cellular respiration a.p. biology. energy –flows into an ecosystem as sunlight and leaves as heat...
TRANSCRIPT
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Cellular Respiration
A.P. Biology
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Energy
– Flows into an ecosystem as sunlight and leaves as heat
Light energy
ECOSYSTEM
CO2 + H2O
Photosynthesisin chloroplasts
Cellular respiration
in mitochondria
Organicmolecules
+ O2
ATP
powers most cellular work
Heatenergy
Figure 9.2
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Catabolic pathways yield energy by oxidizing organic
fuels• The breakdown of organic molecules is
exergonic
• Fermentation– Is a partial degradation of sugars that occurs
without oxygen
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Catabolic pathways yield energy by oxidizing organic
fuels
• Cellular respiration– Is the most prevalent and efficient
catabolic pathway– Consumes oxygen and organic
molecules such as glucose– Yields ATP
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Redox Reactions: Oxidation and Reduction
• Catabolic pathways yield energy– Due to the transfer of electrons
• Redox reactions– Transfer electrons from one reactant to another by
oxidation and reduction
• Oxidation– A substance loses electrons, or is oxidized
• Reduction– A substance gains electrons, or is reduced
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becomes oxidized(loses electron) becomes reduced
(gains electron)
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Some redox reactions
– Do not completely exchange electrons– Change the degree of electron sharing in
covalent bonds
CH4
H
H
HH
C O O O O OC
H H
Methane(reducingagent)
Oxygen(oxidizingagent)
Carbon dioxide Water
+ 2O2 CO2 + Energy + 2 H2O
becomes oxidized
becomes reduced
Reactants Products
Figure 9.3
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Oxidation of Organic Fuel Molecules During Cellular
Respiration
• During cellular respiration– Glucose is oxidized and oxygen is
reduced
C6H12O6 + 6O2 6CO2 + 6H2O + Energy
becomes oxidized
becomes reduced
G = -686 kcal/mol
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Step by step catabolism of glucose
• If electron transfer is not stepwise– A large release
of energy occurs– As in the
reaction of hydrogen and oxygen to form water
(a) Uncontrolled reaction
Fre
e en
ergy
, G
H2O
Explosiverelease of
heat and lightenergy
Figure 9.5 A
H2 + 1/2 O2
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ETC
• The electron transport chain– Passes electrons in
a series of steps – Uses the energy
from the electron transfer to form ATP
2 H 1/2 O2
(from food via NADH)
2 H+ + 2 e–
2 H+
2 e–
H2O
1/2 O2
Controlled release of energy for synthesis of
ATP ATP
ATP
ATP
Electro
n tran
spo
rt chain
Fre
e en
ergy
, G
(b) Cellular respiration
+
Figure 9.5 B
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NAD – Nicotinamide Adenine Dinucleotide
(Electron Acceptor)• Electrons from organic compounds
– Are usually 1st transferred to NAD+, a coenzyme
NAD+
HO
O
O O–
O
O O–
O
O
O
P
P
CH2
CH2
HO OHH
HHO OH
HO
H
H
N+
C NH2
HN
H
NH2
N
N
Nicotinamide(oxidized form)
NH2+ 2[H]
(from food)
Dehydrogenase
Reduction of NAD+
Oxidation of NADH
2 e– + 2 H+
2 e– + H+
NADH
OH H
N
C +
Nicotinamide(reduced form)
N
Figure 9.4
Dehydragenase – removes 2 hydrogen atoms
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NADH
• NADH, the reduced form of NAD+
– Passes the electrons to the electron transport chain
– Electrons are ultimately passed to a molecule of oxygen (Final electron acceptor)
G = -53 kcal/mol
Food NADH ETC Oxygen
Electron path in respiration
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Cellular Respiration
• Respiration is a cumulative function of three metabolic stages– Glycolysis– The citric acid cycle (TCA or Krebbs)– Oxidative phosphorylation
C6H12O6 + 6O2 <----> 6 CO2 + 6 H20 + e- ---> 36-38 ATP G = -686 Kc/mole 263Kc = 38%
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Respiration
• Glycolysis– Breaks down glucose into two molecules of
pyruvate
• The citric acid cycle– Completes the breakdown of glucose
• Oxidative phosphorylation– Is driven by the electron transport chain– Generates ATP
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Respiration Overview
Figure 9.6
Electronscarried
via NADH
GlycolsisGlucos
ePyruvate
ATP
Substrate-levelphosphorylation
Electrons carried via NADH and
FADH2
Citric acid cycle
Oxidativephosphorylation:
electron transport and
chemiosmosis
ATPATP
Substrate-levelphosphorylation
Oxidativephosphorylation
MitochondrionCytosol
2 ATP 2 ATP 34 ATP
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Substrate Level Phosphorylation
• Both glycolysis and the citric acid cycle– Can generate ATP by substrate-level
phosphorylation
Figure 9.7
Enzyme Enzyme
ATP
ADP
Product
SubstrateP
+
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Glycolysis• Harvests energy by oxidizing
glucose to pyruvate
• Glycolysis– Means “splitting of sugar”– Breaks down glucose into
