What Happens When Electrons Are Passed From One Atom to a More Electronegative Atom

Empathise the role motility of electrons plays in free energy exchanges in cells

Energy product within a cell involves many coordinated chemical pathways. Well-nigh of these pathways are combinations of oxidation and reduction reactions. Oxidation and reduction occur in tandem. An oxidation reaction strips an electron from an cantlet in a compound, and the addition of this electron to another chemical compound is a reduction reaction. Considering oxidation and reduction commonly occur together, these pairs of reactions are chosen oxidation reduction reactions, orredox reactions.

Learning Objectives

• Chronicle the move of electrons to oxidation-reduction (redox) reactions
• Describe how cells store and transfer free free energy using ATP

 Electrons and Free energy

Allow'south imagine that you are a prison cell. You've just been given a big, juicy glucose molecule, and you'd like to catechumen some of the free energy in this glucose molecule into a more than usable form, 1 that you can utilize to power your metabolic reactions. How can you become almost this? What's the best way for yous to clasp every bit much energy as possible out of that glucose molecule, and to capture this energy in a handy form?

Fortunately for usa, our cells—and those of other living organisms—are splendid at harvesting energy from glucose and other organic molecules, such as fats and amino acids. Here, we'll become through a quick overview of how cells break down fuels, then look at the electron transfer reactions (redox reactions) that are key to this process.

Overview of fuel breakdown pathways

The reactions that let energy to be extracted from molecules such as glucose, fats, and amino acids are called catabolic reactions, meaning that they involve breaking a larger molecule into smaller pieces. For example, when glucose is broken down in the presence of oxygen, it'due south converted into six carbon dioxide molecules and six water molecules. The overall reaction for this process tin be written every bit:

[latex]\text{C}_6\text{H}_{12}\text{O}_6+6\text{O}_2\to{vi}\text{CO}_2+6\text{H}_2\text{O}\,\,\,\,\,\,\,\,\,\,\Delta{One thousand}=-686\text{kcal/mol}[/latex]

This reaction, as written, is only a combustion reaction, similar to what takes identify when you fire a piece of woods in a fireplace or gasoline in an engine. Does this mean that glucose is continually combusting inside of your cells? Thankfully, non quite! The combustion reaction describes the overall process that takes place, but within of a cell, this process is broken down into many smaller steps. Energy independent in the bonds of glucose is released in small bursts, and some of it tin be captured in the form of adenosine triphosphate (ATP), a minor molecule that is used to ability reactions in the cell. Much of the energy from glucose is nevertheless lost as estrus, but enough is captured to keep the metabolism of the cell running.

Every bit a glucose molecule is gradually broken downwardly, some of the breakdowns steps release free energy that is captured straight as ATP. In these steps, a phosphate group is transferred from a pathway intermediate straight to ADP, a process known every bit substrate-level phosphorylation. Many more steps, nonetheless, produce ATP in an indirect style. In these steps, electrons from glucose are transferred to small-scale molecules known equally electron carriers. The electron carriers take the electrons to a grouping of proteins in the inner membrane of the mitochondrion, chosen the electron transport chain. As electrons move through the electron ship concatenation, they become from a higher to a lower energy level and are ultimately passed to oxygen (forming h2o). Energy released in the electron transport chain is captured as a proton gradient, which powers production of ATP by a membrane protein chosen ATP synthase. This process is known as oxidative phosphorylation. A simplified diagram of oxidative and substrate-level phosphorylation is shown below.

Figure 1. Paradigm modified from "Etc4" by Fvasconcellos (public domain).

When organic fuels similar glucose are broken downward using an electron transport chain that ends with oxygen, the breakdown process is known as aerobic respiration (aerobic = oxygen-requiring). Nigh eukaryotic cells, likewise as many bacteria and other prokaryotes, can acquit out aerobic respiration. Some prokaryotes have pathways similar to aerobic respiration, merely with a different inorganic molecule, such every bit sulfur, substituted for oxygen. These pathways are not oxygen-dependent, so the breakup process is called anaerobic respiration(anaerobic = non-oxygen-requiring). Officially, both processes are examples of cellular respiration, the breakdown of downwardly organic fuels using an electron transport chain. However, cellular respiration is commonly used as a synonym for aerobic respiration, and we'll use information technology that style here^[1].

