UNIT 2: Metabolic Processes
Chapter 4: Cellular Respiration
pg. 166 - 209
4.2: Aerobic Respiration: The Details
pg. 172 - 182
Aerobic Respiration (cellular respiration) is the beak down of glucose in the
presence of oxygen to release free energy to create ATP, primary energy
source of the cell. Aerobic respiration is made up of four energy releasing
pathways; glycolysis, pyruvate oxidation, critic acid cycle (Krebs cycle), and
the electron transport chain.
Glycolysis
Glycolysis is the first pathway used to release free energy from a molecule
of glucose. Oxygen is not required at this point of aerobic respiration.
Glycolysis occurs in the cytosol of the cell.
The Reaction of Glycolysis
Glycolysis consists of 10 steps. Each of the steps is enzyme catalyzed.
Glucose (6 carbon molecule) is broken down into two molecules of pyruvate
(3 carbon molecule), 2 ATP are used, while 4 ATP are synthesized,
producing a 2 ATP net gain. Two co-enzymes (NAD+) are reduced to two
molecules of NADH.
Glycolysis can be sub-divide into two phases. Steps 1 – 5 is the initial
energy investment phase (2 ATP), and steps 6 – 10 is the energy pay off
phase (4 ATP).
Three Key Points:
1. Initially, 2 ATP are consumed as glucose and fructose-6-phosphate
become phosphorylated. In the energy investment phase, 2 ATP
increase the free energy of the chemicals in the glycolytic pathway.
However, even more free energy is released in the payoff phase, 4
ATP and 2 NADH molecules are synthesized.
2. Besides yielding a net of 2 ATP and 2 NADH for each molecule of
glucose that is oxidized, no carbon is lost. All six carbons in
glucose are accounted for in the two molecules of pyruvate.
However, since glucose has been partially oxidized, the potential
energy in two molecules of pyruvate is less than the potential
energy in one molecule of glucose. Although two water molecules
were produced in step 9, they are not usually included in the
overall equation for glycolysis because they are later consumed in
the hydrolysis of the 2 ATP molecules and the reforming of 2 ADP
and Pi.
3. During glycolysis, ATP is produced using substrate-level
phosphorylation. In this mode of ATP synthesis, an enzyme
transfers a phosphate group from a high energy substrate molecule
to adenosine di-phosphate (ADP), producing ATP. Substrate-level
phosphorylation is also the mode of ATP synthesis that is used
during the citric acid cycle.
Net Equations for Glycolysis:
Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 pyruvate + 2 ATP +2 NADH + 2 H+
The synthesis 2 moles of ATP from glycolysis only stores 62 KJ/mol.
The complete oxidation of glucose releases 2870 KJ/mol of energy.
Therefore the percent of energy release in glycolysis is 62/2870 x 100 =
2.2%
This is not sufficient enough to keep an organism alive. The rest of the
energy is still trapped in the two molecules of pyruvate and 2 molecules of
NADH.
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
∆G = -2870 KJ/mol
Figure 1: summary of glycolysis showing the energy inputs and outputs, pg. 172.
Steps of Glycolysis:
1. Glucose (6 carbon molecule) is phosphorylated by one molecule of
ATP, creating a molecule of glucose-6-phosphate (G6P). This process
is mediated by hexokinase enzyme.
2. Glucose-6-phosphate is rearranged into fructose-6phosphate (F6P) by
an isomerization reaction. This process is mediated by phosphoglucomutase enzyme.
3. Fructose-6-phosphate is phosphorylated by one molecule of ATP,
creating a molecule of fructose-1,6-bisphosphate. The process is
mediated by phosphofructokinase enzyme.
4. Fructose-1,6-bisphosphate (6carbon molecule) is split into one
molecule of glyceraldehyde-3-phosphate (G3P) and one molecule of
dihydroxyacetone phosphate (DHAP). Each of these molecules
consists of a 3 carbon chain. This process is mediated by aldolase
enzyme. G3P moves directly to step 6.
