Intro Energy Cell Respiration 2

Cell Respiration - 1

All cells need energy to stay alive and maintain an ordered cellular environment.Cell growth, development and reproduction all require energy. Movement ofmaterials through membranes often requires energy, as do intracellularmovements.

Cells obtain the energy to do work by oxidizing organic molecules, a process calledcellular respiration*. Although many organic molecules can be oxidized,glucose, the main product of photosynthesis, is the primary ffuel molecule for thecells of living organisms. Cell respiration pathways are catabolic – the endproducts have less energy than the reactants. Some of the energy released duringcell respiration is heat energy; the rest is used to make molecules of AATP.

*The term, cellular respiration, in some references is restricted to the aerobicrespiratory pathways of glucose metabolism that occur in the mitochondria ofeukaryotic organisms. It is used to describe all pathways involved in fuelmetabolism here.

All organisms, autotrophs and heterotrophs, must do cell respiration. Recall thatorganisms that do photosynthesis (or properly, manufacture their own fuelmolecules) are called aautotrophs. HHeterotrophs obtain their fuel molecules"pre-formed" by other organisms. Animals, fungi, many protists and manybacteria are heterotrophs. Plants and some protists are autotrophs, as are somebacteria, in particular, the cyanobacteria.

The cell respiration processes of all organisms have common elements ofmetabolic pathways:

• The chemical reactions of cell respiration involve metabolic pathways.• Each chemical reaction in the pathway is catalyzed by a specific enzyme.• The pathways of cell respiration are remarkably uniform in all organisms.• Eukaryotic organisms compartmentalize the respiratory reactions.• Respiration is regulated by feedback mechanisms at key enzyme points.

The metabolic pathways of cell respiration vary depending on the type of organism,the enzymes the organism has, oxygen use, and what the final product molecule inthe cell respiration process is. We will focus on the metabolism of glucose in cellrespiration, but we shall also discuss how alternative fuel molecules fit into the cellrespiration pathways.

Most eukaryotic organisms are aaerobic (oxygen requiring). In aaerobic cellularrespiration, which is the complete metabolism of glucose, electrons removedfrom glucose move down an electron transport system through a series ofoxidation-reduction reactions to a final electron acceptor, ooxygen, hence, theemphasis on oxygen in cell respiration. Most organisms are oobligate aerobes.They cannot survive without the oxygen needed for aerobic cell respiration.


In complete aerobic respiration, glucose is broken down into water and carbondioxide. This process requires oxygen.

C6H12O6 + 6O2 6H2O + 6CO2 + 686 kcal (ATP + Heat)

Not all cell respiration is aerobic. Organisms that do cell respiration without oxygenare said to be aanaerobic. Fuel molecules can be oxidized without oxygen to yieldsmaller amounts of ATP. The ffermentations involve the partial breakdown ofglucose without using oxygen. Many prokaryotes have a variety of fermentationpathways, using a number of different fuel molecules. The final electron acceptorfor the fermentations is an oorganic molecule.

In addition, if the final electron acceptor is an iinorganic molecule other thanoxygen, the process is called aanaerobic respiration.

All organisms do some type of anaerobic respiration or fermentation during timesof oxygen deficit, although it may not be sufficient to sustain the organism's ATPneeds. Some organisms are oobligate anaerobes. They can not survive in thepresence of oxygen. Other anaerobes are mmetabolic anaerobes; they lack theenzymes needed to do aerobic cell respiration. Some organisms will survive nicelyin the absence of oxygen but will do aerobic respiration when oxygen is available.


Aerobic Cell Respiration - An OverviewAs with many metabolic processes, aerobic cell respiration has a number of stages(three or four depending on who is describing the process) and can be used toobtain energy from a number of fuel molecules. By convention, and because it isthe primary fuel molecule for most cellular respiration, we use glucose as our fuelto illustrate the respiration pathway.

