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Transcript of Chem i Osmosis Video
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Chemiosmotic Synthesis of
ATP in the Mitochondria
Dr. Carol Hardy
& Dr. Tyson SaccoCornell University
This video is designed to help you understand how ATP is produced within the cell.
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Chemiosmotic Synthesis of
ATP in the Mitochondria
Dr. Carol Hardy
& Dr. Tyson SaccoCornell University
In particular. it will help you to understand the process of chemiosmosis.
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ATP can be synthesized by
two processes:1. Substrate-level phosphorylation
! Substrate-P + ADP!Substrate + ATP
!
Glycolysis: net gain 2 ATP! Krebs: net gain 2 ATP
There are two ways that ATP can be produced within the cell.
One is by the process known as substrate level phosphorylation. The name gives you a clue as to whatis happening. When something is phosphorylated, a phosphate group is added and, in this case, the
substrate is involved.
A substrate group which has a phosphate group bound to it will transfer that phosphate group to an
ADP molecule to form ATP.
There are two places where substrate-level phosphorylation occurs within the cell: 1) in the process of
glycolysis, where there is a net gain of 2 ATP; and 2) in the Krebs cycle where a molecule of glucosewill produce an additional 2 ATP by substrate-level phosphorylation.
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ATP can be synthesized by
two processes:1. Substrate-level phosphorylation
2. Chemiosmosis (oxidative phosphorylation)
If we assume that the complete metabolism of a molecule of glucose will yield a net gain
of 30 ATP, where does the bulk of the ATP come from? (remember, 4 were produced by
substrate-level phosphorylation)
Chemiosmosis, or oxidative phosphorylation, is the major source of ATP (26/30 ATP per
glucose molecule).
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Metabolism of
GlucoseI. Glycolysis
I.
Takes place in cytosol.
II. Yields 2 pyruvate (pyruvic acid),2 ATP (substrate-level), and 2
reduced carrier NADHmolecules.
III. Does not requireoxygen butcan take place in aerobicconditions.
Before looking at the details of chemiosmosis, letsreview the basics of glucose metabolism.
Shown here are the three stages in which glucoseis metabolized into carbon dioxide and water.
The first stage is Glycolysis.
Glycolysis takes place in the cytoplasm of the celland the 2 pyruvate molecules produced byglycolysis will move into the mitochondrion wheremetabolism will be completed.
Redox reactions in glycolysis reduce NAD+ toNADH molecules that will be used inchemiosmosis.
Of course, 2 molecules of ATP (net) are producedby substrate-level phosphorylation during glycolysisas well.
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Metabolism of
GlucoseI. Glycolysis
II. Pyruvate!Acetyl-CoAI. Takes place in mitochondrion.
II.
2 pyruvate are oxidized, yielding2 Acetyl-CoA molecules.
III. This redox reaction produces 2more NADHs.
Stage 2 begins with the pyruvate from glycolysismoving into the mitochondrion.
This stage is essentially a redox reaction in which 2pyruvate are oxidized and NAD+ molecules arereduced, yielding 2 molecules of Acetyl Coenzyme
A and 2 NADHs.
2 molecules of CO2 are also released when thepyruvate molecules enter the mitochondrion andgives up their carboxyl groups.
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Metabolism of
GlucoseI. Glycolysis
II. Pyruvate!Acetyl-CoA
III.
Krebs cycle
1.
Occurs in mitochondrion.2.
2 Acetyl-CoA molecules lead to2 turnsof the cycle.
3. Each turn produces: 1 ATP (substrate-level)
3 NADHs
1 FADH2
Stage 3 is the Krebs (or citric acid) cycle.
This stage uses each of the 2 acetyl-CoA
molecules produced earlier to convert a 4-carboninto the 6-carbon compound citrate.
Citrate is oxidized over the course of the cycle toproduce a number of reduced carriers that will beused in chemiosmosis.
In addition, 2 ATP are produced by substrate-levelphosphorylation during the Krebs cycle.
4 molecules of CO2 are also released in the courseof the oxidation of the 2 citrate molecules that passthrough the cycle for each glucose molecule.
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Metabolism of
GlucoseSummary:
I.
Glycolysis! 2 ATP
!
2 NADHII. Pyruvate!Acetyl-CoA
! 2 NADH
III. Krebs cycle!
2 ATP
! 6 NADH
! 2 FADH2
In summary, our main products (so far) are: 4 ATPproduced by substrate-level phosphorylation and 12
reduced carrier molecules.
The reduced carrier molecules store a great deal of
energy in them (in a sense) and it is this energy that
will be used (indirectly) to make ATP via
chemiosmosis.
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The Mitochondrion
Where does chemiosmotic ATP production take place? In the mitochondrion.
