Photosynthesis CSC (1) PDF

MODULE 5 PHOTOSYNTHESIS Maria Adela J Lacao, CS LIGHT ENERGY → CHEMICAL ENERGY  Almost all plants are photosynthetic AUTOTROPHS, as are some bacteria and protists Autotrophs generate their own organic matter through photosynthesis Sunlight energy is transformed to energy stored in the...

MODULE 5 PHOTOSYNTHESIS Maria Adela J Lacao, CS LIGHT ENERGY → CHEMICAL ENERGY  Almost all plants are photosynthetic AUTOTROPHS, as are some bacteria and protists Autotrophs generate their own organic matter through photosynthesis Sunlight energy is transformed to energy stored in the form of chemical bonds (c) Euglena (d) Cyanobacteria (b) Kelp (a) Mosses, ferns, and flowering plants  PHOTOSYNTHESIS process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Light Energy 6 CO2 + 6 H2O C6H12O6 + 6 O2  OVERVIEW of Photosynthesis LIGHT REACTIONS Light -convert solar E to Chloroplast chemical E NADP+ Produce ATP & ADP NADPH Light +P Calvin reactions cycle DARK REACTIONS - Calvin Cycle Produce sugar from CO2 ATP provides the E NADPH – reducing power CHLOROPLASTS: Sites of Photosynthesis LEAF CROSS SECTION MESOPHYLL CELL CHLOROPLAST CHLOROPLAST: Site of Photosynthesis Thylakoid vesicle/ Intermembrane membrane Outer membrane space Inner membrane Granum (stack of STROMA thylakoids THYLAKOID Lamella Chloroplast contains several pigments Chlorophyll a Chlorophyll b carotenoids PORPHYRIN RING Chl b = -CHO PHYTOL CHAIN - Excellent light absorbers (due to presence of conjugated double bonds)  Reflected light Light Absorbed light Transmitted Chloroplast light  Absorption spectra of chlorophyll a and b Action spectra of photosynthesis Absorbance 400 500 600 700 Wavelength (nm)  Excitation of electrons in a chloroplast Excited 2 state Light Photon Heat Light e− (fluorescence) Chlorophyll Ground molecule state Excitation of electrons in a chloroplast e− Excited If E is not dissipated 2 state Light Heat - May be utilized Photon - Transferred to another E level Light (fluorescence) e− Ground state until it reaches a Chlorophyll molecule PHOTOCHEMICAL REACTION CENTER (PRC) PHOTOCHEMICAL REACTION CENTERS P700 P680  OVERVIEW of Photosynthesis LIGHT REACTIONS Light -convert solar E to Chloroplast chemical E NADP+ Produce ATP & ADP NADPH Light +P Calvin reactions cycle DARK REACTIONS - Calvin Cycle Produce sugar from CO2 ATP provides the E NADPH – reducing power A. LIGHT REACTIONS E is used to drive e-s from H2O to form NADPH Light is absorbed H+ are driven across the membrane H+ return through ATP synthase to make ATP Z SCHEME: Generation of NADPH e-s are transferred/ shuttled stepwise from one photochemical reaction center to another From PSII (P680) to PSI (P700). Ph = Pheophytin Pc = Plastocyanin QA, QB = Plastoquinone A, B Fx, FA/FB = Fe-S centers Plants produce O2 gas by splitting H2O  The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)  ATP mill Water-splitting NADPH-producing photosystem photosystem STOICHIOMETRY of the Light Reactions Absorption of 4 photons by PSII 2 H2O + 2 NADP+ + 10 H+(stroma) → O2 + 2 NADPH + 12 H+(lumen) ATP synthase 3 ADP + 3 Pi + 3H+ + 12 H+(lumen) → 3 ATP + 3 H2O + 12 H+(stroma) 2 NADP+ + 3 ADP + 3 Pi + H+ → O2 + 2 NADPH + 3 ATP + H2O From LIGHT to DARK REACTIONS From LIGHT to DARK REACTIONS O2 evolved CO2 fixation into sugars B. DARK REACTIONS In STROMA of chloroplasts NOTE: light is still needed for activation of some enzymes Calvin-Benson Cycle /C3 Photosynthesis Reduction of C from CO2 to a more reduced state as HEXOSE Requires NADPH from the light rxns DARK REACTIONS STAGES of the Calvin-Benson Cycle 1. CO2 fixation by Ribulose 1,5-bisphosphate (RubP) to form 2 molecules of 3-phosphoglycerate (3-PGA) 2. Red’n