll a • Is the main pigment • Chlorophyll is a large molecule with a central Magnesium atom held in a porphyrin ring (remarkably similar to Fe in ring in hemoglobin) • Chlorophyll b • Is an accessory pigment • Are substances that absorb visible light

12. Chlorophyll and Other Pigments • When pigments absorb light, electrons within the pigment molecules are boosted to a higher energy level. This is what powers photosynthesis. • Pigments absorb light of certain wavelengths (reflect some wave lengths back, or transmit other wavelengths). Different pigments absorb light energy at different wavelengths = “absorption spectrum” • In plants, chlorophyll a is directly involved in the transformation of light energy into chemical energy • Most photosynthetic cells also contain “accessory pigments”, chlorophyll b and/or carotenoids (red, orange, yellow)…most prevalent is beta-carotene • Xanthophylls = yellow • Anthocyanins = red purple • The presence of the " secondary“ (accessory) pigments allow photosynthetic cells to capitalize on the greatest range of l's • These other pigments absorb different wavelengths of light and pass the energy to chlorophyll a

13. 1 m 106 nm 10–5 nm 106 nm 1 nm 10–3 nm 103 nm 103 m Micro- waves Radio waves Gamma rays X-rays UV Infrared Visible light 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Lower energy Higher energy Figure 10.6 The Nature of Sunlight • Light Is a form of electromagnetic energy, which travels in waves • The electromagnetic spectrum • Is the entire range of electromagnetic energy, or radiation

14. Light • Wavelengthl • Is the distance between the crests of waves • Determines the type of electromagnetic energy • Isaac Newton - discovered prism divides white light into the visible light spectrum • The visible light spectrum includes the colors of light we can see; includes the wavelengths that drive photosynthesis • Range is from about 380-750 nm • James Maxwell - discovered light (visible) is only a small part of a larger electromagetic spectrum • all light travels at approx. 300,000 km/sec • variation in this WAVELENGTH makes different lights. • visible light l is measured in VERY small units, called nanometers • 1 nm = 10-9 m

15. Light • Albert Einstein (1905) proposed that light travels in both waves and particles "photons" (packets of energy) • Photons for different light are inversely energetic to the wavelength • ex: Violet light has short l, but large amounts of energy in each photon

16. "Why does so narrow a region of the EM spectrum have such an important impact on life?" • Biorhythms, seasons, photosynthesis, visible colors, etc • Life forms are held together with weak H-bonds, etc. Easily disrupted by strong light (such as high photon energy or low l, such as UV light) drives e- out of atoms. Conversely, low photon energy or high l (such as IR light) is absorbed by H2O in cells to heat up 2. Most radiation that reaches thru earths atmosphere is in the spectrum higher energy is filtered out by O2 & O3 lower energy l screened by CO2 and H2O in clouds

17. Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7 Absorption of light by pigments • Some light l are reflected by pigments, which include the colors we see • Others are absorbed

18. Refracting prism Chlorophyll solution Photoelectric tube White light Galvanometer 2 3 1 0 100 4 Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 100 The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue light Figure 10.8 The spectrophotometer • Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength

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

20. 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

21. Aerobic bacteria Filament of alga 500 600 700 400 (c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. CONCLUSION “Action Spectrum” experiment: • The action spectrum for photosynthesis • Was first demonstrated by Theodor W. Engelmann

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

23. The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of two processes • The “light” reactions: • "Light-Dependent Reactions" (aka Energy-Capturing Rxns) need light energy to occur. • trap light energy by exciting electrons in chlorophyll --> energy is used to form ATP from ADP, and to reduce NADP+ to NADPH. Water molecules also broken down. • Occurs in the thylakoids. II. The Calvin cycle: • "Light-Independent Reactions" (aka the Carbon-Fixing Rxns) are enzymatic; can take place in/out of light, but need the products of light rxns to work. • Energy in the form of ATP & NADPH (from previous set of rxns) used to reduce carbon (from CO2) into sugar molecules (" carbon fixation") • Occurs in the stroma.

24. A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes In the thylakoids: • Chlorophyll a (reaction center) and other molecules (antennas) are packed into units called photosystems, made up of 250-400 pigment molecules each. • Photons of light are absorbed by pigments and passed to the reaction center. • When a reaction-center chlorophyll molecule absorbs energy, one of its electrons gets bumped up to a primary electron acceptor

25. 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

26. Light is used to produce energy (ATP) and NADPH • There are 2 different photosystems, based on optimal absorbance of "antennae molecules" • PS I = chlor.a molecule (actually a dimer of two molecules) is called P700, because peak absorbance is at 700nm l • PS II = P680 e- H+ e- H+ NADPH e- ATP Light-dependent rxns in the chloroplast here's the way it's laid out:  animation

27. e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem I Photosystem II Figure 10.14  A mechanical analogy for the light reactions

