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Cellular Respiration
A.P. Biology
Energy
– Flows into an ecosystem as sunlight and
leaves as heat
Light energy
ECOSYSTEM
Photosynthesis
in chloroplasts
Organic
CO2 + H2O
+ O2
Cellular
molecules
respiration
in mitochondria
ATP
powers most cellular work
Figure 9.2
Heat
energy
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
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
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
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
Some redox reactions
– Do not completely exchange electrons
– Change the degree of electron sharing in
covalent bonds
Products
Reactants
becomes oxidized
+
CH4
CO
2O2
+
Energy
2 H2O
becomes reduced
O
O
C
O
H
O
O
H
H
H
C
+
2
H
H
Methane
(reducing
agent)
Figure 9.3
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
Oxidation of Organic Fuel Molecules During
Cellular Respiration
• During cellular respiration
– Glucose is oxidized and oxygen is reduced
becomes oxidized
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
becomes reduced
G = -686 kcal/mol
Step by step catabolism of
glucose
• If electron
transfer is not
stepwise
Figure 9.5 A
Free energy, G
– A large release
of energy occurs
– As in the
reaction of
hydrogen and
oxygen to form
water
H2 + 1/2 O2
Explosive
release of
heat and light
energy
H2O
(a) Uncontrolled reaction
ETC
2H
1/
+
2
O2
1/
O2
(from food via NADH)
• The electron
transport chain
Free energy, G
– Passes electrons in a
series of steps
– Uses the energy
from the electron
transfer to form
ATP
2 H+ + 2 e–
Controlled
release of
energy for
synthesis of
ATP
ATP
ATP
ATP
2 e–
2
H+
H2O
Figure 9.5 B
(b) Cellular respiration
2
NAD – Nicotinamide Adenine Dinucleotide
(Electron Acceptor)
• Electrons from organic compounds
– Are usually 1st transferred to NAD+, a coenzyme
2 e– + 2 H+
NAD+
Dehydrogenase
O
NH2
H
C
CH2
O
O–
O
O P
O
H
–
O P O HO
O
N+ Nicotinamide
(oxidized form)
H
OH
HO
CH2
H
HO
N
H
OH
N
NADH
H O
C
H
N
NH2
+
Nicotinamide
(reduced form)
Dehydragenase – removes
2 hydrogen atoms
N
H
O
Reduction of NAD+
+ 2[H]
(from food) Oxidation of NADH
NH2
N
2 e– + H+
H
Figure 9.4
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
Electron path in respiration
Food  NADH  ETC  Oxygen
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
DG = -686 Kc/mole
+ e- --->
36-38 ATP
263Kc = 38%
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
Respiration Overview
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Glycolsis
Pyruvate
Glucose
Cytosol
ATP
Figure 9.6
Substrate-level
phosphorylation
2 ATP
Citric
acid
cycle
Oxidative
phosphorylation:
electron
transport and
chemiosmosis
Mitochondrion
ATP
Substrate-level
phosphorylation
2 ATP
ATP
Oxidative
phosphorylation
34 ATP
Substrate Level Phosphorylation
• Both glycolysis and the citric acid cycle
– Can generate ATP by substrate-level
phosphorylation
Enzyme
Enzyme
ADP
P
Substrate
+
Figure 9.7
Product
ATP
Glycolysis
Glycolysis
• Harvests energy by oxidizing
glucose to pyruvate
ATP
Citric
acid
cycle
Oxidative
phosphorylation
ATP
ATP
Energy investment
phase
Glucose
• Glycolysis
– Means “splitting of sugar”
– Breaks down glucose into
pyruvate
– Occurs in the cytoplasm of
the cell
2 ATP + 2 P
2 ATP used
Energy payoff phase
4 ADP + 4 P
2 NAD+ + 4 e- + 4
H+
4 ATP formed
2
+ 2 H+
NADH
2 Pyruvate + 2
H 2O
• Two major phases
– Energy investment phase
– Energy payoff phase
Figure 9.8
Glucose
4 ATP formed – 2 ATP
used
2 NAD+ + 4 e– + 4
H+
2 Pyruvate + 2 H2O
2 ATP + 2 H+
2 NADH
Energy Investment Phase
Energy Payoff Phase
Glycolysis Summary
The First Stage of Glycolysis
•Glucose (6C) is broken down into 2 PGAL's (3C
Phosphoglyceraldehyde)
•This requires two ATP's
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
Citric Acid Cycle
a.k.a. Krebs Cycle
• Completes the energy-yielding oxidation of
organic molecules
• The citric acid cycle
– Takes place in the matrix of the mitochondrion
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
Fate of Pyruvate
Carboxyl (coo-) is cleaved
CO2 is released
Before the citric acid cycle can begin
–Pyruvate must first be converted to acetyl CoA,
which links the cycle to glycolysis
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+
O–
S
CoA
C
O
2
C
C
O
O
1
3
CH3
Pyruvate
Transport protein
Figure 9.10
CH3
Acetyle CoA
CO2
Coenzyme A
An overview of the citric acid cycle
Pyruvate
(from glycolysis,