pyruvate– Occurs in the cytoplasm of the cell
• Two major phases– Energy investment phase– Energy payoff phase
Glycolysis Citricacidcycle
Oxidativephosphorylation
ATP ATP ATP
2 ATP
4 ATP
used
formed
Glucose
2 ATP + 2
P
4 ADP + 4 P
2 NAD+ + 4 e- + 4 H +
2 NADH
+ 2 H+
2 Pyruvate + 2 H2O
Energy investment phase
Energy payoff phase
Glucose 2 Pyruvate + 2 H2O4 ATP formed – 2 ATP
used 2 ATP
2 NAD+ + 4 e– + 4 H +
2 NADH
+ 2 H+
Figure 9.8
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Energy Investment Phase
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Energy Payoff Phase
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Glycolysis Summary
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The First Stage of Glycolysis •Glucose (6C) is broken down into 2 PGAL's (3C Phosphoglyceraldehyde) •This requires two ATP's
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The Second Stage of Glycolysis •2 PGAL's (3C) are converted to 2 pyruvates •This creates 4 ATP's and 2 NADH's •The net ATP production of Glycolysis is 2 ATP's
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Citric Acid Cyclea.k.a. Krebs Cycle
• Completes the energy-yielding oxidation of organic molecules
• The citric acid cycle– Takes place in the matrix of the
mitochondrion
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Krebs's Cycle (citric acid cycle, TCA cycle)
•Goal: take pyruvate and put it into the Krebs's cycle, producing NADH and FADH2
•Where: the mitochondria
•There are two steps •The Conversion of Pyruvate to Acetyl CoA •The Kreb's Cycle proper
•In the Krebs's cycle, all of Carbons, Hydrogens, and Oxygeng in pyruvate end up as CO2 and H2O
•The Krebs's cycle produces 2 ATP's, 8 NADH's, and 2FADH2's per glucose molecule
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Fate of Pyruvate
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Carboxyl (coo-) is cleaved CO2 is released
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–Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis
Before the citric acid cycle can begin
CYTOSOL MITOCHONDRION
NADH + H+NAD+
2
31
CO2 Coenzyme APyruvate
Acetyle CoA
S CoA
C
CH3
O
Transport protein
O–
O
O
C
C
CH3
Figure 9.10
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ATP
2 CO2
3 NAD+
3 NADH
+ 3 H+
ADP + P i
FAD
FADH2
Citricacidcycle
CoA
CoA
Acetyle CoA
NADH+ 3 H+
CoA
CO2
Pyruvate(from glycolysis,2 molecules per glucose)
ATP ATP ATP
Glycolysis Citricacidcycle
Oxidativephosphorylation
Figure 9.11
An overview of the citric acid cycle
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Acetyl CoA
NADH
Oxaloacetate
CitrateMalate
Fumarate
SuccinateSuccinyl
CoA
-Ketoglutarate
Isocitrate
Citricacidcycle
S CoA
CoA SH
NADH
NADH
FADH2
FAD
GTP GDP
NAD+
ADP
P i
NAD+
CO2
CO2
CoA SH
CoA SH
CoAS
H2O
+ H+
+ H+ H2O
C
CH3
O
O C COO–
CH2
COO–
COO–
CH2
HO C COO–
CH2
COO–
COO–
COO–
CH2
HC COO–
HO CHCOO–
CH
CH2
COO–
HO
COO–
CH
HC
COO–
COO–
CH2
CH2
COO–
COO–
CH2
CH2
C O
COO–
CH2
CH2
C O
COO–
1
2
3
4
5
6
7
8
Glycolysis Oxidativephosphorylation
NAD+
+ H+
ATP
Citricacidcycle
Figure 9.12
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The Kreb's Cycle•6 NADH's are generated •2 FADH2 is generated •2 ATP are generated •4 CO2's are released
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Net Engergy Production from Aerobic Respiration
•Glycolysis: 2 ATP •Kreb's Cycle: 2 ATP •Electron Transport Phosphorylation: 32 ATP
•Each NADH produced in Glycolysis is worth 2 ATP (2 x 2 = 4) - the NADH is worth 3 ATP, but it costs an ATP to transport the NADH into the mitochondria, so there is a net gain of 2 ATP for each NADH produced in gylcolysis •Each NADH produced in the conversion of pyruvate to acetyl COA and Kreb's Cycle is worth 3 ATP (8 x 3 = 24) •Each FADH2 is worth 2 ATP (2 x 2 = 4) •4 + 24 + 4 = 32
•Net Energy Production: 36 ATP
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Energy Yields:
•Glucose: 686 kcal/mol
•ATP: 7.5 kcal/mol
•7.5 x 36 = 270 kcal/mol for all ATP's produced
•270 / 686 = 39% energy recovered from aerobic respiration
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•During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
•NADH and FADH2
–Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
After the Krebs Cycle…
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The Pathway of Electron Transport
•In the electron transport chain
–Electrons from NADH and FADH2 lose energy in several steps
•At the end of the chain
–Electrons are passed to oxygen, forming water
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H2O
O2
NADH
FADH2
FMN
Fe•S Fe•S
Fe•S
O
FAD
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
2 H + + 12
I
II
III
IV
Multiproteincomplexes
0
10
20
30
40
50
Fre
e e
ner
gy
(G)
rela
tive
to O
2 (k
cl/m
ol)
Figure 9.13
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Chemiosmosis: The Energy-Coupling Mechanism
•ATP synthase
–Is the enzyme that actually makes ATP
INTERMEMBRANE SPACE
H+
H+
H+
H+
H+
H+ H+
H+
P i
+ADP
ATP
A rotor within the membrane spins clockwise whenH+ flows past it down the H+
gradient.
