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Photosynthesis: Life from Light
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How are they connected? Heterotrophs and Autotrophs Autotrophs
making energy & organic molecules from ingesting organic molecules glucose + oxygen carbon + water + energy dioxide C6H12O6 6O2 6CO2 6H2O ATP + exergonic Where’s the ATP? Autotrophs So, in effect, photosynthesis is respiration run backwards powered by light. Cellular Respiration oxidize C6H12O6 CO2 & produce H2O fall of electrons downhill to O2 exergonic Photosynthesis reduce CO2 C6H12O6 & produce O2 boost electrons uphill by splitting H2O endergonic making energy & organic molecules from light energy + water + energy glucose + oxygen carbon dioxide 6CO2 6H2O C6H12O6 6O2 light energy + endergonic
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Plant structure Obtaining raw materials sunlight CO2 H2O Nutrients
leaves = solar collectors CO2 stomates = gas exchange Found under leaves H2O uptake from roots Nutrients N, P, K, S, Mg, Fe…
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Plant structure Chloroplasts double membrane stroma thylakoid sacs
grana stacks Chlorophyll & ETC in thylakoid membrane H+ gradient built up within thylakoid sac Photosynthetic prokaryotes use specialized cell membrane regions for photosynthesis A typical mesophyll cell has chloroplasts, each about 2-4 microns by 4-7 microns long. Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma are membranous sacs, the thylakoids. These have an internal aqueous space, the thylakoid lumen or thylakoid space. Thylakoids may be stacked into columns called grana. H+
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Pigments of photosynthesis
Why does this structure make sense? chlorophyll & accessory pigments “photosystem” embedded in thylakoid membrane structure function Orientation of chlorophyll molecule is due to polarity of membrane.
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A photosystem Is composed of a reaction center surrounded by a number of light-harvesting complexes Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 e– The light-harvesting complexes consist of pigment molecules bound to particular proteins Funnel the energy of photons of light to the reaction center Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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Light: absorption spectra
Photosynthesis gets energy by absorbing wavelengths of light chlorophyll a (dominant pigment) absorbs best in red & blue wavelengths & least in green other pigments with different structures absorb light of different wavelengths Why are plants green?
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It’s the Dark Reactions!
Photosynthesis Light reactions light-dependent reactions energy production reactions convert solar energy to chemical energy ATP & NADPH Calvin cycle light-independent reactions sugar production reactions uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6 It’s the Dark Reactions!
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Light reactions Electron Transport Chain (like cell respiration!)
membrane-bound proteins in organelle electron acceptor NADPH proton (H+) gradient across inner membrane ATP synthase enzyme Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.
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Photosystems 2 photosystems in thylakoid membrane reaction center
act as light-gathering “antenna complex” Photosystem II chlorophyll a P680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light reaction center Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a. This starts the light reactions. Don’t compete with each other, work synergistically using different wavelengths.
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ETC of Photosynthesis ETC produces from light energy
ATP & NADPH NADPH (stored energy) goes to Calvin cycle PS II absorbs light excited electron passes from chlorophyll to “primary electron acceptor” at the REACTION CENTER. splits H2O (Photolysis!!) O2 released to atmosphere Provides “replacement” electrons to chlorophyll ATP is produced for later use
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Electron transport chain
Produces NADPH, ATP, and oxygen Figure 10.13 Photosystem II (PS II) Photosystem-I (PS I) ATP NADPH NADP+ ADP CALVIN CYCLE CO2 H2O O2 [CH2O] (sugar) LIGHT REACTIONS Light Primary acceptor Pq Cytochrome complex PC e P680 e– + 2 H+ Fd reductase Electron Transport chain Electron transport chain P700 + 2 H+ + H+ 7 4 2 8 3 5 1 6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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ETC of Photosynthesis Photosystem II Photosystem I
Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide
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Noncyclic Photophosphorylation
Light reactions elevate electrons in 2 steps (PS II & PS I) PS II generates energy as ATP PS I generates reducing power as NADPH 1 photosystem is not enough. Have to lift electron in 2 stages to a higher energy level. Does work as it falls. First, produce ATP -- but producing ATP is not enough. Second, need to produce organic molecules for other uses & also need to produce a stable storage molecule for a rainy day (sugars). This is done in Calvin Cycle!
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In cyclic electron flow
Only ATP is produced Calvin Cycle uses more ATP than NADPH Primary acceptor Pq Fd Cytochrome complex Pc NADP+ reductase NADPH ATP Figure 10.15 Photosystem II Photosystem I Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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From Light reactions to Calvin cycle
Chloroplast stroma Need products of light reactions to drive synthesis reactions ATP NADPH What is there left to do? Make sugar!
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From CO2 C6H12O6 CO2 has very little chemical energy
fully oxidized C6H12O6 contains a lot of chemical energy reduced endergonic Reduction of CO2 C6H12O6 proceeds in many small uphill steps each catalyzed by specific enzyme using energy stored in ATP & NADPH
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Calvin cycle 3C 5C 6C 3C 3C 3C (3) 6x x6 6x CO2 1. Carbon fixation
ribulose bisphosphate 1. Carbon fixation 3. Regeneration of RuBP 5C RuBP (3) Rubisco 6C 3 ADP 3 ATP -enzyme that Binds CO2 to RuBP G3P to make glucose 3C x6 G3P 3C 6x sucrose cellulose etc. RuBP = ribulose bisphosphate Rubisco = ribulose bisphosphate carboxylase PGA = phosphoglycerate PGAL = phosphoglyceraldehyde 2. Reduction 6 NADP 6 NADPH 6 ADP 6 ATP 3C 6x
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Calvin cycle Six turns of Calvin Cycle = 1 glucose
G3P glucose carbohydrates lipids amino acids nucleic acids
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Summary Light reactions Calvin cycle produced ATP produced NADPH
consumed H2O produced O2 as by product Calvin cycle consumed CO2 produced PGAL regenerated ADP regenerated NADP ADP NADP
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The Importance of Photosynthesis: A Review
A review of photosynthesis The Importance of Photosynthesis: A 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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Summary of photosynthesis
6CO2 6H2O C6H12O6 6O2 light energy + Where did the CO2 come from? Where did the CO2 go? Where did the H2O come from? Where did the H2O go? Where did the energy come from? What’s the energy used for? What will the C6H12O6 be used for? Where did the O2 come from? Where will the O2 go? What else is involved that is not listed in this equation?
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An Evolutionary Quirk As long as plants can keep their stomata open and exchanging gases with the environment, there is no problem. Rubisco evolved in conditions of low O2 concentrations. As a result, its active site has an affinity for oxygen This is a problem for plants that only fix carbon using the Calvin Cycle! (C3 Plants – rice, wheat, soybeans)
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Photorespiration: An Evolutionary Relic?
On hot, dry days, C3 plants close their stomata Conserving water but limiting access to CO2 Causing oxygen to build up In photorespiration O2 substitutes for CO2 in the active site of the enzyme rubisco Uses ATP, but produces no sugar Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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C4 Plants- carbon is fixed twice
C4 plants minimize the cost of photorespiration when CO2 levels are lower than O2 levels By incorporating CO2 into four carbon compounds in mesophyll cells using a different enzyme These four carbon compounds Are exported to bundle sheath cells, where they are broken down to release CO2 to the Calvin cycle Ex: corn, sugarcane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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CAM Plants- stomata reversed
Open their stomata at night incorporate CO2 into 4C organic acid and store in vacuole During the day, the stomata close And the CO2 is released from the organic acids for use in the Calvin cycle Ex: cacti, pineapples, aloe, jade plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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The 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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