28. The Light-trapping part: a synopsis • The two photosystems work independently and continuously • The boosted electrons are passed down Electron Transport Chains (remember the last chapter?) and the energy released is used to turn ADP + Pi--->ATP ="phosphorylation“ • Also, WATER is split into its components • The oxygen is let off and the H+ gets passed through photosystems • This accumulation of hydrogen ions builds up a Chemiosmotic Gradient inside the thylakoid

29. Noncyclic Electron Flow • Noncyclic electron flow • Is the primary pathway of energy transformation in the light reactions

30. Cyclic Electron Flow • Under certain conditions, photoexcited electrons take an alternative path • In cyclic electron flow • Only photosystem I is used • Only ATP is produced

31. How ATP is generated from PS-II • The H+ ions (from the splitting of water)build up • Redox reactions of electron transport chains generate a H+ gradient across a membrane • ATP synthase (enzyme) • Uses this proton-motive force to make ATP

32. 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

33. Key Higher [H+] Lower [H+] Chloroplast Mitochondrion CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE H+ Diffusion Thylakoid space Intermembrance space Electron transport chain Membrance ATP Synthase Stroma Matrix ADP+ P ATP H+ Figure 10.16 • Chemiosmosis is used to generate ATP in chloroplasts and mitochondria

34. The Light-Independent Reactions of Photosynthesis • 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 (Krebs) • Occurs in the stroma Trivia: Cycle was deduced in 1960 by Calvin & Benson, using 14C isotope tracer to label CO2 path thru sugarcane 1961 Nobel

35. The Calvin Cycle • The Calvin cycle has three phases • Carbon fixation • Reduction • Regeneration of the CO2 acceptor • The starting (and ending) compound is a 5-C sugar with 3 phosphates attached = RuBP Ribulose biphosphate In the initial step, CO2 binds to RuBP ------------> RuBPCO2 …which then splits into 2 molecules of PGAL (phosphoglyceraldehyde) 3-C* each (enzyme: RuBP carboxylase) * = this is why it's called the "three carbon pathway" rubisco

36. 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 sum of reactions in the Calvin cycle is the following: 6 CO2 + 12 NADPH + 12 H+ + 18 ATP → C6H12O6 + 6 H2O + 12 NADP+ + 18 ADP + 18 Pi Phase 1: Carbon fixation C3 pathway Phase 3:Regeneration ofthe CO2 acceptor(RuBP) Phase 2:Reduction

37. Calvin Cycle • 6 turns of the cycle assimilates enough carbon to produce one 6-C molecule of sugar (glucose) overall equation: 6RuBP + 6 CO2 + 18 ATP + 12 NADPH + 12 H+ + 12 H2O ends up as: 6RuBP + glucose + 18 Pi + 18ADP + 12 NADP+ + H2O

38. C3 photosynthesis: Problems with C3 photosynthesis: • oxygen competes with carbon dioxide for the active site on RuBP carboxylase (“rubisco”) enzyme • So…rubisco has relatively low affinity for carbon dioxide, esp. at low concentrations • Rice, wheat, and soybeans are three examples of agriculturally important C3 plants

39. Concept 10.4: Alternative mechanisms of carbon fixation • …have evolved in hot, arid climates • On hot, dry days, plants must close their stomata • Conserving water but limiting access to CO2 • Causing oxygen to build up

40. Photorespiration: An “Evolutionary Relic”*? • In photorespiration • O2 substitutes for CO2 in the active site of the enzyme rubisco • The photosynthetic rate is reduced • “photo” = occurs in light • “respiration” = consumes O2 while producing CO2 • However, unlike cellular respiration, it produces no ATP (in fact, consumes it) • Produces no sugar, actually inhibits Calvin Cycle *Remember- the young atmosphere contained little O2, so rubisco was less-likely to be inhibited

41. C4 Plants have evolved a better “means to an end” • 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

42. C4-adapted plants While most plants bind CO2 to RuBP in 1st step of the light-independent rxns (3-C pathway), some plants can go through a 4-C pathway • …but not until it goes through an additional series of reactions called the "Hatch-Slack Pathway“ • catalyzed by enzyme PEP carboxylase; has higher CO2 affinity; keeps CO2 gradient in leaf • 1st step binds CO2 to PEP (phosphoenolpyruvate) to form a 4-carbon compound called oxaloacetic acid (like Krebs!) • These four-carbon compounds are then exported to bundle sheath cells, where the CO2 is then transferred to RuBP and enters the Calvin Cycle. • C-4 is better in drought-ridden areas or with "crowded" leaves (little gas exchange) • Maximizes the minimal CO2 PEP binds CO2 faster at lower conc. • Examples: sugarcane, corn, many grasses

43. 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 you must be able to COMPARE & CONTRAST the pathways ex: compare response of C3 Kentucky bluegrass -vs- C4 crabgrass

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

45. 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

46. 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

47. Summary • Organic compounds produced by photosynthesis • Provide the energy and building material for ecosystems STILL CONFUSED?  Tutorials:  check out some photosynthesis animations