Information

How do the High-Energy Electrons During Chemiosmosis Come from Water?

How do the High-Energy Electrons During Chemiosmosis Come from Water?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

On Pg. 199 of Campbell, this is stated.

Both* work by way of chemiosmosis, but in chloro- plasts, the high-energy electrons dropped down the transport chain come from water, while in mitochondria, they are extracted from organic molecules (which are thus oxidized).

*Both refers to the to chemiosmosis in mitochondria and in chloroplasts (during photosynthesis).

My question is, how do the high-energy electrons in the transport chain for chemiosmosis come from water? Aren't these high-energy electrons derived from light harvesting done by the light-harvesting complexes?


The light harvesting complexes use light energy to "lift" electrons to a higher energy state from which they can be used to reduce organic molecules (by way of an electron transport chain and NADPH), leaving a "hole" or cation behind in the LHC and thereby oxidizing it. If these complexes were not re-reduced by something, they would become non-functional after absorption of a single photon. So, they are re-reduced by removing electrons from water to generate molecular oxygen, which happens in photosystem II. So the photosystems I and II act as a series of "pumps" that make otherwise thermodynamically "uphill" (unfavorable) electron transfer from water to NADPH spontaneous, but they do not act as a net source of electrons--water does.


How do the High-Energy Electrons During Chemiosmosis Come from Water? - Biology

The electron transport chain uses the electrons from electron carriers to create a chemical gradient that can be used to power oxidative phosphorylation.

Learning Objectives

Describe how electrons move through the electron transport chain

Key Takeaways

Key Points

  • Oxidative phosphorylation is the metabolic pathway in which electrons are transferred from electron donors to electron acceptors in redox reactions this series of reactions releases energy which is used to form ATP.
  • There are four protein complexes (labeled complex I-IV) in the electron transport chain, which are involved in moving electrons from NADH and FADH2 to molecular oxygen.
  • Complex I establishes the hydrogen ion gradient by pumping four hydrogen ions across the membrane from the matrix into the intermembrane space.
  • Complex II receives FADH2, which bypasses complex I, and delivers electrons directly to the electron transport chain.
  • Ubiquinone (Q) accepts the electrons from both complex I and complex II and delivers them to complex III.
  • Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes.
  • Complex IV reduces oxygen the reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water.

Key Terms

  • prosthetic group: The non-protein component of a conjugated protein.
  • complex: A structure consisting of a central atom, molecule, or protein weakly connected to surrounding atoms, molecules, or proteins.
  • ubiquinone: A lipid soluble substance that is a component of the electron transport chain and accepts electrons from complexes I and II.

Oxidative phosphorylation is a highly efficient method of producing large amounts of ATP, the basic unit of energy for metabolic processes. During this process electrons are exchanged between molecules, which creates a chemical gradient that allows for the production of ATP. The most vital part of this process is the electron transport chain, which produces more ATP than any other part of cellular respiration.

Electron Transport Chain

The electron transport chain is the final component of aerobic respiration and is the only part of glucose metabolism that uses atmospheric oxygen. Electron transport is a series of redox reactions that resemble a relay race. Electrons are passed rapidly from one component to the next to the endpoint of the chain, where the electrons reduce molecular oxygen, producing water. This requirement for oxygen in the final stages of the chain can be seen in the overall equation for cellular respiration, which requires both glucose and oxygen.

A complex is a structure consisting of a central atom, molecule, or protein weakly connected to surrounding atoms, molecules, or proteins. The electron transport chain is an aggregation of four of these complexes (labeled I through IV), together with associated mobile electron carriers. The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and the plasma membrane of prokaryotes.

The electron transport chain: The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to molecular oxygen. In the process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water.

Complex I

To start, two electrons are carried to the first complex aboard NADH. Complex I is composed of flavin mononucleotide (FMN) and an enzyme containing iron-sulfur (Fe-S). FMN, which is derived from vitamin B2 (also called riboflavin), is one of several prosthetic groups or co-factors in the electron transport chain. A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups can be organic or inorganic and are non-peptide molecules bound to a protein that facilitate its function.