5. The one molecule of dihydroxyacetone phosphate is converted into
glyceralehyde-3-phsophate by an isomerization reaction. This process
is mediated by triose-phosphate isomerase. G3P now moves into step
6.
6. 2 molecules of glyceraldehydes-3-phosphate, one from step 4 and the
other from step 5, now release one hydrogen atom each (one electron
and one proton). The electrons and protons reduce 2 NAD+ to 2
NADH (redox reaction). One hydrogen proton is released into the in
cytosol.
An inorganic phosphate group from the cytosol is bonded to each of
the newly created 1,3-bisphosphoglycerate.
This process is mediated by triosephosphate Dehydrogenase.
7. The 2 molecules of 1,3-bisphosphoglycerate now will release one
phosphate each to phosphorylate ADP to ATP (substrate-level
phosphorylation). The produces 2 molecules of 3-phosphoglycerate,
and 2 ATP. This process is mediated by phosphoglycerate kinase.
8. The 2 molecules of 3 phosphoglycerate is rearranged, moving the
phosphate from 3-carbon to 2-carbon, creating 2 molecules of 2phosphoglycerate. The process is mediated by phosphoglucomutase.
9. The 2 molecules of 2-phosphoglycerate loses a water molecule each.
This produces 2 molecules of phosphoenolpyruvate (PEP). This is
mediated by enolase enzyme.
10.Finally the 2 molecules of phosphoenolpyruvate now release a
phosphate group each to phosphorylate ADP to ATP (substrate-level
phosphorylation). The final product is 2 molecules of pyruvate (3
carbon molecule) and 2 molecules of ATP. This process is mediated
by pyruvate kinase enzyme.
Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 pyruvate + 2 ATP +2 NADH + 2 H+
Figure 2: Glycolysis, the reactions of glycolysis, including the initial five-step energy investment
phase followed by the five step energy output phase. Because two molecules of G3P are produced in
reaction 5, all the reactions from 6 to 10 are doubled (not shown). The names of the enzymes that
catalyze each reaction are listed. pg. 173
Pyruvate Oxidation and the Citric Acid Cycle
The free energy that still remains in the 2 molecules of pyruvate, represent
about 75% of the energy found in one glucose molecules. Pyruvate oxidation
occurs in the mitochondrion as the pyruvate travels from the cytosol to the
matrix. At this point pyruvate is oxidized to acetyl-Coenzyme A, carbon
dioxide and one molecule of NADH.
Acetyl-CoA from pyruvate oxidation travels to the matrix and the critic acid
cycle. Here glucose is complete broken down to 2 carbon dioxide molecules,
3 molecules of NADH, and 1 molecule of FADH2. Also 2 molecules of ATP
are phosphorylated.
Pyruvate Oxidation
Decarboxylation reaction – is a chemical reaction that removes a carboxyl
group, to form CO2.
Dehydrogenation – is the removal of a hydrogen atom from a molecule.
The two molecules of pyruvate produced in glycolysis (in the cytosol) now
must travel to the matrix, passing through the outer membrane, intermembrane space, and the inner membrane. As the pyruvate passes into the
matrix, it will be changed to two molecules of Acetyl-CoA. This is a one
step process.
The first thing that occurs is the decarboxylation reaction. Here one carbon
atom is removed from each pyruvate molecule (3 carbons). The carbon is
removed in the form of carbon dioxide (CO2).
The remaining two carbons of each molecule then undergo a
dehydrogenation reaction, where 2 electrons and 1 hydrogen proton are
removed and reduce NAD+ to NADH. This creates an acetyl group.
The acetyl group will react with coenzyme A to create acetyl CoA.
