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Chapter 5: Harvesting energy Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-1 Harvesting chemical energy • Organisms convert chemical energy of fuel molecules to useable energy in the form of adenosine triphosphate (ATP) – ATP is used to drive cellular processes • Energy is released along metabolic pathways – carbohydrates processed by glycolysis – lipids processed by β–oxidation • Products of pathways act as substrate for cellular respiration Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-2 Fig. 5.2: Overview of metabolic pathways Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-3 Glycolysis • One of the earliest biochemical pathways to evolve • Glucose from polysaccharides processed in cytosol by glycolysis • Glycolysis is a net producer of energy (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-4 Glycolysis (cont.) • First stage uses energy – two ATP molecules used to phosphorylate and change glucose before splitting it into two 3-carbon molecules (glyceraldehyde 3-phosphate) • Second stage – oxidation of glyceraldehyde 3-phosphate to pyruvate is coupled to ATP synthesis – four ATP molecules produced (giving net energy profit of two molecules) – four electrons and two hydrogen atoms transferred to NAD+ to produce two molecules of NADH Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-5 Fig. 5.3: Glycolysis (top) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-6 Fig. 5.3: Glycolysis (bottom) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-7 β-oxidation • Lipids hydrolysed into free fatty acids and glycerol – fatty acids are substrate for β-oxidation • β-oxidation takes place inside mitochondria – carbon atom backbone broken down two carbon atoms at a time – four reactions oxidise carbon and produce acetyl CoA – energy from C–C bond conserved in C–H bond in acetyl CoA – acetyl CoA enters citric acid cycle Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-8 Fig. 5.4: β-oxidation Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-9 Citric acid cycle • Also known as Krebs cycle – acetyl CoA from lipids (by β-oxidation) and pyruvate (by glycolysis) combines with oxaloacetate releasing coenzyme A and forming 6-carbon citrate – citrate is rearranged into isocitrate – isocitrate stripped of electrons and H+, which are transferred to NAD+ to form NADH – CO2 released – resulting 5-carbon α-ketoglutarate undergoes removal of electrons and H+ and release of CO2 – succinyl-CoA (4-carbon product) converted in four steps to oxaloacetate – electrons and H+ transferred to form FADH2 and NADH – ATP produced Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-10 Fig. 5.5: Citric acid cycle Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-11 Electron transport system • During glycolysis and the citric acid cycle, electrons are temporarily stored in NADH and FADH2 • Energy conserved in these molecules converted into ATP via electron transport system • NADH and FADH2 transfer electrons to carrier proteins • Electron transport system embedded in – plasma membrane of prokaryote cells – inner membrane of eukaryote mitochondria (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-12 Electron transport system (cont.) • • Cytochrome c oxidase uses four e– and four H+ to reduce one molecule of O2 to two molecules of H2O H+ concentration gradient provides electrochemical force driving ATP synthesis – process catalysed by transmembrane enzyme complex ATP synthase • Action of ATP synthase – channel allows H+ to move freely down electrochemical gradient – movement is source of energy for ATP synthesis Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-13 Fermentation • ATP produced in absence of oxygen by fermentation – additional reactions consume NADH produced in glycolysis for reduction of pyruvate • End products – lactate (animals) – ethanol (plants) – lactate and ethanol (bacteria, yeasts) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-14 Photosynthesis • Light energy is harvested and stored in chemical bonds of ATP and carbohydrates, made from CO2 and H2O Visible light 6CO2 from atmosphere + 12H2O water → C6H12O6 + sugar 6O2 from original water molecule + 6H2O water (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-15 Photosynthesis (cont.) • Absorption of energy from sunlight by pigments – absorbed light energy is passed from pigments to reaction centres of photosystems I and II in thylakoid membranes of chloroplasts • Reactivation of reaction centres – electrons are stripped from water to reactivate reaction centres of photosystems • Carbon fixation to produce carbohydrates in dark reaction – energy stored in ATP and NADPH used to synthesise sucrose and starch Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-16 Photosynthetic pigments • Pigments absorb photons of particular wavelengths of light and reflect or transmit others – chlorophyll absorbs red and blue wavelengths and reflects green light • Pattern of absorption of a pigment is absorption spectrum – absorption spectrum of chlorophyll is similar to the wavelengths that activate photosynthesis (activation spectrum) (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-17 Photosynthetic pigments (cont.) • Chlorophyll molecules are formed from a central magnesium atom surrounded by alternating single and double bonds forming a porphyrin ring – absorption of photons excites magnesium electrons – energy directed through bonds of porphyrin ring • Pigments – chlorophyll a – chlorophyll b – carotenoids Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-18 Chloroplasts • • In eukaryotes, chlorophyll and other photosynthetic pigments are located in chloroplasts Chloroplast structure – double membrane – third inner membrane (thylakoid membrane) – matrix (stroma) • Protein