The First Energy-releasing Pathways

8/14/2019 The First Energy-releasing Pathways

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Chapter 7
http://c/BioCh7Energy-Releasing%20Pathways/csMito.mov


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Some cells in your own body can use an

anaerobic route for short periods, but they doso only when they don't receive enoughoxygen.

Your cells use mainly aerobic respiration.With each breath you take, you provide youractively respiring cells with a fresh supply of

oxygen.

energy_releasing.swf
http://c/BioCh7Energy-Releasing%20Pathways/Ch8.01/energy_releasing.swfhttp://c/BioCh7Energy-Releasing%20Pathways/Ch8.01/energy_releasing.swf


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The main energy-releasing pathways allstart in the cytoplasm with glycolysis, apathway in which enzymes cleave andrearrange each glucose molecule into twopyruvate molecules. Once this stage is over,the energy-releasing pathways differ.


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When a glucose molecule is the starting

material, aerobic respiration can besummarized this way:

C6H

12O

6--------> 6CO

2+ 6H

2O


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However, this summary equation tells usonly what the substances are at the startand finish of the pathway.

In between are three reaction stages. Thesestages are briefly described in the clip

aerobic_stages.swf


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Each glucose molecule has six carbon atoms,twelve hydrogen atoms, and six oxygen atomscovalently bonded to one another.

During glycolysis, glucose or some othercarbohydrate in the cytoplasm partially breaks

down to pyruvate, a molecule with a backboneof three carbon atoms.


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Glycolysis takes place in two major parts.The first part requires energy input. Thesecond part releases energy.

The Figure shows a schematic of theenergy changes during glycolysis.

glycolysis_two_stages.swf
http://c/BioCh7Energy-Releasing%20Pathways/Ch8.02/glycolysis_two_stages.swfhttp://c/BioCh7Energy-Releasing%20Pathways/Ch8.02/glycolysis_two_stages.swf


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Glycolysis begins when ATP molecules eachtransfer a phosphate group to glucose and sodonate energy to it.

Such a transfer is known as phosphorylation.In this case, phosphorylation raises the energycontent of glucose to a level that is highenough to enable entry into the second part,

the energy-releasing steps of glycolysis.


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The first energy-releasing step cleaves theactivated glucose into two molecules,which we can call PGAL

(phosphoglyceraldehyde).

Each PGAL gives up two electrons and ahydrogen to the coenzyme NAD+, reducing

it to NADH.


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The first energy-releasing step cleaves theactivated glucose into two molecules,which we can call PGAL

(phosphoglyceraldehyde).

Each PGAL gives up two electrons and ahydrogen to the coenzyme NAD+, reducing

it to NADH.


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As a result of these donations, each PGALis converted to an unstable intermediate.

This intermediate enables ATP to form bygiving up a phosphate group to ADP. Thenext intermediate in the sequence does thesame thing.


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A total of four ATP form by substrate-levelphosphorylation. This metabolic event is definedas the direct transfer of a phosphate group froma substrate of a reaction to some other

molecule, such as ADP.

Remember that two ATP were invested to startthe reactions, so the net energy yield fromglycolysis is only two ATP. Two NADH also form.


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The animation provides a step-by-steplook at the reactions of glycolysis.

glycolysis.swf


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Suppose two pyruvate molecules, formedby glycolysis, leave the cytoplasm andenter a mitochondrion.

In this organelle, both the second andthird stages of the aerobic pathway runto completion. The Figure reviews thestructure of a mitochondrion.

mitochondrion.swf


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preparatory_rx.swf

The second stage occurs in the innercompartment of the mitochondrion.

It starts with a few preparatory steps in whichan enzyme removes a carbon atom from eachpyruvate molecule.

A coenzyme, known as coenzyme A, becomesacetyl-CoA when it combines with the remainingtwo-carbon fragment.


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The second-stage reactions continue when

acetyl-CoA transfers the two-carbonfragment to oxaloacetate, the entry pointof the Krebs cycle.
http://../Bio-%20Unity%20in%20Diversity/ICIB%20UDL10%20(E)/Media/Unit1/Ch08/03/glosshttp://../Bio-%20Unity%20in%20Diversity/ICIB%20UDL10%20(E)/Media/Unit1/Ch08/03/gloss


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The combination of acetyl-CoA andoxaloacetate forms citrate. The Krebs cyclebreaks down citrate into carbon dioxide and

water in a stepwise fashion. All the carbonmolecules of pyruvate eventually end up incarbon dioxide.

