Photosynthesis and Carbohydrate Synthesis Lecture

ANABOLISM I Carbohydrates Photosynthesis and Carbohydrate Synthesis in Plants 1 Introduction to Anabolic Pathways So far, we were mainly concerned with CATABOLISM: how to extract energy from biomolecules. The rest of the semester we’ll be concerned with ANABOLISM: how to build biomolecules. Biosynthesis Photosynthesis TCA/OxPhos LIGHT ANN Photosynthesis CO2 + H2O ⇌ CH2O + O2 Use heavy isotope of oxygen (18O) to ”label” the oxygen separately in CO2 and H2O, then test the O2 that is produced. 2 Photosynthesis CO2 + H2O ⇌ CH2O + O2 Light Energy is Converted to ATP in Plant Chloroplasts Photophosphorylation 2H2O ⇌ O2 + 4e– + 4H+ (2NADPH + 2H+) 4e– + 4H+ + CO2 ⇌ CH2O + H2O Both of these half-reactions are VERY unfavorable!! Photosynthesis The Electromagnetic Spectrum Photo-pigments 3 Photosynthesis – If Light Energy is Converted to ATP, how much light is needed? – What is the energy in ONE photon of light? – There is enough Light Energy to make >5 ATP – How do you convert photons to chemical energy? – You go through electrons…. – So we have to do re-dox – – ox-phos in reverse! Energy of 1928: Walther Nernst, Albert Einstein, Max Planck, Robert Millikan, and Max Laue Max Planck was a physicist who initiated the particle theory of light; its not just electromagnetic waves. Einstein developed the concept of a photon, or a quanta of light. Thus, quantum mechanics was born. Electromagnetic energy could be emitted only in quantized form, in other words, the energy could only be a multiple of an elementary unit: E = h! ! = c/l = h•c/l where h is Planck's constant, also known as Planck's action quantum, and ! is the frequency of the radiation. Note that the units of energy are represented by h! and not simply by !. So, the energy comes in units and not just a spectrum or continuum. one mole of photons (1 einstein): h = Planck’s constant (6.626 x 10–34 J•s) c = 3 x108 m/s l = 700 x 10–9 m E = 2.84 x 10–19 J/photon × 6.022 x 1023 photons/einstein E = 17.1 x 104 J/einstein E = 171 kJ/einstein E = 41 kcal per mole of photons….In terms of volts: E = +1.8 volts per mole of photons Photosynthesis – If we have to do re-dox, what is it that must be done? CO2 + H2O ⇌ CH2O + O2 We need an electron acceptor with Eo more positive than water We need an electron donor with Eo more negative than NADPH – First thing is to absorb the light. What does this? ⊝ NADP+ NADPH –0.32 V Need >1.8 V Because we know that oxygen comes from the water, using the energy from the light, and we know that NADPH is the reductant…..we have a problem! DEo for Mito (–1.14V) +0.82 V ⊕ 4 Photosynthesis Various Pigments Harvest the Light Energy Photosynthesis –Light Absorption The excited state has several choices for dissipating the energy: internal conversion (heat), fluorescence (emit light), or energy transfer. e– hole! Which occurs depends on several factors: ⊕ ⊝ Excited state * ⇀ Energy transfer e– gained! ⇀ special Chl (Acceptor) ⇀ ⇀ ⇀ Vibrational states ⇀ Ground state ⇀ ⇀ ⇀ We have chemistry! special Chl (Donor) Reaction Center (1) The overlap of the fluorescence band of the donor and the absorption band of the acceptor (Förster's overlap integral). (2) The distance between the two molecules; essentially a dipole-dipole interaction obeying Coulomb’s law proportional to r-6. They have to be close. (3) The orientation of the dipoles of the donor and the acceptor molecules. 