Glycolys isThe initial stage of glucose metabolism, or cell respiration, is a process calledglycolysis, which splits a glucose molecule into two molecules of pyruvate, a3-carbon compound. Glycolysis occurs in the cytosol of the cell.

What happens after glycolysis depends on the presence or absence of oxygenand/or the enzymes needed.

• If oxygen is not available, or if the organism lacks enzymes needed foraerobic respiration, the pyruvate molecules will proceed withfermentations, or for some prokaryotes, anaerobic respiration, which wewill discuss later.

• If oxygen is available and the organism has the enzymes to do aaerobicrespiration, the pyruvate molecules will be oxidized in the next stages ofaerobic respiration.

During the second (and third) stages of aaerobic respiration:• Pyruvate molecules are oxidized and lose a CO2.• The two-carbon molecules then enter the KKrebs, or CCitric Acid CCycle,

where more oxidations occur, releasing two more CO2. The Krebs cycleoccurs in the mitochondrial matrix.


The final stage of aerobic respiration is the eelectron transport chain and thechemiosmotic synthesis of ATP. Since the energy to synthesize ATP is from theoxidation-reduction reactions, such synthesis is called ooxidativephosphorylation.

• Oxygen is the final electron acceptor for the oxidation-reductions that startwith NADH in the electron transport system.

• The electron transport system takes place in the inner membrane of themitochondria.

• When oxygen is available, as much as 38 ATP can be generated from oneglucose molecule

Cellular Respiration - The PathwaysGlycolys is

• Glucose is “activated” by two ATP-consuming reactions. The glucosemolecule is phosphorylated in these reactions. The phosphorylated bondingsites are sufficiently unstable to start what is, from that point, a series ofexergonic oxidations.

• Glucose is then broken into two molecules of the 3-carbon compound,Pyruvate .

• In addition:o Two molecules of NADH are producedo A net of two molecules of ATP are produced

(Four molecules of ATP are made during Glycolysis, but 2molecules are consumed in activating the glucose)

• Glycolysis always occurs in the cytosol of the cell.• Incidentally, Glycolysis is the most widespread metabolic pathway in living

organisms, today and evolutionarily. The earliest prokaryotes probably hadthe glycolysis pathway.


Glycolys is


Summary of Glycolysis

Glucose + 2ATP + 2NAD+ + 2ADP + 2Pi --> 2 Pyruvate + 2NADH + 4ATP** Net gain of 2ATP

InputsGlucose2 ATP*2 NA D+

2 ADP + 2Pi

Outputs2 Pyruvate2 NADH4 ATP*

* Therefore thenet energy yieldis 2 ATP

Notes:• The ATP generated is by ssubstrate-level phosphorylation

• All steps are catalyzed by specific enzymes• Glycolysis occurs in the cytosol of the cell• Glycolysis is the initial cell respiratory pathway of aall eukaryotic

organ isms.

After GlycolysisPyruvate is a critical intermediate in the cellular respiratory pathways. As stated,virtually all organisms do glycolysis. The respiratory pathways diverge atpyruvate. If oxygen is available, and the organism has the appropriate enzymes,pyruvate is oxidized and follows the aerobic respiratory pathway. In the absenceof oxygen, pyruvate will be reduced in a fermentation pathway or the anaerobicrespiratory pathway. Pyruvate is also an important intermediate in the use of fuelmolecules other than glucose.


Aerobic Respiration PathwayAerobic Cellular Respiration is has two or three stages following glycolysis.(Some references consider the oxidation of pyruvate to be a part of the Krebscycle; others a separate, preparatory step.)

• Oxidation of Pyruvate to Acetyl-CoA• The Krebs (Citric Acid) Cycle• Electron Transport Chain and Oxidative phosphorylation

Revisiting the MitochondrionThe aerobic respiration reactions occur within the mmitochondria of the cell. Priorto discussing the Krebs cycle and electron transport, let's review the structure ofthe mitochondrion. Recall that the mitochondrion has an smooth outer membraneand a deeply folded inner membrane. The folds are called cristae. The internalspace of the mitochondrion is called the matrix. The space between the outermembrane and the inner membrane is the intermembrane space.

