Whereas building and maintaining the turbine are energy-dependent processes, the flow of water works with gravity so long as there is water upstream. Similarly, although producing and maintaining the mitochondrial enzymes, cell membranes, and cofactors are energy-dependent processes, fuel oxidation and respiratory electron flow are exothermic that is, they liberate heat.
Electrons flow in cellular respiration precisely as they flow in other electrical circuits, toward acceptors of higher electron affinity. Just as the cost of turning a water turbine is paid for by water flowing downriver, the cost of pumping protons is paid for by electrons flowing from higher-energy states to lower-energy states. See also: Proton. Small stepwise increases in electron affinity are manifested by small drops in electron free energy along the respiratory electron chain.
Damage produced by reactive oxygen species ROS is an obvious cost of aerobic metabolism, and ROS in the form of hydrogen peroxide H 2 O 2 and phospholipid hydroperoxides are controlled by glutathione reductases and glutathione peroxidases, which depend on NADPH as the reducing agent to reactivate oxidized glutathione. Protons return through NNT in order to drive this catalytic process in a manner that is directly competitive with production of ATP and heat Fig.
See also: Free energy ; Free radical ; Hydrogen peroxide ; Superoxide chemistry. Respiratory demands vary by type of fuel, by the balance between catabolism and anabolism in which a cell is engaged, and by the degree to which the cell produces cytosolic NADPH anaerobically through processes such as the pentose phosphate pathway in which glucose is metabolized or transformed into NADPH.
See also: Citric acid cycle. In contrast to glucose oxidation, the complete oxidation of triglycerides neutral lipids consisting of three fatty acyl chains esterified to a glycerol backbone is almost entirely aerobic Fig.
The ratio of fatty-acid carbons to glycerol carbons in a triglyceride provides an indication of how aerobically demanding triglyceride oxidation is. Considering that the cytosolic NADH can be effectively reoxidized aerobically via the malate-aspartate shuttle or the glycerolphosphate shuttle and that the glycerol-derived pyruvate can also be oxidized in mitochondria, complete oxidation of a typical triglyceride can demand sufficient oxygen to reoxidize approximately mitochondrial NADH and FADH 2 equivalents.
See also: Lipid ; Lipid metabolism ; Triglyceride triacylglycerol. It should also be pointed out that amino acid oxidation is intermediate in its O 2 requirement between glycolysis and mitochondrial fatty-acid oxidation because some reduced cofactors are produced in the cytosol and others are produced in the mitochondria. See also: Amino acid ; Amino acid metabolism.
The other consideration that guides the magnitude of a cellular O 2 requirement is the degree to which a cell is busy with reactions that demand the hydride carried on NADH and NADPH and whether reducing equivalents can be produced cytosolically.
Unlike a fireplace, whose purpose is to combust fuel fully to generate heat Fig. Thus, the logic of life is such that the relatively low energy electrons carried on cytochrome C in the inner mitochondrial membrane have much less power to do meaningful work than the electrons carried on cytosolic NADPH. Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can catalyze both forward and reverse reactions.
Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and two NADH molecules for its use. If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose.
Mature mammalian red blood cells are not capable of aerobic respiration —the process in which organisms convert energy in the presence of oxygen—and glycolysis is their sole source of ATP. If glycolysis is interrupted, these cells lose their ability to maintain their sodium-potassium pumps, and eventually, they die. The last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is not available in sufficient quantities.
In this situation, the entire glycolysis pathway will proceed, but only two ATP molecules will be made in the second half. Thus, pyruvate kinase is a rate-limiting enzyme for glycolysis.
Glycolysis is the first pathway used in the breakdown of glucose to extract energy. It was probably one of the earliest metabolic pathways to evolve and is used by nearly all of the organisms on earth. Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars.
ATP is invested in the process during this half to energize the separation. Two ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation during the second half. If oxygen is available, aerobic respiration will go forward.
In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration. There, pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called coenzyme A CoA. The resulting compound is called acetyl CoA.