Prosthetic groups include co-enzymes, which are the prosthetic groups of enzymes. The enzyme in complex I is NADH dehydrogenase, a very large protein containing 45 amino acid chains. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space it is in this way that the hydrogen ion gradient is established and maintained between the two compartments separated by the inner mitochondrial membrane.

Q and Complex II

Complex II directly receives FADH2, which does not pass through complex I. The compound connecting the first and second complexes to the third is ubiquinone (Q). The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced to QH2, ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the electrons derived from NADH from complex I and the electrons derived from FADH2 from complex II, including succinate dehydrogenase. This enzyme and FADH2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass, and thus do not energize, the proton pump in the first complex, fewer ATP molecules are made from the FADH2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.

Complex III

The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic heme group. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe 2+ (reduced) and Fe 3+ (oxidized). The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, which makes each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes. Cytochrome c is the acceptor of electrons from Q however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time.

Complex IV

The fourth complex is composed of cytochrome proteins c, a, and a3. This complex contains two heme groups (one in each of the cytochromes a and a3) and three copper ions (a pair of CuA and one CuB in cytochrome a3). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to produce water (H2O). The removal of the hydrogen ions from the system also contributes to the ion gradient used in the process of chemiosmosis.


7.4B: Chemiosmosis and Oxidative Phosphorylation

  • Contributed by Boundless
  • General Microbiology at Boundless
  • Describe how the energy obtained from the electron transport chain powers chemiosmosis and discuss the role of hydrogen ions in the synthesis of ATP

During chemiosmosis, electron carriers like NADH and FADH donate electrons to the electron transport chain. The electrons cause conformation changes in the shapes of the proteins to pump H+ across a selectively permeable cell membrane. The uneven distribution of H + ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient) owing to the hydrogen ions&rsquo positive charge and their aggregation on one side of the membrane.

Figure (PageIndex<1>): Chemiosmosis: In oxidative phosphorylation, the hydrogen ion gradient formed by the electron transport chain is used by ATP synthase to form ATP.

If the membrane were open to diffusion by the hydrogen ions, the ions would tend to spontaneously diffuse back across into the matrix, driven by their electrochemical gradient. However, many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through a membrane protein called ATP synthase. This protein acts as a tiny generator turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of this molecular machine harnesses the potential energy stored in the hydrogen ion gradient to add a phosphate to ADP, forming ATP.

Figure (PageIndex<1>): ATP Synthase: ATP synthase is a complex, molecular machine that uses a proton (H+) gradient to form ATP from ADP and inorganic phosphate (Pi).

Chemiosmosis is used to generate 90 percent of the ATP made during aerobic glucose catabolism. The production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. It is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen attract hydrogen ions (protons) from the surrounding medium and water is formed.


Examples of Chemiosmosis

Although chemiosmosis is often generally defined as the movement of ions across a membrane, it is really only used in the context of talking about the movement of H + ions during the production of ATP. The most common method involving chemiosmosis in the production of ATP is cellular respiration in the mitochondria, the process of which is discussed above. All eukaryotic organisms have mitochondria, so chemiosmosis is involved in ATP production through cellular respiration in the vast majority of different types of organisms, from animals to plants to fungi to protists. However, even though archaea and bacteria do not have mitochondria, they also use chemiosmosis to produce ATP through photophosphorylation. This process also involves an electron transport chain, proton gradient, and chemiosmosis of H + , but it takes place across the inner membrane of the bacterium or archaeon, since they have no mitochondria.

Plants produce ATP during photosynthesis in the chloroplast in addition to the ATP they generate through cellular respiration in mitochondria. The process is again similar: during photosynthesis, light energy excites electrons, which flow down an electron transport chain, which in turn allows H + ions to travel through a membrane in the chloroplast. Some bacteria, such as cyanobacteria, also use photosynthesis.


