In this tutorial, we have seen that nonspontaneous reactions in the body occur by coupling them with a very spontaneous reaction usually the ATP reaction shown in Equation 3. But we have not yet answered the question: by what mechanism are these reactions coupled? Every day your body carries out many nonspontaneous reactions.
As discussed earlier, if a nonspontaneous reaction is coupled to a spontaneous reaction, as long as the sum of the free energies for the two reactions is negative, the coupled reactions will occur spontaneously. How is this coupling achieved in the body? Living systems couple reactions in several ways, but the most common method of coupling reactions is to carry out both reactions on the same enzyme.
Consider again the phosphorylation of glycerol Equations Glycerol is phosphorylated by the enzyme glycerol kinase, which is found in your liver. The product of glycerol phosporylation, glycerolphosphate Equation 2 , is used in the synthesis of phospholipids. Glycerol kinase is a large protein comprised of about amino acids.
X-ray crystallography of the protein shows us that there is a deep groove or cleft in the protein where glycerol and ATP attach see Figure 6, below. Because the enzyme holds the ATP and the glycerol in place, the phosphate can be transferred directly from the ATP to glycerol. Instead of two separate reactions where ATP loses a phosphate Equation 3 and glycerol picks up a phosphate Equation 2 , the enzyme allows the phosphate to move directly from ATP to glycerol Equation 4.
The coupling in oxidative phosphorylation uses a more complicated and amazing! This is a schematic representation of ATP and glycerol bound attached to glycerol kinase. The enzyme glycerol kinase is a dimer consists of two identical subuits. There is a deep cleft between the subunits where ATP and glycerol bind.
Since the ATP and phosphate are physically so close together when they are bound to the enzyme, the phosphate can be transferred directly from ATP to glycerol. Hence, the processes of ATP losing a phosphate spontaneous and glycerol gaining a phosphate nonspontaneous are linked together as one spontaneous process.
Neglecting any differences in difficulty synthesizing or accessing these molecules by biological systems, rank the molecules in order of their efficiency as a free-energy currency i.
In order to couple the redox and phosphorylation reactions needed for ATP synthesis in the body, there must be some mechanism linking the reactions together. In cells, this is accomplished through an elegant proton-pumping system that occurs inside special double-membrane-bound organelles specialized cellular components known as mitochondria. A number of proteins are required to maintain this proton-pumping system and catalyze the oxidative and phosphorylation reactions.
There are three key steps in this process:. Note: Steps a and b show cytochrome oxidase, the final electron-carrier protein in the electron-transport chain described above. When this protein accepts an electron green from another protein in the electron-transport chain, an Fe III ion in the center of a heme group purple embedded in the protein is reduced to Fe II.
Cells use a proton-pumping system made up of proteins inside the mitochondria to generate ATP. Before we examine the details of ATP synthesis, we shall step back and look at the big picture by exploring the structure and function of the mitochondria, where oxidative phosphorylation occurs. The mitochondria Figure 8 are where the oxidative-phosphorylation reactions occur.
Mitochondria are present in virtually every cell of the body. They contain the enzymes required for the citric-acid cycle the last steps in the breakdown of glucose , oxidative phosphorylation, and the oxidation of fatty acids. This is a schematic diagram showing the membranes of the mitochondrion.
The purple shapes on the inner membrane represent proteins, which are described in the section below. An enlargement of the boxed portion of the inner membrane in this figure is shown in Figure 8, below. The mitochondrial membranes are crucial for this organelle's role in oxidative phosphorylation. As shown in Figure 8, mitochondria have two membranes, an inner and an outer membrane. The outer membrane is permeable to most small molecules and ions, because it contains large protein channels called porins.
The inner membrane is impermeable to most ions and polar molecules. The inner membrane is the site of oxidative phosphorylation. Recall the discussion of protein channels in the " Maintaining the Body's Chemistry: Dialysis in the Kidneys " Tutorial. As shown in Figure 8, inside the inner membrane is a space known as the matrix ; the space between the two membranes is known as the intermembrane space.
This charge difference is used to provide free energy G for the phosphorylation reaction Equation 8. Electrons are not transferred directly from NADH to O 2 , but rather pass through a series of intermediate electron carriers in the inner membrane of the mitochondrion. This allows something very important to occur: the pumping of protons across the inner membrane of the mitochondrion.
As we shall see, it is this proton pumping that is ultimately responsible for coupling the oxidation-reduction reaction to ATP synthesis. Two major types of mitochondrial proteins see Figure 9, below are required for oxidative phosphorylation to occur. Both classes of proteins are located in the inner mitochondrial membrane.
The electron carriers can be divided into three protein complexes NADH-Q reductase 1 , cytochrome reductase 3 , and cytochrome oxidase 5 that pump protons from the matrix to the intermembrane space, and two mobile carriers ubiquinone 2 and cytochrome c 4 that transfer electrons between the three proton-pumping complexes. Gold numbers refer to the labels on each protein in Figure 9, below. Because electrons move from one carrier to another until they are finally transferred to O 2 , the electron carriers shown in Figure 9,below are said to form an electron-transport chain.