Recall that the mitochondrion has a double membrane - an outer membrane that
is relatively permeable and an inner, highly folded membrane that has veryselective permeability. These membranes separate the mitochondrion into outer
and inner compartments.
The reactions of stages II and III (shown previously) take place in the inner
compartment and chemiosmosis occurs on the inner mitochondrial membrane.
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Chemiosmotic Synthesis of ATP
Note the two mitochondrial membranes and the twocompartments they form. The cytosol (where
glycolysis has taken place) is at the top of thediagram, beyond the outer membrane.
Cytosol
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Chemiosmotic Synthesis of ATP
Both mitochondrial membranes are normal lipidbilayers with proteins imbedded in and on the
membranes.
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Chemiosmotic Synthesis of ATP
Focus on the protein complexes labeled I, II, III, andIV on the inner membrane. They are groups of
proteins that are anchored together in the membrane.These complexes are linked by mobile carriers.
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Chemiosmotic Synthesis of ATP
The mobile carrier ubiquinone, labeled Q, linkscomplexes I, II, and III. It can move back and forth
through the plane of the membrane. Cytochrome Cmoves back and forth on the outer surface of the
inner membrane, connecting complexes III and IV.
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Chemiosmotic Synthesis of ATP
Collectively, the 4 protein complexes and their mobilecarriers make up the Electron Transport Chain.
Electron Transport Chain
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Chemiosmotic Synthesis of ATP
Each element in the chain is electronegative,meaning that it has a strong attraction for electrons.
A gradient of increasing electronegativity exists withinthe electron transport chain. In terms of their
electronegativities: I < Q < II < III and so on.
Low High
Electronegativity
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Chemiosmotic Synthesis of ATP
The final electron acceptor in the series (and themost electronegative component of the chain) is
oxygen.
O2
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Chemiosmotic Synthesis of ATP
Electrons transferred from NADH and FADH2will bepassed along the chain, finally reaching oxygen.
When oxygen receives those extra electrons it takeson a very strong negative charge and attracts
protons. This results in the formation of water.
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Chemiosmotic Synthesis of ATP
Remember that as they move down the chain(towards O2), the electrons release (lose) energy.
That energy will be used indirectly to create ATP.Well now look at the details of this indirect exchange
of energy
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Chemiosmotic Synthesis of ATP
Molecules labeled in red above are electron transportmolecules that must accept protons (hydrogen ions)
along with the electrons they pick up. For example,ubiquinone (Q) can accept 2 electrons and must also
accept two protons, so it becomes QH2
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Chemiosmotic Synthesis of ATP
The molecules labeled in orange above (Complex IIIand Cytochrome C) are electron only carriers. They
do not accept protons with the electrons theytransport.
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Chemiosmotic Synthesis of ATP
Notice that Complex I is going to take electrons awayfrom NADH. When it accepts the electrons it will pick
up hydrogen ions as well. However, some of themolecules that make up Complex I are electron-only
carriers, so something has to happen to the hydrogen
ions that have been taken up.
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Chemiosmotic Synthesis of ATP
Hydrogen ions picked up by Complex I are releasedinto the outer compartment of the mitochondrion and
electrons are passed along the electron transportchain to Q. Notice that one effect of Complex I has
been the movement of H+ across the mitochondrialmembrane from the inner to the outer compartment.
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Chemiosmotic Synthesis of ATP
Like Complex I, ubiquinone (Q) is a hydrogen andelectron carrier, when it picks up the electrons from
Complex I it also picks up a hydrogen ion from theinner compartment. It then becomes QH2and moves
through the membrane until it finds complex III.
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Chemiosmotic Synthesis of ATP
Complex III is an electron-only carrier, so the H+ ionscarried by Q are again released into the outer
compartment.
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Chemiosmotic Synthesis of ATP
Next, Cytochrome C moves across the innermembrane, carrying electrons from Complex III to
Complex IV. Like Complex I, Complex IV is ahydrogen and electron carrier, so as it accepts
electrons from Cytochrome C it also picks up H+ ionsfrom the inner compartment and deposits them in the
outer compartment.
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Chemiosmotic Synthesis of ATP
To review, there are three sites where hydrogen ionsare moved from the inner to the outer compartment of
the mitochondrion. The movement of these ionscreates a very strong concentration gradient of
hydrogen ions across the inner membrane.
1 2 3
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Chemiosmotic Synthesis of ATP
Note that the movement of hydrogen ions from theinner to the outer compartment is active transport
because the ions are being moved against theirconcentration gradient.
1 2 3
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Chemiosmotic Synthesis of ATP
Recall that the electrons picked up by Complex IV arefinally passed to oxygen, forming water.