The Electron Transport Chain

NADH and FADH2 convey their electrons to the electron transport chain. This transport chain is composed of a number of molecules (mostly proteins) that are located in the inner membrane of the mitochondrion. Each membrane protein has a particular electronegativity (affinity for electrons). The more electronegative the molecule, the more energy required to keep the electron away from it. In this way, a slightly electronegative membrane protein will pull electrons away from reduced electron carriers. In the presence of an even-more electronegative molecule, these electrons will be oxidized from the first membrane protein, and so on. Finally the electrons reduce oxygen, and along with the addition of hydrogen ions, water is produced as a waste product. This stepwise movement, whereby an electron from one protein is transferred to another in the chain, is also reflective of the overall decrease in the amount of energy that the electron possesses. Importantly, each step has a negative delta G. Therefore with each oxidation/reduction reaction, energy is made available to do work. This work involves the movement of protons.

Oxygen is one of the most electronegative atoms. This is important because the relative change in electronegativity determines how much energy is available to do work. When oxygen acts as the terminal electron acceptor, there is a maximal amount of free energy released hence, more protons can be transported, which means that a greater charge buildup occurs across the inner mitochondria membrane. This figure illustrates the energetic relationship between various members of the electron transport chain when oxygen serves as the electron acceptor.


Figure 3. The Electron Transport Chain and Free Energy Change. (Click to enlarge)


Summary

  1. Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid (Krebs) cycle, and an electron transport chain and chemiosmosis.
  2. During various steps in glycolysis and the citric acid cycle, the oxidation of certain intermediate precursor molecules causes the reduction of NAD + to NADH + H + and FAD to FADH2. NADH and FADH2 then transfer protons and electrons to the electron transport chain to produce additional ATPs by oxidative phosphorylation.
  3. The electron transport chain consists of a series of electron carriers that eventually transfer electrons from NADH and FADH2 to oxygen.
  4. The chemiosmotic theory states that the transfer of electrons down an electron transport system through a series of oxidation-reduction reactions releases energy. This energy allows certain carriers in the chain to transport hydrogen ions (H + or protons) across a membrane.
  5. As the hydrogen ions accumulate on one side of a membrane, the concentration of hydrogen ions creates an electrochemical gradient or potential difference (voltage) across the membrane called proton motive force.
  6. This proton motive force provides the energy necessary for enzymes called ATP synthases, also located in the membranes mentioned above, to catalyze the synthesis of ATP from ADP and phosphate.
  7. During aerobic respiration, the last electron carrier in the membrane transfers 2 electrons to half an oxygen molecule (an oxygen atom) that simultaneously combines with 2 protons from the surrounding medium to produce water as an end product.

Biology 171

By the end of this section, you will be able to do the following:

  • Discuss the importance of electrons in the transfer of energy in living systems
  • Explain how ATP is used by cells as an energy source

Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are combinations of oxidation and reduction reactions, which occur at the same time. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions .

Electrons and Energy

The removal of an electron from a molecule (oxidizing it), results in a decrease in potential energy in the oxidized compound. The electron (sometimes as part of a hydrogen atom) does not remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound. The shift of an electron from one compound to another removes some potential energy from the first compound (the oxidized compound) and increases the potential energy of the second compound (the reduced compound). The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of high-energy electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy from food you will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.

Electron Carriers

In living systems, a small class of compounds functions as electron shuttles: they bind and carry high-energy electrons between compounds in biochemical pathways. The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) ((Figure)) is derived from vitamin B3, niacin. NAD + is the oxidized form of the molecule NADH is the reduced form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron). Note that if a compound has an “H” on it, it is generally reduced (e.g., NADH is the reduced form of NAD).

NAD + can accept electrons from an organic molecule according to the general equation:

When electrons are added to a compound, it is reduced. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent, and NAD + is reduced to NADH. When electrons are removed from a compound, it is oxidized. A compound that oxidizes another is called an oxidizing agent. In the above equation, NAD + is an oxidizing agent, and RH is oxidized to R.

Similarly, flavin adenine dinucleotide (FAD + ) is derived from vitamin B2, also called riboflavin. Its reduced form is FADH2. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD + and FAD + are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis in plants.

