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Tracking of oxygen molecules in glucose oxidation

Tracking of oxygen molecules in glucose oxidation


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For this reaction, found in typical biochemistry textbook:

$C_6H_{12}O_6 + 6O_2 o 6CO_2 + 6H_2O$

I am interested in where do the oxygen atoms of $6O_2$ go. I think they go to $6H_2O$, but this is not sufficient for balance. So Does oxygen atom from $6CO_2$ also get into the carbon dioxide. Experimentally these can be confirmed by iostope labeling experiments. Reference to any such experiments will be appreciated.

I am interested in this question because the central metabolism can be understood as a electron transfer process where glucose is donor and oxygen is acceptor. We need to calculate, effectively how many electrons glucose transfers to oxygen in case of full oxidation.


I think this experiment (PDF file) will help you understand the basic concept about the fate of oxygen in aerobic respiration. Basically the result is:

  1. The oxygen of respiratory carbon dioxide is in exchange equilibrium with body water.
  2. Utilized molecular oxygen is converted to body water.

In respect to calculation of electrons donated to oxygen, just calculating how many NADH2 and how many FADH2 is reduced in how many steps during ETC will be able to answer your question.


Glucose catabolism is a multistep process involving a series of reactions. The reaction you gave is simply the overall, balanced equation; it doesn't actually happen like that in living cells. All diatomic oxygen is converted to water in the electron transport chain, but water is also consumed and produced throughout the preceding steps, which is why the equation doesn't balance that way.

Check out these images from Wikipedia and note all of the compounds entering and exiting the process.

Glycolysis

TCA Cycle

Mitochondrial Electron Transport Chain


What is Glucose Oxidation? (with pictures)

Glucose oxidation is a chemical process that provides energy for an organism to carry out all of its required activities. During this process, glucose, a simple sugar molecule obtained from food, is broken down into carbon dioxide and water. This reaction releases energy and stores it in a chemical form for the cell to use. There are three separate stages of glucose oxidation: glycolysis, the citric acid cycle, and the electron transport system.

Glucose

Molecules of glucose are used to build more complex carbohydrates, like starch and cellulose. The chemical formula for this molecule is C6H12O6, meaning that it is made up of six carbon atoms, 12 hydrogen atoms, and six oxygen atoms. Found in plants and many types of food, glucose is absorbed into the bloodstream during digestion.

Oxidation

Glucose oxidation is an aerobic process, a chemical reaction that requires oxygen. The term "oxidation," in fact, refers to any reaction where oxygen is combined with another molecule, which is then said to be oxidized. During the process, one glucose molecule combines with six oxygen molecules to produce six carbon dioxide molecules, six water molecules, and adenosine triphosphate (ATP), a molecule that cells use to store or transfer energy.

Glycolysis

The first step in the oxidation process is glycolysis, which takes place within a cell’s cytoplasm, the gel-like substance that fills the cell and surrounds the other cellular organs. During this stage, the glucose molecule is broken down into two molecules of pyruvate, an organic acid that can supply cells with energy. This breakdown also releases energy, which is used to add a phosphate ion to adenosine diphosphate (ADP) to create ATP. ADP, in turn, is formed with ATP is broken down to release its energy.

Glycolysis of a single glucose molecule consumes two ATP molecules, and produces four in total, leading to a net energy gain of two ATP. The energy from the process is also used to produce two NADH, a form of an enzyme used to transfer electrons to power cellular chemical reactions.

The Citric Acid Cycle

To begin the citric acid cycle, also called the Krebs cycle, pyruvate molecules produced by glycolysis are moved to the mitochondria, a cellular organ involved in metabolic processes. Once there, the molecules are converted into acetyl CoA, the molecule that powers the citric acid cycle. Acetyl CoA is made up of carbon from the pyruvate and coenzyme A, a molecule that assists in biological processes. The conversion process produces one NADH.

Acetyl CoA releases the carbon portion of the molecule into the citric acid cycle, which runs constantly, producing ATP, high energy electrons, and carbon dioxide. Most of the energy produced is stored in the form of high energy electrons, and one turn of the cycle will result in three NADH and one FADH2. Like NADH, FADH2 stores the captured electrons. The cycle also produces two ATP, and gives off the rest of the energy as heat.

