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Cellular Respiration
Cellular respiration
Aerobic respiration
Requires molecular oxygen
Includes redox reactions
Anaerobic
Anaerobic respiration
Fermentation
Do not require oxygen
All exergonic
Energy flows into an ecosystem as sunlight and
leaves as heat
Photosynthesis generates O2 and organic
molecules, which are used in cellular respiration
Cells use chemical energy stored in organic
molecules to regenerate ATP, which powers work
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 O2
Aerobic respiration consumes organic
molecules and O2 and yields ATP
Anaerobic respiration is similar to aerobic
respiration but consumes compounds
other than O2
Catabolic Pathway
Complex organic
molecules
Catabolic Pathway
Simpler waste
products with
less E
Some E used to do
work and
dissipated as heat
Cellular respiration includes both
aerobic and anaerobic respiration but is
often used to refer to aerobic respiration
Although carbohydrates, fats, and
proteins are all consumed as fuel, it is
helpful to trace cellular respiration with
the sugar glucose
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy
(ATP + heat)
Redox Reactions:
Oxidation and Reduction
The transfer of electrons during chemical
reactions releases energy stored in
organic molecules
This released energy is ultimately used to
synthesize ATP
The Principle of Redox
Chemical reactions that transfer electrons
between reactants are called oxidationreduction reactions, or redox reactions
• In oxidation, a substance loses
electrons, or is oxidized
In reduction, a substance gains
electrons, or is reduced (the amount of
positive charge is reduced)
During cellular respiration, the fuel (such
as glucose) is oxidized, and O2 is reduced
Cellular respiration includes both
aerobic and anaerobic respiration but is
often used to refer to aerobic respiration
Although carbohydrates, fats, and
proteins are all consumed as fuel, it is
helpful to trace cellular respiration with
the sugar glucose
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy
(ATP + heat)
Stepwise Energy Harvest via
NAD+ and the Electron
Transport Chain
In cellular respiration, glucose and other organic
molecules are broken down in a series of steps
Electrons from organic compounds are usually
first transferred to NAD+, a coenzyme
As an electron acceptor, NAD+ functions as an
oxidizing agent during cellular respiration
Each NADH (the reduced form of NAD+)
represents stored energy that is tapped to
synthesize ATP
Four stages of aerobic respiration
Takes place in the cytosol
Glycolysis
Takes place in the mitochondrion
Formation of acetyl CoA
Citric acid cycle
Electron transport chain/chemiosmosis
Four stages of aerobic respiration
Summary of aerobic respiration
Glycolysis
“Sugar splitting”
Does not require oxygen
Divided into two major phases
Energy investment phase
Energy capture phase
Each glucose molecule produces net yield of
two NADH molecules and two ATP molecules
Substrate-level
phosphorylation
Glycolysis
INPUTS
Glucose
2 ATP
4 ADP
2 P
2 NAD+
Outputs
2 Pyruvate
2 ADP
4 ATP
2 NADH + H+
Glycolysis
Glycolysis Occurs in the cytoplasm.
Glycolysis is anaerobic - requires no O2.
Energy in the form of 2 ATP is gained.
Energy in the form of 2NADH + H+ is
gained. Each NADH + H+ yields 3 ATP at
the ETC for a total of 6 ATP.
2 Pyruvate are gained which are modified
where further energy can be extracted.
Glycolysis
In the presence of O2, pyruvate enters the
mitochondrion (in eukaryotic cells) where
the oxidation of glucose is completed
The Mitochondrion
Outer membrane contains large-pore
channel proteins.
Highly folded selective inner membrane
contains the mitochondrial matrix.
Many enzymes of the CAC are dissolved in
the matrix with the rest being attached to
the inner face of the inner membrane.
This membrane also contains ET
molecules.
Mito. Cont.
Inner membrane helps to create an
electrochemical gradient by pushing H+
ions outside the membrane.
Inner membrane contains ATP synthetase
enzymes.
Aerobic bacteria membrane molecules
occur in the plasma membrane
Conversion of Pyruvate to
Acetyl Co-A
Pyruvate crosses the inner mitochondrial
membrane into the matrix.
Pyruvate is oxidized and decarboxylated.
NAD picks up a hydrogen atom and
electron and an H+ follows in solution.
