cell respiration part ii
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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Electron Transport Chain
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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Stepwise Energy Harvest via NAD+
and the Electron Transport Chain
In glycolysis and Krebs Cycle, glucose moleculesare broken down in a series of steps. Electronsfrom some intermediate products are transferred
to NAD+, a coenzyme; hence, NAD+ is thee- acceptor.
NADH and FADH2 account for most of the
energy extracted from food.
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NADH, together with FADH2 passes the electronsto the electron transport chain.
Unlike an uncontrolled reaction, the electron
transport chain passes electrons in a series ofsteps instead of one controlled, unexplosivereaction.
Oxygen (terminal e-acceptor) pulls electrons downthe chain in an energy-yielding tumble.
The energy yielded is used to regenerate ATP.
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2 H+ + 2 e
2 H
(from food via NADH & FADH2)
Controlled
release ofenergy for
synthesis ofATP ATP
ATP
ATP
2 H+
2 e
H2O
+ 1/2 O21/2 O2H2 +
1/2 O2
H2O
Explosiverelease of
heat and lightenergy
Cellular respirationUncontrolled reaction
Freeen
ergy,
G
Freeen
ergy,
G
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The Electron Transport Chain
In the ETC, high-energy electrons of NADH and FADH2are passed step-by-step to the low energy level of oxygen
through a series (chain) of electron carriers.
Each carrier is capable of accepting or donating one or twoelectrons.
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The Malate-Aspartate Shuttle
Steps:In the cytosol,NADH powers the conversion of OAA to malate.
Malate crosses to the mitochondrial matrix and powers the formation of anew NADH molecule.
Malate is converted to OAA, then to aspartateAspartate is transported back to the cytoplasm, where it is converted to
OAA again.
Cytosolic NADHdrives the formation of amatrix NADHand theconsequentoxidative phosphorylation of 3 ADPs to 3 ATPs.
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The Glycerol Phosphate Shuttle for FADH2
Steps:
Reduction of DHAP to G3P by cytosolicNADH.
Transport of G3P across the inner membraneto the matrix.
Conversion of G3P back to DHAP, resultingin the reduction of FAD to FADH2.
Each cytosolic NADH results in the formation of 1matrix FADH2, which drives the formation of only2 ATPs.
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NADH Transport without a Shuttle?
------------ In some plants, NADH can cross theouter mitochondrial membrane and a shuttlemechanism is not necessary.
NADH reacts directly with ubiquinone (orCoQ) at the outer surface of the innermembrane.
However, the step of proton pumping by FMNis bypassed, decreasing the amount of ATPthat can be generated.
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NADH as Electron Carrier
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FADH2 as Electron Carrier
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The Principal Electron Carriers in the ETC
Flavin mononucleotide (FMN) - similar in structure to FAD, with 1phosphate group.
- accepts 2 electrons from NADH and passes them to CoQ.
- reduced form: FMNH2, hence 2 protons are also transferred.
Ubiquinone or coenzyme Q (CoQ) can donate or accept 2electrons simultaneously; can carry 2 protons in its reduced form;small molecule compared to other e- carriers.
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Electron Carriers in the ETC. contn
Cytochromes (fig. a & b) heme proteins with FE-containingporphyrin ring;
They pick up electrons on their Fe atoms, which can be reversiblyreduced from Fe3+ to Fe2+.
In their reduced forms, cyt. carry only 1 e-, without a proton.
Fe-S proteins also involved in e- transfer.
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The Pathway of Electron Transport
The electron transport chain is in the cristae of themitochondrion.
Most of the chains components are proteins,which exist in multiprotein complexes.
The carriers alternate reduced and oxidized statesas they accept and donate electrons.
Electrons drop in free energy as they go down thechain and are finally passed to O2, forming water.
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ATP ATP ATP
GlycolysisOxidative
phosphorylation:electron transportand chemiosmosis
Citricacidcycle
NADH
50
FADH2
40 FMN
FeS
I FAD
FeS II
IIIQ
FeS
Cyt b
30
20
Cyt c
Cyt c1
Cyt a
Cyt a3
IV
10
0
Multiproteincomplexes
Freeenergy(G)relativetoO2(kcal/mol)
H2O
O22 H+ + 1/2
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Oxidative Phosphorylation
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Mitochondrion
Glycolysis
PyruvateGlucose
Cytosol
ATP
Substrate-levelphosphorylation
ATP
Substrate-levelphosphorylation
Citricacid
cycle
ATP
Oxidativephosphorylation
Oxidativephosphorylation:electron transport
andchemiosmosis
Electrons
carriedvia NADH
Electrons carried
via NADH andFADH2
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Substrate-Level Phosphorylation
- ATP formation by enzymatic transfer of a phosphategroup from an intermediate to ADP.