ATP in Living Systems

A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP). ATP is often called the “energy currency” of the cell, and, like currency, this versatile compound can be used to fill any energy need of the cell. How? It functions similarly to a rechargeable battery.

When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The energy is used to do work by the cell, usually when the released phosphate binds to another molecule, thereby activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium. In this way, the cell performs work, pumping ions against their electrochemical gradients.

ATP Structure and Function

At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group ((Figure)). Ribose is a five-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in the formation of adenosine diphosphate (ADP) the addition of a third phosphate group forms adenosine triphosphate (ATP).

The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called dephosphorylation , releases energy.

Energy from ATP

Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or lysed, and the resulting hydrogen atom (H + ) and a hydroxyl group (OH – ), or hydroxide, are added to the larger molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (Pi), and the release of free energy. To carry out life processes, ATP is continuously broken down into ADP, and like a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group. Water, which was broken down into its hydrogen atom and hydroxyl group (hydroxide) during ATP hydrolysis, is regenerated when a third phosphate is added to the ADP molecule, reforming ATP.

Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come from? In nearly every living thing on Earth, the energy comes from the metabolism of glucose, fructose, or galactose, all isomers with the chemical formula C6H12O6 but different molecular configurations. In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.

Phosphorylation

Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other on the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it allows one of the substrates (such as ATP) and reactants to more readily react with each other in reactions involving ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation. Phosphorylation refers to the addition of the phosphate (

P). This is illustrated by the following generic reaction, in which A and B represent two different substrates:

When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are available for recycling through cell metabolism.

Substrate Phosphorylation

ATP is generated through two mechanisms during the breakdown of glucose. A few ATP molecules are generated (that is, regenerated from ADP) as a direct result of the chemical reactions that occur in the catabolic pathways. A phosphate group is removed from an intermediate reactant in the pathway, and the free energy of the reaction is used to add the third phosphate to an available ADP molecule, producing ATP ((Figure)). This very direct method of phosphorylation is called substrate-level phosphorylation .

Oxidative Phosphorylation

Most of the ATP generated during glucose catabolism, however, is derived from a much more complex process, chemiosmosis, which takes place in mitochondria ((Figure)) within a eukaryotic cell or the plasma membrane of a prokaryotic cell. Chemiosmosis , a process of ATP production in cellular metabolism, is used to generate 90 percent of the ATP made during glucose catabolism and is also the method used in the light reactions of photosynthesis to harness the energy of sunlight. The production of ATP using the process of chemiosmosis is called oxidative phosphorylation because of the involvement of oxygen in the process.

Mitochondrial Disease Physician What happens when the critical reactions of cellular respiration do not proceed correctly? This may happen in mitochondrial diseases, which are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells. In type 2 diabetes, for instance, the oxidation efficiency of NADH is reduced, impacting oxidative phosphorylation but not the other steps of respiration. Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases. Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial diseases, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disorders.

Section Summary

ATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and either ADP or AMP is produced. Energy derived from glucose catabolism is used to convert ADP into ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation. The two processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process of chemiosmosis.

Free Response

Why is it beneficial for cells to use ATP rather than energy directly from the bonds of carbohydrates? What are the greatest drawbacks to harnessing energy directly from the bonds of several different compounds?

ATP provides the cell with a way to handle energy in an efficient manner. The molecule can be charged, stored, and used as needed. Moreover, the energy from hydrolyzing ATP is delivered as a consistent amount. Harvesting energy from the bonds of several different compounds would result in energy deliveries of different quantities.

Glossary


33 Energy in Living Systems

By the end of this section, you will be able to do the following:

  • Discuss the importance of electrons in the transfer of energy in living systems
  • Explain how ATP is used by cells as an energy source

Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are combinations of oxidation and reduction reactions, which occur at the same time. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions .

Electrons and Energy

The removal of an electron from a molecule (oxidizing it), results in a decrease in potential energy in the oxidized compound. The electron (sometimes as part of a hydrogen atom) does not remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound. The shift of an electron from one compound to another removes some potential energy from the first compound (the oxidized compound) and increases the potential energy of the second compound (the reduced compound). The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of high-energy electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy from food you will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.