CoA is made from vitamin B5, pantothenic acid. 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 stage of the pathway in glucose catabolism.
In order for pyruvate which is the product of glycolysis to enter the Citric Acid Cycle the next pathway in cellular respiration , it must undergo several changes. The conversion is a three-step process Figure 5. Figure 5. Upon entering the mitochondrial matrix, a multi-enzyme complex converts pyruvate into acetyl CoA. In the process, carbon dioxide is released and one molecule of NADH is formed.
A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase. This is the first of the six carbons from the original glucose molecule to be removed.
This step proceeds twice remember: there are two pyruvate molecules produced at the end of glycolysis for every molecule of glucose metabolized; thus, two of the six carbons will have been removed at the end of both steps.
An acetyl group is transferred to conenzyme A, resulting in acetyl CoA. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA. Note that during the second stage of glucose metabolism, whenever a carbon atom is removed, it is bound to two oxygen atoms, producing carbon dioxide, one of the major end products of cellular respiration.
In the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups; this pathway will harvest the remainder of the extractable energy from what began as a glucose molecule. This single pathway is called by different names, but we will primarily call it the Citric Acid Cycle.
In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism.
During the conversion of pyruvate into the acetyl group, a molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide accounts for two conversion of two pyruvate molecules of the six carbons of the original glucose molecule. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized.
Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs. Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria. This single pathway is called by different names: the citric acid cycle for the first intermediate formed—citric acid, or citrate—when acetate joins to the oxaloacetate , the TCA cycle since citric acid or citrate and isocitrate are tricarboxylic acids , and the Krebs cycle , after Hans Krebs, who first identified the steps in the pathway in the s in pigeon flight muscles.
Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion. Unlike glycolysis, the citric acid cycle is a closed loop: The last part of the pathway regenerates the compound used in the first step.
This is considered an aerobic pathway because the NADH and FADH 2 produced must transfer their electrons to the next pathway in the system, which will use oxygen.
If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur. Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.
Figure 6. In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle. Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants. Prior to the start of the first step, pyruvate oxidation must occur.
Then, the first step of the cycle begins: This is a condensation step, combining the two-carbon acetyl group with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available.
If ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases. In step two, citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate.
Steps 3 and 4. CoA binds the succinyl group to form succinyl CoA. In step five, a phosphate group is substituted for coenzyme A, and a high-energy bond is formed. This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP.
There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found.
One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver. Chapter 6: Cell Signaling. Chapter 7: Metabolism. Chapter 9: Photosynthesis. Chapter Cell Cycle and Division. Chapter Meiosis. Chapter Classical and Modern Genetics. Chapter Gene Expression.
Chapter Biotechnology. Chapter Viruses. Chapter Nutrition and Digestion. Chapter Nervous System. Chapter Sensory Systems. Chapter Musculoskeletal System. Chapter Endocrine System. Chapter Circulatory and Pulmonary Systems. Chapter Osmoregulation and Excretion.
Chapter Immune System. Chapter Reproduction and Development. Chapter Behavior. Chapter Ecosystems. Chapter Population and Community Ecology. Chapter Biodiversity and Conservation. Chapter Speciation and Diversity. Chapter Natural Selection. Chapter Population Genetics. Chapter Evolutionary History. These products are generated per single molecule of pyruvate. The products of the Krebs cycle power the electron transport chain and oxidative phosphorylation. Acetyl CoA enters the Krebs cycle after the transition reaction has taken place conversion of pyruvate to acetyl CoA.
See figure 9. There are 8 steps in the Krebs cycle. Below reviews some of the principal parts of these steps and the products of Krebs cycle:. Acetyl CoA joins with oxaloacetate releasing the CoA group and producing citrate, a six-carbon molecule. The enzyme involved in this process is citrate synthase. Citrate is converted to isocitrate by the enzyme aconitase. This involves the removal then the addition of water.