The Electron Transport System

The final stage of glucose oxidation also takes place within the mitochondria, where a group of proteins, called the electron transport system, help transform the energy of the electrons captured by NADH and FADH2 into ATP. This process is modeled by chemiosmotic theory, which describes the way these electrons pass along the transport system, releasing energy as they move through.

The released energy is used to move positively charged hydrogen ions back and forth across the membrane separating two parts of the mitochondria. Energy from this movement is stored in ATP. This process is called oxidative phosphorylation, because oxygen is necessary for the final step, accepting electrons and hydrogen atoms to become H2O, or water. The energy yield from this stage is 26 to 28 ATP.

Energy Gained

When a single molecule of glucose is oxidized, the cell gains about 30 to 32 ATP. This number can vary, because often a mitochondrion does not work at full capacity. Some energy may be lost as the NADH molecules formed in glycolysis transfer their electrons through the membrane separating the mitochondria and the cytoplasm.

ATP is present in all living organisms and plays a critical role in cell metabolism, as it is the main way cells store and transfer energy. Plants produce it by photophosphorylation, a process that converts sunlight to energy. ATP can also be produced in an anaerobic process, a reaction that does not require oxygen. Fermentation, for example, can take place with no oxygen present, but this and other anaerobic metabolic processes tend to be much less efficient ways of making this molecule.

A large number of cellular functions require ATP. The cell breaks down these molecules into ADP and phosphate ions, releasing the stored energy. This energy is then used to do things like move large molecules in and out of the cell or to help create proteins, DNA, and RNA. ATP is also involved in muscle movement and is essential for maintaining the cell’s cytoskeleton, the structure within the cytoplasm that supports the cell and holds it together.


MATERIALS AND METHODS

Animals

In 2008 and 2009 house sparrows, Passer domesticus L. (N=60 25.3±2 g mean ± s.d.) were captured with mist nets at Midreshet Ben-Gurion, Israel. The birds were banded with uniquely numbered aluminum or plastic leg bands and quarantined in a large, permanent outdoor aviary (4 m×3 m×2 m length×width×height), where they were fed a diet of mixed millet seeds (approx. 12% protein and 5% lipid dry mass) (Williams and Ternan, 1999) and provided with tap water ad libitum for a minimum of 45 days. Crushed chicken egg shells, vitamin-supplemented water, and fresh lettuce were also provided once a week. Males and females were housed together, but reproduction was not observed.

At least one month before experiments, the birds were administered two deworming treatments 1 week apart to eliminate intestinal parasites that might influence oxidative dynamics of tracers. Birds were gavaged with an oral dose of Ivermectin (220 μg kg −1 in 0.5 ml water), followed a week later by a dose of Fenbendazole (30 mg kg −1 in 0.5 ml water). After deworming, the sparrows were transferred into neighboring, smaller outdoor aviaries (1.5 m×1.5 m×2.5m length×width×height) with 8-12 individuals in each.

Metabolic rates

Sparrows with full crops were removed from the aviaries and weighed to ±0.1 g. Rates of oxygen consumption () and carbon dioxide production () were then measured every 30 min between 10:00-15:00 h at 24±1°C (N=8 females N=14 males) by open-flow, indirect calorimetry using the multiplexing respirometry system (Qubit Systems, Kingston, Ontario, Canada) previously described by Marom et al. (Marom et al., 2006).

VcGBzw1hCZBfvcMFrsNTJZWlFpw__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA" /> and were calculated as ml gas min −1 using Eqns 1 and 2:


Isomers

Isomers are the molecules that have the same molecular formula but differ with respect to the arrangement of atom in the three-dimensional space. The number of isomers of a molecule depends on the number of chiral carbons in it. A chiral carbon is the one that is attached to four different groups of atoms. The formula to find the number of isomers based on chiral carbons is as follows

Number of isomers = 2 n (here, n=number of chiral carbons)

Except for the first and the last carbon atom, the other four carbon atoms in glucose are chiral. Thus, glucose has 2 4 =16 isomers.

When in ring form, each of these 16 isomers can have one of the two possible orientations alpha or beta. Thus, glucose actually has 32 isomers.

The two different structural forms of glucose are as follows

When dissolved in solution, each of them can have one of the following ring structure.

D and L isomers

These are the two isomeric forms of glucose that differ in the optical properties.

The D-glucose is dextrorotatory. When a ray of light is passed through a solution of D-glucose, it bends the light in the right direction. It is the most abundant form of glucose present in nature. When the structure of D-glucose is drawn on a paper, the -OH groups are written on the right side of carbon atoms, except the 3 rd carbon atom that has the -OH group on its left side.