Redox Reactions

Cellular respiration involves many reactions in which electrons are passed from one molecule to another. Reactions involving electron transfers are known as oxidation-reduction reactions (or redox reactions), and they play a central role in the metabolism of a cell. In a redox reaction, 1 of the reacting molecules loses electrons and is said to exist oxidized, while some other reacting molecule gains electrons (the ones lost by the kickoff molecule) and is said to existreduced. You can recall what oxidation and reduction hateful with the handy mnemonic "LEO goes GER": Lose Electrons, Oxidized; Gain Electrons,Reduced. The formation of magnesium chloride is 1 simple case of a redox reaction:

[latex]\text{Mg}+\text{Cl}_2\to\text{Mg}^{two+}+2\text{Cl}^{-}[/latex]

In this reaction, the magnesium cantlet loses two electrons, then it is oxidized. These two electrons are accustomed by chlorine, which is reduced. The cantlet or molecule that donates electrons (in this case, magnesium) is chosen thereducing agent, because its donation of electrons allows another molecule to become reduced. The atom or molecule that accepts the electrons (in this case, chlorine) is known as the oxidizing agent, because its acceptance of electrons allows the other molecule to go oxidized.

Redox reactions with carbon-containing molecules

When a reaction involves the germination of ions, as in the instance with magnesium and chlorine higher up, it's relatively easy to see that electrons are being transferred. Not all redox reactions involve the complete transfer of electrons, though, and this is particularly true of reactions important in cellular metabolism. Instead, some redox reactions only change the corporeality of electron density on a particular atom by altering how it shares electrons in covalent bonds. As an example, let'southward consider the combustion of butane:

Figure ii. Butane: [latex]2\text{C}_4\text{H}_{10}+13\text{O}_2\to8\text{CO}_2+x\text{H}_2\text{O}[/latex]

What's the electron-sharing situation at the kickoff of the reaction? In butane, the carbon atoms are all bonded to other carbons and hydrogens. In [latex]\text{C}-\text{C}[/latex] bonds, electrons are shared equally, and in [latex]\text{C}-\text{H}[/latex] bonds, the [latex]\text{C}[/latex] cantlet has a very slight negative charge (since information technology's a bit more electronegative than hydrogen). Similarly, when oxygens are bonded to one some other in [latex]\text{O}_2[/latex], first subscript, 2, finish subscript, electrons are shared very equally. After the reaction, however, the electron-sharing picture looks quite different. Oxygen is much more electronegative than carbon, so the in the [latex]\text{C}=\text{O}[/latex] bonds of carbon dioxide, oxygen volition "hog" the bond electrons. In the [latex]\text{O}-\text{H}[/latex] bonds of water, oxygen will similarly pull electrons away from the hydrogen atoms. Thus, relative to its state before the reaction, carbon has lost electron density (because oxygen is now hogging its electrons), while oxygen has gained electron density (because it tin at present sus scrofa electrons shared with other elements). Information technology's thus reasonable to say that carbon was oxidized during this reaction, while oxygen was reduced. (Hydrogen arguably loses a little electron density too, though its electrons were beingness hogged to some degree in either case.) Biologists frequently refer to whole molecules, rather than individual atoms, every bit being reduced or oxidized; thus, we can say that butane—the source of the carbons—is oxidized, while molecular oxygen—the source of the oxygen atoms—is reduced.

It's important to sympathise that oxidation and reduction reactions are fundamentally most the transfer of electrons. In the context of biology, nonetheless, you may find information technology helpful to use the gain or loss of H and O atoms as a proxy for the transfer of electrons. Every bit a general rule of thumb, if a carbon-containing molecule gains H atoms or loses O atoms during a reaction, it's likely been reduced (gained electrons). Conversely, if it loses H atoms or gains O atoms, it'south probably been oxidized (lost electrons). For example, let'southward go back to the reaction for glucose breakdown,[latex]\text{C}_6\text{H}_{12}\text{O}_6+6\text{O}_2\to{half-dozen}\text{CO}_2+six\text{H}_2\text{O}[/latex]. In glucose, carbon is associated with H atoms, while in carbon dioxide, no Hs are nowadays. Thus, we would predict that glucose is oxidized in this reaction.

We can confirm this if we expect at the actual electron shifts involved, every bit in the video below:

Free energy in Redox Reactions

Effigy 3. Click on the epitome for a larger view. Image based on similar diagram past Ryan Gutierrez.