of 3-PGA to form hexose sugars 3. Regeneration of RubP so that more CO2 could be fixed. RubisCO 3- phosphoglycerate kinase NADP+-GAP dH  1. CO2 Fixation by RubisCO RubisCO - RubP carboxylase/oxygenase - most abundant enzyme on earth - a complex (L8S8) of - 8 large catalytic subunits - 8 small subunits  1. CO2 Fixation by RubisCO RubisCO - Requires Mg2+ as cofactor 2. Reduction of 3-PGA to GAP  C H O 2- CH2O PO3 H OH O 2- CH O PO CH 2 3 2OH Glyceraldehyde 3-P (GAP) DHAP aldolase C H 2O P O 32- CH2OH O Fru 1,6- O H O H bisphosphatas HO H H O H e H OH H O H H OH C H 2O P O 32- CH2OPO 32- Fru 1,6-BP Fruc 6-P 3. Regeneration of RubP GAP and Fru 6-P are rearranged by enzymes similar to the PPP to regenerate Ribulose 1,5-bisphosphate (RubP)  3 CO2 1C 3 molecules 6 molecules Ribulose 1,5-bisP 3C 3-Phosphoglycerate 3C 3 ADP 6 ATP 3 ATP 3 molecules 6 ADP 6 molecules Ribulose 5-P 3C 1,3-bisPglycerate 3C 2 Pi 6 NADPH 5 molecules 6 NADP+ 6 molecules 6 Pi Glyceraldehyde 3-P 3C Glyceraldehyde 3-P 3C 1 molecule Glyceraldehyde 3-P 3C 3C HEXOSES, Fatty Acids, Amino acids To fix 1 molecule of CO2 2 NADPH 3 ATP To synthesize 1 molecule of glucose (C6), six rounds of Calvin cycle is needed. Thus, 12 NADPH 18 ATP 6 CO2 + 18 ATP + 12 NADPH + 12 H2O C6H12O6 + 18 ADP + 18 Pi + 12 NADP+ + 6 H+ LIGHT Rxns: 2 NADP+ + 3 ADP + 3 Pi + H+ → O2 + 2 NADPH + 3 ATP + H2O x6 LIGHT Rxns: 12 NADP+ + 18 ADP + 18 Pi + 6H+  6O2 + 12 NADPH + 18 ATP + 6H2O DARK Rxns: 6 CO2 + 18 ATP + 12 NADPH + 12 H2O  C6H12O6 + 18 ADP + 18 Pi + 12 NADP+ + 6 H+ Light 6 CO2 + 6 H2O C6H12O6 + 6 O2  RubisCO also catalyzes a WASTEFUL Oxygenase Reaction CARBOXYLASE Reaction: C H 2 O -P C H 2 O -P HO C H C O - COO H C OH + H 2O + CO 2 + - COO H C OH H C OH C H 2 O -P C H 2 O -P RubP 2 molecules 3-PGA  RubisCO also catalyzes a WASTEFUL Oxygenase Reaction OXYGENASE Reaction: COO- CH2O-P H C OH C O CH2O-P + O2 + H2O 3-PGA H C OH + H C OH CH2O-P CH2O-P C - O O RubP phosphoglycolate PHOTORESPIRATION - Occurs when [CO2]/[O2] is LOW NOT a versatile phospho CO2, metabolite glycolate H2O, Pi RubisCO O2 RubP High levels O2 phospho glycolate PHOTORESPIRATION / Peroxisome  Why is photorespiration wasteful? 1. NOT coupled to oxidative phosphorylation - NO ATP nor NADPH is formed 2. CO2 that has already been fixed is released again as CO2 3. Consumes O2  THE C4 CYCLE - Alternative strategy of some plant species for favoring the carboxylase over the oxygenase activity of RubisCO - Occurs in leaves with Krantz anatomy - Accelerates photosynthesis by concentrating CO2 by making use of 2 Carbon-fixing enzymes: PEP carboxylase & RubisCO  C3 plant C4 plant - Scattered chloroplasts - Close association of bundle - Only mesophyll cells sheath (BS) & MC (MC) contain - BS also contain chloroplasts chloroplasts - MORE efficient photosynthesis 1. CO2 is added to PEP to form OAA (PEP carboxylase) 2. MCs export OAA to the bundle sheath cells through malate. 3. In the BS, OAA is broken back down to OAA and CO2. 4. CO2 is converted into sugar through the regular Calvin cycle. ADVANTAGES of C4 over C3 Plants 1. NO photorespiration 2. At  [CO2], PEP carboxylase concentrates CO2 in BS cells for RubisCO to function as carboxylase 3. Closeness of MC and BS cells – easy transfer of photosynthates 4. Both MC and BS cells contain chloroplasts  CAM Photosynthesis CAM – Crassulacean Acid Metabolism - Observed in Crassulaceae (succulents) -Thick cuticles - H2O content -er # of stomates DAYTIME - Stomates are CLOSED to prevent H2O loss -CO2 cannot be absorbed for Glc synthesis NIGHTIME - Stomates OPEN -CO2 fixed (via C4 cycle) into malate (stored in vacuole) DAYTIME - Malate is decarboxylated and CO2 is fixed in the Calvin cycle C4 vs. CAM Photosynthesis