The reactions of the Krebscycle in more detail


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There are three other important points to

understand about the Krebs cycle:

1. Hydrogen and electrons are transferredto the coenzymes NAD+ and FAD (flavin

adenine dinucleotide, a different coenzyme)to produce NADH and FADH2.

2. Substrate-level phosphorylations produce

more ATP.

3. Oxaloacetate regenerates. (That's whythe process is called a cycle.)


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The animation shows the reactions and

intermediates of the Krebs cycle indetail.

krebs_cycle_reactions.swf


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In total, the second-stage reactions producetwo ATP, eight NADH, and two FADH2 for eachmolecule of glucose. The coenzymes go to theelectron transport system for the final stage of

the aerobic pathway.

The animation will show a final overview of thesecond-stage reactions

krebs_telecourse.mov
http://c/BioCh7Energy-Releasing%20Pathways/Ch8.03/krebs_telecourse.movhttp://c/BioCh7Energy-Releasing%20Pathways/Ch8.03/krebs_telecourse.mov


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ATP production goes into high gear in thethird stage of the aerobic pathway, electrontransport phosphorylation.

During the earlier stages, hydrogen andelectrons were stripped from reactants andloaded onto the coenzymes NAD and FAD,reducing them to NADH and FADH2.


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In the final stage, these coenzymes deliverhydrogen ions and electrons to an electron

transfer chain in the inner mitochondrialmembrane.

The electrons are transferred from one

molecule of the chain to the next moleculein line.


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When certain molecules accept and thendonate electrons, they also pick uphydrogen ions in the inner compartment.

Quickly afterward, they release the ions tothe outer compartment.


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This shuttling action sets up H+concentration and electric gradients acrossthe inner mitochondrial membrane.

Nearby in the membrane, H+ ions followthe gradients and flow back to the innercompartment, through the interior of ATP

synthases.


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The H+ flow through these transport proteins

drives the formation of ATP from ADP andunbound phosphate.

Free oxygen keeps ATP production going. When

it withdraws electrons at the end of thetransport systems and then combines with H+,water is the result.

This process is illustrated in the followinganimations


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electron_transport.mov

mito_chemiosmosis.swf


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Thirty-two ATP typically form during thethird stage of aerobic respiration.

Add these to the net yield from the

preceding stages, and the total harvest isthirty-six ATP from one glucose molecule.

However, the exact yield varies depending

on the type of cell and prevailingconditions. See Figure.

energy_harvest.swf
http://c/BioCh7Energy-Releasing%20Pathways/Ch8.04/energy_harvest.swfhttp://c/BioCh7Energy-Releasing%20Pathways/Ch8.04/energy_harvest.swf


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The human body has many alternative sourcesof energy.

Complex carbohydrates, fats, and proteinscannot enter the aerobic pathway directly.

The digestive system and individual cells mustfirst break apart these molecules into simplerdegradable subunits.

The Figure shows the reaction sites where avariety of organic compounds can enter the

stage of aerobic respiration.

alt_energy_sources.swf


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In common usage, the term fermentation isused to describe the process by which alcoholicbeverages such as beer and wine are produced.

This is alcoholic fermentation, an energy-releasing pathway. It is an example of ananaerobic pathway because, unlike aerobicrespiration, it does not require oxygen.


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Many microorganisms rely entirely onfermentation pathways to meet their modest

energy requirements.

Other microorganisms switch back and forthbetween aerobic and anaerobic pathways as

the oxygen levels in their environmentschange.


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The yeast used to brew alcoholicbeverages is an example.

Human muscle cells also can switch backand forth. They utilize a fermentationpathway (lactate fermentation) duringperiods of vigorous exercise, when they are

short of oxygen.


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Like all the main energy-releasing pathways,fermentation begins with the breakdown ofglucose by glycolysis.

However, the reactions do not completelybreak down glucose to carbon dioxide andwater, and produce no more ATP beyond thetiny yield from glycolysis.

The final steps serve only to regenerate NAD+,a coenzyme with central roles in thebreakdown reactions.


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fermentation.swf

Exactly what happens after glycolysis

depends on what type of fermentation istaking place.


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Summary of CellularRespiration

CellRespiration.svg
http://c/BioCh7Energy-Releasing%20Pathways/CellRespiration.svghttp://c/BioCh7Energy-Releasing%20Pathways/CellRespiration.svg


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The First Energy-releasing Pathways