5 Photosynthesis Organization of LightAbsorbing Molecules in Chloroplasts 200 chlorophylls and 50 carotinoids The probability of energy transfer is ~50% when the "critical distances" are of the order of 50 Å. In a chloroplast, the distance between different pigment molecules much less than 50 Å, so that the probability of energy transfer is high. It is actually >90% efficiency and fast <100 ps Photosynthesis Organization of LightAbsorbing Molecules in Chloroplasts 6 Photosynthesis Harvested Light causes charge separation in the Reaction Center Lets look at this in reality Photosynthesis Harvested Light causes charge separation in the Reaction Center Chlorophyll P* Chlorophyll HA All these distances are ~10 Å QA +1.45 V –0.045 V What is PS II? Is there a PS I? Pheophytin (H+ in place of Mg2+) DEo for PSII≈1.5 V ❶The excited chlorophyll special pair passes an electron through a chlorophyll monomer to pheophytin (very very fast). ❷from which the electron moves rapidly to the tightly bound menaquinone, QA. ❸This quinone passes electrons much more slowly to the diffusible ubiquinone, QB, through the nonheme Fe. 7 Photosynthesis RED-DROP Experiment 680 nm Overall photosynthesis Conclusion is that there are two photosystems: PS I (P700) & PS II (P680) AND, PS II is responsible for O2 production Action Spectrum –––– Keep yellow light on. Give flashes of higher and higher wavelengths Oxygen production – – – – How do we know there were two PSs? Red-Drop Experiment: No O2 after 680 nm ∴ below 680 nm there is photosynthesis system that generates oxygen But, above 680 nm there must be another photosystem because the overall photosynthesis still goes on. Photosynthesis Photosystem II (P680) 8 Photosynthesis ⊕ e– hole! ⊝ e– gained! Photosystem II Evolves Oxygen from Water Photosynthesis Mn4CaO5 Goes through a neutral free radical: Y-O• + H• The water-splitting complex offers single electrons to photosystem II to fill the hole after charge separation. After four electrons are freed from two waters, a single O2 is released. Overall reaction: 2H2O ⇌ O2 + 4e– + 4H+ 9 Photosynthesis Photosystem II Transfers Electrons to Plastoquinones [NOTE on what side of the thylakoid membrane is the water splitting machine…… 2H2O ⇌ O2 + 4e– + 4H+ (2NADPH + 2H+) e– hole! ⊕ 2 PQH2 ⊝ e– gained! 4H+ Recall that the DEo is enough to provide an electron hole deep enough to dissociate water (E’o +1.45V), but not a reductant with the ability to reduce NADP+ (PQ, like CoQ, likes e– more than NADH). Plastoquinones are structurally and functionally similar to ubiquinones (CoQ) found in the mitochondria. These lipid-soluble small molecules transport two electrons to the cytochrome b6f complex. Photosynthesis x2 • Structurally and functionally similar to Complex III in the electron-transport chain • Protons are removed from plastoquinones (PQH2) as plastoquinones are oxidized (PQ). • The PQ cycle is similar to the Q cycle in the ETC. • Electrons are funneled to plastocyanin, a single electron carrier, which carries the electron to PS-I. Cytochrome b6f Complex Translocates Protons into the Lumen 10 Photosynthesis Cu2+ ⇌ Cu1+ Plastocyanin (E’o is +0.37) What do we do with all the reduced Plastocyanin? Cytochrome b6f Complex Translocates Protons into the Lumen Photosynthesis e– hole! ⊕ ⊝ e– gained! A1 (QK) A1 (QK) PSII PSI Photosystem I creates the strongest biological reductant known: A0– Photosystem I (P700) (E’o is ~–1.4) 11 Photosynthesis A0– e– hole! ⊕ ⊝ Plastocyanin (E’o is +0.37) e– gained! Shuttling the electron quickly out: A1 (QK) A1 (QK) PSII PSI Photosystem I creates the strongest biological reductant known: A0– Ferredoxin (E’o is –0.4) Photosystem I (P700) (E’o is ~–1.4) Photosynthesis NADPH is produced by the NADPH Reductase in the Stroma Photosystem I created reduced ferrodoxin (Fd) (E’o is –0.4) Fd is a 1-e– carrier, so 2 Fd transfer their electrons, one at a time, to the FAD cofactor in the reductase. The fully reduced FADH2 then transfers 2e– to NADP+ to make each NADPH. 12 –1.7 (+1.45) (–0.4) –0.32 V +P 700 (+0.4) DEo for Mito (–1.14V) +0.82 V ⊝ Need >1.8 V > hn PS-I ⟿ +P 680 ⟿ > > > > > > > > > > hn PS-II ⧛ ⧛ ⧛ ⧛ ⧛ ⧛ (+0.37) > > > > > > > > > > 2x PC 2x Fd >> >> >> > ⟿ ⟿ ⟿ PQ (+0.08) Cytochrome b6f Complex ⧛ ⧛ ⧛ ⧛ ⧛ ⧛ ⧛ NADP+ NADPH QA (–0.05) A0 (–1.4) ⟿ NADH Reductase > > > > > > > > > > > > > > Photosynthesis ⊕ +1.6 13