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Chapter 6: Harvesting energy Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-1 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-2 Fig. 6.2: Overview of metabolic pathways Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-3 Glycolysis • One of the earliest biochemical pathways to evolve • Glucose from polysaccharides processed in cytosol by glycolysis • Glycolysis is a net producer of energy Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-4 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-5 Fig. 6.3: Glycolysis (top) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-6 Fig. 6.3: Glycolysis (bottom) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-7 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-8 Fig. 6.4: β-oxidation Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-9 Citric acid cycle (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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-10 Fig. 6.5: Citric acid cycle Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-11 Electron transport system • During glycolysis and the citric acid cycle, electrons are temporarily stored in NADH and FADH2 • Energy conserved in these molecules is converted into ATP via electron transport system • NADH and FADH2 transfer electrons to carrier proteins • Electron transport system is embedded in – plasma membrane of prokaryote cells – inner membrane of eukaryote mitochondria Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-12 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-13 Question 1: Which process in eukaryotic cells will proceed normally whether oxygen (O2) is present or absent? a) electron transport b) glycolysis c) the citric acid cycle d) oxidative phosphorylation e) chemiosmosis Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-14 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-15 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 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University + 6H2O water 6-16 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-17 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) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-18 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-19 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-20 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-21 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-22 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-23 Fig. 6.17: Thylakoid membrane complexes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-24 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-25 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-26 Question 2: Photosynthetic plant cells differ from animal cells because: a) they don’t contain mitochondria. b) ATP for all cellular processes is produced by chemiosomosis in the chloroplast. c) they do not contain enzymes. d) they are capable of producing carbohydrate from light energy, that can be metabolised. Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-27 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-28 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-29 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-30 Photorespiration • O2 competes with CO2 for binding site on Rubisco – Rubisco has higher affinity for CO2 than for O2 • Photorespiration – process occurs only in light – consumes O2 and produces CO2 • CO2 produced in photorespiration reduces amount of carbohydrate manufactured – also uses ATP Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-31 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 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-32 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 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-33 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 they are decarboxylated – CO2 released then fixed normally via RuBP and Calvin– Benson cycle Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-34 Question 3: What implications do you think global warming would have for plants? a) b) c) d) e) Decreased plant growth Increased plant growth Increased desertification Increase in tropical rainforest It depends on latitude Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-35 Summary • All organisms, both autotrophs and heterotrophs, extract energy from the oxidation of glucose • Electrons drive proton pumps that power ATP synthesis • All energy supporting life on earth originates from the sun and is trapped by photosynthetic organisms, energising electrons that drive reactions • There are two types of reaction centre associated with the two photosystems (PSI and PSII) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-36 Summary (cont.) • ATP and NADPH generated in the light-dependent reactions drive the Calvin–Benson cycle and the synthesis of sugar from CO₂ • C4 photosynthesis and CAM are special adaptations of the carbon-fixing process in some plants Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 6-37