Transcript Chapter 5

Chapter 5: Harvesting energy
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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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
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Fig. 5.2: Overview of metabolic pathways
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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.)
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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
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Fig. 5.3: Glycolysis (top)
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Fig. 5.3: Glycolysis (bottom)
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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
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Fig. 5.4: β-oxidation
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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
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Fig. 5.5: Citric acid cycle
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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.)
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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
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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)
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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.)
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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
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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.)
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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
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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
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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)
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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
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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
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Fig. 5.17: Thylakoid membrane complexes
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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
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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
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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
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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
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(cont.)
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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
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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
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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.)
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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
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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
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