The reverse of Glycolysis step 10 takes two steps. Loss of muscle mass is the equivalent of reduced strength, which tends to inhibit seniors from engaging in sufficient physical activity. Enzymes located in certain tissues can add a phosphate group to fructose. Glycolysis consists of ten steps, split into two phases. These electrons, O2, and H+ ions from the matrix combine to form new water molecules. Glucose is the body’s most readily available source of energy. Fructose must undergo certain extra steps in order to enter the glycolysis pathway. Glycosaminoglycan metabolism and xylulose-5-phosphate synthesis from glucuronate are also annotated as parts of carbohydrate metabolism. zIn strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic It also functions to maintain a concentration gradient with higher glucose levels in the blood than in the tissues. The first step of carbohydrate catabolism is glycolysis, which produces pyruvate, NADH, and ATP. ), Eds. When oxygen is limited or absent, pyruvate enters an anaerobic pathway. The process of anaerobic respiration converts glucose into two lactate molecules in the absence of oxygen or within erythrocytes that lack mitochondria. Hormones released from the pancreas regulate the overall metabolism of glucose. As will be discussed as part of lipolysis, fats can be broken down into glycerol, which can be phosphorylated to form dihydroxyacetone phosphate or DHAP. Gluconeogenesis is not simply the reverse of glycolysis. The pentose phosphate pathway is an alternative method of oxidizing glucose. This equation states that glucose, in combination with ATP (the energy source), NAD+ (a coenzyme that serves as an electron acceptor), and inorganic phosphate, breaks down into two pyruvate molecules, generating four ATP molecules—for a net yield of two ATP—and two energy-containing NADH coenzymes. Glucose and fructose are examples of simple sugars, and starch, glycogen, and cellulose are all examples of complex sugars. Carbohydrate metabolism is the whole of the biochemical processes responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms. Hexokinase is found in nearly every tissue in the body. A molecule of NADH can produce 1.5–2.5 molecules of ATP, whereas a molecule of FADH2 yields 1.5 molecules of ATP. This section will focus first on glycolysis, a process where the monosaccharide glucose is oxidized, releasing the energy stored in its bonds to produce ATP. The ETC couples the transfer of electrons between a donor (like NADH) and an electron acceptor (like O2) with the transfer of protons (H+ ions) across the inner mitochondrial membrane, enabling the process of oxidative phosphorylation. The pyruvate molecules generated during glycolysis are transported across the mitochondrial membrane into the inner mitochondrial matrix, where they are metabolized by enzymes in a pathway called the Krebs cycle (Figure 4). Glycolysis begins with the phosphorylation of glucose by hexokinase to form glucose-6-phosphate. Glucose constitutes about 80% of the products and is the primary structure that is distributed to cells in the tissues, where it is broken down or stored as glycogen. In the presence of oxygen, energy is passed, stepwise, through the electron carriers to collect gradually the energy needed to attach a phosphate to ADP and produce ATP. In summary, one glucose molecule breaks down into two pyruvate molecules, and creates two net ATP molecules and two NADH molecules by glycolysis. Carbohydrate metabolism. The regulation also allows for the storage of glucose molecules into fatty acids. The enolase enzyme then acts upon the 2-phosphoglycerate molecules to convert them into phosphoenolpyruvate molecules. Glucoregulation is the maintenance of steady levels of glucose in the body. They depend on glycolysis and lactic acid production for rapid ATP production. This means that once the electrons have passed through the entire ETC, they must be passed to another, separate molecule. The last step in glycolysis produces the product pyruvate. The electrons released from NADH and FADH2 are passed along the chain by each of the carriers, which are reduced when they receive the electron and oxidized when passing it on to the next carrier. This is an effective pathway of ATP production for short periods of time, ranging from seconds to a few minutes. This helps the cell to regulate glycolysis and gluconeogenesis independently of each other. During the first phase, it requires the breakdown of two ATP molecules. These measures can help keep energy levels from dropping and curb the urge for increased calorie consumption from excessive snacking. During the first phase, it requires the breakdown of two ATP molecules. Plants synthesize carbohydrates from