L6 2025 Photosynthesis I PDF

1/20/21 Photosynthesis 1 – light reactions What are ’light’ and ‘dark’ reactions? How is light energy captured for biochemistry? What light is important? Where does photosynthesis take place? What is the ‘Z’ scheme?...

1/20/21 Photosynthesis 1 – light reactions What are ’light’ and ‘dark’ reactions? How is light energy captured for biochemistry? What light is important? Where does photosynthesis take place? What is the ‘Z’ scheme? Biology, Campbell&Reese 7th ed., Chap. 10 1 What is photosynthesis? Photosynthesis: a process that converts atmospheric CO2 and H2O to carbohydrates Solar energy is captured in chemical form as ATP and NADPH ATP and NADPH are used to convert CO2 to hexose phosphates Phototrophs: photosynthetic organisms (some bacteria, algae, higher plants) 2 1  1/20/21 Photosynthesis divides between ‘light’ and ‘dark’ reactions 3 Light and Dark reactions Light (-dependent) reactions... Are membrane-bound Release O2 from splitting 2H2O molecules, with H+ from H2O used in the Chemiosmotic synthesis of ATP, and Hydride ion (H:-) from H2O reduces NADP+ to NADPH Dark (light-independent reactions... Occur in solution Reduce gaseous CO2 to carbohydrate Require energy of NADPH and ATP 4 2  1/20/21 Chloroplast structure Chloroplasts 4-8 mm diameter organelle two surrounding membranes internal membrane stacking 5 Chloroplast structure Chloroplasts 4-8 mm diameter organelle two surrounding membranes internal membrane stacking EM of chloroplast Freeze-fracture EM of grana 6 3  1/20/21 Chlorophyll and other pigments capture light Chlorophylls are most abundant and most important in light harvesting... contain tetrapyrrole ring (chlorin) similar to heme, but contains Mg2+ chlorophylls a (Chl a) and b (Chl b) in plants bacteriochlorophylls a (BChl a) and b (BChl b) are similar and major pigments in anaerobic photosynthetic bacteria 7 Chlorophyll and other pigments capture light ‘Antenna’ pigments include carotenoids (carotene pigments), phycocyanin and phycoerythrin 8 4  1/20/21 Chlorophyll and other pigments capture light Antenna pigments extend the range of light capture, especially in the green (where chlorophyll absorbs poorly) Antenna pigments and chlorophylls exchange light energy (excited electrons) until captured 9 Light absorption is organised between two Photosystems Photosystems I (PSI) and II (PSII): contain many proteins and pigments embedded in the thylakoid membrane PSI and PSII operate in series, connected by cytochrome bf complex Electrons are conducted from H2O to NADP+ Z-scheme 10 5  1/20/21 Light absorption is organised between two Photosystems PSI and PSII each contain a reaction centre (site of the photochemical reaction) chlorophylls in each reaction center are paired to capture light PSI paired for P700 (absorb light maximally at 700nm) PSII paired for P680 (absorb light maximally at 680nm) Z-scheme 11 Physical layout of light reaction steps 12 6  1/20/21 The Z-scheme Z-scheme: path of electron flow and reduction potentials of the components in photosynthesis Absorption of light energy converts P680 and P700 (poor reducing agents) to excited molecules (good reducing agents) Light energy drives the electron flow uphill NADP+ is ultimately reduced to NADPH Light is captured by antenna complexes For 2 H2O oxidized to O2, 2 NADP+ are reduced to 2 NADPH 13 Chemiososmosis and energy conversion Mitchell’s Chemiosmosis Theory – conversion of energy from electron transport via formation of a transmembrane electrochemical gradient Requires e- (H+) transfer across a ‘coupling’ membrane to generate DµH DµH = DY + DpH (DµH also sometime called the ‘proton motive force’) DµH used to drive protons through the ATP synthase (H+-ATPase) to make ATP 14 7  1/20/21 Photophosphorylation - H+-ATP synthase Photophosphorylation: synthesis of ATP which is dependant upon light energy Chloroplast ATP synthase consists of two major particles: CFo and CF1 CFo spans the membrane, forms a pore for H+ CF1 protrudes into the stroma and catalyzes ATP synthesis from ADP and Pi 15 Photophosphorylation - H+-ATP synthase Mitchell’s Chemiosmosis Theory – conversion of energy from electron transport via formation of a transmembrane electrochemical gradient Requires e- (H+) transfer across a ‘coupling’ membrane to generate DµH 16 8  1/20/21 Photophosphorylation - H+-ATP synthase Mitchell’s Chemiosmosis Theory – conversion of energy from electron transport via formation of a transmembrane electrochemical gradient Requires e- (H+) transfer across a ‘coupling’ membrane to generate DµH 17 Cyclic electron flow and phosphorylation For 4e- transferred to 2 NADPH, 2 ATP are produced from the proton gradient However, for each CO2 reduced to (CH2O) in carbohydrate synthesis, 2 NADPH and 3 ATP are required Cyclic electron transport yields ATP but not NADPH, thus balancing the need for 3 ATP for every 2 NADPH Ferridoxin donates e- not to NADP+, but back to the PQ pool via a specialized cytochrome Cyclic flow increases the protonmotive force and increases ATP production, but no NADP+ is produced 18 9  1/20/21 Summary Photosynthesis uses light energy to fix carbon into sugar Photosynthesis takes place in the chloroplast Light reactions capture and use light energy to reduce NADP and phosphorylate ADP Light drives photosynthetic electron transport to generate µH The µH is used by H+-ATP synthase to phosphorylate ADP ATP and NADPH energy is used for cellular reactions 19 10