Enzyme
ADP
P
Substrate
Product
Enzyme
ATP+
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During oxidative phosphorylation,
chemiosmosis couples electron transport toATP synthesis.
NADH and FADH2 (electron carriers) donateelectrons to the electron transport chain, whichpowers ATP synthesis via oxidativephosphorylation.
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Oxidative Phosphorylation
A small amount of ATP is formed in glycolysis and the citricacid cycle by substrate-level phosphorylation.
~ 90% of the ATP generated by cellular respiration isthrough oxidative phosphorylation.
This is a process of ATP production powered by energy
derived from redox reactions of an ETC.
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Chemiosmosis: The Energy-Coupling Mechanism
Electron transfer in the electron transport chaincauses proteins to pump H+ from themitochondrial matrix to the intermembrane space,creating a proton gradient.
H+ then moves back across the membrane,passing through channels in ATP synthase.
ATP synthase uses the exergonic flow of H+ to
drive phosphorylation of ATP.
This is an example of chemiosmosis, the use ofenergy in a H+ gradient to drive cellular work.
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The energy stored in a H+
gradient across amembrane couples the redox reactions of theelectron transport chain to ATP synthesis.
The H+
gradient (also a pH gradient) is referred toas a proton-motive force, emphasizing its capacityto do work.
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Protein complexof electron
carriers
H+
ATP ATP ATP
GlycolysisOxidative
phosphorylation:electron transportand chemiosmosis
Citricacidcycle
H+
Q
IIII
II
FADFADH2
+ H+NADH NAD+
(carrying electronsfrom food)
Innermitochondrialmembrane
Innermitochondrialmembrane
Mitochondrialmatrix
Intermembrane
space
H+
H+
Cyt c
IV
2H+ + 1/2 O2 H2O
ADP +
H+
ATP
ATPsynthase
Electron transport chainElectron transport and pumping of protons (H+),
Which create an H+ gradient across the membrane
P i
ChemiosmosisATP synthesis powered by the flow
of H+ back across the membrane
Oxidative phosphorylation
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ATP SynthaseINTERMEMBRANE SPACE
H+ H+
H+H+
H+
H+
H+
H+
ATP
MITOCHONDRAL MATRIX
ADP
+
Pi
A rotor within themembrane spinsas shown whenH+ flows pastit down the H+
gradient.
A stator anchoredin the membraneholds the knobstationary.
A rod (or stalk)
extending intothe knob alsospins, activatingcatalytic sites inthe knob.
Three catalyticsites in thestationary knobjoin inorganicphosphate toADP to makeATP.
A A i f A i
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An Accounting of ATP Productionby Cellular Respiration
During cellular respiration, most energy flows inthis sequence:
glucose NADH electron transport chain
proton-motive force ATP
About 40% of the energy in a glucose molecule istransferred to ATP during cellular respiration,
making about 38 ATP.
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CYTOSOL Electron shuttlesspan membrane 2 NADH
or
2 FADH2
MITOCHONDRION
Oxidativephosphorylation:electron transport
andchemiosmosis
2 FADH22 NADH 6 NADH
Citricacidcycle
2AcetylCoA
2 NADH
Glycolysis
Glucose2
Pyruvate
+ 2 ATP
by substrate-levelphosphorylation
+ 2 ATP
by substrate-levelphosphorylation
+ about 32 or 34 ATP
by oxidation phosphorylation, dependingon which shuttle transports electronsform NADH in cytosol
About36 or 38 ATPMaximum per glucose:
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How Many ATP?----------------------------------------------------------------------------------------------
Source FADH2 NADH ATP Yield
------------------------------------------------------------------------------------------------
Glycolysis 2 ATP
Glycolysis 2 NADH = 4 (6) ATP*
PyruvateAceylCoA 2 NADH = 6 ATPKrebs Cycle 2 ATP
Krebs Cycle 6 NADH = 18 ATP
Krebs Cycle 2 FADH2 = 4 ATP
-----------------------------------------------------------------------------------------------
TOTAL 36 (38) ATP*
*NADH transport across the mitochondrial membrane requires ATP
(1 ATP per 1 NADH).
