photosynthesis. the sun - ultimate energy 1.5 x 10 22 kj falls on the earth each day 1% is absorbed...
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Photosynthesis
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The Sun - Ultimate Energy
• 1.5 x 1022 kJ falls on the earth each day
• 1% is absorbed by photosynthetic organisms and transformed into chemical energy
• 6CO2 + 6H2O C6H12O6 + 6O2
• 1011 tons (!) of CO2 are fixed globally per year
• Formation of sugar from CO2 and water requires energy
• Sunlight is the energy source!
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Photosynthesis: Light Reactions and Carbon
Fixation• The light reactions capture light energy and
convert it to chemical energy in the form of reducing potential (NADPH) and ATP with evolution of oxygen
• During carbon fixation (dark reactions) NADPH and ATP are used to drive the endergonic process of hexose sugar formation from CO2 in a series of reactions in the stroma
Light: H2O + ADP + Pi + NADP+ + light O2 + ATP + NADPH + H+
CF: CO2 + ATP + NADPH + H+ Glucose + ADP + Pi + NADP+
Sum: CO2 + light Glucose + O2
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Chloroplast• Inner and outer membrane = similar to
mitochondria, but no ETC in inner membrane.• Thylakoids = internal membrane system.
Organized into stromal and granal lammellae.• Thylakoid membrane - contains
photosynthetic ETC• Thylakoid Lumen – aqueous interior of
thylkoid. Protons are pumped into the lumen for ATP synthesis
• Stroma – “cytoplasm” of chloroplast. Contains carbon fixation machinery.
• Chloroplasts possess DNA, RNA and ribosomes
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Conversion of Light Energy to Chemical Energy
• Light is absorbed by photoreceptor molecules (Chlorophylls, carotenoids)
• Light absorbed by photoreceptor molecules excite an electron from its ground state (low energy) orbit to a excited state (higher energy) orbit .
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• The high energy electron can then return to the ground state releasing the energy as heat or light or be transferred to an acceptor.
• Results in (+)charged donor and (–)charged acceptor = charge separation
• Charge separation occurs at photocenters. • Conversion of light NRG to chemical NRG
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Photosynthetic Pigments
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Chlorophyll• Photoreactive,
isoprene-based pigment • A planar, conjugated
ring system - similar to porphyrins
• Mg in place of iron in the center
• Long chain phytol group confers membrane solubility
• Aromaticity makes chlorophyll an efficient absorber of light
• Two major forms in plants Chl A and Chl B
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Accessory Pigments
• Absorb light through conjugated double bond system • Absorb light at different wavelengths than Chlorophyll• Broaden range of light absorbed
Carotenoid
Phycobilin
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Absorption Spectra of Major Photosynthetic
Pigments
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Harvesting of Light and Transfer of Energy to
Photosystems• Light is absorbed by
“antenna pigments” and transferred to photosystems.
• Photosystems contain special-pair chlorophyll molecules that undergo charge separation and donate e- to the photosynthetic ETC
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Resonance Transfer• Energy is transfer through antenna
pigment system by resonance transfer not charge separation.
• An electron in the excited state can transfer the energy to an adjacent molecule through electromagnetic interactions.
• Acceptor and donor molecule must be separated by very small distances.
• Rate of NRG transfer decreases by a factor of n6 (n= distance betwn)
• Can only transfer energy to a donor of equal or lower energy
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Photosynthetic Electron Transport and
Photophosphorylation • Analogous to respiratory ETC and oxidative
phosphorylation
• Light driven ETC generates a proton gradient which is used to provide energy for ATP production through a F1Fo type ATPase.
• The photosynthetic ETC generates proton gradient across the thylakoid membrane.
• Protons are pumped into the lumen space.
• When protons exit the lumen and re-enter the stroma, ATP is produced through the F1Fo ATPase.