carbon dioxide and water through photosynthesis, allowing them to store energy absorbed from the sunlight internally. a) one step b) two steps c) three steps d) four steps. In this step, the formation of branches is brought about by the action of a branching enzyme, namely branching enzyme (amylo-[1—>4]—>[1—>6]-transglucosidase).. Glycolysis only requires the input of one molecule of ATP when the glucose originates in glycogen. During the second phase, chemical energy from the intermediates is transferred into ATP and NADH. As the terminal step in the electron transport chain, oxygen is the terminal electron acceptor and creates water inside the mitochondria. In addition, the Krebs cycle supplies the starting materials to process and break down proteins and fats. After separation from glucose, galactose travels to the liver for conversion to glucose. The aconitase enzyme converts citrate into isocitrate. Carbohydrate Metabolism. Cellular respiration oxidizes glucose molecules through glycolysis, the Krebs cycle, and oxidative phosphorylation to produce ATP. Increased levels of glucagon activates the enzymes that catalyze glycogenolysis, and inhibits the enzymes that catalyze glycogenesis. For example, because erythrocytes (red blood cells) lack mitochondria, they must produce their ATP from anaerobic respiration. Amalyses are the enzymes that degrade starch for assisting metabolism. During the second phase, chemical energy from the intermediates is transferred into ATP and NADH. Some simple carbohydrates have their own enzymatic oxidation pathways, as do only a few of the more complex carbohydrates. During part 1, we talk about carbohydrate metabolism. Two molecules of glyceraldehyde-3-phosphate then combine to form fructose-1-6-bisphosphate, which is converted into fructose 6-phosphate and then into glucose-6-phosphate. The last step of glycolysis involves the dephosphorylation of the two phosphoenolpyruvate molecules by pyruvate kinase to create two pyruvate molecules and two ATP molecules. The enzyme phosphofructokinase-1 then adds one more phosphate to convert fructose-6-phosphate into fructose-1-6-bisphosphate, another six-carbon sugar, using another ATP molecule. To start the Krebs cycle, citrate synthase combines acetyl CoA and oxaloacetate to form a six-carbon citrate molecule; CoA is subsequently released and can combine with another pyruvate molecule to begin the cycle again. Following are some of the most common ones – Diabetes Mellitus: It occurs due to the lack of insulin or resistance to it, which further leads to either hyper or hypoglycemia. This route achieves the oxidative decarboxylation of glucose to give ribose, as the 5-phosphate ester. Blood sugar concentrations are controlled by three hormones: insulin, glucagon, and epinephrine. Step 6: Glycogen Branches formation. Electrons from NADH and FADH2 are transferred through protein complexes embedded in the inner mitochondrial membrane by a series of enzymatic reactions. Test Bank Chapter 24: Carbohydrate Metabolism 281. Ninja Nerds,Join us for this three part series on the various metabolic pathways. Oxaloacetate is then ready to combine with the next acetyl CoA to start the Krebs cycle again (see Figure 4). Figure 2. Under anaerobic conditions, the pyruvate can be converted into lactate to keep glycolysis working. Glucokinase, on the other hand, is expressed in tissues that are active when blood glucose levels are high, such as the liver. Effectively, it is a turbine that is powered by the flow of H+ ions across the inner membrane down a gradient and into the mitochondrial matrix. The most important carbohydrate is glucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms. Carbohydrates are central to many essential metabolic pathways. Match. The family of carbohydrates includes both simple and complex sugars. A net of two ATP are produced through glycolysis (four produced and two consumed during the energy-consuming stage). Carbohydrate metabolism is significantly different in dogs with cancer; tumour cells preferentially metabolize glucose (carbohydrate) for energy and make lactate (lactic acid) as an end product. Gluconeogenesis is inhibited by AMP, ADP, and insulin. Carbohydrates are central to many essential metabolic pathways. Pyruvate is a common starting material for gluconeogenesis. PLAY. Anaerobic respiration occurs when no oxygen is present. In this form, glucose-6-phosphate is trapped in the cell. Flashcards. It produces products that are used in other cell processes, while reducing NADP to NADPH. Some experts also suggest avoiding sugar, which can lead to excess fat storage. Click to view a larger image. The lactic acid produced diffuses into the plasma and is carried to the liver, where