Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next pathway in glucose catabolism. Like the conversion of pyruvate to acetyl CoA, the citric acid cycle in eukaryotic cells takes place in the matrix of the mitochondria.
Unlike glycolysis, the citric acid cycle is a closed loop: The last part of the pathway regenerates the compound used in the first step. Part of this is considered an aerobic pathway oxygen-requiring because the NADH and FADH 2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. If oxygen is not present, this transfer does not occur. Two carbon atoms come into the citric acid cycle from each acetyl group. Two carbon dioxide molecules are released on each turn of the cycle; however, these do not contain the same carbon atoms contributed by the acetyl group on that turn of the pathway.
The two acetyl-carbon atoms will eventually be released on later turns of the cycle; in this way, all six carbon atoms from the original glucose molecule will be eventually released as carbon dioxide. It takes two turns of the cycle to process the equivalent of one glucose molecule. These high-energy carriers will connect with the last portion of aerobic respiration to produce ATP molecules. One ATP or an equivalent is also made in each cycle. Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is both anabolic and catabolic.
You have just read about two pathways in glucose catabolism—glycolysis and the citric acid cycle—that generate ATP. Most of the ATP generated during the aerobic catabolism of glucose, however, is not generated directly from these pathways.
Rather, it derives from a process that begins with passing electrons through a series of chemical reactions to a final electron acceptor, oxygen. These reactions take place in specialized protein complexes located in the inner membrane of the mitochondria of eukaryotic organisms and on the inner part of the cell membrane of prokaryotic organisms.
The energy of the electrons is harvested and used to generate a electrochemical gradient across the inner mitochondrial membrane.
The potential energy of this gradient is used to generate ATP. The entirety of this process is called oxidative phosphorylation. The electron transport chain Figure 4.
Oxygen continuously diffuses into plants for this purpose. In animals, oxygen enters the body through the respiratory system.
Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where oxygen is the final electron acceptor and water is produced. There are four complexes composed of proteins, labeled I through IV in Figure 4. The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of prokaryotes.
In each transfer of an electron through the electron transport chain, the electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions across the inner mitochondrial membrane into the intermembrane space, creating an electrochemical gradient.
The reactions that allow 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.
The overall reaction for this process can be written as:. This reaction, as written, is simply a combustion reaction, similar to what takes place when you burn a piece of wood in a fireplace or gasoline in an engine. Does this mean that glucose is continually combusting inside of your cells? Thankfully, not quite! The combustion reaction describes the overall process that takes place, but inside of a cell, this process is broken down into many smaller steps.
Energy contained in the bonds of glucose is released in small bursts, and some of it can be captured in the form of adenosine triphosphate ATP , a small molecule that is used to power reactions in the cell. Much of the energy from glucose is still lost as heat, but enough is captured to keep the metabolism of the cell running. As a glucose molecule is gradually broken down, some of the breakdowns steps release energy that is captured directly as ATP. In these steps, a phosphate group is transferred from a pathway intermediate straight to ADP, a process known as substrate-level phosphorylation.
Many more steps, however, produce ATP in an indirect way. In these steps, electrons from glucose are transferred to small molecules known as electron carriers. The electron carriers take the electrons to a group of proteins in the inner membrane of the mitochondrion, called the electron transport chain. As electrons move through the electron transport chain, they go from a higher to a lower energy level and are ultimately passed to oxygen forming water.
Energy released in the electron transport chain is captured as a proton gradient, which powers production of ATP by a membrane protein called ATP synthase. This process is known as oxidative phosphorylation. A simplified diagram of oxidative and substrate-level phosphorylation is shown below.
Most eukaryotic cells, as well as many bacteria and other prokaryotes, can carry out aerobic respiration. Some prokaryotes have pathways similar to aerobic respiration, but with a different inorganic molecule, such as sulfur, substituted for oxygen.
Officially, both processes are examples of cellular respiration , the breakdown of organic fuels using an electron transport chain. 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, one of the reacting molecules loses electrons and is said to be oxidized , while another reacting molecule gains electrons the ones lost by the first molecule and is said to be reduced.
The formation of magnesium chloride is one simple example of a redox reaction:. In this reaction, the magnesium atom loses two electrons, so it is oxidized. Figure 4. FAD is a similar type of molecule, although its functional groups are different. They deposit their electrons at or near the beginning of the transport chain, and the electrons are then passed along from one protein or organic molecule to the next in a predictable series of steps. In redox terms, this means that each member of the electron transport chain is more electronegative electron-hungry that the one before it, and less electronegative than the one after [2].
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.
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 by the released phosphate binding to another molecule, 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. Figure 5. ATP adenosine triphosphate has three phosphate groups that can be removed by hydrolysis to form ADP adenosine diphosphate or AMP adenosine monophosphate. The negative charges on the phosphate group naturally repel each other, requiring energy to bond them together and releasing energy when these bonds are broken.
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 5.
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. The release of one or two phosphate groups from ATP, a process called dephosphorylation , releases energy. Hydrolysis is the process of breaking complex macromolecules apart.
Water, which was broken down into its hydrogen atom and hydroxyl group 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. 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.
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.
This is illustrated by the following generic reaction:. 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. 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 6. This very direct method of phosphorylation is called substrate-level phosphorylation. Figure 6. In phosphorylation reactions, the gamma phosphate of ATP is attached to a protein. Most of the ATP generated during glucose catabolism, however, is derived from a much more complex process, chemiosmosis, which takes place in mitochondria Figure 7 within a eukaryotic cell or the plasma membrane of a prokaryotic cell.
Figure 7. The mitochondria Credit: modification of work by Mariana Ruiz Villareal. 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.
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