Electron Carriers

In living systems, a small class of compounds functions as electron shuttles: they bind and carry high-energy electrons between compounds in biochemical pathways. The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) ((Figure)) is derived from vitamin B3, niacin. NAD + is the oxidized form of the molecule NADH is the reduced form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron). Note that if a compound has an “H” on it, it is generally reduced (e.g., NADH is the reduced form of NAD).

NAD + can accept electrons from an organic molecule according to the general equation:

When electrons are added to a compound, it is reduced. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent, and NAD + is reduced to NADH. When electrons are removed from a compound, it is oxidized. A compound that oxidizes another is called an oxidizing agent. In the above equation, NAD + is an oxidizing agent, and RH is oxidized to R.

Similarly, flavin adenine dinucleotide (FAD + ) is derived from vitamin B2, also called riboflavin. Its reduced form is FADH2. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD + and FAD + are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis in plants.

ATP in Living Systems

A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP). ATP is often called the “energy currency” of the cell, and, like currency, this versatile compound can be used to fill any energy need of the cell. How? It functions similarly to a rechargeable battery.

When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The energy is used to do work by the cell, usually when the released phosphate binds to another molecule, thereby activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium. In this way, the cell performs work, pumping ions against their electrochemical gradients.

ATP Structure and Function

At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group ((Figure)). Ribose is a five-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in the formation of adenosine diphosphate (ADP) the addition of a third phosphate group forms adenosine triphosphate (ATP).

The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called dephosphorylation , releases energy.

Energy from ATP

Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or lysed, and the resulting hydrogen atom (H + ) and a hydroxyl group (OH – ), or hydroxide, are added to the larger molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (Pi), and the release of free energy. To carry out life processes, ATP is continuously broken down into ADP, and like a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group. Water, which was broken down into its hydrogen atom and hydroxyl group (hydroxide) during ATP hydrolysis, is regenerated when a third phosphate is added to the ADP molecule, reforming ATP.

Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come from? In nearly every living thing on Earth, the energy comes from the metabolism of glucose, fructose, or galactose, all isomers with the chemical formula C6H12O6 but different molecular configurations. In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.

Phosphorylation

Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other on the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it allows one of the substrates (such as ATP) and reactants to more readily react with each other in reactions involving ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation. Phosphorylation refers to the addition of the phosphate (

P). This is illustrated by the following generic reaction, in which A and B represent two different substrates:

When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are available for recycling through cell metabolism.

Substrate Phosphorylation

ATP is generated through two mechanisms during the breakdown of glucose. A few ATP molecules are generated (that is, regenerated from ADP) as a direct result of the chemical reactions that occur in the catabolic pathways. A phosphate group is removed from an intermediate reactant in the pathway, and the free energy of the reaction is used to add the third phosphate to an available ADP molecule, producing ATP ((Figure)). This very direct method of phosphorylation is called substrate-level phosphorylation .

Oxidative Phosphorylation

Most of the ATP generated during glucose catabolism, however, is derived from a much more complex process, chemiosmosis, which takes place in mitochondria ((Figure)) within a eukaryotic cell or the plasma membrane of a prokaryotic cell. Chemiosmosis , a process of ATP production in cellular metabolism, is used to generate 90 percent of the ATP made during glucose catabolism and is also the method used in the light reactions of photosynthesis to harness the energy of sunlight. The production of ATP using the process of chemiosmosis is called oxidative phosphorylation because of the involvement of oxygen in the process.

Mitochondrial Disease Physician What happens when the critical reactions of cellular respiration do not proceed correctly? This may happen in mitochondrial diseases, which are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells. In type 2 diabetes, for instance, the oxidation efficiency of NADH is reduced, impacting oxidative phosphorylation but not the other steps of respiration. Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases. Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial diseases, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disorders.

Section Summary

ATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and either ADP or AMP is produced. Energy derived from glucose catabolism is used to convert ADP into ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation. The two processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process of chemiosmosis.