2 pyruvate + 2 NAD+ + 2 CoA
→
2 acetyl CoA + 2 NADH + 2 H+ + 2 CO2
Figure 5: Pyruvate Oxidation, pyruvate is oxidized to an acetyl group, which is carried to the citric
acid cycle by CoA. The reactions that are catalyzed by the pyruvate dehydrogenase complex include
(1) decarboxylation, followed by (2) a dehydrogenation, and finally (3) a reaction with coenzyme A
(CoA) that produces acetyl-CoA. pg. 175
The Citric Acid Cycle
The citric acid cycle was discovered by Sir Han Krebs (1900 – 1981). The
critic acid cycle consists of eight steps, each catalyzed by an enzyme. 7 of
the eight steps occur in the matrix while the first step occurs as acetyl CoA
crosses the inner mitochondrial membrane.
The break down of acetyl CoA requires two turns of the cycle, one turn for
each acetyl CoA that enters. The end products are CO2, NADH, FADH2,
and ATP.
The steps of the Citric Acid Cycle:
1. The acetyl CoA (2 carbon) entering the matrix will combine with a
molecule of oxaloacetate (4 carbons), forming a citrate molecule (6
carbons). This reaction is facilitated by citrate synthase enzyme.
2. The citrate molecule undergoes an isomerization reaction,
rearranging the molecule into isocitrate (6 carbons). This
isomerization is facilitated by the enzyme aconitase.
3. Citrate undergoes a dehydrogenization reaction where two electrons
and two hydrogen protons are e released to form NADH and H+. Also
a CO2 molecule is released, creating a molecule of α-ketoglutarate (5
carbons). This reaction is facilitated by the enzyme isocitrate
dehydrogenase.
4. The α-ketoglutarate is oxidized, here another NADH and H+ are
produced and CO2. A molecule of coenzyme CoA is attached creating
a molecule of succinyl CoA (4 carbons). This reaction is facilitated by
enzyme α-ketoglutarate dehydrogenase.
5. The succinyl CoA undergoes a substrate-level phosphorylation
reaction creating one molecule of ATP, and the CoA group is released.
The end product is a molecule of succinate (4 carbons). This reaction
is facilitated by the enzyme succinyl CoA synthetase.
6. The succinate molecule undergoes a dehydrogenation reaction,
releasing one molecule of FADH2, and creating a molecule of
fumerate (4 carbons). This reaction is facilitated by the enzyme
succinate dehydrogenase.
7. A water molecule is added to fumerate, creating a molecule of
malate (6 carbons). This reaction is facilitated by the enzyme
fumerase.
8. The final step is the regeneration of oxaloacetate (4 carbons), so it
may participate in the citric acid cycle again. Malate is oxidized
releasing electrons and hydrogen protons to reduce NAD+ to NADH
and H+. This reaction is facilitated by the enzyme malate
dehydrogenase.
After two turns of the citric acid cycle the original glucose molecule (6
carbons) has been completely broken down and 6 molecules of CO2 have
been released.
Acetyl-CoA + 3 NAD+ +FAD + ADP + Pi → 2 CO2 + 3 NADH + 3 H+ + FADH2 + ATP + CoA
Figure 6: Citric Acid Cycle, The reactions in the citric acid cycle: Enzyme names are in red. The CoA
that is released in reaction 1 can cycle back for another turn in pyruvate oxidation, pg. 176
Summary:
- Two acetyl-CoA molecules enter the citric acid cycle from glycolysis
and the pyruvate oxidation of one glucose molecule.
- In step 1, the acetyl group enters the cycle as it reacts with
oxaloacetate to form one molecule of citrate. This is why the process
is called the citric acid cycle.
- In steps 3, 4, 5, 6, and 8 some of the released energy is captured and
used to form NADH, ATP, and FADH2.
- In steps 3, 4, and 8, NAD+ is reduced to form NADH.
- Step 5 produces ATP from ADP and Pi by substrate-level
phosphorylation.
- Step 6 reduces FAD to form FADH2.
- Because one glucose molecule yields two pyruvate molecules, each
glucose molecule generates two turns of the citric acid cycle.
The Electron Transport Chain and Chemiosmosis
Most of the potential energy found in the glucose molecule at the beginning
is now captured in molecules of NADH and FADH2.
