1 Lecture 16 Oct 7, 2005 Photosynthesis I. Light Reactions

2 Lecture Outline 1.Importance of Photosynthesis to all life on earth - primary producer, generates oxygen, ancient 2. What needs to be accomplished in photosynthesis 3. Structure of the chloroplast – 3 functional spaces 4.How light energy is harvested – antenna complex, pigments, light spectrum - splitting of water, excitation of e - 5. What work is done with capture light energy - “light” reactions - noncyclic e - transport - ATP and NADPH + H + - cyclic electron transport – primarily ATP, limit O 2 6. How ATP and NADPH + H + power anabolic pathways - “dark” reactions – the Calvin Cycle

3 Figure 10.1 Photoautotrophs Make their own “food” by light Heterotrophs Obtain “food” from “other” sources ∆G < 0 Light energy Heat Motion Photo Auto trophs Hetero trophs

4 Photosynthesis is the ultimate energy source for almost all life on earth for almost all life on earth Solar Energy Input Herbivore Biomass Production (Reflection/Heat) (Heat Motion) Carnivore Omnivore Biomass (Heat) Plant Biomass Production Net Energy Absorbed And utilized Eat producers Net Energy utilized Net utilized

5Photosynthesis H2OH2OH2OH2O CO 2 O2O2O2O2 C 6 H 12 O 6 Carbohydrate Light Oxidized Carbon Input Reduced Carbon Output Waste Product Basis for Heterotroph Respiration

6 Photosynthesis is a remarkably similar process at the molecular/cell biology level in a wide diversity of organisms Evolutionarily Related Process, or an Evolutionarily Conserved Process “ancient”

7 (a) Plants (b) Multicellular algae (c) Unicellular protist 10  m 40  m (d) Cyanobacteria 1.5  m (e) Pruple sulfur bacteria Figure 10.2 Cyanobacteria “blue-green algae” Prokaryotes single cells stick together as mats (but no cooperation) EuglenaChlamydamonas Photosyntheic Protists (Eukaryotes) single cell aquatic Plants An entire Kingdom Non-Vascular Plants true algae bryophytes -liverworts -mosses Vascular Plants Ferns Gymnosperms -conifers Angiosperms -monocots - dicots Photosynthetic Organisms

8 Photosynthesis – is comprised of TWO Distinct Processes which occur simultaneously (in most photosynthetic organisms) Energy Capture Processes Energy Utilization Processes Make Carbohydrate NEED ATP and NADPH NOTE: ONLY OCCUR IN THE PRESENCE OF AN ENERGY SOURCE use light to Make ATP, NADPH O 2 gas made as by-product O 2 gas made as by-product “Light” Reactions “Dark” Reactions Calvin Cycle

9 H2OH2O CO 2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH 2 O] (sugar) NADPH NADP  ADP + P O2O2 ATP Figure 10.5 LightReactions(energycapture) “Dark Reactions” Calvin Cycle (energy utilization) Interdependent

10 Structures all Photosynthetic Eukaryotes have in common The organelle called the Chloroplast This organelle is the SITE of photosynthesis where ALL photosynthetic reactions occur Blue green algae (cyanobacteria) do not have internal membranes (they are prokaryotes!) but they themselves resemble chloroplasts The extensively folded plasma membrane of cyanobacteria lays the same role as thylakoid membrane in chloroplasts

11 Chloroplast Mesophyll Cell 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid Granum Stroma 1 µm Vein Leaf cross section Figure 10.3 Mesophyll CO 2 O2O2 Stomata

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13 Chloroplasts -Contain their own DNA -Contain bacterial-like ribosomes -Believed derived from prokaryotic ancestor cyanobacterium = blue-green alga -Double membrane organelle three functional spaces defines three functional spaces

14 Stroma Thylakoid Space Thylakoid Membrane Intermembrane Space (transports things in and out of the chloroplast, but not central to photosynthesis itself Inner Chlorplast Membrane OuterChlorplast Membrane 3 Central Players

15 H+H+ Thylakoid Space Thylakoid Membrane - Site of Light Harvesting is where ATP and NADPH are made Stroma Stroma - is where all the carbon fixation reactions take place Thylakoid Space - is the transient energy storage shed for H + ions generated in the in the light lightreactions pH5.5 pH 8.5

16 Thylakoid Membrane Thylakoid Membrane – Light Harvesting Complex Photosystem II - Antenna Complex - Water-Splitting Complex - Reaction Center “Excitation Complex”

