cell respiration part ii

8/9/2019 Cell Respiration Part II

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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Electron Transport Chain


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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.


7/43Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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.

------------



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.



NADH as Electron Carrier



FADH2 as Electron Carrier



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.



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.



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


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


Glycolysis

Glucose

Pyruvate

Acetyl CoA

Fructose-6-phosphate

Phosphofructokinase

Fructose-1,6-bisphosphate

Inhibits

ATP Citrate

Inhibits

Stimulates

AMP

+

cell respiration part ii

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