Six molecules of CO2 have been released back into the atmosphere. Four
molecules of substrate-level phosphorylated ATP have been released into the
cytosol for uses by the cell. What remains are ten molecules of NADH and
two molecules of FADH2, which now enter the electron transport change to
synthesize ATP.
The Electron Transport Chain
The electron transport chain is found in the inner mitochondrial membrane.
Here the coenzymes NADH and FADH2 deliver their electrons and protons
to drive the synthesis of ATP through oxidative phosphorylation. The final
electron and proton acceptor is oxygen (O2). This process is made up of four
protein complexes, which release energy in a series of stages, as to not
damage the cell with an increase in temperature.
Phase of the Electron Transport Chain:
a)
b)
c)
d)
Complex I: NADH dehydrogenase
Complex II: Succinate dehydrogenase
Complex III: Cytochrome Complex
Complex IV: Cytochrome Oxidase
** There are two electron shuttle enzymes involve in the electron transport
chain, Ubiquinone (UQ), which shuttles electrons from complex I to
complex II, and Cytochrome c (cyt c) which shuttles electrons from
complex III to complex IV, and is found in the inter-membrane space.
These complexes represent a series of redox reactions, responsible for
shuttle electrons to the final destination, an oxygen molecule.
The electron transport chain is a series of steps, where electrons are used to
reduce and oxidize proteins, release small amounts of energy along the way.
When a protein loses an electron it is oxidized and when it receives an
electron it is reduced.
When oxygen is present in the mitochondrion (matrix), it will remove a pair
of electrons from complex IV, and attract 2 hydrogen protons (H+) to form a
water molecule.
Complex IV must replace the lost electrons therefore it will receive 2
electrons from complex III. Complex III will replace its lost electrons by
drawing them away from complex I.
All hydrogen protons release from NADH and FADH2, travel from the
matrix, inner mitochondrial membrane, into the inter-membrane space,
creating a proton gradient.
** FADH2 enters the electron transport chain at complex II and passes its
electrons onto complex III.
Figure 7: Electron Transport Chain, a) during the electron transport chain, electrons flow through a
series of proton (H+) pumps. The energy released builds an H+ gradient across the inner
mitochondrial membrane. b) During oxidative phosphorylation, ATP Synthase catalyzes ATP
synthesis using energy from the H+ gradient across the membrane (Chemiosmosis). pg. 178
Figure 8: Redox reaction, redox components of the electron transport chain are organized from high
to low energy. Without the driving force of oxygen, the entire chain would stop. pg. 179
Chemiosmosis
Proton gradient – is a difference in proton (H+ ion) concentration across a
membrane.
Proton-motive force – is a force that moves protons because of a chemical
gradient (often referred to as an electrochemical gradient) of protons across a
membrane.
Chemiosmosis – is a process in which ATP is synthesized using the energy
of an electrochemical gradient and the ATP synthase enzyme.
The Free Energy found in the NADH and FADH2 is used to transport the
hydrogen protons (H+ ions) into the inter-membrane space, creating a proton
gradient. Here the H+ ion concentration is greater in the inter-membrane
space then the H+ ion concentration in the matrix. The difference in
concentration is called the proton gradient and is the driving force for
oxidative phosphorylation of ATP.
Since the concentration of H+ ions in the inter-membrane space is greater
these ions try to create equilibrium, but they are unable to pass across the
membrane. These ions also have a positive charge and therefore are
repelling each other. The concentration gradient and the electrical potential
gradient across the membrane produce a force known as the proton-motive
force.
When the cell uses the proton-motive force to do work it is called
Chemiosmosis. At this point the H+ ions can reenter the matrix through a
special protein channel called ATP synthase. It is at this point that ATP is
synthesized (oxidative phosphorylation) and oxygen combines with
hydrogen to form water.
ATP Synthase: A Molecular Motor
Uncoupling Electron Transport and Chemiosmosis