Figure 9, below, is a schematic representation of the proteins involved in oxidative phosphorylation. To see an animation of oxidative phosphorylation, click on "View the Movie.
This is a schematic diagram illustrating the transfer of electrons from NADH, through the electron carriers in the electron transport chain, to molecular oxygen. In plant and other photosynthetic cells, chloroplasts also have an electron transport chain that harvests solar energy. Even though they do not contain mithcondria or chloroplatss, prokaryotes have other kinds of energy-yielding electron transport chains within their plasma membranes that also generate energy. When energy is abundant, eukaryotic cells make larger, energy-rich molecules to store their excess energy.
The resulting sugars and fats — in other words, polysaccharides and lipids — are then held in reservoirs within the cells, some of which are large enough to be visible in electron micrographs. Animal cells can also synthesize branched polymers of glucose known as glycogen , which in turn aggregate into particles that are observable via electron microscopy. A cell can rapidly mobilize these particles whenever it needs quick energy.
Athletes who "carbo-load" by eating pasta the night before a competition are trying to increase their glycogen reserves. Under normal circumstances, though, humans store just enough glycogen to provide a day's worth of energy. Plant cells don't produce glycogen but instead make different glucose polymers known as starches , which they store in granules. In addition, both plant and animal cells store energy by shunting glucose into fat synthesis pathways.
One gram of fat contains nearly six times the energy of the same amount of glycogen, but the energy from fat is less readily available than that from glycogen.
Still, each storage mechanism is important because cells need both quick and long-term energy depots. Fats are stored in droplets in the cytoplasm; adipose cells are specialized for this type of storage because they contain unusually large fat droplets.
Humans generally store enough fat to supply their cells with several weeks' worth of energy Figure 7. Figure 7: Examples of energy storage within cells.
A In this cross section of a rat kidney cell, the cytoplasm is filled with glycogen granules, shown here labeled with a black dye, and spread throughout the cell G , surrounding the nucleus N.
B In this cross-section of a plant cell, starch granules st are present inside a chloroplast, near the thylakoid membranes striped pattern. C In this amoeba, a single celled organism, there is both starch storage compartments S , lipid storage L inside the cell, near the nucleus N. Qian H. Letcher P. A Bamri-Ezzine, S. All rights reserved. This page appears in the following eBook. Aa Aa Aa. Cell Energy and Cell Functions. Figure 3: The release of energy from sugar.
Compare the stepwise oxidation left with the direct burning of sugar right. Figure 5: An ATP molecule. ATP consists of an adenosine base blue , a ribose sugar pink and a phosphate chain. Figure 6: Metabolism in a eukaryotic cell: Glycolysis, the citric acid cycle, and oxidative phosphorylation.
Glycolysis takes place in the cytoplasm. Cells need energy to accomplish the tasks of life. Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, eukaryotic cells make energy-rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation.
Rodwell, et al. Harper's Illustrated Biochemistry, 31e. McGraw Hill; Accessed November 11, McGraw Hill.
Download citation file: RIS Zotero. Reference Manager. Autosuggest Results. Describe the reaction of pyruvate dehydrogenase and its regulation. Explain how inhibition of pyruvate metabolism leads to lactic acidosis. Subscribe: Institutional or Individual. Username Error: Please enter User Name. Password Error: Please enter Password. Forgot Password?
Pop-up div Successfully Displayed This div only appears when the trigger link is hovered over. 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 down 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. These two electrons are accepted by chlorine, which is reduced.
The atom or molecule that donates electrons in this case, magnesium is called the reducing 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 become oxidized.
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 simply change the amount of electron density on a particular atom by altering how it shares electrons in covalent bonds.
Figure 2. In butane, the carbon atoms are all bonded to other carbons and hydrogens. After the reaction, however, the electron-sharing picture looks quite different. 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 can now hog electrons shared with other elements.
Hydrogen arguably loses a little electron density too, though its electrons were being hogged to some degree in either case. Biologists often refer to whole molecules, rather than individual atoms, as 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.
In the context of biology, however, you may find it helpful to use the gain or loss of H and O atoms as a proxy for the transfer of electrons. In glucose, carbon is associated with H atoms, while in carbon dioxide, no Hs are present.
Thus, we would predict that glucose is oxidized in this reaction. Figure 3. Click on the image for a larger view. Image based on similar diagram by Ryan Gutierrez. Like other chemical reactions, redox reactions involve a free energy change. Reactions that move the system from a higher to a lower energy state are spontaneous and release energy, while those that do the opposite require an input of energy.
In redox reactions, energy is released when an electron loses potential energy as a result of the transfer. Electrons have more potential energy when they are associated with less electronegative atoms such as C or H , and less potential energy when they are associated with a more electronegative atom 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 away from carbon and hydrogen and onto oxygen.
Instead, cells harvest energy from glucose in a controlled fashion, capturing as much of it as possible in the form of ATP. This is accomplished by oxidizing glucose in a gradual, rather than an explosive, sort of way. There are two important ways in which this oxidation is gradual:.
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.
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