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Chapter 7 - Photosynthesis - Photosynthesis Chapter 7 Leaf...

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Unformatted text preview: Photosynthesis Chapter 7 Leaf Cross Section upper leaf surface photosynthetic cells (see next slide) Cutaway section of leaf two outer membranes inner membrane system (thylakoid disks connected by channels) stroma channel stacked part of thylakoid membrane, one granum Leaf Cross Section Stomates (also called stomata) Guard Cells Photons • Packets of light energy • Each type of photon has fixed amount of energy • Photons having most energy travel as shortest wavelength (blue-violet light) Electromagnetic (EMR) Spectrum (Ana lo g s ig na l) http://www.dnr.sc.gov/ael/personals/pjp b/lecture/spectrum.gif FYI Substances Either… Absorb Reflect, or Transmit… Radiant energy of some/any given wavelength Variety of Pigments Chlorophylls a and b Carotenoids Anthocyanins Phycobilins Chlorophylls Wavelength absorption (%) Main pigments in most photoautotrophs chlorophyll a chlorophyll b Wavelength (nanometers) Figure 7.6a Page 119 Figure 7.7 Page 120 Accessory Pigments percent of wavelengths absorbed Carotenoids, Phycobilins, Anthocyanins beta-carotene phycoerythrin (a phycobilin) wavelengths (nanometers) incoming light reaction center Remember, light actually hits molecules all over the leaf surface all the time (unless it’s night) PHOTOSYSTEM II Reaction Center LE 10-17 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome complex Photosystem II Light Photosystem I Light NADP+ reductase 2 H+ NADP+ + 2H+ Fd NADPH + H+ Pq H2O THYLAKOID SPACE (High H+ concentration) /2 O2 +2 H+ Pc 1 2 H+ To Calvin cycle Thylakoid membrane STROMA (Low H+ concentration) ATP synthase ADP + Pi ATP H+ Where do the atoms go? Reactants: 12H2O 6CO2 Products: 6 O2 C6H12O6 6H2O Light Dependent Reactions Reactions Where do the atoms go? Reactants: 12H2O 6CO2 Products: 6 O2 C6H12O6 6H2O Energy Changes – “Z” Scheme Energy Level e– first transfer chain e – e – second transfer chain e– NADPH sunlight (Photosystem I) (Photosystem II) H2O sunlight 1/2O2 + 2H+ Figure 7.13b Page 123 Chemiosmotic Model of ATP Formation • Electrical and H+ concentration gradients are created between thylakoid compartment and stroma • H+ flows down gradients into stroma through ATP synthesis • Flow of ions drives formation of ATP Chemiosmotic Model for ATP Formation Photolysis in the thylakoid compartment splits water H2O e– H+ is shunted across membrane by some components of the first electron transfer chain Gradients propel H+ through ATP synthases; ATP forms by phosphate-group transfer acceptor ATP SYNTHASE PHOTOSYSTEM II Figure 7.15 Page 124 ADP + Pi ATP Chemiosmosis Water is split INSIDE the thylakoid disk (interior) Chemiosmosis Photosystem I Chemiosmosis • Photosystem I can run in two modes: in the presence of NADP+ it will run in the mode described above (noncyclic flow of e-), generating NADPH. • When the NADPH/NADP+ ratio is high, the complex can run in a manner that generates a proton gradient (and thus ATP) without producing NADPH. Because photosystem II is not involved, oxygen is not produced. Cyclic Electron Flow • Electrons – are donated by P700 in photosystem I to acceptor molecule – flow through electron transfer chain and back to P700 • Electron flow drives ATP formation - no NADPH is formed Cyclic… • Done when cells need additional ATP or there is no NADP+ at the time. Cyclic Electron Flow (follow pink arrows) (follow Fd Photosynthesis Overview Light-independent Light-dependent 12H2O + 6CO2 Water Carbon Dioxide 6O2 + C6H12O6 + 6H2O Oxygen Glucose Water Where do the atoms go? Reactants: 12H2O 6CO2 Products: 6 O2 C6H12O6 6H2O Calvin Cycle: Light Independen Light-Independent Reactions • Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma • Calvin-Benson cycle Calvin-Benson Cycle Overall reactants Carbon dioxide ATP NADPH Overall products Glucose ADP + Pi NADP+ 6 CalvinBenson Cycle CO2 (from the air) CARBON FIXATION 6 6 RuBP unstable