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Anaerobic Respiration
(Fermentation)
F t ti bl ll t d ATP
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Fermentation enables some cells to produce ATPwithout the use of oxygen.
Glycolysis can produce ATP with or without O2(i.e.,in aerobic or anaerobic conditions).
In the absence of O2, glycolysis couples withfermentation to produce ATP.
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Pyruvate
Glucose
CYTOSOL
No O2 presentFermentation
Ethanolor
lactate
Acetyl CoA
MITOCHONDRION
O2 presentCellular respiration
Citricacidcycle
T f F t ti
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Types of Fermentation
Fermentation consists of glycolysis plus reactionsthat regenerate NAD+, which can be reused byglycolysis.
Two common types:
--alcohol fermentation
--lactic acid fermentation
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CO2+ 2 H+
2 NADH2 NAD+
2 Acetaldehyde
2 ATP2 ADP + 2 P i
2 Pyruvate
2
2 Ethanol
Alcohol fermentation
Glucose Glycolysis
LE 9-17b
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+ 2 H+2 NADH2 NAD+
2 ATP2 ADP + 2 P i
2 Pyruvate
2 Lactate
Lactic acid fermentation
Glucose Glycolysis
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In alcohol fermentation, pyruvate is converted toethanol in two steps, with the first releasing CO2.
enzymes: pyruvate decarboxylase; alcoholdehydrogenase.
In lactic acid fermentation, pyruvate is reducedto NADH, forming lactate as an end product, withno release of CO2.
Enzyme: lactic acid dehygrogenase
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Fermentation & Cellular Respiration Compared
Both processes use glycolysis to oxidize glucoseand other organic fuels to pyruvate.
The processes have different final electronacceptors: an organic molecule (such as pyruvate)in fermentation and O2 in cellular respiration.
Cellular respiration produces much more ATP.
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Yeast and many bacteria are facultativeanaerobes, meaning that they can survive usingeither fermentation or cellular respiration.
In a facultative anaerobe, pyruvate is a fork in themetabolic road that leads to two alternativecatabolic routes.
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Significance of Fermentation Pathway
Why should the energy in the energy-rich molecule, NADH beremoved and put into the formation of ethanol or lactate?
Oxidative phosphorylation cannot accept the electrons ofNADH without oxygen.
The purpose of fermentation is to release or free someNAD+ for glycolysis to occur (or NAD+ would remainedbottled up in NADH).
Reward: 2 ATP from glycolysis for each 2 convertedpyruvate.
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Plant Metabolic Pathways
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The Versatility of Catabolism
Catabolic pathways funnel electrons from manykinds of organic molecules into cellular respiration.
Glycolysis accepts a wide range of carbohydrates.
Proteins must be digested to amino acids; aminogroups can feed glycolysis or the citric acid cycle.
Fats are digested to glycerol (used in glycolysis)and fatty acids (used in generating acetyl CoA).
An oxidized gram of fat produces more than twiceas much ATP as an oxidized gram ofcarbohydrate.
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Citricacidcycle
Oxidativephosphorylation
Proteins
NH3
Aminoacids
Sugars
Carbohydrates
Glycolysis
Glucose
Glyceraldehyde-3- P
Pyruvate
Acetyl CoA
Fattyacids
Glycerol
Fats
Biosynthesis (Anabolic Pathways)
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Biosynthesis (Anabolic Pathways)
The body uses small molecules to build othersubstances.
These small molecules may come directly from
food (inorganic nutrients in plants), fromglycolysis, or from the citric acid cycle.
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Krebs Cycle is the Metabolic Hub for the
Breakdown and Synthesis of Many DifferentTypes of Molecules.
Regulation of Cellular Respiration
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Regulation of Cellular Respirationvia Feedback Mechanisms
Feedback inhibition is the most commonmechanism for control.
If ATP concentration begins to drop, respirationspeeds up; when there is plenty of ATP,
respiration slows down. Control of catabolism is based mainly on
regulating the activity of enzymes at strategicpoints in the catabolic pathway.
Glucose
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Citricacidcycle
Oxidativephosphorylation
Glycolysis
Glucose
Pyruvate
Acetyl CoA
Fructose-6-phosphate
Phosphofructokinase
Fructose-1,6-bisphosphate
Inhibits
ATP Citrate
Inhibits
Stimulates
AMP
+
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