Photosynthesis 2425 - Tagged PDF

Photosynthesis  Graphical representation of the global biomass distribution by taxa. Yinon M. Bar-On et al. PNAS 2018;115:25:6506-6511 ©2018 by National Academy of Sciences  Energy Stored -CH2 ↔ -CHOH ↔ -CHO ↔ -COOH ↔ CO2 Reduced Oxidised En...

Photosynthesis  Graphical representation of the global biomass distribution by taxa. Yinon M. Bar-On et al. PNAS 2018;115:25:6506-6511 ©2018 by National Academy of Sciences  Energy Stored -CH2 ↔ -CHOH ↔ -CHO ↔ -COOH ↔ CO2 Reduced Oxidised Energy released C6H12O6 + 6 O2 → 6 CO2 + 6 H2O The Chloroplast Prof Edward Wood © University of Leeds Cyanobacteria  “CBB” Cycle CO2 fixed as glyceraldehyd e phosphate Light Dependent Reactions Generates a proton gradient that is used to By Kelvinsong - Own work, CC BY 3.0, synthesise https://commons.wikimedia.org/w/index.php?curid=27715972 ATP In fact about 20% of the products of the light dependent reactions (by energy yield) are not used for the light dependent reactions. https:// www.quia.com/ jg/ 1244058list.htm l Hill Reagents “Hill reagents” were used in elucidating the electron transport chain involve in the light reactions. These are substances that accept electrons transferred from water by the photosynthetic light reactions and thus lead to evolution of oxygen upon illumination (the Hill reaction). In vivo, the Hill reagent is NADP+, but some other chemicals can substitute for it in vitro. Hill reagents have been identified which Dichlorophenolindophenol (DCPIP) The oxidised form is blue and the reduced form is colourless. It was found that when DCPIP was added to “stripped” chloroplasts without NADP+, the reagent was decolourised and the chloroplasts began to produce oxygen even in the absence of CO2. This demonstrated that the light and dark reactions could be separated. The Light-Independent Reactions The Calvin Cycle Benson-Calvin Cycle Calvin-Benson-Bassham Cycle (summary) Phosphoglycerate kinase Glyceraldehyde 3- phosphate dehydrogenase 9ATP and http://www.bio.umass.edu/biology/conn.river/calvin.html The Calvin-Bensen-Bassham Cycle (summary) Phosphoglycerate kinase Glyceraldehyde 3- phosphate dehydrogenase 9ATP and 6NADPH come from Light- dependent reactions http://www.bio.umass.edu/biology/conn.river/calvin.html  TI-Triose phosphate isomerase AL-Aldolase FB-Fructose bisphosphatase TK-Transketolase SB-Sedoheptulose bisphosphatase EP-Epimerase IS-Isomerase PK-Phosphoribulokinase http://www.bio.umass.edu/biology/conn.river/calvin.html  C3 + C3 C6 C3 + C6 C4 + C5 C4 + C3 C7 C7 + C3 2 C5 http://www.bio.umass.edu/biology/conn.river/calvin.html Capture of carbon dioxide is carried out by RuBisCO – Ribulose Bisphosphate carboxylase/oxygenase The most abundant protein on earth. http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm Carboxylase reaction http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm Light Dependent Reactions Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition.  As protons pass through the F0 domain, the c10 cylinder rotates, carrying round the γ and ε subunits of F1. http://en.wikipedia.org/wiki/ATP_synthase_subunit_C Before Cyanobacteria http://oblate-spheroid.blogspot.co.uk/2007/04/light-wavelength-analysis- nets-purple.html Structure of Bacteriorhodopsin http://www.gpcr.org/articles/thdlh/pictures.html Bacteriorhodopsin http://en.wikipedia.org/wiki/Bacteriorhodopsin Generation of ATP Using Rhodopsin  Oxygenic Photosynthesis There are a number of groups of bacterial species that carry out some type of photosynthesis. Cyanobacteria, plants, “algae”, and phytoplankton carry out oxygenic photosynthesis Light Dependent Reactions In the light reactions, light energy is absorbed by chlorophylls a and b and some other pigments. The energy is transferred to a pair of specialised chlorophyll a molecules known as a “photochemical reaction centre”. Activated electrons are transferred to NADP+ via an electron transfer chain producing NADPH. Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition. In higher plants each photosystem is composed of about 200 chlorophyll molecules and about 50 carotenoid molecules. Only two chlorophyll a molecules are capable of conducting photosynthetic reactions – the REACTION CENTER. The remainder are light harvesting or antenna molecules. When an electron is excited in one of the antenna molecules, the energy is transferred rapidly (< 100 ps) to a reaction center. When an electron is excited in one of the antenna molecules, the energy is transferred rapidly (< 100 ps) to a reaction center. http://photoscience.la.asu.edu/Photosyn/education/antenna.html Transfer is >90% efficient https://www.biorxiv.org/content/10.1101/2019.12.13.875344v1.full.pdf In the reaction center, the pair of chlorophyll a molecules are converted to a high energy state noted as P680* (PSII) or P700* (PSI). As a result the redox potentials of the activated pigments are substantially reduced. PSII: P680, Eo 1.00 V P680*, Eo - 0.95 V PSI: P700, Eo 0.45 V P700*, Eo -1.40 V i.e. The excited chlorophyll becomes a Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition. Photosystem II The two protons are taken from the stromal side and are released to the lumen side when PQB is regenerated. This contributes to the proton gradient. Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition. http://www.bio.ic.ac.uk/ research/barber/psIIimages/ PSII.html http://www.bio.ic.ac.uk/ research/nield/psIIimages/ cytb6f.html Q-type reaction centre (PSII like) Photosystem I http://userpage.chemie.fu- berlin.de/saenger/phosys/ Structure.html Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition. Spinach Ferredoxin https://www.rcsb.org/structure/1A70 FeS-type reaction centre (PSI like) There are several lines of photosynthetic bacteria PSII Only PSII Only PSI Only PSI Only PSI Only PSI and PSII Cyanobacteria Cyanobacteria Light Dependent Reactions Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition.  Photosynthesis drives life on Earth It is thought that there was no oxygen in the atmosphere before the origins of photosynthesis It is estimated that photosynthesis increased global production from 0.2-2.0  1012 mol C year-1 to 9000  1012 mol C year-1 – 4500-45000 times. The Great Oxygenation Event / Catastrophe MCQs 1)Which of the following statements is incorrect? a. Photosynthesis reduces carbon dioxide to carbohydrate. b.The Light Dependent Reactions produce ATP and NADPH. c. Photosynthesis is an oxidative process. d.The Light Dependent Reactions take place in the thylakoid membranes. e.The Light Independent Reactions take place in the chloroplast stroma. 2) How much of the oxygen in the atmosphere is produced by photosynthesis? a. 10% b.20% c. 30% d.60% e.100% 3) What is the source of the oxygen produced in photosynthesis? a. ATP. b.Carbon Dioxide. c. Photosystem I. d.Water. e.Plastoquinone. 4) What are the products of photosystem II? a. NADPH and ATP b.Oxygen and PQH2. c. Reduced Ferredoxin. d.PQ and reduced Plastocyanin. e.Phosphoglyceraldehyde. 5) Which of the following is correct? a. The Light Independent Reactions produce oxygen. b. The Light Independent Reactions take place in the thylakoid lumen. c. The Light Independent Reactions produce NADPH and ATP. d. The Light Independent Reactions use water directly. e. RuBisCO catalyses the first step in the CBB cycle in the Light Independent Reactions. 1)Which of the following statements is incorrect? a. Photosynthesis reduces carbon dioxide to carbohydrate. b.The Light Dependent Reactions produce ATP and NADPH. c. Photosynthesis is an oxidative process. d.The Light Dependent Reactions take place in the thylakoid membranes. e.The Light Independent Reactions take place in the chloroplast stroma. 2) How much of the oxygen in the atmosphere is produced by photosynthesis? a. 10% b.20% c. 30% d.60% e.100% 3) What is the source of the oxygen produced in photosynthesis? a. ATP. b.Carbon Dioxide. c. Photosystem I. d.Water. e.Plastoquinone. 4) What are the products of photosystem II? a. NADPH and ATP b.Oxygen and PQH2. c. Reduced Ferredoxin. d.PQ and reduced Plastocyanin. e.Phosphoglyceraldehyde. 