The L-Glucose is levorotatory. It bends the light rays to the left when passed through its aqueous solution. It is normally present in living cells. Its structure is opposite to that of D-glucose i.e. the -OH groups are attached to the left side of carbon atoms except the third carbon.

Alpha and beta isomers

When either D-glucose or L-glucose is dissolved in water, it forms a hexagonal ring structure known as glucopyranose. The ring formed by each of these molecules can have alpha or beta orientation.

In alpha-D-glucose or alpha-L-glucose, the hydroxyl group attached to the carbonyl carbon or the first carbon is on the side of the ring opposite to that of the sixth carbon.

On the other hand, in beta-D-glucose or beta-L-glucose, the hydroxyl group of the carbonyl carbon is on the same side of the ring as the sixth carbon.

Epimers of Glucose

Epimers are the molecules that differ in structure around one carbon atom only. Glucose has eight epimers.

  • Glucose
  • Allose
  • Altrose
  • Mannose
  • Idose
  • Galactose
  • Talose
  • Gulose

All these eight epimers have D-form and L-form making a total of 16 isomers.

Each of the 16-isomers can have either an alpha ring or a beta ring when dissolved in aqueous solution.


Role of Reactive Oxygen Species (ROS) in Therapeutics and Drug Resistance in Cancer and Bacteria

Evading persistent drug resistance in cancer and bacteria is quintessential to restore health in humans, and impels intervention strategies. A distinct property of the cancer phenotype is enhanced glucose metabolism and oxidative stress. Reactive oxygen species (ROS) are metabolic byproducts of aerobic respiration and are responsible for maintaining redox homeostasis in cells. Redox balance and oxidative stress are orchestrated by antioxidant enzymes, reduced thiols and NADP(H) cofactors, which is critical for cancer cells survival and progression. Similarly, Escherichia coli (E. coli) and life-threatening infectious pathogens such as Staphylococcus aureus (SA) and Mycobacterium tuberculosis (Mtb) are appreciably sensitive to changes in the intracellular oxidative environment. Thus, small molecules that modulate antioxidant levels and/or enhance intracellular ROS could disturb the cellular oxidative environment and induce cell death, and hence could serve as novel therapeutics. Presented here are a collection of approaches that involve ROS modulation in cells as a strategy to target cancer and bacteria.


Electron Transport System (ETS) | Microbiology

Catabolism of energy-giving substrates (mostly glucose) up to the stage of pyruvic acid through EMP or EDP yields comparatively small amount of energy in the form of ATP by substrate-level phosphorylation. The anaerobic organisms have to be satisfied with this small amount of energy.

But the aerobic organisms can extract much larger quantity of energy from oxidation of the same amount of substrate through the TCA cycle and the electron transport system in which the H-atoms — transferred to specific acceptors, like NAD and FAD in the TCA cycle — are fully oxidized in course of electron transport.

The ultimate acceptor of H + and electrons is oxygen in oxygen-respiring organisms. In course of the electron transport through several carriers, ATP is generated by oxidative phosphorylation. As it will be seen, from every mole of glucose oxidized, 38 moles of ATP are formed, in contrast to only two moles in EMP and one mole in EDP.

The ETS is a sequence of carrier molecules which are capable of alternately acting as an electron (or hydrogen) acceptor and electron donor, i.e. undergoing alternate reduction and oxidation. The components of ETS are located in the inner membrane of the mitochondria in eukaryotes and in the cytoplasmic membranes in prokaryotes.

There are three classes of carrier molecules. The first of these are flavoproteins which contain FMN (flavin mononucleotide) as coenzyme. An example of this class is NADH2 dehydrogenase which contains, in addition to FMN, a non-haem iron. The function of this enzyme is to transfer hydrogen from NADH2 to ubiquinone.

Ubiquinone or coenzyme Q constitutes another class of carrier molecule which can reversibly accept hydrogen atoms. It is a benzoquinone derivative with a long isoprenoid chain (R).

The third and the most important class of electron carrier molecules are constituted by the cytochromes. There are several different cytochromes, but all of them are not present in all organisms, but some members of this class are present in all aerobic organisms.

All cytochromes are composed of an iron-porphyrin prosthetic group known as haemin ring (also present in haemoglobin) attached to a protein. The haemin group is common to all cytochromes which differ from each other in their protein component. The central iron atom of the haemin ring undergoes reversible valency change with reduction or oxidation.