2 molecules per glucose)
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidative
phosphorylatio
n
ATP
CO2
CoA
NADH
+ 3 H+ Acetyle CoA
CoA
CoA
Citric
acid
cycle
2 CO2
3 NAD+
FADH2
FAD
3 NADH
+ 3 H+
ADP + P i
ATP
Figure 9.11
Glycolysis
Citric
Oxidative
acid phosphorylation
cycle
S
CoA
C
O
CH3
Acetyl CoA
CoA SH
O
NADH
+ H+
C COO–
COO–
1
CH2
COO–
NAD+
8 Oxaloacetate
HO C
COO–
COO–
CH2
COO–
HO CH
H2O
CH2
CH2
2
HC COO–
COO–
Malate
HO
Citrate
CH2
COO–
Isocitrate
COO–
H2O
CO2
Citric
acid
cycle
7
COO–
CH
3
NAD+
COO–
Fumarate
HC
CH2
CoA SH
6
CoA SH
COO–
FAD
CH2
CH2
COO–
C O
Succinate
Pi
S
CoA
GTP GDP Succinyl
CoA
ADP
ATP
4
C O
COO–
CH2
5
CH2
FADH2
COO–
NAD+
NADH
+ H+
NADH
+ H+
a-Ketoglutarate
CH2
COO–
Figure 9.12
CH
CO2
The Kreb's Cycle
•6 NADH's are generated
•2 FADH2 is generated
•2 ATP are generated
•4 CO2's are released
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
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
After the Krebs Cycle…
•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
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
NADH
50
FADH2
Free energy (G) relative to O2 (kcl/mol)
40
I
FMN
Multiprotein
complexes
FAD
Fe•S
Fe•S
II
O
III
Cyt b
30
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
0
Figure 9.13
2 H + + 12
O2
H2 O
Chemiosmosis:
The Energy-Coupling Mechanism
•ATP synthase
INTERMEMBRANE SPACE
H+
H+
–Is the enzyme
that actually
makes ATP
H+
H+
H+
H+
H+
A stator anchored
in the membrane
holds the knob
stationary.
H+
ADP
+
Pi
Figure 9.14
A rotor within the
membrane spins
clockwise when
H+ flows past
it down the H+
gradient.
MITOCHONDRIAL MATRIX
ATP
A rod (for “stalk”)
extending into
the knob also
spins, activating
catalytic sites in
the knob.
Three catalytic
sites in the
stationary knob
join inorganic
Phosphate to ADP
to make ATP.
ETC
–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
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
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP
Inner
Mitochondrial
membrane
ATP
ATP
H+
H+
H+
Intermembrane
space
H+
Cyt c
Protein complex
of electron
carners
Q
I
Inner
mitochondrial
membrane
IV
III
ATP
synthase
II
FADH2
FAD+
NADH+
2 H+ + 1/2 O2
NAD+
H2O
ADP +
(Carrying electrons
from, food)
Mitochondrial
matrix
Figure 9.15
ATP
Pi
H+
Electron transport chain
Electron transport and pumping of protons (H+),
which create an H+ gradient across the membrane
Chemiosmosis
ATP synthesis powered by the flow
Of H+ back across the membrane
Oxidative phosphorylation
An Accounting of ATP
Production by Cellular Respiration
•During respiration, most energy flows in this
sequence
–Glucose to NADH to electron transport chain to
proton-motive force to ATP
Electron shuttles
span membrane
CYTOSOL
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
2 NADH
Glycolysis
Glucose
2
Acetyl
CoA
2
Pyruvate
Citric
acid
cycle
+ 2 ATP
+ 2 ATP
by substrate-level
phosphorylation
by substrate-level
phosphorylation
Maximum per glucose:
Figure 9.16
6 NADH
About
36 or 38 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
+ about 32 or 34 ATP
by oxidative phosphorylation, depending
on which shuttle transports electrons
from NADH in cytosol
Anaerobic Respiration
•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
•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
2 ADP + 2
Glucose
P1
2 ATP
Glycolysis
O–
C
O
C
O
CH3
2 Pyruvate
2 NADH
2 NAD+
H
H
2 CO2
H
C
C
OH
CH3
O
CH3
2 Ethanol
2 Acetaldehyde
(a) Alcohol fermentation
2 ADP + 2
Glucose
P1
O
H
C
O
OH
CH3
2 Lactate
Figure 9.17
O–
Glycolysis
2 NAD+
C
2 ATP
(b) Lactic acid fermentation
2 NADH
C
O
C
O
CH3
Pyruvate is a key juncture in catabolism
Glucose
CYTOSOL
Pyruvate
No O2 present
Fermentation
O2 present
Cellular respiration
MITOCHONDRION
Ethanol
or
lactate
Figure 9.18
Acetyl CoA
Citric
acid
cycle
•Glycolysis
–Occurs in nearly all organisms
–Probably evolved in ancient prokaryotes before
there was oxygen in the atmosphere
Proteins
Amino
acids
Carbohydrates
Sugars
Glycolysis
Glucose
Glyceraldehyde-3- P
NH3
Pyruvate
Acetyl CoA
Citric
acid
cycle
Figure 9.19
Oxidative
phosphorylation
Fats
Glycerol
Fatty
acids
•Cellular respiration
Glucose
AMP
Glycolysis
–Is controlled by
allosteric enzymes at
key points in
glycolysis and the
citric acid cycle
Fructose-6-phosphate
Stimulates
+
–
Inhibits
Phosphofructokinase
–
Fructose-1,6-bisphosphate
Inhibits
Pyruvate
Citrate
ATP
Acetyl CoA
Citric
acid
cycle
Figure 9.20
Oxidative
phosphorylation