intermediate 12 PGA Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated 6 ADP 6 12 12 ATP ATP 12 NADPH 4 Pi 10 PGAL 12 PGAL 2 PGAL Pi Figure 7.16 Page 125 12 ADP 12 Pi 12 NADP+ P glucose ADP, Pi and NADP+ • Byproducts of the Calvin Cycle diffuse through the stroma back to the light-dependent reaction sites • There, they are regenerated Products of the Calvin Cycle “Carbon Fixation” Incorporating a carbon atom from CO2 (an inorganic source) into a stable organic compound The C3 Pathway • In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA (phosphoglycerate) • Because the first intermediate has three carbons, the pathway is called the C3 pathway 6 CalvinBenson Cycle CO2 (from the air) CARBON FIXATION 6 6 RuBP unstable intermediate 12 PGA Carbon fixation 6 ADP 6 12 12 ATP ATP 12 NADPH 4 Pi 12 ADP 12 Pi 12 NADP+ 10 PGAL 12 PGAL 2 PGAL Pi Figure 7.16 Page 125 P glucose Photorespiration in C3 Plants • On hot, dry days stomata close • Inside leaf – Oxygen levels rise – Carbon dioxide levels drop • Rubisco preferentially attaches RuBP (5C) to oxygen instead of carbon dioxide • Only one PGA forms instead of two when the intermediate splits (How many CO2 are needed to make one glucose, then??? How many turns of the Krebs??? What happens if rubisco binds NO CO2???) CO Photorespiration Mitochondrion How many carbons each in RuBP, PGA and glycolate? Oxygen Concentration C4 Photosynthesis: Separation of photosystems and Calvin cycle by compartment, or location location Light dependent reactions (producing O2), and carbon fixation into oxaloacetate (4 carbon molecule) here. Chloroplasts Bundle sheath cells have no PSII activity and therefore no photolysis (or O2) C4 Photosynthesis O n b rig h t, h o t d a ys , s to m a ta c lo s e . O xy g e n b u ild s up in th e m e s o p h yll c e lls , b u t C O 2 is being fixed by an alternate enzyme into 4 carbon compounds which diffuse into the bundle­sheath cells. There, the Calvin cycle proceeds. C4 Plants • Carbon dioxide is fixed twice – In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate – Oxaloacetate is transferred to lowoxygen bundle-sheath cells – Carbon dioxide is released and fixed again in Calvin-Benson cycle which proceeds as normal CAM Photosynthesis Separation of photosystem activity and Calvin-Benson cycle/carbon fixation activity by time of time occurrence (Arid environments) CAM Plants: Crusselacean Acid Metabolism • Carbon is fixed twice (in the same cells) • Night – Photosynthesis does not proceed – Stomata are open, receiving CO2, minimally losing water so there’s net water storage – Carbon dioxide is fixed to form organic acids (malate) • Day – Stomata are closed to prevent water loss – Photosystem II and I are active making ATP, NADPH, and O 2 – CO2 is released from the organic molecules and is fixed in the Calvin-Benson cycle. (It is in relatively high concentration.) Explain why the curves have the shape that they do…. Effect of Temperature… Effect of Light Intensity… Summary of Photosynthesis light LIGHT-DEPENDENT REACTIONS 6O2 12H2O ATP ADP + Pi NADP+ NADPH LIGHT-INDEPENDENT REACTIONS PGA 6CO2 RuBP CALVINBENSON CYCLE PGAL 6H2O P C6H12O6 (phosphorylated glucose) end product (e.g., sucrose, starch, cellulose) Figure 7.21 Page 129 Satellite Images Show Photosynthesis Atlantic Ocean Photosynthetic activity in spring Figure 7.20 Page 128 Please ignore the following slides…. Chemiosmosis Cyclic Electron Flow electron acceptor e– electron e– transfer chain e– Electron flow through transfer chain sets up conditions for ATP formation at other membrane sites. ATP e– Figure 7.12 Page 122 CO2 Levels vs O2 Levels Photorespiration C3 Photosynthesis On bright, hot days, stomata close to prevent water loss. O2 builds up and CO2 depletes. Rubisco bonds with oxygen and photorespiration occurs. LE 10-12 Thylakoid Photosystem Photon Thylakoid membrane Light-harvesting complexes Reaction center STROMA Primary electron acceptor e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) ...
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