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Photosynthetic ETC
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Eukaryotic Photosystems
• PSI (P700) and PSII (P680) • PSI and PSII contain special-pair
chlorophylls• PSI absorbs at 700 nm and PSII absorbs
at 680 nm • PSII oxidizes water (termed “photolysis") • PSI reduces NADP+ • ATP is generated by establishment of a
proton gradient as electrons flow from PSII to PSI
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Z-Scheme
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The Z Scheme• An arrangement of the electron
carriers as a chain according to their standard reduction potentials
• PQ = plastoquinone • PC = plastocyanin • "F"s = ferredoxins
• Ao = a special chlorophyll a
• A1 = a special PSI quinone
• Cytochrome b6/cytochrome f complex is a proton pump
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P680(PSII) to PQ Pool
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Excitation, Oxidation and Re-reduction of
P680• Special pair
chlorophyll in P680 (PS II) is excited by a photon
• P680* transfer energy as a e- to pheophytin A through a charge separation step.
• The oxidized P680+ is re-reduced by e- derived from the oxidation of water
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Oxygen evolution by PSII
• Requires the accumulation of four oxidizing equivalents
• P680 has to be oxidized by 4 photons
• 1 e- is removed in each of four steps before H2O is oxidized to O2 + 4H+
• Results in the accumulation of 4 H+ in lumen
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Electrons are passed from Pheophytin to Plastoquinone
• Plastoquinone is analagous to ubiquinone
• Lipid soluble e- carrier• Can form stable semi-
quinone intermediate• Can transfer 2
electrons on at a time.
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Transfer of e- from PQH2 to Cytbf Complex (another Q-
cycle)• Electrons must be transferred one at a time to Fe-S group.
• Another Q-cycle• First PQH2 transfers one
electron to Fe-S group, a PQ- formed. 2 H+ pumped into lumen
• A second PQH2 transfers one electron to Fe-S group and the one to reduce the first PQ- to PQH2. 2 more H+ pumped into lumen
• 4 protons pumped per PQH2. Since 2 PQH2 produced per O2 evolved 8 protons pumped
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Terminal Step in Photosynthetic ETC
• Electrons are transferred from the last iron sulfur complex to ferredoxin.
• Ferredoxin is a water soluble protein coenzyme
• Very powerful reducing agent.
• Ferredoxin is then used to reduce NADP+ to NADPH by ferredoxin-NADP+ oxidoreductase
• So NADP+ is terminal e- accepter
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Photophosphorylation
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Photophosphorylation• Light-Driven ATP Synthesis • Electron transfer through the
proteins of the Z scheme drives the generation of a proton gradient across the thylakoid membrane
• Protons pumped into the lumen of the thylakoids flow back out, driving the synthesis of ATP
• CF1-CFo ATP synthase is similar to the mitochondrial ATP synthase
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Chloroplast CF1CFo ATPase
• Similar in structure to mitochondrial F1Fo ATPase
• CF1 domain (ATP synthesis) extends into the stroma.
• Many of the protein subunits are encoded by the chloroplast genome
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Chloroplast Proton Motive Force (p)• What contributes more to PMF, or pH?
• In the light pH=3
• is negligible due to counter ion movement in and out of the lumen
• G for export of one mole H+ across thylakoid membrane = -17 kJ/mole
• Go’ for ATP formation = 30.5 kJ/mole
• Since 12 moles of protons gives –200 kJ of energy
• Experiment show that 3 ATPs are generated per mole of O2 produced
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Energy Balance Sheet• 8 photons (4 e-) generate 1 oxygen
and 2 NADPH
• Photosynthetic ETC pumps between 8 and 12 protons across thylakoid membrane to generate proton gradient (pH ~3.5).
• Photophosphorylation produces 3 ATPs per O2 produced
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Non-cyclic photosynthetic ETC
cyclic photosynthetic ETC
•NADPH and ATP produced
•Involves both PSI and PSII
•only ATP produced
•Involves only PSI
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Cyclic Photosynthetic ETC• Involves only PSI
• Reduced ferredoxin transfers e- to Cytobf complex which then re-reduces Plastocyanin and finally the oxidized P700 of PSI
• No NADPH produce.Only ATP• Levels of NADP+ thought to
regulated this process. • Low NADP+ activates cyclic ETC• Observed in vitro.
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Arrangement of photosystems in thylakoid
membrane
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• PSII primarily present in granal lamellae
• Light harvesting antennae complexes (LHC) are also present in the granal lamellae.
• Under low light conditions LHCs are closely associated with PSII, Under high light condition the 2 disassociate.
• PSI and ATPase are in the stroma lamellae.
• Physical separation suggest that mobile electron carrier must be involved (i.e. PQ and Plastocyanin)
Arrangement of photosystems in thylakoid
membrane