The terminal cytochrome of the ETS is cytochrome oxidase. It reacts with oxygen and transfers two electrons to oxygen to form doubly negatively charged oxygen atom which combines with 2H + to form a molecule of water.

In the respiratory chain hydrogen or electrons move from a negative redox-potential to a positive one. For example, NAD/NADH2 has a redox-potential of – 0.32 volts while oxygen has a redox- potential of + 0.81 volts. During the passage from the negative to the positive potential, there is a fall in the free-energy which can be trapped for phosphorylation of ADP to form ATP. This mode of ATP formation is known as oxidative phosphorylation.

The passage of hydrogen and electrons through the ETS is shown in a simplified manner in Fig. 8.49 and in details in Fig. 8.50:

From Fig. 8.50 it may be observed that electron transport chain begins by transfer of high energy protons and electrons which have a redox potential of – 0.32 volt to gradually diminishing redox- potential. The downhill flow of electrons releases free energy which is trapped in ATP molecules.

The production of ATP by oxidative phosphorylation of ADP can be explained by the chemiosmotic mechanism. This theory postulates that the transport of electrons by the carrier molecules which are located in the membrane creates a gradient of H + -ions across the membrane. This is possible because the membrane is impermeable to H + -ions.

The proton-gradient is formed by active transport of H + -ions by the proton pumps outwards. In case of mitochondria, H + -ion concentration increases in the space between the outer and inner membrane compared to the H + -ion concentration in the matrix. In case of bacteria, the H + -ion concentration in the surrounding medium becomes higher compared to that of the cytoplasm.

The proton gradient created by active expulsion of H + -ions by proton pumps also creates an electric charge gradient, because of accumulation of excess of positively charged H + -ions on one side of the membrane. The resulting electrochemical gradient has potential energy which is known as the proton motive force (pmf).

Protons expelled by the proton pumps can cross the membrane only through some special proton channels where an enzyme ATP synthase is located. When the protons pass through these channels the potential energy is released and it is utilized by the enzyme for synthesis of ATP from ADP and inorganic phosphoric acid. Both eukaryotic organisms and prokaryotes use the chemiosmotic mechanism for ATP synthesis.

ATP formation by this mechanism in bacterial cells is shown diagrammatically in Fig. 8.51:

ATP yield in aerobic respiration:

The total yield of ATP when glucose is fully oxidized to CO2 and H2O amounts to 38 moles per mole of glucose. It may now be examined in which steps this amount of ATP is formed (Table 8.3). For calculating ATP yield, it should be noted that NADH2 can generate 3 ATP through ETS and FADH2 can produce 2 ATP (Fig. 8.50).

So, the overall reaction of glucose oxidation in aerobic respiration can be written as:

When 1 mole of glucose is oxidized under non-biological conditions, 674 K calories of energy is liberated as heat. Now, it can be calculated how much of this energy is preserved in the form of ATP.

Taking an average value of energy liberated by ATP hydrolysis producing ADP + Pi as -Δ K calories (ATP ADP + Pi, ∆G = -7 K cal), it is observed that a little less than 40% of the total energy released by glucose oxidation is conserved as ATP in aerobic respiration. Thus the efficiency of biological oxidation is nearly 40%. The rest of the energy is given out in the form of heat.


Aerobic Respiration

Glycolysis can take place without oxygen. This forms the anaerobic part of cell respiration and therefore is called anaerobic cell respiration. However, the pyruvate produced from glycolysis cannot be oxidised further without the presence of oxygen. The oxidisation of the pyruvate forms part of the aerobic respiration and therefore is called aerobic cell respiration. Aerobic respiration occurs in the mitochondria of cells. The first reaction to take place is the link reaction.

The Link Reaction

Mitochondria in cells take up the pyruvate which is formed from glycolysis in the cytoplasm. Once the pyruvate is in the mitochondrion, enzymes within the matrix of the mitochondrion remove hydrogen and carbon dioxide from the pyruvate. This is called oxidation (removal of hydrogen or addition of oxygen) and decarboxylation (removal of carbon dioxide). Therefore, the process is called oxidative decarboxylation. The hydrogen removed is accepted by NAD + . The link reaction results in the formation of an acetyl group. This acetyl group is then accepted by CoA and forms acetyl CoA.

