Photosynthesis The Sun Ultimate Energy 1 5 x

Photosynthesis

The Sun - Ultimate Energy • 1. 5 x 1022 k. J falls on the earth each day • 1% is absorbed by photosynthetic organisms and transformed into chemical energy • 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2 • 1011 tons (!) of CO 2 are fixed globally per year • Formation of sugar from CO 2 and water requires energy • Sunlight is the energy source!

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 CO 2 in a series of reactions in the stroma Light: H 2 O + ADP + Pi + NADP+ + light O 2 + ATP + NADPH + H+ CF: CO 2 + ATP + NADPH + H+ Glucose + ADP + Pi + NADP+ Sum: CO 2 + light Glucose + O 2

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

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.

• 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

Photosynthetic Pigments

Chlorophyll • Photoreactive, isoprenebased 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

Accessory Pigments Carotenoid Phycobilin • Absorb light through conjugated double bond system • Absorb light at different wavelengths than Chlorophyll • Broaden range of light absorbed

Absorption Spectra of Major Photosynthetic Pigments

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

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 n 6 (n= distance betwn) • Can only transfer energy to a donor of equal or lower energy

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 F 1 Fo 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 F 1 Fo ATPase.

Photosynthetic ETC

Eukaryotic Photosystems • PSI (P 700) and PSII (P 680) • 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

Z-Scheme

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 • A 1 = a special PSI quinone • Cytochrome b 6/cytochrome f complex is a proton pump

P 680(PSII) to PQ Pool

Excitation, Oxidation and Re-reduction of P 680 • Special pair chlorophyll in P 680 (PS II) is excited by a photon • P 680* transfer energy as a e- to pheophytin A through a charge separation step. • The oxidized P 680+ is rereduced by e- derived from the oxidation of water

Oxygen evolution by PSII • Requires the accumulation of four oxidizing equivalents • P 680 has to be oxidized by 4 photons • 1 e- is removed in each of four steps before H 2 O is oxidized to O 2 + 4 H+ • Results in the accumulation of 4 H+ in lumen

Electrons are passed from Pheophytin to Plastoquinone • Plastoquinone is analagous to ubiquinone • Lipid soluble e- carrier • Can form stable semiquinone intermediate • Can transfer 2 electrons on at a time.

Transfer of e- from PQH 2 to Cytbf Complex (another Q-cycle) • Electrons must be transferred one at a time to Fe-S group. • Another Q-cycle • First PQH 2 transfers one electron to Fe-S group, a PQformed. 2 H+ pumped into lumen • A second PQH 2 transfers one electron to Fe-S group and the one to reduce the first PQ- to PQH 2. 2 more H+ pumped into lumen • 4 protons pumped per PQH 2. Since 2 PQH 2 produced per O 2 evolved 8 protons pumped

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

Photophosphorylation

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 • CF 1 -CFo ATP synthase is similar to the mitochondrial ATP synthase

Chloroplast CF 1 CFo ATPase • Similar in structure to mitochondrial F 1 Fo ATPase • CF 1 domain (ATP synthesis) extends into the stroma. • Many of the protein subunits are encoded by the chloroplast genome

Chloroplast Proton Motive Force (Dp) • What contributes more to PMF, DY or Dp. H? • In the light Dp. H=3 • DY is negligible due to counter ion movement in and out of the lumen • DG for export of one mole H+ across thylakoid membrane = -17 k. J/mole • DGo’ for ATP formation = 30. 5 k. J/mole • Since 12 moles of protons gives – 200 k. J of energy • Experiment show that 3 ATPs are generated per mole of O 2 produced

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 (Dp. H ~3. 5). • Photophosphorylation produces 3 ATPs per O 2 produced

Non-cyclic photosynthetic ETC • NADPH and ATP produced • Involves both PSI and PSII cyclic photosynthetic ETC • only ATP produced • Involves only PSI

Cyclic Photosynthetic ETC • Involves only PSI • Reduced ferredoxin transfers e- to Cytobf complex which then re-reduces Plastocyanin and finally the oxidized P 700 of PSI • No NADPH produce. Only ATP • Levels of NADP+ thought to regulated this process. • Low NADP+ activates cyclic ETC • Observed in vitro.

Arrangement of photosystems in thylakoid membrane

Arrangement of photosystems in thylakoid membrane • 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)