it is converted back into pyruvate or glucose via the Cori cycle. In the presence of oxygen, pyruvate can enter the Krebs cycle where additional energy is extracted as electrons are transferred from the pyruvate to the receptors NAD+, GDP, and FAD, with carbon dioxide being a “waste product” (Figure 3). So, the question can be raised as to why the body would create something it has just spent a fair amount of effort to break down? The first phase is the energy-consuming phase, so it requires two ATP molecules to start the reaction for each molecule of glucose. Figure 1. The electron transport chain is a series of electron carriers and ion pumps that are used to pump H+ ions out of the inner mitochondrial matrix. The typical example used to introduce concepts of metabolism to students is carbohydrate catabolism. In all phases after glycolysis, the number of ATP, NADH, and FADH, In the ETC, about three ATP are produced for every oxidized NADH. Purine and Pyrimidine Metabolism: MCQ. Therefore, by the end of this chemical- priming or energy-consuming phase, one glucose molecule is broken down into two glyceraldehyde-3-phosphate molecules. The digestion of dietary starch and sugars and the uptake of the resulting monosaccharides into the circulation from the small intestine are annotated as parts of the “Digestion and absorption” pathway. Explain how glucose is metabolized to yield ATP. Galactose and fructose metabolism is a logical place to begin looking at carbohydrate metabolism, before shifting focus to the preferred monosaccharide glucose. However, animals, including humans, lack the necessary enzymatic machinery and so do not synthesize glucose from lipids (with a few exceptions, e.g. The Krebs cycle is also commonly called the citric acid cycle or the tricarboxylic acid (TCA) cycle. This is important when levels of glucose are very low in the body, as it allows glucose to travel preferentially to those tissues that require it more. THE HANDOUT This handout has five parts: 1. a summary of small molecules commonly used in metabolism for the production and utilization of energy by cells (METABOLIC ENERGY), 2. a short synopsis of the metabolic pathways and hormonal regulation of glucose metabolism (SYNOPSIS OF GLUCOSE METABOLISM), 3. In gluconeogenesis (as compared to glycolysis), the enzyme hexokinase is replaced by glucose-6-phosphatase, and the enzyme phosphofructokinase-1 is replaced by fructose-1,6-bisphosphatase. This six-carbon sugar is split to form two phosphorylated three-carbon molecules, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, which are both converted into glyceraldehyde-3-phosphate. Glycogenesis refers to the process of synthesizing glycogen. Anaerobic respiration occurs in most cells of the body when oxygen is limited or mitochondria are absent or nonfunctional. Under aerobic conditions, pyruvate enters the Krebs cycle, also called the citric acid cycle or tricarboxylic acid cycle. Regardless of insulin levels, no glucose is released to the blood from internal glycogen stores from muscle cells. Hexokinase has a higher affinity for glucose than glucokinase and therefore is able to convert glucose at a faster rate than glucokinase. The production of ATP is achieved through the oxidation of glucose molecules. Test. There are several things that can be done to help prevent general declines in metabolism and to fight back against the cyclic nature of these declines. Watch this video to learn about the electron transport chain. Overview of Carbohydrate Metabolism. Figure 3. This reaction is an oxidative decarboxylation reaction. The glucose molecule then splits into two three-carbon compounds, each containing a phosphate. This is the basis for your need to breathe in oxygen. This reaction releases an electron that is then picked up by NAD+ to create an NADH molecule. In these reactions, pyruvate can be converted into lactic acid. Since all digestible forms of carbohydrates are eventually transformed into glucose, it is important to consider how glucose is able to provide ener… This pathway is regulated by multiple different molecules. We explain the process of carbohydrate digestion and how many carbs you should aim to eat daily. The essential steps are In the presence of oxygen, pyruvate continues on to the Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle (TCA), where additional energy is extracted and passed on. This step uses one ATP, which is the donor of the phosphate group. For example, since the poorly developed mitochondrion of the amastigote includes neither a cytochrome system nor a functional TCA cycle, the amastigote processes carbohydrates incompletely by anaerobic metabolism. Carbohydrate metabolism involves glycolysis, the Krebs cycle, and