CO2 leaves
the remaining 2-C acetyl binds to Co-A
Oxidation of Pyruvate to
Acetyl CoA
Before the citric acid cycle can begin,
pyruvate must be converted to acetyl
Coenzyme A (acetyl CoA), which links
glycolysis to the citric acid cycle
This step is carried out by a multienzyme
complex that catalyses three reactions
Conversion of Pyruvate to
Acetyl Co-A
Inputs
2 Pyruvate
2 NAD
2 Co-A
Outputs
2 CO2
2 NADH + H+ (for a
total gain of 6 ATP at
the ETC)
2 acetyl Co-A
Formation
of acetyl CoA
Citric Acid Cycle
The molecules are structurally rearranged to
strip off H+ ions to be used in oxidative
phosphorylation.
A total of 2 ATP molecules are formed by
substrate level phosphorylation.
The cycle oxidizes pyruvate to 6 NADH, and 2
FADH2
The remaining carbon and oxygen are released
as CO2 (4 CO2 total).
Citric Acid Cycle
2
2
2
2
6
2
Inputs
Oxaloacetate
Acetyl Co-A
ADP
P
NAD+
FAD
2
2
4
2
6
2
Outputs
Oxaloacetate
Co-A
CO2
ATP
NADH + H+ 18 ATP
FADH2
4 ATP
Citric acid cycle
Oxidative PhosphorylationChemiosmosis
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 proton, ATP synthase
ATP synthase uses the exergonic flow of H+ to
drive phosphorylation of ATP
This is an example of chemiosmosis, the use
of energy in a H+ gradient to drive cellular work
Accumulation
of protons within
the inter-membrane
space
Electron transport and chemiosmosis
The Electron Transport
System
e- arriving at the end have given up most
of their energy.
O2 takes these e- plus some H+ forming
new water molecules.
4e- + 4H+ + O2
2H2O
The H+ Gradient and
Phosphorylation -??
Electrochemical potential energy
220 millivolts difference - electrical.
H+ pH difference - chemical.
Proton-motive force - hydrogen ions are
protons coupled with the force of them
moving back across the membrane.
Chemiosmotic ATP
Synthesis
Coupling between the flow of H+ down its
gradient and the phosphorylation of ADP.
ATP synthetase can make ATP without the flow
of H+.
It can not release the ATP until H+ ions flow
flow through the chanel toward the matrix,
down their electrochemical gradient.
H+ gradient powers ATP synthesis indirectly by
freeing the enzymes active site.
Summary of Oxidation
 Energy is stockpiled in the form of an H+ electrochemical
gradient across the inner mitochondrial membrane, via a
series of oxidation-reduction reactions.
 The electron transport system accepts hydrogens from
NADH and FADH2.
 The system passes the hydrogens’ electrons through a
series of redox reaction and uses the energy released to
pump H+ through the membrane against its
electrochemical gradient.
 At the end of the system, the e- are accepted by O2,
which also picks up H+ to form water.
Summary of
Phosphorylation
The energy of the H+ gradient is used in
the phosphorylation of ADP to ATP.
This happens as H+ re-crosses the
membrane via channel proteins connected
to ATP synthetase enzymes.
Maximum ATP Yield
Process
Number of
Reduced H
Carriers
Maximum
Number of
ATP per H
Pair
Number of
ATP by
Oxidative
Net ATP by
substrate level
Glycolysis 2 NADH + 3 ATP/2H
6 ATP
2 ATP
Pyruvate
to AcetylCoA
Citric Acid 6 NADH +
+
H
Cycle
6 ATP
H+
2 NADH + 3 ATP/2H
H+
Grand
2 FADH2
Total
3 ATP/2H
2 ATP/2H
Phosphorylation
Phosphorylation
18 ATP
4 ATP
2 ATP
34 ATP 4 ATP
Energy
yield from
complete
oxidation of
glucose by
aerobic
respiration
Many organisms depend on nutrients other than
glucose
Products of protein and lipid catabolism enter
same metabolic pathways as glucose
Amino acids are deaminated
Energy
from
carbohydrates,
proteins, and fats
Fermentation
Electron transport molecules get stuck in
the reduced form, holding electrons.
System stops working.
H+ gradient is used up.
Fermentation uses organic molecules as
final electron acceptors.
Pyruvate can serve as an alternative
electron acceptor to free up NADH
Aerobic respiration versus fermentation
Aerobic respiration
Electrons transferred from fuel molecules to
electron transport chain
Final electron acceptor is inorganic substance
Fermentation
Anaerobic process that does not use electron
transport chain
Comparison of aerobic respiration, anaerobic respiration, and
fermentation
Fermentation
Glycolysis:
energy investment
phase
Glycolysis:
energy capture
phase
Detail of
citric acid cycle