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Chemiosmotic Synthesis of ATP
The net movement of positively charged hydrogenions across the inner mitochondrial membrane
produces an electrochemical gradient- chemicalbecause there is a gradient of hydrogen
concentration and electricalbecause there is agradient of charge (more positive in the outer andmore negative in the inner compartment).
Hi [H+], ++positive charge++
Low [H+], --negative charge--
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Chemiosmotic Synthesis of ATP
Keep in mind that the positively-charged hydrogenions will want to move down this concentration
gradient (from the outer to the inner compartment).The inner membrane is impermeable to H+ ions,
maintaining the electrochemical gradient.
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Chemiosmotic Synthesis of ATP
The tendency of H+ ions to move down theelectrochemical gradient can be thought of as
potential energy that will be harnessed to produceATP.
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An electrochemical gradient provides the
power to make ATP.
Imagine the H+ ions as water trapped behind a talldam. The fact that the water will tend to move down
can be used to do work - as in a hydroelectric powerplant. The inner mitochondrial membrane is like the
wall of the dam with many positively chargedhydrogen ions trapped behind it.
Hi [H+], ++positive charge++
Low [H+], --negative charge--
Glen Canyon Dam outside Page, AZ is 710 tall and over 300
thick at its thickest point. It holds back enough water to supply
over 26 million families of 4 with water for a year!
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Chemiosmotic Synthesis of ATP
There is one place in the membrane that will allowhydrogen ions to pass through and move down the
electrochemical gradient - the ATP synthase complex.
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Chemiosmotic Synthesis of ATP
As the H+ ions flow down the gradient, the energystored in the gradient is used to make ATP.
ATP
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ATP synthase is like a turbine in a dam.
Returning to our dam metaphor, you can think of theATP synthase as a turbine inside a channel cut
through the dam. Just as a turbine converts themechanical energy of water turning its blades to
electricity, the ATP synthase uses the H+ ionspassing through it to power ATP synthesis.
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Chemiosmotic Synthesis of ATP
Weve already seen how NAHD from the Krebs cyclepasses electrons into the electron transport chain and
how hydrogen ions are moved across the innermembrane in three places to create an
electrochemical gradient. Now lets look at howFADH2plays a role in chemiosmosis.
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Chemiosmotic Synthesis of ATP
FADH2produced in the Krebs cycle transfers itselectrons and hydrogen ions to a different carrier than
NADH. FADH2exchanges with Complex II in theelectron transport chain. Complex II is a hydrogen
and electron carrier and passes both to Q, whichpasses them on to Complex III where (as before)electrons are passed on and H+ are moved into the
outer compartment.
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Chemiosmotic Synthesis of ATP
The electrons from FADH2miss the first step in theelectron transport chain entirely. Because of this,
FADH2 is responsible for fewer H+ ions being pumpedacross the membrane, creating less potential
energy, and resulting in fewer ATP being producedper FADH
2
(compared to NADH).
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Chemiosmotic Synthesis of ATP
How much ATP is produced by each NADH orFADH2?
ATP
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Chemiosmotic Synthesis of ATP
On average, each NADHproduced by the conversionof pyruvate to acetyl-CoA or by the Krebs cycle
results in the production of 2.5 ATP.
ATP
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Chemiosmotic Synthesis of ATP
In contrast, each FADH2results in the production of(on average) 1.5 ATP.
ATP
X
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Chemiosmotic Synthesis of ATP
Since the steps in glucose metabolism precedingchemiosmosis produce a number of NADH and
FADH2molecules. a large number of ATP can beproduced by chemiosmosis. How many?
ATP
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Chemiosmotic Synthesis of ATP
Before we do the final accounting for ATP productionby chemiosmosis there is one last aspect that needs
to be addressed - the NADH molecules produced byglycolysis. How are they involved in chemiosmosis?
ATP
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Chemiosmotic Synthesis of ATP
Cytosol
NADH molecules produced by glycolysis are different than
those produced in the inner compartment in that they areout in the cytoplasm. NADH is too large and too polar topass through the outer mitochondrial membrane so the
hydrogen ions and electrons from the NADHs in thecytoplasm are passed through the membrane by a shuttlemolecule. (Note: An alternate shuttle molecule, shown
unshaded, is used to transport the NADH produced inglycolysis in highly metabolic active tissues which results in
the electrons entering the chain at complex I and therefore
resulting in the production of 2.5 ATP.)
ATP
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Chemiosmotic Synthesis of ATP
This shuttle molecule will oxidize NADH, taking awayelectrons and a hydrogen ion and carrying them
across to molecule Q in the inner membrane. Qaccepts both the H+ and the electrons and carries
them to Complex III where (as usual) the electronsare passed along to IV and the H+ is released.
