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Ch. 10 Photosynthesis
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powers most cellular work
Photosynthesis Energy flows into ecosystem as sunlight, Feeds the Biosphere Converts solar E into chemical E Light energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O2 ATP powers most cellular work Heat energy Figure 9.2
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Energy Transformations
Photoautotrophs (producers) Use E of sunlight to make organic molecules from water and CO2 (a) Plants (b) Multicellular algae (c) Unicellular protist 10 m 40 m (d) Cyanobacteria 1.5 m (e) Pruple sulfur bacteria Figure 10.2
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Photosynthesis converts light E to the chemical E of food
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Heterotrophs Obtain organic material f/ other organisms Consumers of the biosphere
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Chloroplasts: Site of Photosynthesis (plants)
Leaf Vein Leaf cross section Figure 10.3 Mesophyll CO2 O2 Stomata
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Leaf Anatomy
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Chloroplasts Chloroplast Structure
Contain grana which consisting of thylakoid stacks Chloroplast Mesophyll 5 µm Outer membrane Intermembrane space Inner Thylakoid Granum Stroma 1 µm
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Chloroplasts
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Photosynthesis summary reaction
6 CO H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O
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Chloroplasts split water into
H2 and O2, incorporating the e- of H2 into sugar molecules 6 CO2 12 H2O Reactants: Products: C6H12O6 6 H2O 6 O2 Figure 10.4
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Photosynthesis as a Redox Process
Water is oxidized, CO2 is reduced Protons and Electron are taken from water and added to CO2
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The Two Stages of Photosynthesis: A Preview
Light Reactions Occurs on thylakoid membranes Converts solar E to chemical E Dark Reaction (Calvin Cycle) Occurs in the stroma Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power
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Overview of photosynthesis
CO2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH2O] (sugar) NADPH NADP ADP + P O2 Figure 10.5 ATP
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Lets Talk about Light Form of electromagnetic E, travels in waves
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Distance between the crests of waves
Wavelength (l) Distance between the crests of waves Determines the type of electromagnetic E More Powerful
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Electromagnetic spectrum
Entire range of electromagnetic E, or radiation Gamma rays X-rays UV Infrared Micro- waves Radio 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m 380 450 500 550 600 650 700 750 nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy
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Visible light spectrum
Colors of light we can see l’s that drive photosynthesis
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Photosynthetic Pigments: The Light Receptors
Substances that absorb visible light
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Reflect light, which include the colors we see
Pigments Reflect light, which include the colors we see Light Reflected Chloroplast Absorbed light Granum Transmitted Figure 10.7
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Transmitted vs. Absorbed Light
Figure 10.8 White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer Slit moves to pass light of selected wavelength Green The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue 1 2 3 4 100
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Absorption spectra of 3 types of pigments
Absorption of light by chloroplast pigments Chlorophyll a Wavelength of light (nm) Chlorophyll b Carotenoids
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Action spectrum of a pigment
Effectiveness of different l of radiation in driving photosynthesis (measured by O2 release) Rate of photosynthesis Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. (b)
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First demonstrated by Theodor W. Engelmann
400 500 600 700 Aerobic bacteria Filament of alga 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. (c) Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. CONCLUSION
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Chlorophylls: Photosynthetic Pigments
Chlorophyll a Main photosynthetic pigment Chlorophyll b Accessory pigment C CH CH2 N H3C Mg H CH3 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
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Other accessory pigments
Assesory proteins Other accessory pigments Absorb different ls of light and pass the E to chlorophyll a
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Excitation of Chlorophyll by Light
When a pigment absorbs light electrons go from their ground state to an excited state (unstable) Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground e– Figure A
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Photosystems I and II NADPH e– Mill makes ATP Photon Photosystem II
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Starter Question Compare and contrast the electron transport chain in cellular respiration with the light reactions in photosynthesis. Be sure to indicate similarities and differences
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Photosystem II and I: Site of Photophosphorylation
Proton Motive Force? Non Cyclic Flow to Calvin Cycle
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Chemiosmosis in Chloroplasts v. Mitochondria
Spatial organization of chemiosmosis Key Higher [H+] Lower [H+] Mitochondrion Chloroplast MITOCHONDRION STRUCTURE Intermembrance space Membrance Matrix Electron transport chain H+ Diffusion Thylakoid Stroma ATP P ADP+ Synthase CHLOROPLAST Figure 10.16
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Light reactions and chemiosmosis: Cyclic Flow
NADPH and O2 Not produce. Does not go to Calvin Cycle LIGHT REACTOR NADP+ ADP ATP NADPH CALVIN CYCLE [CH2O] (sugar) STROMA (Low H+ concentration) Photosystem II H2O CO2 Cytochrome complex O2 1 1⁄2 2 Photosystem I Light THYLAKOID SPACE (High H+ concentration) Thylakoid membrane synthase Pq Pc Fd reductase + H+ NADP+ + 2H+ To Calvin cycle P 3 H+ 2 H+ +2 H+
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Calvin cycle Uses ATP and NADPH to convert CO2 to sugar Similar to the citric acid cycle Occurs in the stroma
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Calvin Cycle Happens in the Stroma
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The “Other” Calvin Cycle
(G3P) Input (Entering one at a time) CO2 3 Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 6 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ Glyceraldehyde-3-phosphate 6 ATP ATP 3 ADP CALVIN CYCLE 5 1 G3P (a sugar) Output Light H2O LIGHT REACTION ATP NADPH NADP+ ADP [CH2O] (sugar) CALVIN CYCLE O2 6 ADP Glucose and other organic compounds Phase 1: Carbon fixation From Light Reactions From Light Reactions Phase 3: Regeneration of the CO2 acceptor (RuBP) Phase 2: Reduction Glyceraldehyde-3-P Can go to sugars, amino acids, fatty acids
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Alternative mechanisms of carbon fixation have evolved in hot, arid climates
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On hot, dry days, plants close their stomata
Conserving water but limiting access to CO2 Causing O2 to build up photorespiration
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Photorespiration: An Evolutionary Relic?
Photosynthetic rate is reduced
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Minimize photorespiration
C4 Plants (e.g. corn) Minimize photorespiration Incorporate CO2 into four carbon compounds in mesophyll cells
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4 carbon compounds in bundle sheath cells
release CO2 CO2 Calvin cycle
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C4 leaf anatomy and the C4 pathway
CO2 Mesophyll cell Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C4 plant leaf Stoma Mesophyll C4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Sheath Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure 10.19
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CAM Plants (e.g. pineapple)
Open their stomata at night, CO2 organic acids
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During the day, stomata close
CO2 is released from the organic acids for use in the Calvin cycle
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CAM pathway is similar to the C4 pathway
Spatial separation of steps. In C4 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 CO2 C4 CAM CO2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 Organic acids release CO2 to Calvin cycle Figure 10.20
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Electron transport chain
Review 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 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 O2 CO2 H2O 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
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Organic compounds produced by photosynthesis
Provide the E and building material for ecosystems
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Extra stuff: Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H+ ions and O-2 ions. These O-2 ions combine to form the diatomic O2 that is released.
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