Figure 8.1.3 - The link reaction

The Krebs Cycle

Step 1 - In the first stage of the Krebs cycle, the acetyl group from acetyl CoA is transferred to a four carbon compound. This forms a six carbon compound.

Step 2 - This six carbon compound then undergoes decarboxylation (CO2 is removed) and oxidation (hydrogen is removed) to form a five carbon compound. The hydrogen is accepted by NAD + and forms NADH + H + .

Step 3 - The five carbon compound undergoes decarboxylation and oxidation (hydrogen is removed) again to form a four carbon compound. The hydrogen is accepted by NAD + and forms NADH + H + .

Step 4 - The four carbon compound then undergoes substrate-level phosphorylation and during this reaction it produces ATP. Oxidation also occurs twice (2 hydrogens are removed). The one hydrogen is accepted by NAD + and forms NADH + H + . The other is accepted by FAD and forms FADH2. The four carbon compound is then ready to accept a new acetyl group and the cycle is repeated.

The carbon dioxide that is removed in these reactions is a waste product and is excreted from the body. The oxidations release energy which is then stored by the carriers when they accept the hydrogen. This energy is then later on used by the electron transport chain to produce ATP.

Carbon dioxide is removed in two reactions

Hydrogen is removed in 4 reactions

NAD + accepts the hydrogen in 3 reactions

FAD accepts the hydrogen in 1 reaction

ATP is produced in one of the reactions

The Electron Transport Chain

Inside the inner membrane of the mitochondria there is a chain of electron carriers. This chain is called the electron transport chain. Electrons from the oxidative reactions in the earlier stages of cell respiration pass along the chain. NADH donates two electrons to the first carrier in the chain. These two electrons pass along the chain and release energy from one carrier to the next. At three locations along the chain, enough energy is released to produce ATP via ATP synthase. ATP synthase is an enzyme that is also found in the inner mitochondrial membrane. FADH2 also donates electrons but at a later stage than NADH. Also, enough energy is released at only two locations along the chain by electrons from FADH2. The ATP production relies on energy release by oxidation and it is therefore called oxidative phosphorylation.

Figure 8.1.5 - Electron transport chain

The Role of Oxygen

Oxygen is important for cell respiration as at the end of the electron transport chain, the electrons are donated to oxygen. This occurs in the matrix at the surface of the inner membrane. At the same time oxygen binds with hydrogen ions and forms water.
If there is no oxygen then electrons can no longer pass through the electron transport chain and NADH + H + can no longer be reconverted into NAD + . Eventually NAD + in the mitochondrion runs out and therefore the link reaction and Krebs cycle no longer take place.


Difference between Glycolysis and Krebs Cycle | Metabolic Engineering

Glycolysis is the sequence of enzymatic reactions which oxidize the six-carbon sugar glucose into two three-carbon compounds with the production of a small amount of adenosine triphosphate (ATP). Glycolysis has two basic functions if the cell.

First, it metabolizes simple six-carbon sugars to smaller three-carbon compounds that are then either fully metabolized by the mitochondria to produce carbon dioxide and a large amount of ATP or used for the synthesis of fat for storage.

Second, glycolysis functions to produce a small amount of ATP, which is essential for some cells solely dependent on that pathway for the generation of energy.

Glycolytic pathway is catalyzed by soluble enzymes located in the cytosol of cells. The glycolytic pathway operates in both the presence (aerobic) and absence of oxygen (anaerobic). The metabolism of fuel molecules in the cell can be thought of as an oxidation process.

In glycolysis, glucose is the fuel molecule being oxidized. As the glucose is oxidized by the glycolytic enzymes, the coenzyme nicotinamide adenine dinucleotide (NAD+) is converted from its oxidized to reduced form (NAD + to NADH).

When oxygen is available (aerobic conditions) mitochondria in the cell can re-oxidize to NADH to NAD + . However, if either oxygen levels are insufficient (anaerobic conditions) or mitochondrial activity is absent, NADH must be re-oxidized by the cell using some other mechanism. In animal cells, the re-oxidation of NADH is accomplished by reducing pyruvate, the end-product of glycolysis, to form lactic acid.

This process is known as anaerobic glycolysis. During vigorous exercise, skeletal muscle relies heavily on it. In yeast, anaerobic conditions result in the production of carbon dioxide and ethanol from pyruvate rather than lactic acid. This process, called as alcoholic fermentation, is the basis of wine production and the reason why bread dough rises.