The enzymes needed to do the Krebs cycle are located in the mitochondrial matrixor embedded in the inner membrane as integral proteins. The enzymes andelectron carrier complex for electron transport in the respiratory chain arelocated in the inner membrane.


Oxidation of Pyruvate to Form Acetyl-CoAThe oxidation of pyruvate uses a multi-enzyme complex within the mitochondriathat completes the three steps while retaining the intermediates within thecomplex. PPyruvate dehydrogenase is among the larges enzymes known,composed of at least 60 polypeptide subunits.

The two Pyruvate molecules are transported into the iinner matrix of themitochondria via facilitated diffusion where is undergoes ooxidativedecarboxylation within the pyruvate dehydrogenase complex.

• CO2 is removed from pyruvate, producing a 2-carbon compound.• The 2-carbon fragment is oxidized releasing H+ to reduce NAD+ to NADH,

leaving AAcetyl• Acetyl combines with Co-enzyme A (formed from the B vitamin, pantothenic

acid and added sulfur groups) to form AAcetyl-CoA, which can enter theKrebs cycle.

Pyruvate dehydrogenase

For one glucose molecule(two pyruvate molecules), we obtain:

• 2 CO2

• 2 NADH• 2 Acetyl C0-A

Note: When the level of ATP is high in a cell, the cell can convert acetyl-CoA intolipid molecules that can be stored for later energy use. This is one way thatexcess calories, no matter the nutrient source, are converted to fat.


The Krebs Cycle (Citric Acid or TCA Cycle)The Krebs cycle is a means to remove energy rich H+ (with its electrons) from theremnants of the original glucose molecule (or other fuel molecules). The H+ andelectrons removed can subsequently be used to generate ATP in the electrontransport chain via chemiosmosis. This is done via a series of oxidation-reductions.

The acids of the Krebs cycle under the right conditions (i.e., The Krebs Cycle) canbe ooxidized. They donate H+ and its electron to the appropriate energy transfermolecule. Once the hydrogen is removed, carbon can also be removed as the wasteproduct, CO2.

In addition, for each original glucose molecule, two ATP are produced in the Krebscycle by ssubstrate-level phosphorylation, one for each acetyl Co-A moleculethat enters the Krebs cycle. (Remember that the glucose molecule has alreadygone through glycolysis and been converted to two molecules of pyruvate in thecytoplasm prior to starting the Krebs cycle.)

In biology, a cycle is a metabolic pathway that starts and ends with the samemolecule. The Krebs cycle starts with OOxaloacetic acid (A 4-carbon acid), whichis regenerated at the end of the cycle. OOxaloacetic acid combines with AAcetyl-CoA to begin the Krebs cycle. Most of the enzymes needed to do the Krebs cycleare located in the mmitochondrial matrix. Two, succinate dehydrogenase and -

ketodehydrogenase, are integral membrane proteins of the inner mitochondrialmembrane.

Note: The acids in this process are ionized, and the naming convention is to usethe suffix -ate. For example, oxaloacetic acid may be called oxaloacetate in thecycle.

For each turn of the Krebs cycle we will get:• 2 CO2

• 1 ATP produced (by substrate phosphorylation)• 1 FADH2

• 3 NADH

The Krebs cycle must turn two times to oxidize the two molecules of Acetyl-CoAthat are what's left of the original glucose molecule.

It should be noted, for preciseness, that a molecule of H2O is also consumed in theKrebs cycle. Don't worry about it.


The Krebs Cycle

The Krebs cycle will turn ttwo times for each glucose molecule, since glycolysisproduces two pyruvate molecules. Therefore, for each glucose molecule that westart with, at the completion of the Krebs cycle's two turns, including thepreparation step of pyruvate acetyl we have:

• 6 CO2

• 2 ATP• 2 FADH2

• 8 NADH

For the curious, the enzymes of the Krebs cycle are:Citrate synthaseAconitaseIsocitrate dehydrogenase-Ketodehydrogenase

Succinyl-CoA synthetaseSuccinate dehydrogenaseFumaraseMalate dehydrogenase


To provide an idea of the potential energy gained from the Krebs cycle, we can lookat the change in free energy as the reactions of the Krebs cycle take place.