complexes integrated into thylakoid membranes – photosystems I and II Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-19 Photosystems I and II • Photosystems are photosynthetic electron transport systems – light-harvesting complexes – electron transport complexes – ATP-synthesising complexes • Pigment molecules in light-harvesting complexes arranged so excitation energy is channelled to a specific pair of chlorophyll molecules, the reaction centre – P700 (PS I) – P680 (PS II) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-20 Reaction centres • • As a response to excitation, reaction centre expels electron Electron expelled from P680 accepted by electron acceptor on opposite side of photosystem – loss of e– creates positive charge in reaction centre – electron donor provides e– to neutralise reaction centre – donor itself is neutralised by e– stripped from H2O, which produces O2 and four H+ for every four e– displaced from reaction centre – e– on electron acceptor is passed to cytochrome b/f complex, which passes it on to electron donor molecule of PS I Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-21 Reaction centres • Light-harvesting complex associated with PS I absorbs photon – energy allows e– from P700 to move to an electron acceptor – e– removed from PS I and passed to ferredoxin, which passes them to NADP+ – NADP+ reduced to NADPH • H+ gradient provides potential energy used in ATP synthesis – for every three H+, one ATP molecule is synthesised from ADP and phosphate Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-22 Fig. 5.17: Thylakoid membrane complexes Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-23 Photophosphorylation • Non-cyclic electron transport in photosynthesis – H2O → PS II → PS I → NADP+ • Non-cyclic photophosphorylation – ATP synthesis coupled to non-cyclic electron transport • Cyclic phosphorylation – e– can be transported back to PS I by ferredoxin and cytochrome b/f complex not used for NADPH production – ATP synthesised Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-24 Photosynthesis in prokaryotes • Earliest photosynthetic organisms were anoxygenic photoautotrophs – used H2S or organic molecules instead of H2O as source of e– for NADPH – O2 not produced as by-product • Evolution of PSII in cyanobacteria provided mechanism for using H2O as source of e– – production of O2 as by-product changed composition of atmosphere Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-25 Carbon fixation • • In the process of carbon fixation, atmospheric CO2 is incorporated into carbohydrates CO2 reduction – CO2 is attached to 5-carbon ribulose biphosphate (RuBP) • Carboxylation of RuBP is part of Calvin-Benson cycle in which carbohydrates are formed Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-26 Calvin-Benson cycle • • Carboxylation of RuBP by ribulose biphosphate carboxylase-oxygenase (Rubisco) produces unstable 6-carbon intermediate Intermediate splits into two 3-carbon molecules of phosphoglyceric acid (PGA) – PGA phosphorylated by ATP – intermediate compound reduced and dephosphorylated with NADPH to form glyceraldehyde 3-phosphate (PGAL) • PGAL can follow three paths – sucrose production – starch production – RuBP production Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint (cont.) 5-27 Calvin-Benson cycle (cont.) • Sucrose production – up to two molecules in every twelve exported from chloroplast to cytoplasm – combined and rearranged to form fructose and glucose phosphates – these compounds condensed to form sucrose – inorganic phosphate imported to replace that lost as part of PGAL • Starch production – up to two PGAL molecules combined, rearranged and used in synthesis of starch – starch stored in chloroplasts • RuBP production – in stroma, remaining ten PGAL molecules used to form six RuBP molecules to complete cycle Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-28 Photorespiration • O2 competes with CO2 for binding site on Rubisco – although Rubisco has higher affinity for CO2, O2 is more abundant • Photorespiration – process occurs only in light – consumes O2 and produces CO2 • CO2 produced in photorespiration reduces amount of carbohydrate manufactured – also uses ATP Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-29 C4 pathway • Photosynthetic pathways are adaptations to environmental conditions – tropical and subtropical grasses and other plants use C4 pathway – stomata generally not as wide open as in C3 plants – concentrate CO2 in bundle sheath cells inhibiting photorespiration • Leaf anatomy – vascular bundles surrounded by cylinder of bundle sheath cells – bundle sheath and mesophyll cells contain chloroplasts that differ in structure and function (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-30 C4 pathway (cont.) • • In C4 pathway, the first stable product of carbonfixation is a 4-carbon compound Cytoplasm of leaf mesophyll cells – additional enzyme, phosphoenolpyruvate (PEP) carboxylase, catalyses carboxylation of PEP – produces oxaloacetate – oxaloacetate converted into malate • Chloroplasts of bundle sheath cells – malate decarboxylated to CO2 and pyruvate – CO2 fixed into carbohydrates by Calvin-Benson cycle – pyruvate transported back to mesophyll cells Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-31 Crassulacean acid metabolism • • Crassulacean acid metabolism (CAM) evolved independently in Crassulaceae, Bromeliaceae and other plant families CAM is a variation on the C4 pathway – C4 and Calvin-Benson cycle reactions occur at different times • Stomata open at night reducing moisture loss – 4-carbon compounds produced in darkness and stored until daylight when it is decarboxylated – CO2 released then fixed normally via RuBP and CalvinBenson cycle Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 5-32