Photosynthesis: The Chemical Process of Converting Light Energy into Carbohydrates - Prof., Study notes of Agricultural engineering

Download Photosynthesis: The Chemical Process of Converting Light Energy into Carbohydrates - Prof. and more Study notes Agricultural engineering in PDF only on Docsity! PHOTOSYNTHESIS THYLAKOID REACTIONS (Chapter 7) 1. 6CO2 + 6H2O ------- C6H12O6 + 6O2 2. CO2 + H2O ------- (CH2O) + O2 Light, chlorophyll (plant) Simpler form, 2 is sort of one “turn” of 1, but neither is actually correct chemically, a complex of several dozen reactions but the bottom line is that CO2 is reduced to carbohydrate and water is oxidized and O2 released, all by capture and transfer of solar energy. Note: #1 above is highly unfavorable energetically and WILL NOT happen randomly. G is about +467 kJ mol-1 for each turn of the cycle Historical perspective – read about it, interesting. Several basic questions we need to answer: What absorbs light and what happens when it is absorbed? 1 How is energy transferred? What are the products? How are the products used and for what? (part 2) How can we evaluate this process? What absorbs light and what happens when it is absorbed by a pigment molecule? Primary photosynthetic pigments – chlorophylls Accessory pigments – carotenoids Fig. 7.6 p. 129. 2 Fluorescence – photon reemitted, but at a longer , why? (Fig. 7.5) Transferred – to another molecule, inductive resonance coupling, this is rapid and efficient – solid state system. Transfers from chl to chl or accessory to chl, etc. Photochemistry – light driven chemical reactions, really just a specific way light energy is transferred, photons to electrons, in this case through a chain of redox reactions The light energy absorbed makes the chl reaction center a strong reducing agent – i.e. “wants” to give this loosely-held e- to an acceptor molecule, this is the transfer of light to reducing energy How do we know the fate of any given absorbed photon? Speed (photochemistry must be very fast), availability of a molecule to transfer energy to – e.g. are there any open reaction centers, a place for the electron to go? 5 How is the energy transferred? Light absorbed by pigments and energy transferred to one of two reaction centers. Fig. 7.10. p. 131. Each consists of an organization of antenna pigments linked to the reaction center molecule, old name “trap chlorphyll” that is where the photochemistry begins. Antenna complexes vary in size and composition and in the types of accessory pigments. These absorb just inside the primary pigments so help capture light and help protect the system from too much light (later). Fig. 7.6 and 7.7 p. 129. 6 (B) Carotenoids (C) Bilin pigments H 3 H.C CH 3 YY" CH H,C—CH H3C Uy cH cH, HOOC—CH,—CH, Ncw HOOC—CH,—CH, Y = w i = Zy/ SA = xr ww a a YY mix a Hc 3 ScH | na G = HyC—HCC H,C=CH r HeZ CH, H3€ Phycoerythrobilin H3C B-Carotene PLANT PHYSIOLOGY,, Third Edition, Figure 7.6 (Part 2) @ 2002 Sinauer Associates, Ine. Question: Would you expect all plants to have the same antenna composition and configuration?? How would alteration in the ratio of chla to chlb affect light absorbance?? The reaction centers are organized into 2 distinct regions or photosystems – these differ in their max, proteins structure, energetics and 10 chemistry and their function but both work together to perform their total function PSI: P700, primarily far-red light, this produces a strong reductant that reduces NADP+ (in the stroma) when excited and a weak oxidant that oxidizes a cytochrome complex via PC, when unexcited PSII: P680, primarily red light, this produces a strong oxidant that oxidizes water and a weak reductant that reduces the cytochrome complex. They work in conjunction with each other in the commonly referred to “Z-scheme.” (Fig. 7.14, p. 134) 11 Each Photosystem is associated by noncovalent bonds with integral proteins and PSI and PSII are spatially separted along the thylalkoid membranes 12 I. PSII (photosystem II) LHCII – Fig. 7.20, several proteins, D1 and D2, etc. Antenna