Cellular Respiration

Cellular respiration in the presence of oxygen (aerobic respiration) is the process by which energy-rich organic substrates are broken down into carbon dioxide and water, with the release of a considerable amount of energy in the form of adenosine triphosphate (ATP). Anaerobic respiration breaks down glucose in the absence of oxygen, and produces pyruvate, which is then reduced to lactate or to ethanol and CO2. Anaerobic respiration releases only a small amount of energy (in the form of ATP) from the glucose molecule.

Respiration occurs in three stages. The first stage is glycolysis, which is a series of enzyme-controlled reactions that degrades glucose (a 6-carbon molecule) to pyruvate (a 3-carbon molecule) which is further oxidized to acetylcoenzyme A (acetyl CoA). Amino acids and fatty acids may also be oxidized to acetyl CoA as well as glucose.

In the second stage, acetyl CoA enters the citric acid (Krebs) cycle, where it is degraded to yield energy-rich hydrogen atoms which reduce the oxidized form of the coenzyme nicotinamide adenine dinucleotide (NAD + ) to NADH, and reduce the coenzyme flavin adenine dinucleotide (FAD) to FADH2. (Reduction is the addition of electrons to a molecule, or the gain of hydrogen atoms, while oxidation is the loss of electrons or the addition of oxygen to a molecule.) Also in the second stage of cellular respiration, the carbon atoms of the intermediate metabolic products in the Krebs cycle are converted to carbon dioxide.

The third stage of cellular respiration occurs when the energy-rich hydrogen atoms are separated into protons [H + ] and energy-rich electrons in the electron transport chain. At the beginning of the electron transport chain, the energy-rich hydrogen on NADH is removed from NADH, producing the oxidized coenzyme, NAD + and a proton (H+) and two electrons (e-). The electrons are transferred along a chain of more than 15 different electron carrier molecules (known as the electron transport chain). These proteins are grouped into three large respiratory enzyme complexes, each of which contains proteins that span the mitochondrial membrane, securing the complexes into the inner membrane. Furthermore, each complex in the chain has a greater affinity for electrons than the complex before it. This increasing affinity drives the electrons down the chain until they are transferred all the way to the end where they meet the oxygen molecule, which has the greatest affinity of all for the electrons. The oxygen thus becomes reduced to H2O in the presence of hydrogen ions (protons), which were originally obtained from nutrient molecules through the process of oxidation.

During electron transport, much of the energy represented by the electrons is conserved during a process called oxidative phosphorylation. This process uses the energy of the electrons to phosphorylate (add a phosphate group) adenosine diphosphate (ADP), to form the energy-rich molecule ATP.

Oxidative phosphorylation is driven by the energy released by the electrons as they pass from the hydrogens of the coenzymes down the respiratory chain in the inner membrane of the mitochondrion. This energy is used to pump protons (H + ) across the inner membrane from the matrix to the intermediate space. This sets up a concentration gradient along which substances flow from high to low concentration, while a simultaneous current of OH - flows across the membrane in the opposite direction. The simultaneous opposite flow of positive and negative ions across the mitochondrial membrane sets up an electrochemical proton gradient. The flow of protons down this gradient drives a membrane-bound enzyme, ATP synthetase, which catalyzes the phosphorylation of ADP to ATP.

This highly efficient, energy conserving series of reactions would not be possible in eukaryotic cells without the organelles called mitochondria. Mitochondria are the "powerhouses" of the eukaryotic cells, and are bounded by two membranes, which create two separate compartments: an internal space and a narrow intermembrane space. The enzymes of the matrix include those that catalyze the conversion of pyruvate and fatty acids to acetyl CoA, as well as the enzymes of the Krebs cycle. The enzymes of the respiratory chain are embedded in the inner mitochondrial membrane, which is the site of oxidative phosphorylation and the production of ATP.

In the absence of mitochondria, animal cells would be limited to glycolysis for their energy needs, which releases only a small fraction of the energy potentially available from the glucose.