17 Photon Thylakoid Photosystem II STROMA Thylakoidmembrane Transfer of energy Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure AntennaLight-harvestingcomplexes Primary election acceptorReactioncenter Special chlorophyll a molecules e–e– e-e- H 2 O – O 2 WaterSplittingComplex

18 Photosystem Antenna Complex - chlorophyll & accessory pigments

19 The Antenna Complex proteins which hold PIGMENTS Pigments: Chlorophylls - absorb all but greens Xanthophylls - absorb all but yellows Carotenoids - absorb all but orange/reds Phycocyanin - absorb all but blue-green

20 Reflected light - the colors we see Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7

21 The electromagnetic spectrum the higher the energy, the shorter the wavelength Gamma rays X-raysUVInfrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy Figure 10.6

22 Absorption Spectra of Antenna Pigments Absorption of light by chloroplast pigments Chlorophyll a Wavelength of light (nm) Chlorophyll b Carotenoids Figure 10.9

23 Excitation of Chlorophyll by Light 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 Excited state Energy of election Chlorophyll molecule Ground state Photon e–e– Figure A Heat Photon(fluorescence)

24 Figure B chlorophyll Isolated chlorophyll when illuminated fluoresce red – will fluoresce red, giving off light and heat Blue light absorbed e-e- Red light Emitted With Heat

25 Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground state Photon e–e– Figure A Reaction Center Chlorophyll electron boosted to high energy level Capturex x need replacement electron e - transferred to an electron transport chain

26 Water splitting complex (a protein in thylakoid membrane) H O H H O H O=O H+H+ H+H+ H+H+ H+H+ e-e- e-e- e-e- e-e- Discard this, yuk These e - go to replace electron lost by chlorophyll We’ll save H + in the thylakoid space

27 Stroma Thylakoid Space Thylakoid Membrane H+H+ e-e- H O H 2 O=O (a gas) H+H+ H+H+ H+H+ e-e- e-e- e-e- H O H H O-O- H+H+ H O-O- H O-O- H O-O- e-e- e-e- PS II e-e- PS I e-e- NADP + NADPH H + ATPase H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ An “H + pump” ADP + P i ATP pH 5.5 pH 8.5

28 Key Players in the light reactions a. photosystem II: captures light energy “boost” e - to a higher energy level, splits water into H + e - and O 2 b. Electron transport H + pump: lets e - “fall” to lower energy level, uses energy to form H + gradient

29 c. another photosystem: photosystem I: captures light energy re-“boosts” e - to a higher energy level – forms NADPH + H + *makes reducing equivalents* d. ATP synthase (H + ATPase): uses H + gradient to power ATP synthesis

30 Produces NADPH, ATP, and oxygen Figure 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 H + + H Photosystem II -Light Energy used to Form H + gradient (ATP Synthesis) Photosystem I -Light Energy used to make reducing equivalents (NADPH + H + ) Non-Cyclic Electron Flow

31 Non-Cyclic Electron Flow Mill makes ATP e–e– e–e– e–e– e–e– e–e– Photon Photosystem II Photosystem I e–e– e–e– NADPH Photon Figure Photosystem I -Light Energy can also be used to make H + gradient)

32 cyclic electron flow –photosystem I is used primarily –Primarily ATP is produced –Little O 2 produced Primary acceptor Pq Fd Cytochrome complex Pc Primary acceptor Fd NADP + reductase NADPH ATP Figure Photosystem II Photosystem I NADP +

33 Cyclic e - flow Photosystem II II Photosystem I ElectronTransport H + gradient (ATP synth) NADP + Reductase

34 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 Light Dependent Reactions ProduceNADPH AndATP To power The Calvin Cycle

35 Next Time: the DARK Side the Light independent reactions The Calvin Cycle

36 Summary 1.Photosynthesis ultimate source of energy for life On earth 2.Ancient Process – highly conserved 3.Thylakoid membrane, Thylakoid Space, Stroma 4.Photosynthetic light reactions -capture energy from sunlight – light harvesting pigments -use energy to “split” water -use energy to boost electron to high energy level (PS II) -electron transport lets electron fall to low energy state, energy used to make H + gradient (ATP) -electron re-boosted by light absorption to high energy state (PS I) - high energy electron used to reduce NADP + to NADPH + H + 5. Can vary relative amount of ATP/NADPH made by cyclic electron flow