the electron transport chain. The liver is the organ that breaks down the various non-carbohydrate molecules and sends them out to other organs and tissues to be used in Gluconeogenesis. Metabolic enzymes catalyze catabolic reactions that break down carbohydrates contained in food. It converts the three-carbon pyruvate into a two-carbon acetyl CoA molecule, releasing carbon dioxide and transferring two electrons that combine with NAD+ to form NADH. There is an alternative route, called the pentose phosphate pathway, by which glucose enters the glycolytic sequence to pyruvate. However, the strong affinity of most carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex. organisms (primarily plants) to manufacture carbohydrate from fatty acids, is considered. A single glucose molecule is cleaved from a branch of glycogen, and is transformed into glucose-1-phosphate during this process. Carbohydrates give your body energy to do everyday tasks. The final step is the conversion of glucose-6-P to glucose that is catalyzed by glucose-6-phosphatase. For each molecule of glucose that is processed in glycolysis, a net of 36 ATPs can be created by aerobic respiration. Importantly, by the end of this process, one glucose molecule generates two pyruvate molecules, two high-energy ATP molecules, and two electron-carrying NADH molecules. These can then be transported across the intestinal membrane into the bloodstream and then to body tissues. This pathway is regulated through changes in the activity of glucose-6-phosphate dehydrogenase. The NADH that is produced in this process will be used later to produce ATP in the mitochondria. Insulin It is secreted by the beta-cells of the pancreas in response to a high blood glucose. In the liver, hepatocytes either pass the glucose on through the circulatory system or store excess glucose as glycogen. The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates. In this reaction, lactic acid replaces oxygen as the final electron acceptor. The energy for this endergonic reaction is provided by the removal (oxidation) of two electrons from each three-carbon compound. Therefore, the net production of ATP during glycolysis is zero. Changes in body composition, including reduced lean muscle mass, are mostly responsible for this decrease. Because there are two glyceraldehyde-3-phosphate molecules, two NADH molecules are synthesized during this step. Gravity. The human body’s metabolic rate decreases nearly 2 percent per decade after age 30. This regulation is to ensure that the body is not over-producing pyruvate molecules. The release of glucagon is precipitated by low levels of blood glucose, whereas high levels of blood glucose stimulates cells to produce insulin. Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms. Glucose regulation and product use are the primary categories in which these pathways differ between organisms. Disorders of Carbohydrate Metabolism. Respond using the list on the right. The NADH and FADH2 pass electrons on to the electron transport chain, which uses the transferred energy to produce ATP. Both animals and plants temporarily store the released energy in the form of high-energy molecules, such as ATP, for use in various cellular processes. Glycolysis is the process of breaking down a glucose molecule into two pyruvate molecules, while storing energy released during this process as ATP and NADH. The phosphorylated galactose is then converted to glucose-1-phosphate, and then eventually glucose-6-phosphate, which can be broken down in glycolysis. The breakdown of one molecule of glucose results in two molecules of pyruvate, which can be further oxidized to access more energy in later processes. Figure 5. This pathway is common to both anaerobic and aerobic respiration. The following discussions of glycolysis include the enzymes responsible for the reactions. glycogen, starch). When animals and fungi consume plants, they use cellular respiration to break down these stored carbohydrates to make energy available to cells. This process occurs when there are lowered amounts of glucose. Alternatively, glucose-6-phosphate can be converted back into glucose in the liver and the kidneys, allowing it to raise blood glucose levels if necessary. Energy produced during metabolism of one glucose molecule. Lactate must then be converted back to glucose, which consumes energy and gives a net energy gain to the tumour and a net loss to the animal, contributing to cancer cachexia. By the end of this section, you will be able to: Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms. Write. Succinate dehydrogenase then converts succinate into fumarate, forming a molecule of FADH2. NADH is a high-energy molecule, like ATP, but unlike ATP, it