and accessory, chl b, chl a, reaction center (P680) P680 receives E through the LHC and is oxidized (loses an electron to Pheophyton – a chlorophyll – like molecule w/o the Mg ions.) It is then re-reduced (picks the e- back up) from the oxidation of water (ultimate electron donor) through an electron acceptor YZ a tyrosine in the D1 protein complex of PSII. The water splitting process is very complex and may involve several oxidation states of a Mn containing enzyme (note: a requirement for Mn in plants). Fig. 7.25: Note, several proteins, integral protein complex 15 Result: H+ released into the lumen by the splitting of water, 2 electrons are used so far for each water or 4 to make one oxygen. Now – back to the electron flow: Electron released from P680 into the Pheophyton /Plastoquinones QA and QB system Pheophytin ---- PQ ---- PQH2 Picks up H from the stroma 16 This is a 2-electron gate that accepts the 2 electrons (Fig. 7.26, p. 145) and picks up another 2 H + from the stroma to be released into the lumen. Chemically its - Quinone --+e--- plastosemiquinone (Q-) +e-, +2H+-- QH2 (PQH2 or plastohydroquinone) The nonpolar PQH2 (fully reduced hydroquinone) dissociates from the complex and transfers electrons and protons through the hydrophoebic portion of the membrane to the cytochrome complex – protein complex 2. Note that the systems aren’t sitting there side by side like in the diagrams. The systems are more complex and fluid. A “carrier” is required to “connect” them. The carrier is PQH2 17 The cytochrome complex is large and presumably not mobile so it is likely that PQ and PC serve as the electron carriers that link PSI and PSII by way of the cytochrome complex The electron donor to PSI is likely to be PC (plastocyanin), a small water soluble (polar so moves on the outside of the membrane), copper protein, that transfers reducing power from the cytochrome complex to PSI. Compare PC with QH2: Polarity? – PQ nonpolar PC - polar Where does each travel? PQ – inside, PC – outside the membrane 20 PSII – QH2 – Cyto – PC - PSI Critical links between the photosytems. III: PSI: (photosystem I) Like PSII in that there are 2 primary proteins (PsaA and PsaB), with other associated smaller proteins, cofactors, etc. Fig. 7.29 p. 147 and “Z- scheme” Figs again. 21 What happens in PSI then? Electrons travel toward the stroma beginning with excited P700 which reduces a series of compounds (a chlorophyll, quninones and phylloquinone (vitamin K1), and membrane- bound FeS proteins) to ferredoxin (Fd), where a flavoprotein (a ferredoxin-protein complex, Fp) then finally reduces NADP+ to NADPH in the 22 Now we have NADPH+. What about ATP? IV: ATP Synthase: The energy stored in the proton gradient is then used to couple ADP + Pi ----- ATP (photophosphorylation) The ATP Synthase complex does this and uses the power of the chemical potential gradient (pH gradient) built up from the processes we’ve already discussed. This was developed by Mitchell in the 1960s as the chemiosmotic hypothesis which produced what is called the proton motive force (pmf or the power produced by the pH gradient). This is proportional to the trans-membrane electric potential and pH gradient, which “powers” the pmf. Exp. By Jagendorf (Fig. 7.31) demonstrated that Mitchell was correct and you could produce ATP in the dark using a pH gradient. 25 Remember you get energy from a concentration gradient (laws of thermodynamics) and also in this case you get a greater pmf from the charge differential (i.e. greater pH gradient = larger pmf, 1 pH unit = 59 mV). It takes 4 H+ to get 1 ATP. The protein complex consists of 2 primary coupling factors, CF0 and CF1. and several other smaller proteins (Fig. 7.32 p. 149). CF0 forms the channel through the membrane (a hydrophoebic protein within the membrane and the CF1 forms the catalytic site (sticks out of the membrane) and produces ATP. 26 This system is similar to mitochondria and bacteria and thus is said to be highly conserved across organelles and organisms! What does this mean? Who cares? 27