Similar other chemic reactions, redox reactions involve a free free energy change. Reactions that motion the system from a higher to a lower energy state are spontaneous and release energy, while those that do the contrary require an input of free energy. In redox reactions, energy is released when an electron loses potential energy every bit a upshot of the transfer. Electrons have more potential energy when they are associated with less electronegative atoms (such every bit C or H), and less potential energy when they are associated with a more electronegative cantlet (such as O). Thus, a redox reaction that moves electrons or electron density from a less to a more electronegative atom will be spontaneous and release energy. For instance, the combustion of butane (above) releases energy because there is a net shift of electron density abroad from carbon and hydrogen and onto oxygen. If you lot've heard it said that molecules like glucose have "high-energy" electrons, this is a reference to the relatively high potential energy of the electrons in their [latex]\text{C}-\text{C}[/latex] and [latex]\text{C}-\text{H}[/latex] bonds.

Quite a bit of energy can be released when electrons in [latex]\text{C}-\text{C}[/latex] and [latex]\text{C}-\text{H}[/latex] bonds are shifted to oxygen. In a jail cell, still, it's non a great idea to release all that energy at once in a combustion reaction. Instead, cells harvest energy from glucose in a controlled manner, capturing every bit much of it as possible in the grade of ATP. This is accomplished by oxidizing glucose in a gradual, rather than an explosive, sort of style. There are 2 important ways in which this oxidation is gradual:

• Rather than pulling all the electrons off of glucose at the same time, cellular respiration strips them abroad in pairs. The redox reactions that remove electron pairs from glucose transfer them to small molecules chosen electron carriers.
• The electron carriers deposit their electrons in the electron send chain, a series of proteins and organic molecules in the inner mitochondrial membrane. Electrons are passed from one component to the next in a series of energy-releasing steps, allowing energy to exist captured in the class of an electrochemical slope.

We'll look at both redox carriers and the electron transport concatenation in more detail below.

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 non remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second chemical compound, reducing the 2nd chemical compound. The shift of an electron from one compound to some other removes some potential energy from the beginning chemical compound (the oxidized compound) and increases the potential energy of the 2nd compound (the reduced compound). The transfer of electrons between molecules is important because about of the free energy stored in atoms and used to fuel cell functions is in the class of loftier-energy electrons. The transfer of energy in the form of electrons allows the cell to transfer and use energy in an incremental fashion—in pocket-sized packages rather than in a single, destructive flare-up. This module focuses on the extraction of energy from food; you will see that as you track the path of the transfers, y'all are tracking the path of electrons moving through metabolic pathways.

Electron Carriers

Electron carriers, sometimes called electron shuttles, are small organic molecules that readily cycle between oxidized and reduced forms and are used to send electrons during metabolic reactions. There are 2 electron carriers that play particularly important roles during cellular respiration: NAD⁺  (nicotinamide adenine dinucleotide, shown below) and FAD (flavin adenine dinucleotide). Both NAD⁺ and FAD tin serve equally oxidizing agents, accepting a pair of electrons, along with 1 or more than protons, to switch to their reduced forms. NAD⁺  accepts two electrons and i H⁺ to become NADH, while FAD accepts two electrons and two H⁺ to get FADH₂. NAD⁺ is the principal electron carrier used during cellular respiration, with FAD participating in just one (or 2 sometimes two) reactions.

Figure 4. The oxidized form of the electron carrier (NAD⁺) is shown on the left and the reduced form (NADH) is shown on the right. The nitrogenous base in NADH has 1 more hydrogen ion and two more than electrons than in NAD⁺.

As shown in the image above, NAD⁺ is a minor organic molecule whose construction includes the RNA nucleotide adenine. (FAD is a similar type of molecule, although its functional groups are different.) Both molecules are B vitamin derivatives, with NAD⁺ produced from niacin and FAD produced from riboflavin. NAD⁺  and FAD are coenzymes, organic molecules that serve as helpers during enzyme-catalyzed reactions, and they receive electrons and protons every bit part of these reactions. Specifically, both NAD⁺  and FAD serve as cofactors for enzymes called dehydrogenases, which remove 1 or more hydrogen atoms from their substrates.

The Electron Transport Chain

In their reduced forms, NADH and FADH₂ deport electrons to the electron send chain in the inner mitochondrial membrane. They deposit their electrons at or near the showtime of the transport chain, and the electrons are then passed forth from one poly peptide or organic molecule to the side by side in a predictable series of steps. Importantly, the movement of electrons through the transport chain is energetically "downhill," such that free energy is released at each pace. In redox terms, this means that each member of the electron transport concatenation is more electronegative (electron-hungry) that the one before it, and less electronegative than the i after^[2]. NAD⁺, which deposits its electrons at the showtime of the chain as NADH, is the least electronegative, while oxygen, which receives the electrons at the end of the chain (forth with H⁺) to form water, is the most electronegative. As electrons trickle "downhill" through the transport concatenation, they release energy, and some of this energy is captured in the form of an electrochemical gradient and used to make ATP.