is not used as energy currency by the cell. The energy production phase involves the next five steps during which the two molecules of glyceraldehyde-3-phosphate are converted to two pyruvate molecules with the production of two NADH molecules and four ATP molecules. This step utilizes the enzyme aldolase, which catalyzes the cleavage of FBP to yield two 88 89. Photosynthesis, a process in which light energy is captured to drive carbohydrate synthesis, is described in Chapter 13. Glycogen is a highly branched structure, consisting of glucose, in the form of glucose-6-phosphate, linked together. Glycogenesis occurs primarily in the liver, skeletal muscles, and kidney. There are various enzymes that are used throughout glycolysis. polysaccharides: complex carbohydrates made up of many monosaccharides, monosaccharide: smallest, monomeric sugar molecule, salivary amylase: digestive enzyme that is found in the saliva and begins the digestion of carbohydrates in the mouth, cellular respiration: production of ATP from glucose oxidation via glycolysis, the Krebs cycle, and oxidative phosphorylation, glycolysis: series of metabolic reactions that breaks down glucose into pyruvate and produces ATP, pyruvate: three-carbon end product of glycolysis and starting material that is converted into acetyl CoA that enters the, Krebs cycle: also called the citric acid cycle or the tricarboxylic acid cycle, converts pyruvate into CO2 and high-energy FADH2, NADH, and ATP molecules, citric acid cycle or tricarboxylic acid cycle (TCA): also called the Krebs cycle or the tricarboxylic acid cycle; converts pyruvate into CO2 and high-energy FADH2, NADH, and ATP molecules, energy-consuming phase, first phase of glycolysis, in which two molecules of ATP are necessary to start the reaction, glucose-6-phosphate: phosphorylated glucose produced in the first step of glycolysis, Hexokinase: cellular enzyme, found in most tissues, that converts glucose into glucose-6-phosphate upon uptake into the cell, Glucokinase: cellularenzyme, found in the liver, which converts glucose into glucose-6-phosphate upon uptake into the cell, energy-yielding phase: second phase of glycolysis, during which energy is produced, terminal electron acceptor: ATP production pathway in which electrons are passed through a series of oxidation-reduction reactions that forms water and produces a proton gradient, electron transport chain (ETC): ATP production pathway in which electrons are passed through a series of oxidation-reduction reactions that forms water and produces a proton gradient, oxidative phosphorylation: process that converts high-energy NADH and FADH2 into ATP, ATP synthase protein: pore complex that creates ATP, Gluconeogenesis: process of glucose synthesis from pyruvate or other molecules, http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25, Describe the pathway of a pyruvate molecule through the Krebs cycle, Explain the transport of electrons through the electron transport chain, Describe the process of ATP production through oxidative phosphorylation. Glycolysis can be regulated at different steps of the process through feedback regulation. There are some important differences (Figure 7). Along the way, each citrate molecule will produce one ATP, one FADH2, and three NADH. This enzyme transfers a small fragment of five to eight glucose residues from the non-reducing end of the glycogen chain. CARBOHYDRATE METABOLISM Warren Jelinek I. Under aerobic conditions, pyruvate enters the Krebs cycle, also … Carbohydrate catabolism. The triosephosphate isomerase enzyme then converts dihydroxyacetone phosphate into a second glyceraldehyde-3-phosphate molecule. Each carbon of pyruvate is converted into CO2, which is released as a byproduct of oxidative (aerobic) respiration. Galactokinase uses one molecule of ATP to phosphorylate galactose. to another glucose residue where it is linked by the alpha-1,6 bond. In the muscles, glycogen ensures a rapidly accessible energy source for movement. In the next step of the first phase of glycolysis, the enzyme glucose-6-phosphate isomerase converts glucose-6-phosphate into fructose-6-phosphate. Carbohydrate is a general term for sugars and related compounds with the general formula Cn(H2O)n. The smallest are monosaccharides (e.g. The accumulation of these protons in the space between the membranes creates a proton gradient with respect to the mitochondrial matrix. The role of molecular oxygen, O2, is as the terminal electron acceptor for the ETC. The second phase of glycolysis, the energy-yielding phase, creates the energy that is the product of glycolysis. glucose); polysaccharides (e.g. glycerol). Certain key organs, including the brain, can use only glucose as an energy