ATP
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Chemiosmotic Synthesis of ATP
As you can see, the electrons and H+ ions from theNADHs produced by glycolysis enter the electron
transport chain at essentially the same point aselectrons and H+ ions from FADH2. As a result, each
NADH from glycolysis results in the production (onaverage) of 1.5 ATP, just like FADH2.
ATP
X
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Chemiosmotic Synthesis of ATP
ATP
Chemiosmosis
ATP accounting:
Glycolysis 2 NADH/glucose 1.5 ATP/NADH
= 3 ATP
Stage II (pyruvate to
acetyl-CoA)
2 NADH/glucose
2.5 ATP/NADH
= 5 ATP
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Chemiosmotic Synthesis of ATP
ATP
Chemiosmosis
ATP accounting:
Glycolysis
2 NADH/glucose
1.5 ATP/NADH
= 3 ATP
Stage II (pyruvate to
acetyl-CoA)
2 NADH/glucose
2.5 ATP/NADH
= 5 ATP
Krebs cycle
6 NADH/glucose 2.5 ATP/NADH
= 15 ATP&
2 FADH2/glucose
1.5 ATP/FADH2
= 3 ATP
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Final ATP accounting:
Glycolysis 2 NADH/glucose 1.5 ATP/NADH = 3 ATP
Stage II (pyruvate to acetyl-CoA)
2 NADH/glucose 2.5 ATP/NADH = 5 ATP
Krebs cycle 6 NADH/glucose 2.5 ATP/NADH = 15 ATP
2 FADH2/glucose 1.5 ATP/FADH2 = 3 ATP
SubTotal = 26 ATP/glucose
Adding up the numbers at left (3 + 5
+ 15+ 3)we find that 26 ATPare
produced via chemiosmosisfor
each glucose molecule metabolized.
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Final ATP accounting:
Glycolysis 2 NADH/glucose 1.5 ATP/NADH = 3 ATP
Stage II (pyruvate to acetyl-CoA)
2 NADH/glucose 2.5 ATP/NADH = 5 ATP
Krebs cycle 6 NADH/glucose 2.5 ATP/NADH = 15 ATP
2 FADH2/glucose 1.5 ATP/FADH2 = 3 ATP
SubTotal = 26 ATP/glucose
Adding up the numbers at left (3 + 5+ 15+ 3)we find that 26 ATPareproduced via chemiosmosisforeach glucose molecule metabolized.
Recall that an additional 4 ATP(net)per glucose molecule wereproduced by substrate-levelphosphorylationduring glycolysisand the Krebs cycle.
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Final ATP accounting:
Glycolysis 2 NADH/glucose 1.5 ATP/NADH = 3 ATP
Stage II (pyruvate to acetyl-CoA)
2 NADH/glucose 2.5 ATP/NADH = 5 ATP
Krebs cycle 6 NADH/glucose 2.5 ATP/NADH = 15 ATP
2 FADH2/glucose 1.5 ATP/FADH2 = 3 ATP
SubTotal = 26 ATP/glucose
Adding up the numbers at left (3 + 5+ 15+ 3)we find that 26 ATPareproduced via chemiosmosisforeach glucose molecule metabolized.
Recall that an additional 4 ATP(net)per glucose molecule wereproduced by substrate-levelphosphorylationduring glycolysisand the Krebs cycle.
This gives us a final net productionof 30 ATP per glucose molecule(on average).
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Final ATP accounting:
Glycolysis 2 NADH/glucose 1.5 ATP/NADH = 3 ATP
Stage II (pyruvate to acetyl-CoA)
2 NADH/glucose 2.5 ATP/NADH = 5 ATP
Krebs cycle 6 NADH/glucose 2.5 ATP/NADH = 15 ATP
2 FADH2/glucose 1.5 ATP/FADH2 = 3 ATP
SubTotal = 26 ATP/glucose
Adding up the numbers at left (3 + 5+ 15+ 3)we find that 26 ATPareproduced via chemiosmosisforeach glucose molecule metabolized.
Recall that an additional 4 ATP(net)per glucose molecule wereproduced by substrate-levelphosphorylationduring glycolysisand the Krebs cycle.
This gives us a final net productionof 30 ATP per glucose molecule(on average).
Grand Total = 30 ATP/glucose
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Closing Notes:
Keep in mind that ATP production described here assumes that aerobicrespiration is taking place.
As youll learn, the number of ATP produced differs dramatically in anaerobicsituations.
It is also important to realize that the exact number of ATP produced per glucose
molecule may actually differ depending on the exact cellular conditions.
Total numbers, whether 30 or 32 represent average ATP production.
The most important concept is that chemiosmosis produces many more ATP
than substrate-level phosphorylation.
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Need More Help? Talk to a TA or Dr.
Campbell, theyll be
happy to answeryour questions.
ATP