The reactions of glycolysis require the input of two ATP molecules and produce four ATP molecules for a net gain of only two molecules per molecule of glucose. These ATP molecules are formed when phosphate groups are removed from phosphorylated intermediate products of glycolysis and transferred to ADP, a process called substrate level phosphorylation (synthesis of ATP by direct transfer of a high-energy phosphate group from a molecule in a metabolic pathway to ADP).

In contrast, mitochondria supplied with oxygen produce about 36 molecules of ATP for each molecule of glucose oxidized. Procaryotic cells, such a bacteria, lack mitochondria as well as nuclear membranes. Fatty acids and amino acids when transported into the mitochondria are degraded into the two-carbon acetyl group on acetyl CoA, which then enters the Krebs cycle. In animals, the body stores fattyacids in the form of fats, and glucose in the form of glycogen in order to ensure a steady supply of these nutrients for respiration.

While the Krebs cycle is an integral part of aerobic metabolism, the production of NADH and FADH 2 is not dependent on oxygen. Rather, oxygen is used at the end of the electron transport chain to combine with electrons removed from NADH and FADH2 and with hydrogen ions in the cytosol to produce water.

Although the production of water is necessary to keep the process of electron transport chain in motion, the energy used to make ATP is derived from a different process called chemiosmosis.

Chemiosmosis is a mechanism that uses the proton gradient across the membrane to generate ATP and is initiated by the activity of the electron transport chain. Chemiosmosis represents a link between the chemical and osmotic processes in the mitochondrion that occur during respiration.

The electrons that are transported down the respiratory chain on the mitochondrion's inner membrane release energy that is used to pump protons (H + ) across the inner membrane from the mitochondrial matrix into the intermembrane space. The resulting gradient of protons across the mitochondrial inner membrane creates a backflow of protons back across the membrane. This flow of electrons across the membrane, like a waterfall used to power an electric turbine, drives a membrane-bound enzyme, ATP synthetase. This enzyme catalyzes the phosphorylation of ADP to ATP, which completes the part of cellular respiration called oxidative phosphorylation. The protons, in turn, neutralize the negative charges created by the addition of electrons to oxygen molecules, with the resultant production of water.

Cellular respiration produces three molecules of ATP per pair of electrons in NADH, while the pair of electrons in FADH2 generate two molecules of ATP. This means that 12 molecules of ATP are formed for each acetyl CoA molecule that enters the Krebs cycle and since two acetyl CoA molecules are formed from each molecule of glucose, a total of 24 molecules of ATP are produced from each molecule of this sugar. When added to the energy conserved from the reactions occurring before acetyl CoA is formed, the complete oxidation of a glucose molecule gives a net yield of about 36 ATP molecules. When fats are burned, instead of glucose, the total yield from one molecule of palmitate, a 16-carbon fatty, is 129 ATP.


What is the correct sequence of electron travels during aerobic respiration?

I Write for myself and many other clients. Blog post, Article writing and writing for ProProfs is my daily thing.

Another name for aerobic respiration is cellular respiration this is when there is a conversion of nutrients to energy. The energy produced that is within the cell is called adenosine triphosphate or ATP. The correct sequence of electron travel during aerobic respiration is Food- NADH-electron transport chain &ndash oxygen. The process is explained as follows.

Electrons are picked up from the food we eat by the NADH and FADH through the electron pump, the electron is transferred to the electron transport chain due to the activity of pump electrochemical gradient is created.

ATP is generated from the electrochemical gradient with the help of an enzyme called the ATP synthase. The last process in aerobic respiration occurs when an electron is donated to oxygen to produce water.



Comments:

  1. Rhoecus

    Long ago I was looking for such an answer

  2. Cesario

    This information is not true

  3. Jessey

    It's a pity that I can't speak right now - I'm very busy.But I will return - I will definitely write what I think.

  4. Vugrel

    Oh, how I liked it! :)

  5. Ciodaru

    I'm sorry, but I think you are wrong. I'm sure. Let's discuss this. Email me at PM, we'll talk.

  6. Bradene

    That's the beauty of it!

  7. Oceanus

    There's something about that, and it's a great idea. I am ready to support you.



Write a message