Although some cells are highly dependent on glycolysis for the generation of ATP, the amount of ATP generated per glucose molecule is actually quite small. Under anaerobic conditions, the metabolism of each glucose molecule yields only two ATPs. In contrast, the complete aerobic metabolism of glucose to carbon dioxide by glycolysis and the Krebs cycle yields up to thirty-eight ATPs.

Therefore, in the majority of cells the most important function of glycolysis is to metabolize glucose to generate three-carbon compounds that can be utilized by other pathways. The final product of aerobic glycolysis is pyruvate. Pyruvate can be metabolized by pyruvate dehydrogenase to form acetyl coenzyme A (acetyl CoA). Under conditions where energy is needed, acetyl CoA is metabolized by the Krebs cycle to generate carbon dioxide and a large amount of ATP. Acetyl CoA can be used to synthesize fats or amino acids, when the cell does not need energy.

Krebs Cycle:

Krebs cycle is a set of enzymatic reactions that catalyzes the aerobic metabolism of fuel molecules to carbon dioxide and water, thereby producing energy for the production of adenosine triphosphate (ATP) molecules. The Krebs cycle is so named because much of its elucidation was the work of the British biochemist Hans Krebs.

Many types of fuel molecules can be drawn into and utilized by the cycle, including acetyl coenzyme A (acetyl CoA), derived from glycolysis or fatty acid oxidation . Some amino acids are metabolized via the enzymatic reactions of the Krebs cycle. In eukaryotic cells, all but one of the enzymes catalyzing the reactions of the Krebs cycle is found in the mitochondrial matrixes.

The sequence of events known as the Krebs cycle is indeed a cycle oxaloacetate is both the first reactant and the final product of the metabolic pathway (creating a loop). Because the Krebs cycle is responsible for the ultimate oxidation of metabolic intermediates produced during the metabolism of fats, proteins, and carbohydrates, it is the central mechanism for metabolism in the cell.

In the first reaction of the cycle, acetyl CoA condenses with oxaloacetate to form citric acid. Acetyl CoA utilized in this way by the cycle has been produced either via the oxidation of fatty acids, the breakdown of certain amino acids, or the oxidative decarboxylation of pyruvate (a product of glycolysis).

The citric acid produced by the condensation of acetyl CoA and oxaloacetate is a tri Carboxylic acid containing three carboxylate groups. (Hence, the Krebs cycle is also referred to as the citric acid cycle or tri-carboxylic acid cycle.)

After citrate has been formed, the cycle machinery continues through seven distinct enzyme-catalyzed reactions that produce, in order, iso-citrate, a – ketoglutarate, succinyl coenzyme A, succinate, fumarate, malate, and oxaloacetate.

The freshly produced oxaloacetate, in turn, reacts with yet another molecule of acetyl CoA, and the cycle begins again. Each turn of the Krebs cycle produces two molecules of carbon dioxide, one guanosine triphosphate molecule (GTP), and enough electrons to generate three molecules of NADH and one molecule of FADH2.

The Krebs cycle is present in virtually all eukaryotic cells that contain mitochondria, but functions only as part of aerobic metabolism (when oxygen is available). This oxygen requirement is owing to the close relationship between the mitochondrial electron transport chain and the Krebs cycle. In the Krebs cycle, four oxidation-reduction reactions occur.

A high energy phosphate bond in the form of GTP is also generated. (This high energy phosphate bond is later transferred to adenosine di-phosphate [ADP] to form adenosine triphosphate [ATP].) As the enzymes of the Krebs cycle oxidize fuel molecules to carbon dioxide, the coenzymes NAD + , FAD, and coenzyme Q (also known as ubiquinone) are reduced.

In order for the cycle to continue, these reduced coenzymes must become re-oxidized by transferring their electrons to oxygen, thus producing water. Therefore, the final acceptor of the electrons produced by the oxidation of fuel molecules as part of the Krebs cycle is oxygen. In the absence of oxygen, the Krebs cycle is inhibited.

The citric acid cycle is an amphibolic pathway, meaning that it can be used for both, the synthesis and degradation of biomolecules. Besides acetyl CoA (generated from glucose, fatty acids, or ketogenic amino acids), other biomolecules are metabolized by the cycle.

Several amino acids are degraded to become what are intermediates of the cycle. Likewise, odd-chain fatty acids are metabolized to form succinyl coenzyme A, another intermediate of the cycle. Krebs cycle intermediates are also used by many organisms for the synthesis of other important biomolecules.