Special Note on Vitamins and the Krebs CycleSeveral vitamins function as precursors to coenzymes and energy transfermolecules involved in the Krebs cycle as well as in nutrient interconversions sothat fuel molecules other than glucose can be used in cell respiration.

Here are a few:• Coenzyme A is made from pantothenic acid• NAD is made from niacin• FAD is made from riboflavin• Cobalamin (B12) is needed for amino acid interconversion• Biotin is used for conversion of fats for fuel molecules• Pyroxidine (B6) is used for amino acid interconversion and converting

glycogen to glucose• Thiamin is a coenzyme use for removing CO2 molecules


Electron Transport Chain and ATP Synthesis by ChemiosmosisThe molecules of the electron transport chain, also called the respiratory chain,and a protein complex, AATP Synthase, are found in the iinner membrane of themitochondria.

• The molecules of the electron transport chain are a set of four large integralmembrane protein complexes: NNADH-Q reductase, ssuccinatereductase, ccytochrome c reductase and ccytochrome oxidase, theperipheral protein, ccytochrome c, and the lipid, uubiquinone (or QQ).

• Electrons enter the respiratory chain from NADH and FADH2.

The Respiratory Chain

• Electrons travel down the electron transport chain by oxidation-reductionreactions releasing their energy in controlled bits.

• The redox reactions of the electron transport chain are used to move, byactive transport, Hydrogen ions (H+) from the mitochondrial matrix throughthe inner membrane into the intermembrane space. Some of the carrierspick up both electrons and H+ and release the H+ on the opposite side of themembrane. Others move just electrons.

Electron Transport Chain


• The concentration of H+ in the intermembrane space establishes aconcentration, pH and electrical gradient that has an inherent (potential)energy value. There can be as much as a 1000 X difference H+ concentrationon the different sides of the mitochondrion inner membrane.

The accumulated H+ ions, known as the pproton-motive force, diffusethrough the channels of the ATP synthase protein complex back into themitochondrial matrix. The protein complex of ATP synthase uses theexergonic flow of H+ ions to phosphorylate ADP, forming ATP in themitochondrial matrix, a process called cchemiosmosis.

Peter Mitchell won the 1978 Nobel prize in chemistry "for his contribution tothe understanding of biological energy transfer through the formulation ofthe chemiosmotic theory". ATP is synthesized in the thylakoid membranesof the chloroplast by a similar mechanism.

• Each NADH that enters the ETS provides sufficient energy to synthesize 3ATP molecules by chemiosmosis. FADH2 provides energy for 2 ATPs.

ATP Synthase

• Oxygen is required as the final electron (and Hydrogen) acceptor, producingwater as the end product of aerobic cellular respiration as the H+ and e-

passed off the carriers combine with oxygen. (Recall that CO2 is also aproduct of aerobic cellular respiration.)

Certain ppoisons work by blocking electron transport. Rotenone blocks NADH.Cyanide and carbon monoxide block cytochrome c from reducing oxygen. Oligomycinblocks the flow of H+ through the ATP synthase pump.


Summary of ATP production through the Electron Transport System8 NADH 24 ATP2 FADH2 4 ATP2 NADH (moved in from cytosol) 4 ATPTotal ATP production from ETS: 32 ATP

Total ATP Yield from Aerobic Cellular RespirationATP from Chemiosmosis using the Electron Transport System

• The potential energy of the electrons and H+ from each NADH produced in the

Krebs cycle and the oxidation of pyruvate provides energy to produce amaximum of 3 ATP by chemiosmosis using the electron transport chain.