A stator anchoredin the membraneholds the knobstationary.
A rod (for “stalk”)extending into the knob alsospins, activatingcatalytic sites inthe knob.
Three catalytic sites in the stationary knobjoin inorganic Phosphate to ADPto make ATP. MITOCHONDRIAL MATRIXFigure 9.14
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–Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space
•The resulting H+ gradient
–Stores energy
–Drives chemiosmosis in ATP synthase
–Is referred to as a proton-motive force
ETC
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Chemiosmosis
–Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work
H+ gradient = Proton motive force
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Oxidativephosphorylation.electron transportand chemiosmosis
Glycolysis
ATP ATP ATP
InnerMitochondrialmembrane
H+
H+H+
H+
H+
ATPP i
Protein complexof electron carners
Cyt c
I
II
III
IV
(Carrying electronsfrom, food)
NADH+
FADH2
NAD+
FAD+ 2 H+ + 1/2 O2
H2O
ADP +
Electron transport chainElectron transport and pumping of protons (H+),
which create an H+ gradient across the membrane
ChemiosmosisATP synthesis powered by the flowOf H+ back across the membrane
ATPsynthase
Q
Oxidative phosphorylation
Intermembranespace
Innermitochondrialmembrane
Mitochondrialmatrix
Figure 9.15
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•During respiration, most energy flows in this sequence
–Glucose to NADH to electron transport chain to proton-motive force to ATP
An Accounting of ATP Production by Cellular Respiration
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Electron shuttlesspan membrane
CYTOSOL 2 NADH
2 FADH2
2 NADH 6 NADH 2 FADH22 NADH
Glycolysis
Glucose2
Pyruvate
2AcetylCoA
Citricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
MITOCHONDRION
by substrate-levelphosphorylation
by substrate-levelphosphorylation
by oxidative phosphorylation, dependingon which shuttle transports electronsfrom NADH in cytosol
Maximum per glucose:About
36 or 38 ATP
+ 2 ATP + 2 ATP + about 32 or 34 ATP
or
Figure 9.16
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•Fermentation enables some cells to produce ATP without the use of oxygen
•Glycolysis
–Can produce ATP with or without oxygen, in aerobic or anaerobic conditions
–Couples with fermentation to produce ATP
Anaerobic Respiration
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•Fermentation consists of
–Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis
•Alcohol fermentation
–Pyruvate is converted to ethanol in two steps, one of which releases CO2
•Lactic acid fermentation
–Pyruvate is reduced directly to NADH to form lactate as a waste product
Anaerobic Respiration
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2 ADP + 2 P1 2 ATP
GlycolysisGlucose
2 NAD+ 2 NADH
2 Pyruvate
2 Acetaldehyde 2 Ethanol
(a) Alcohol fermentation
2 ADP + 2 P1 2 ATP
GlycolysisGlucose
2 NAD+ 2 NADH
2 Lactate
(b) Lactic acid fermentation
H
H OH
CH3
C
O –
OC
C O
CH3
H
C O
CH3
O–
C O
C O
CH3O
C O
C OHH
CH3
CO22
Figure 9.17
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Glucose
CYTOSOL
Pyruvate
No O2 presentFermentation
O2 present Cellular respiration
Ethanolor
lactate
Acetyl CoA
MITOCHONDRION
Citricacidcycle
Figure 9.18
Pyruvate is a key juncture in catabolism
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•Glycolysis
–Occurs in nearly all organisms
–Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere
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Amino acids
Sugars Glycerol Fattyacids
Glycolysis
Glucose
Glyceraldehyde-3- P
Pyruvate
Acetyl CoA
NH3
Citricacidcycle
Oxidativephosphorylation
FatsProteins Carbohydrates
Figure 9.19
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•Cellular respiration
–Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle
Glucose
Glycolysis
Fructose-6-phosphate
Phosphofructokinase
Fructose-1,6-bisphosphateInhibits Inhibits
Pyruvate
ATPAcetyl CoA
Citricacidcycle
Citrate
Oxidativephosphorylation
Stimulates
AMP
+
– –
Figure 9.20