Download Document

Cellular Respiration - Conclusion
The Electron Transport Chain
and Oxidative Phosphorylation
• Oxidative phosphorylation: when electron transport is
coupled to ATP synthesis through chemiosmosis
• NADH and FADH2 (from glycolysis and the citric acid cycle)
donate electrons to the electron transport chain, which
powers ATP synthesis via oxidative phosphorylation.
The electron transport chain is in the inner
membrane (cristae) of the mitochondrion.
Figure 9.13
NADH
50
The carriers alternate
reduced and oxidized states
as they accept and donate
electrons.
Electrons drop in free
energy as they go down the
chain and are finally passed
to O2, forming H2O.
NAD
FADH2
2 e
Free energy (G) relative to O2 (kcal/mol)
Most of the chain’s
components are proteins,
which exist in multiprotein
complexes.
2 e
40
FMN
FeS
FeS
II
Q
III
Cyt b
30
Multiprotein
complexes
FAD
I
FeS
Cyt c1
IV
Cyt c
Cyt a
20
10
0
Cyt a3
2 e
(originally from
NADH or FADH2)
2 H + 1/2 O2
H2O
• Electrons are passed through a number of
proteins including cytochromes (each
with an iron atom) to O2.
• The electron transport chain generates no
ATP directly.
• What is its purpose then?
Chemiosmosis: The Energy-Coupling
Mechanism
• Electron transfer in the electron
transport chain causes proteins to
pump H+ from the mitochondrial
matrix to the intermembrane
space.
• H+ then moves back across the
membrane, passing through the
protein, ATP synthase.
• ATP synthase uses the exergonic
flow of H+ to drive phosphorylation
of ATP.
• This is an example of
chemiosmosis, the use of
potential energy in a H+ gradient to
drive cellular work.
Figure 9.14
INTERMEMBRANE SPACE
H
Stator
Rotor
Internal
rod
Catalytic
knob
ADP
+
Pi
ATP
MITOCHONDRIAL MATRIX
Figure 9.15
H
H

H
Protein
complex
of electron
carriers
Cyt c
Q
I
IV
III
II
FADH2 FAD
NADH
H
2 H + 1/2O2
ATP
synthase
H2O
NAD
ADP  P i
(carrying electrons
from food)
ATP
H
1 Electron transport chain
Oxidative phosphorylation
2 Chemiosmosis
• The H+ gradient is
referred to as a
proton-motive
force,
emphasizing its
capacity to do
work.
• The potential
energy from
diffusion of H+
across the
membrane
powers the
synthesis of ATP.
Summary
• During cellular respiration, most energy flows
in this sequence:
glucose  NADH  electron transport chain
 proton-motive force  ATP
• About 34% of the energy in a glucose
molecule is transferred to ATP during cellular
respiration, making about 36 ATP.
• What happens to the rest of the energy?
It’s given off as heat.
Cellular Respiration
What if there’s no oxygen?
• Without O2, the electron transport chain
will cease to operate.
• In that case, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP.
– Anaerobic respiration: electron transport chain
with an electron acceptor other than O2 (often
sulfate)
– Fermentation: substrate-level phosphorylation
(like glycolysis)
Fermentation
• Fermentation = glycolysis + recycling of
NAD+ (to use for more glycolysis)
• alcohol or lactic acid
Compare + Contrast
• Both do glycolysis
• Both reduce NAD+ (electron acceptor)
• Final electron receptor is different
– Cellular Respiration: O2
– Fermentation: pyruvate or acetaldehyde
• Produce different amounts of ATP
– Cellular respiration = 32 ATP per glucose
– Fermentation = 2 ATP per glucose
Figure 9.18
Glucose
CYTOSOL
Glycolysis
Pyruvate
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
MITOCHONDRION
Ethanol,
lactate, or
other products
Acetyl CoA
Citric
acid
cycle
Other Fuel Molecules
• Catabolic pathways funnel electrons from many
kinds of organic molecules into cellular respiration
– not just glucose.
• Carbohydrates  glycolysis
• Proteins (amino acids)  glycolysis or the citric
acid cycle
• Fats
– Glycerol  glycolysis
– Fatty acids  acetyl CoA (Citric Acid Cycle)
• An oxidized gram of fat produces more than twice
as much ATP as an oxidized gram of
carbohydrate.
Figure 9.19
Proteins
Carbohydrates
Amino
acids
Sugars
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3
Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Fats
Glycerol Fatty
acids
With 1 molecule of glucose, cellular
respiration produces 36-38 ATP molecules.