o The 8 NADH yield energy for 24 ATP

• The electrons and H+ from each FADH2 produced in the Krebs cycle provides

energy to produce a maximum of 2 ATP by chemiosmosis in the ETS. (FAD isa lower energy electron transfer molecule and enters the transport chain inmid-chain, rather than at the start.)

o The 2 FADH2 yield energy for 4 ATP• The electrons and hydrogen from each NADH from Glycolysis provides energy

to produce 2 ATP in the ETS. (The NADH molecules have to be transferredfrom the cytoplasm to the mitochondria.)

o The 2 NADH yield energy for 4 ATP

ATP from Substrate Phosphorylation• 2 ATP net gain are produced in Glycolysis• 2 ATP are produced in the Krebs cycle

Total ATP = 36

The maximum ATP may not be realized, since the inner mitochondrial membrane isleaky to protons and some energy is used to move pyruvate from the cytoplasminto the mitochondrial matrix. Cells may get about .5 ATP less per reduced carrierthat enters electron transport than the maximum.


Aerobic (Cellular) Respiration Summary• The complete aerobic respiration of glucose requires four stages:

o Glycolysiso Pyruvate Oxidationo The Krebs cycleo Electron transport phosphorylation

• Oxygen is the final electron acceptor in the electron transport system TheO2 combines with Hydrogen to form water

• Carbon Dioxide (CO2) is released during aerobic respiration• As much as 36 ATP can be produced from each glucose molecule• Oxidation of pyruvate, the Krebs cycle and Electron transport occur in the

mitochondria; GGlycolysis occurs in the cytosol.• All steps are catalyzed by enzymes

Generating Heat From Cell Respiration - ThermogenesisThere are times when generating heat rather than ATP is desired. Organisms,such as bats, that experience torpor (a reduced metabolic state that results inlowered body temperature) need to increase body temperature rapidly when theywake from torpor. Heat generation requires separating the H+ proton flow fromATP synthase. Mitochondria in special fat containing cells (the "brown" fat) have aH+ "uncoupling" protein, tthermogenin, which moves protons through themembrane into the mitochondrial matrix bypassing the ATP synthase pump.Energy released by the oxidations generates heat instead.

Plants also have uncoupling proteins for heat generation. Arum lilies attractcarrion beetles and flies for pollination by exuding odors that smell like carrion.The voodoo lily increases the temperature of the flower so that the odorsvolatilize and disperse better. Skunk cabbage elevates its body temperature asmuch as 10 - 12° C above ambient air temperature for flowering.

Arum thermogenesis


The Fermentations: Fate of Pyruvate in the Absence of OxygenThe overwhelming majority of living organisms must do aerobic cellular respirationto stay alive. Fermentations and anaerobic respiration pathways provideinsufficient ATP to sustain life for most organisms. However, when oxygen is notavailable for aerobic cell respiration, eukaryotic organisms and some prokaryotes,will complete glucose metabolism with the ffermentation reactions, which areessentially an extension of glycolysis. Some prokaryotes do aanaerobicrespirat ion.

For some prokaryotes and eukaryotes, fermentation is a way of life. Some lackthe enzymes to do the Krebs cycle or oxidative phosphorylation; for others,oxygen is toxic. These are the sstrict (or obligate) anaerobes. Others, such asyeasts and E. coli are ffacultative organisms. When oxygen is available, they doaerobic respiration. When oxygen is not, they perform a fermentation.

NADH must be recycled constantly in cells. Like ATP, it cannot be stockpiled.NADH's oxidation reaction is highly spontaneous. NADH must use its electrons toreduce something and recover NAD+ for more glycolysis. However, NADH's veryhigh energy electrons can be used to make ATP only in the presence of oxygen.

In the fermentations the NADH electrons produced in glycolysis are used to reducepyruvate to some other organic molecule, which becomes the final electronacceptor. NNo more ATP is obtained in the fermentation processes beyond thetwo ATP produced during the glycolysis pathway. However, in cells that normallydo aerobic respiration, the rate of glycolysis increases when oxygen is unavailable,to produce as much ATP as possible.