The ketone is then decarboxylated i. CO 2 removed by isocitrate dehydrogenase leaving behind alpha-ketoglutarate which is a 5-carbon molecule. Isocitrate dehydrogenase, is central in regulating the speed of the Krebs cycle citric acid cycle. Oxidative decarboxylation takes place by alpha-ketoglutarate dehydrogenase. Succinyl-CoA is converted to succinyl phosphate, and then succinate.
Succinate thiokinase other names include succinate synthase and Succinyl coenzyme A synthetase , converts succinyl-CoA to succinate, and free coenzyme A. Firstly, the coenzyme A at the succinyl group is substituted by a hydrogen phosphate ion.
Succinyl phosphate then transfers its phosphoric acid residue to guanosine diphosphate GDP so that GTP and succinate are produced.
Succinate is oxidized to fumarate by succinate dehydrogenase. Flavin adenine dinucleotide FAD is the coenzyme bound to succinate dehydrogenase. FADH 2 is formed by the removal of 2 hydrogen atoms from succinate. This releases energy that is sufficient to reduce FAD. FADH remains bound to succinate dehydrogenase and transfers electrons directly to the electron transport chain.
Succinate dehydrogenase performs this process inside the mitochondrial inner membrane which allows this direct transfer of the electrons. L-malate is formed by the hydration of fumarate. The enzyme involved in this reaction is fumarase. In the final step, L-malate is oxidized to form oxaloacetate by malate dehydrogenase. Where is oxygen used in cellular respiration? It is in the stage involving the electron transport chain. The electron transport chain is the final stage in cellular respiration.
It occurs on the inner mitochondrial membrane and consists of several electron carriers. The purpose of the electron transport chain is to form a gradient of protons that produces ATP.
It moves electrons from NADH to FADH 2 to molecular oxygen by pumping protons from the mitochondrial matrix to the intermembrane space resulting in the reduction of oxygen to water. Therefore, the role of oxygen in cellular respiration is the final electron acceptor. It is worth noting that the electron transport chain of prokaryotes may not require oxygen. Other chemicals including sulfate can be used as electron acceptors in the replacement of oxygen. Four protein complexes are involved in the electron transport chain.
These electrons are then shuttled down the remaining complexes and proteins. They are passed into the inner mitochondrial membrane which slowly releases energy. The electron transport chain uses the decrease in free energy to pump hydrogen ions from the matrix to the intermembrane space in the mitochondrial membranes.
This creates an electrochemical gradient for hydrogen ions. Overall, the end products of the electron transport chain are ATP and water. See figure The process described above in the electron transport chain in which a hydrogen ion gradient is formed by the electron transport chain is known as chemiosmosis. After the gradient is established, protons diffuse down the gradient through ATP synthase.
Chemiosmosis was discovered by the British Biochemist, Peter Mitchell. In fact, he was awarded the Nobel prize for Chemistry in for his work in this area and ATP synthesis. How much ATP is produced in aerobic respiration? What are the products of the electron transport chain? Glycolysis provides 4 molecules of ATP per molecule of glucose; however, 2 are used in the investment phase resulting in a net of 2 ATP molecules.
Finally, 34 molecules of ATP are produced in the electron transport chain figure Only 2 molecules of ATP are produced in fermentation. This occurs in the glycolysis phase of respiration. Therefore, it is much less efficient than aerobic respiration; it is, however, a much quicker process.
And so essentially, this is how in cellular respiration, energy is converted from glucose to ATP. And by glucose oxidation via the aerobic pathway, more ATPs are relatively produced. What are the products of cellular respiration? The biochemical processes of cellular respiration can be reviewed to summarise the final products at each stage. Mitochondrial dysfunction can lead to problems during oxidative phosphorylation reactions.
These mutations can lead to protein deficiencies. For example, complex I mitochondrial disease is characterized by a shortage of complex I within the inner mitochondrial membrane. This leads to problems with brain function and movement for the individual affected.
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