ATP in Living Systems

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

When ATP is broken downwardly, commonly past the removal of its final phosphate group, energy is released. The energy is used to do work by the cell, unremarkably by the released phosphate binding to another molecule, activating it. For instance, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins. Recall the active ship work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral poly peptide that functions equally the pump, irresolute its affinity for sodium and potassium. In this manner, the cell performs work, pumping ions against their electrochemical gradients.

ATP Structure and Role

Figure v. ATP (adenosine triphosphate) has three phosphate groups that tin be removed past hydrolysis to grade ADP (adenosine diphosphate) or AMP (adenosine monophosphate).The negative charges on the phosphate group naturally repel each other, requiring free energy to bond them together and releasing energy when these bonds are broken.

At the centre 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 (Effigy 5). Ribose is a v-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this cadre molecule results in the formation of adenosine diphosphate (ADP); the addition of a 3rd 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 bundled in serial, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or 2 phosphate groups from ATP, a procedure calleddephosphorylation, releases free energy.

Free energy from ATP

Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, h2o is split, or lysed, and the resulting hydrogen cantlet (H⁺) and a hydroxyl group (OH^–) are added to the larger molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (P _i ), and the release of complimentary free energy. To carry out life processes, ATP is continuously broken down into ADP, and similar a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a 3rd phosphate grouping. Water, which was broken down into its hydrogen atom and hydroxyl group during ATP hydrolysis, is regenerated when a tertiary 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 up from? In nearly every living affair on earth, the energy comes from the metabolism of glucose. In this mode, ATP is a direct link between the express fix 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 demark to several substrates that react with each other on the enzyme, forming an intermediate complex. An intermediate circuitous is a temporary structure, and information technology allows one of the substrates (such equally 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 3rd phosphate group, with its energy, to the substrate, a process called phosphorylation.Phosphorylation refers to the add-on of the phosphate (~P). This is illustrated by the following generic reaction:

A + enzyme + ATP → [A − enzyme − ~P] → B + enzyme + ADP + phosphate ion

When the intermediate complex breaks autonomously, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a gratuitous phosphate ion are released into the medium and are available for recycling through prison 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 directly upshot of the chemical reactions that occur in the catabolic pathways. A phosphate group is removed from an intermediate reactant in the pathway, and the gratis energy of the reaction is used to add together the third phosphate to an available ADP molecule, producing ATP (Figure half dozen). This very straight method of phosphorylation is calledsubstrate-level phosphorylation.

Figure half-dozen. In phosphorylation reactions, the gamma phosphate of ATP is attached to a protein.

Oxidative Phosphorylation

Most of the ATP generated during glucose catabolism, nevertheless, is derived from a much more complex procedure, chemiosmosis, which takes place in mitochondria (Figure 7) within a eukaryotic jail cell or the plasma membrane of a prokaryotic cell.

Figure 7. The mitochondria (Credit: modification of piece of work by Mariana Ruiz Villareal)

Chemiosmosis, a process of ATP production in cellular metabolism, is used to generate ninety 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 procedure of chemiosmosis is called oxidative phosphorylation because of the involvement of oxygen in the procedure.

Mitochondrial Disease Dr.

What happens when the disquisitional reactions of cellular respiration practise non go along correctly? Mitochondrial diseases are genetic disorders of metabolism. Mitochondrial disorders tin can arise from mutations in nuclear or mitochondrial DNA, and they outcome in the production of less energy than is normal in trunk cells. In type 2 diabetes, for instance, the oxidation efficiency of NADH is reduced, impacting oxidative phosphorylation but non 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 training for this profession requires a higher didactics, followed by medical schoolhouse with a specialization in medical genetics. Medical geneticists can be board certified past the American Board of Medical Genetics and keep to become associated with professional person organizations devoted to the report of mitochondrial diseases, such as the Mitochondrial Medicine Society and the Club for Inherited Metabolic Disease.

In Summary: ATP in Living Systems

ATP functions as the free energy currency for cells. It allows the prison cell to shop energy briefly and ship information technology within the jail cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with 3 phosphates fastened. As ATP is used for free energy, a phosphate group or two are discrete, and either ADP or AMP is produced. Free 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 procedure chosen phosphorylation. The 2 processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process of chemiosmosis.

Bank check Your Understanding

Reply the question(s) below to see how well you lot empathise the topics covered in the previous section. This curt quiz doesnot count toward your grade in the class, and you lot can retake it an unlimited number of times.

Use this quiz to bank check your understanding and decide whether to (1) written report the previous section further or (two) move on to the next section.

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