In the Fermentations:• Organic molecules serve as the electron acceptors for NADH.• Among the Prokaryotes there are several different fermentation pathways.• However only two pathways are found in Eukaryotic organisms:

Alcoholic FermentationLactic Acid Fermentation


Details of the Fermentations• Pyruvate functions as the electron acceptor for the NADH produced in

glycolysis.• NADH is used to reduce Pyruvate to some stable organic molecule, freeing

the NAD+ (or regenerating NAD+) for the reduction step in glycolysis.• No additional ATP is produced.• Two fermentation pathways are common in eukaryotes. The fermentation

pathways are genetically determined. Humans, for example, do lactic acidfermentation; yeasts do alcohol fermentation.

Anaerobic Electron Transport in Prokaryotes (Anaerobic Respiration)Some bacteria have an electron transport system but oxygen is not the finalelectron acceptor. An inorganic substance, such as a sulfur or nitrogen-containingmolecule, becomes the final electron acceptor. Of note are the mmethanogens, asignificant source of methane production on earth. They reduce CO2 to CH4 usinghydrogen from a number of organic molecules, including some acids. The sulfurbacteria can reduce sulfates to hydrogen sulfide, and the first photosynthesis onearth oxidized H2S for its source of hydrogen rather than water. Some bacteriatoday still use H2S for photosynthesis. Nitrogen and iron molecules also providereducing power for anaerobic respiration. These processes are studied inmicrobiology.


Versatility of Metabolic Pathways – Alternative Fuel MoleculesFats, proteins and even nucleic acids can be utilized for fuel in cell respiration.

• Other carbohydrates Glucose Glycolysis• Lipids Glycerol and Fatty Acids

o Glycerol Glycolysis (Glyceraldehyde 3 Phosphate)o Fatty Acids Acetyl Krebs Cycle

• Alcohol Acetyl Krebs Cycle• Proteins Amino Acids

All amino acids must be deaminated prior to being used for fuel.o Amino Acids Pyruvate Krebs Cycleo Amino Acids Acetyl Krebs Cycleo Amino Acids Krebs Cycleo Amino Acids*** Pyruvate Glycolysis Glucose

* *The gglucogenic amino acids in this group are converted topyruvate and can be metabolized "back" to glucose to provideglucose to brain and nervous system cells and developing red bloodcells. Amino acids that are converted to acetyl are calledketogenic.


Any ccarbohydrate that can be digested will be converted to glucose. It's just amatter of time needed to digest and rate of absorption. Polysaccharides anddisaccharides are digested to monosaccharides in the digestive tract. Allmonosaccharides absorbed are converted in the liver to glucose for use inglycolysis.

Fats are energy rich. A gram of fat potentially can produce two times as muchATP as a gram of carbohydrate or protein. oxidation, which occurs in the

mitochondrial matrix, converts fatty acids to the 2-carbon acetyl. oxidation

uses one ATP, and produces one FADH2 and one NADH along with each acetyl Co-Aformed. A 16-carbon fatty acid can produce 8 acetyl Co-A.

oxidation of fatty acids

Most cells routinely use a mix of fats and carbohydrates for fuel. The brain, nervecells and red blood cells, however, have an absolute glucose requirement; fatty acidfragments cannot normally cross the brain membrane barriers so that the braindoes not use fats for fuel. The use of fatty acids for fuel is also a strictly aerobicprocess. All anaerobic respiration requires glucose.

Two important places for alternative fuel molecules to enter our respirationpathway are pyruvate and acetyl-CoA. When we have insufficient glucose for ourbrain and nerve cells, any molecule that can be converted to pyruvate canultimately be used to form glucose, although it is an energy consuming process to"reverse" glycolysis. However, the step from pyruvate to acetyl is not reversible.Fuel molecules that are converted to acetyl, or to acids that are a part of theKrebs cycle are not only unavailable for conversion to glucose, but are useful onlyin aerobic respiration.


