Chem i Osmosis Video

7/25/2019 Chem i Osmosis Video

1/55

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.


2/55

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.


3/55

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.


4/55

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).


5/55

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.


6/55

Metabolism of


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.


7/55

Metabolism of


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.


8/55

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.


9/55

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.


10/55

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


11/55


Both mitochondrial membranes are normal lipidbilayers with proteins imbedded in and on the

membranes.


12/55


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.


13/55


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.


14/55


Collectively, the 4 protein complexes and their mobilecarriers make up the Electron Transport Chain.

Electron Transport Chain


15/55


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


16/55


The final electron acceptor in the series (and themost electronegative component of the chain) is

oxygen.

O2


17/55


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.


18/55


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


19/55


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


20/55


The molecules labeled in orange above (Complex IIIand Cytochrome C) are electron only carriers. They

do not accept protons with the electrons theytransport.


21/55


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.


22/55


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.


23/55


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.


24/55


Complex III is an electron-only carrier, so the H+ ionscarried by Q are again released into the outer

compartment.


25/55


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.


26/55


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


27/55


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


28/55


Recall that the electrons picked up by Complex IV arefinally passed to oxygen, forming water.


29/55


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--


30/55


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.


31/55


The tendency of H+ ions to move down theelectrochemical gradient can be thought of as

potential energy that will be harnessed to produceATP.


32/55

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!


33/55


There is one place in the membrane that will allowhydrogen ions to pass through and move down the

electrochemical gradient - the ATP synthase complex.


34/55


As the H+ ions flow down the gradient, the energystored in the gradient is used to make ATP.

ATP


35/55

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.


36/55


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.


37/55


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.


38/55


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).


39/55


How much ATP is produced by each NADH orFADH2?

ATP


40/55


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


41/55


In contrast, each FADH2results in the production of(on average) 1.5 ATP.

ATP

X


42/55


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


43/55


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


44/55


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


45/55


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


46/55


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


47/55


48/55


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


49/55


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


50/55

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.


51/55








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.


52/55










This gives us a final net productionof 30 ATP per glucose molecule(on average).


53/55










This gives us a final net productionof 30 ATP per glucose molecule(on average).

Grand Total = 30 ATP/glucose


54/55

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.


55/55

Need More Help? Talk to a TA or Dr.

Campbell, theyll be

happy to answeryour questions.

ATP

Documents

Chem i Osmosis Video