5) Which of the following is correct? a. The Light Independent Reactions produce oxygen. b. The Light Independent Reactions take place in the thylakoid lumen. c. The Light Independent Reactions produce NADPH and ATP. d. The Light Independent Reactions use water directly. e. RuBisCO catalyses the first step in the CBB cycle in the Light Independent Reactions. Slides from here are for reference Oxygenase reaction Photosystem II Activated electrons are transferred via Pheophytin, and two quinones to reduce one of the Fe(III) ions of the cytochrome b6f complex. The electron transferred to pheophytin is replaced by the O2 evolving complex (from water). This contributes to a proton gradient across the thylakoid membrane which is used to synthesise ATP in the same way as in oxidative phosphorylation. 1. P680* donates the activated electron to Pheophytin and becomes P680+. 2. Pheophytin (a chlorophyll without a central Mg2+ ion) is converted to a Pheophytin- radical. 3. The pheophytin- radical in turn transfers the electron to a tightly bound plastoquinone (PQA) to form a semiquinone radical. 4. The PQA- radical transfers the electron to a loosely bound plastoquinone (PQB). 5. Two electron transfers to PQB convert Because the reaction center of PSI has a lower activation energy than that for PSII, the two photosystems must be spatially separated or excitons from the PSII antennae pigments would lose them to PSI – “exciton larceny”. Therefore, electron carriers that can move from granal lamellae rich in PSII to stromal lamellae rich in PSI are needed. These carriers are PQBH2, the cytochrome b6f complex and plastocyanin. 1. PQBH2 Diffuses freely through the thylakoid membrane. 2. One of the two electrons it carries is transferred to a Rieske iron-sulfur protein, then to the Cyt f heme Fe(III) ion and then to plastocyanin. 3. The second electron is transferred via the two heme ions of Cyt b6 back to PQB. 4. Therefore overall, each each re- oxidation of a molecule of PQBH2 transfers up to FOUR H+ ions to the PQB + 2H+stroma + 2e- PQBH2 PQB + 2H+lumen + 2e- Actually Q Cycle. Analogous to PSII cytochrome bc1 complex PQB + 2e- + 2H+stroma PQBH2 in oxidative phosphorylation. Cyt b6f Qo (lumen side) PQBH2 PQB - + e-FeS + 2H+lumen PQB - PQB + e-Cyt b Cyt b6f Qi (stromal side) PQB + e-Cyt b PQB - + e-Fd or Cyt b + 2H+stroma The Cytochrome b6f compex transfers electrons via the Rieske Fe-S centre to cytochrome f and then to Plastocyanin. Plastocyanin is a soluble low Mw protein 10.4 kDa with a copper ion. It can act as a single electron carrier. Because plastocyanin can diffuse freely through the lumen and into and out of the thylakoid membrane, it can transport electrons from PSII to PSI.  PSI behaves similarly to PSII. It also has a pair of chlorophyll a molecules forming a reaction center. The reaction center is P700 because of the slightly lower energy photons required to activate it. Excitons convert P700 to p700*, which reduces A0 (thought to be a specialised chlorophyll) and forming P700+. P700+ is reduced back to P700 by plastocyanin, i.e. using electrons from PSII (we will come to an exception to A0- transfers an electron to phylloquinone (A1). A1- further transfers its electron through three Fe-S centers (FX, FA and FB) to Ferredoxin (Fd). Ferredoxin is a low Mw (10.7 kDa) protein with a single electron carrying Fe-S (2Fe-2S). Ferredoxin travels through the stroma to Ferredoxin:NADP+ oxidoreductase – a flavoprotein that transfers single electrons from two reduced ferrodoxin + Some chloroplast F0 domains have 14 c subunits. Does this mean that more protons are required per ATP using this ATP synthase? 4⅔ H+ per ATP, but Pi enters by a triose phosphate/phosphat e exchanger (so no The Fdred generated by PSI can be used by the Cyt b6f complex to reduce PQB, which then (again via the Cyt b6f complex) can generate reduced plastocyanin. This reduced plastocyanin can be used by PSI to regenerate P700 from P700+. This can allow PSI to perform CYCLIC PHOTOPHOSPHORYLATION. This generates a proton gradient (and therefore ATP), but NO NADPH, and DOES NOT GENERATE OXYGEN.