source; therefore, it is essential that the body maintain a minimum blood glucose concentration. These include eating breakfast, eating small meals frequently, consuming plenty of lean protein, drinking water to remain hydrated, exercising (including strength training), and getting enough sleep. Therefore, glycolysis generates energy for the cell and creates pyruvate molecules that can be processed further through the aerobic Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle); converted into lactic acid or alcohol (in yeast) by fermentation; or used later for the synthesis of glucose through gluconeogenesis. Several hormones regulate carbohydrate metabolism. In accounting for the total number of ATP produced per glucose molecule through aerobic respiration, it is important to remember the following points: Therefore, for every glucose molecule that enters aerobic respiration, a net total of 36 ATPs are produced (see Figure 6). Under anaerobic conditions, ATP production is limited to those generated by glycolysis. In aerobic respiration, the main form of cellular respiration used by humans, glucose and oxygen are metabolized to release energy, with carbon dioxide and water as byproducts. … Carbohydrate Metabolism 1. chitin, cellulose) or for energy storage (e.g. Aerobic respiration is the oxygen-requiring degradation of food molecules and production of ATP, and is the one we shall be concerned with in carbohydrate metabolism. The acetyl CoA is systematically processed through the cycle and produces high- energy NADH, FADH2, and ATP molecules. Its functions are: 1. The step that is regulated the most is the third step. Because all of the glucose has been phosphorylated, new glucose molecules can be transported into the cell according to its concentration gradient. Click to view a larger image. The breakdown of one molecule of glucose results in two molecules of pyruvate, which can be further oxidized to access more energy in later processes. 31 Carbohydrate Metabolism BIOCHEMISTRY MODULE Biochemistry Notes Site of reaction: All the reaction steps take place in the cytoplasm. The non-carbohydrate molecules that are converted in this pathway include pyruvate, lactate, glycerol, alanine, and glutamine. 2.Citric Acid Cycle (Kreb's Cycle) The complex sugars are also called polysaccharides and are made of multiple monosaccharide molecules. The electron transport chain consists of a series of four enzyme complexes (Complex I – Complex IV) and two coenzymes (ubiquinone and Cytochrome c), which act as electron carriers and proton pumps used to transfer H+ ions into the space between the inner and outer mitochondrial membranes (Figure 5). Conversely, glycogenesis is enhanced and glycogenolysis inhibited when there are high levels of insulin in the blood. Any discussion of carbohydrate metabolism focuses on the synthesis and usage of glucose, a major fuel for most organisms. The most dramatic loss of muscle mass, and consequential decline in metabolic rate, occurs between 50 and 70 years of age. At this point, a second ATP donates its phosphate group, forming fructose-1,6-bisphosphate. … This process, called gluconeogenesis, is almost the reverse of glycolysis and serves to create glucose molecules for glucose-dependent organs, such as the brain, when glucose levels fall below normal. During the energy-consuming phase of glycolysis, two ATPs are consumed, transferring two phosphates to the glucose molecule. Glycolysis can be expressed as the following equation: $\text{Glucose}+2\text{ATP}+2\text{NAD}^{+}+4\text{ADP}+2\text{P}_{i}\to\text{Pyruvate}+4\text{ATP}+2\text{NADH}+2\text{H}^{+}$. The energy released is used to power the cells and systems that make up your body. Carbohydrate digestion begins in the mouth with the action of salivary amylase on starches and ends with monosaccharides being absorbed across the epithelium of the small intestine. The ten pathways/cycles of carbohydrate metabolism are: (1) Glycolysis (2) Conversion of Pyruvate to Acetyl COA (3) Citric Acid Cycle (4) Gluconeogenesis (5) Glycogen Metabolism (6) Glycogenesis (7) Glycogenolysis (8) Hexose Monophosphate Shunt … Liver is the terminal step in the body glycolysis consists of one molecule FADH2! Finally Here body ’ s metabolic rate decreases nearly 2 percent per after... To two ADPs to form two phosphorylated three-carbon molecules, including reduced lean muscle mass are..., consists of one molecule of ATP synthase to encourage ADP and Pi to create an molecule. This helps the cell in metabolic rate decreases nearly 2 percent per decade after age 30,... Zglycolysis is the ATP yield per glucose molecule in each of the glycogen chain last part of the reaction four... 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