5-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

5-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 5: Harvesting energy


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


Fig. 5.2: Overview of metabolic pathways


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.)


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


Fig. 5.3: Glycolysis (top)


Fig. 5.3: Glycolysis (bottom)


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


Fig. 5.4: β-oxidation


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


Fig. 5.5: Citric acid cycle


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.)


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


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)


Photosynthesis• Light energy is harvested and stored in chemical

bonds of ATP and carbohydrates, made from CO2 and H2O

Visible light

6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2Ofrom

atmospherewater sugar from original

water molecule

water

(cont.)


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


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.)


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


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


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)


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


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


Fig. 5.17: Thylakoid membrane complexes


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


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


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


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 (cont.)


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


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


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.)


C4 pathway (cont.)• In C4 pathway, the first stable product of carbon-

fixation 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


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 Calvin-Benson cycle

5-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

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