Acetyl is also a major point for the conversion of all excess fuel molecules to fatsas well. If we have sufficient energy, acetyl need not enter the Krebs cycle and isdiverted to the formation of fatty acids for adipose storage. This acetyl cancome from any fuel molecule: glucose, fatty acids or amino acids. All excesscalories consumed, no matter the source, will be converted to adipose.

Nutrient inter-conversion adds versatility to metabolic pathways. Acids from theKrebs cycle can be used to synthesize some amino acids, and acetyl can be used tosynthesize fatty acids. -ketoglutarate is an intermediate for amino acid, purine

and chlorophyll synthesis. About half the amino acids can be synthesized fromdifferent amino acids or from other acids in the cells. We maintain a mmetabolichomeostasis or a pool of metabolic intermediates within our cells and tissuesthat remains constant so long as our diet provides the appropriate mix of basicnutrients.

During sstarvation or ffasting, or when there is insufficient carbohydrate forenergy needs, the body uses proteins from body tissues to supply fuel moleculesto the brain, nerve cells and red blood cells. If fat reserves are diminished, proteinfrom body tissue will supply metabolic needs in all cells and tissues. The body willliterally degrade itself to maintain essential cell activity.

When fat reserves are mobilized in response to insufficient calories or insufficientcarbohydrate in the diet, some of the fatty acid fragments combine to formketone bodies rather than acetyl. These ketone bodies enter into circulation.Muscle and some other tissues can use ketone bodies for fuel, and ketone bodiescan provide energy to some brain cells. However, some ketone bodies containcarboxyl groups forming keto acids that can cause ketosis, a condition that lowersthe pH of the blood and impairs health.


Regulating Cell RespirationCells regulate cell respiration just as they regulate other metabolic activities.Cells that are metabolically more active will do more cell respiration (and generallyhave more mitochondria) to provide the ATP needed. When activity drops, the rateof ATP formation likewise diminishes. Fuel molecules used in cell respiration arealso regulated.

In general:• When levels of carbohydrate are high, glucose is metabolized more than fats.

o As levels of glucose fall, stored glycogen (in animals) will be convertedto glucose.

o When glucose supplies diminish, more fat is mobilized to supplementmetabolic needs.

• Protein will be removed from body tissues when carbohydrate is unavailableto provide glucose for brain and nerve cells.

• When specific nutrients are high, biosynthesis pathways related to nutrientinter-conversion that would produce those nutrients are stopped.

All excess calories are converted to fat. Fat to fat conversion is efficient, andany fat consumed not needed for structural or fuel purposes is readily convertedto adipose for storage. Excess carbohydrate beyond the maximum glycogenstores is also converted to fat in an endergonic process. Some of the caloricvalue of the carbohydrate is lost in the conversion. Excess amino acids will beconverted either to glucose, if carbohydrate reserves are low, or to adipose. Muchof this excess nutrient conversion occurs in the respiratory pathway at acetyl.Just as we convert fatty acids to acetyl to "feed" the Krebs cycle, acetyl notneeded for the Krebs cycle is readily converted to fat.

The mechanisms for most of these regulations involve feedbackinhibition/activation. The relative amounts of ATP/ADP, NADH, and someintermediates in the Krebs cycle regulate aerobic respiration rate by feedback.For example:

• High ADP stimulates the enzyme, phosphofructokinase, that convertsfructose 6-phosphate to fructose 1,6-bisphosphate, enhancing the rate ofglycolysis.

• High ATP levels inhibit the conversion of isocitrate to -ketoglutarate in the

Krebs cycle as well as inhibit phosphofructokinase.• Low citrate levels in the Krebs cycle also stimulate phosphofructokinase;

high levels inhibit the enzyme. High citrate levels also promote conversion offatty acids to acetyl.

• High NADH inhibits the enzyme, pyruvate decarboxylase, the enzyme thatoxidizes pyruvate to acetyl, stopping the Krebs cycle.


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Intro Energy Cell Respiration 2