Some organisms use the Krebs cycle intermediates a -ketoglutarate and oxaloacetate in the synthesis of several amino acids. Succinyl coenzyme A is utilized in the synthesis of porph5Tin rings, used in home manufacture and chlorophyll biosynthesis. Oxaloacetate and maltase are utilized in the synthesis of glucose, in a process called as gluconeogenesis.


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So, Acetyl-CoA (a 2 carbon molecule attached to Co-enzyme A) is the form in which the carbons enter the Krebs Cycle. Oxaloacetate is a Krebs cycle intermediate and is produced elsewhere in the cell via anaplerotic reactions (reactions that generate metabolic intermediates), so it's a bit misleading to state that it is the entry molecule. Also, the Krebs cycle itself does not produce much energy at all. 1 net ATP from the cycle is nothing. What it does produce are the reducing agents (NADH and FADH2) which carry the electrons captured from your initial Pyruvate (which was turned into Acetyl-CoA) to the Electron Transport Chain, which is the portion of oxidative metabolism that is the most important, and entirely overlooked here.

I understand you are probably going for a very simplified look at the process, but you are leaving out the most crucial portion of the process. Also, the products of the Krebs cycle continue down the catabolic pathway in order to generate ATP, the only products that don't are the 1 ATP (which is actually generated as GTP) and two CO2's. BambooForest December 3, 2011

@vogueknit17- I have a friend who studied biology and hopes to do nutritional research. She thinks that things like a person's ability to perform proper oxidation and metabolism of food will turn out to be a big part of obesity's rise, and that it might be something that we're adapting to as a result of the "modern" diet. I don't understand much of it beyond that, but I agree that it's complicated, from what I do know. vogueknit17 December 3, 2011

Metabolism is so complicated. I know a lot of people argue a lot of different ways about things like weight gain and loss and nutrition, but it really can be a little different for everyone, because there are so many different cycles for metabolizing food. We think of it as "food in, energy out", but there are so many steps in a person's body.


What is Aerobic Glycolysis? (with pictures)

Aerobic glycolysis is the first of three stages that make up aerobic cellular respiration. Cellular respiration is the process that takes place within all cells to release energy stored in glucose molecules. There are two forms of cellular respiration, aerobic and anaerobic, meaning requires oxygen and doesn’t require oxygen.

All living organisms need energy to survive. That energy is received through food, which for plants also includes energy captured from the sun. Whatever the form of food that is taken in by the organism, it is converted to carbohydrates, glucose in particular. During cellular respiration, glucose is converted to carbon dioxide and water with energy being released into the cell. Breaking down the glucose molecules is an oxidation reaction, so oxygen is required for the process to go ahead.

The three stages of aerobic respiration are aerobic glycolysis, the Krebs cycle and the electron transport system. During each stage, a number of chemical reactions take place which form the cellular respiration overall process. The outcome of aerobic glycolysis is that the glucose molecule is broken down into two pyruvate, or pyruvic acid, molecules, which are broken down further in the Krebs cycle, and two water molecules.

The energy that is released by cellular respiration does not happen all at once. In fact, some energy is released through each of the three main stages. When the energy is released from the glucose molecule, it is not released as free energy. The energy is stored in adenosine triphosphate (ATP) molecules, which are short term energy storage molecules that are easily transported within and between cells.

The energy production begins during aerobic glycolysis. During this process, two of the 36 total ATP molecules are created. All the stages of cellular respiration are made up of a number of complex chemical reactions. Aerobic glycolysis is actually made up of a number of different stages that the glucose molecule moves through. The energy necessary to produce the eight ATP molecules is released at different stages of the process.

During aerobic glycolysis, two ATP molecules are initially used to make the glucose molecule sufficiently reactive. The glucose molecule is phosphorylated, meaning that phosphate molecules are added to the glucose molecule from the ATP molecules. After the glucose has been phosphorylated, it splits from a six carbon sugar molecule into two three carbon sugar molecules. Hydrogen atoms are removed from the resulting three carbon sugars and two phosphates are lost from each, forming four new ATP molecules. After the glucose has gone through all these steps, the final outcome is two three carbon pyruvate molecules, two water molecules and two ATP molecules.


Watch the video: Η γλώσσα του ψεύδους - Νόα Ζαντάν (July 2022).


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