# carbohydrate metabolism steps

Test Bank Chapter 24: Carbohydrate Metabolism 281. 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. (Eliseev MS et al.) 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. This results in a positive-feedback system where the reduced physical activity leads to even more muscle loss, further reducing metabolism. Cells in the body take up the circulating glucose in response to insulin and, through a series of reactions called glycolysis, transfer some of the energy in glucose to ADP to form ATP (Figure 2). Because there are two glyceraldehyde-3-phosphate molecules, two NADH molecules are synthesized during this step. The regulation also allows for the storage of glucose molecules into fatty acids. Acetyl CoA enters the Krebs cycle by combining with a four-carbon molecule, oxaloacetate, to form the six-carbon molecule citrate, or citric acid, at the same time releasing the coenzyme A molecule. 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). The cofactors NAD+ and FAD are sometimes reduced during this process to form NADH and FADH2, which drive the creation of ATP in other processes. In these reactions, pyruvate can be converted into lactic acid. This process takes place primarily in the liver during periods of low glucose, that is, under conditions of fasting, starvation, and low carbohydrate diets. The accumulation of these protons in the space between the membranes creates a proton gradient with respect to the mitochondrial matrix. This pathway is regulated through changes in the activity of glucose-6-phosphate dehydrogenase. During the Krebs cycle, each pyruvate that is generated by glycolysis is converted into a two-carbon acetyl CoA molecule. This rotation enables other portions of ATP synthase to encourage ADP and Pi to create ATP. Flashcards. Most of the fructose and galactose travel to the liver, where they can be converted to glucose. After separation from glucose, galactose travels to the liver for conversion to glucose. In other words, glucose will be completely degraded to pyruvate after this reaction has taken place. The human body’s metabolic rate decreases nearly 2 percent per decade after age 30. Ninja Nerds,Join us for this three part series on the various metabolic pathways. zGlycolysis is the only source of energy in erythrocytes. 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. The amount of insulin released in the blood and sensitivity of the cells to the insulin both determine the amount of glucose that cells break down. Gluconeogenesis is the synthesis of new glucose molecules from pyruvate, lactate, glycerol, or the amino acids alanine or glutamine. In some tissues and organisms, glycolysis is the sole method of energy production. Glycogenesis occurs primarily in the liver, skeletal muscles, and kidney. 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. This is the basis for your need to breathe in oxygen. Excess or unutilized energy is stored as fat or glycogen for later use. The production of ATP is achieved through the oxidation of glucose molecules. This means that once the electrons have passed through the entire ETC, they must be passed to another, separate molecule. The first step of carbohydrate catabolism is glycolysis, which produces pyruvate, NADH, and ATP. The essential steps are However, only about two ATP are produced for every oxidized FADH. Figure 24.2.6 – Carbohydrate Metabolism: Carbohydrate metabolism involves glycolysis, the Krebs cycle, and the electron transport chain. 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. 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. Disorders of Carbohydrate Metabolism. Increased levels of glucagon activates the enzymes that catalyze glycogenolysis, and inhibits the enzymes that catalyze glycogenesis. Changes in body composition, including reduced lean muscle mass, are mostly responsible for this decrease. Any discussion of carbohydrate metabolism focuses on the synthesis and usage of glucose, a major fuel for most organisms. During the first phase, it requires the breakdown of two ATP molecules. Glucose and fructose are examples of simple sugars, and starch, glycogen, and cellulose are all examples of complex sugars. 24.61 a … A net of two ATP are produced through glycolysis (four produced and two consumed during the energy-consuming stage). chitin, cellulose) or for energy storage (e.g. Also embedded in the inner mitochondrial membrane is an amazing protein pore complex called ATP synthase. Plants synthesize carbohydrates from carbon dioxide and water through photosynthesis, allowing them to store energy absorbed from the sunlight internally. This is an effective pathway of ATP production for short periods of time, ranging from seconds to a few minutes. 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. The pentose phosphate pathway is an alternative method of oxidizing glucose. ... Carbohydrate Metabolism: MCQ on Glycogen Synthesis and Breakdown. Some simple carbohydrates have their own enzymatic oxidation pathways, as do only a few of the more complex carbohydrates. Glucagon, adrenocorticotropic hormone, and ATP encourage gluconeogenesis. Step 6: Glycogen Branches formation. There are three basic steps involved with carbohydrate metabolism: 1.Glycolysis. The glyceraldehyde-3-phosphate is further phosphorylated with groups donated by dihydrogen phosphate present in the cell to form the three-carbon molecule 1,3-bisphosphoglycerate. In the next step of the first phase of glycolysis, the enzyme glucose-6-phosphate isomerase converts glucose-6-phosphate into fructose-6-phosphate. 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. The aconitase enzyme converts citrate into isocitrate. Carbohydrate metabolism begins with digestion in the small intestine where monosaccharides are absorbed into the blood stream. Bastyr40. In the liver, muscles, and the kidney, this process occurs to provide glucose when necessary. glycerol). In summary, one glucose molecule breaks down into two pyruvate molecules, and creates two net ATP molecules and two NADH molecules by glycolysis. Enzymes located in certain tissues can add a phosphate group to fructose. Adrenaline stimulates the breakdown of glycogen in the skeletal muscle during exercise. 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. The release of glucagon is precipitated by low levels of blood glucose, whereas high levels of blood glucose stimulates cells to produce insulin. These electrons, O2, and H+ ions from the matrix combine to form new water molecules. to another glucose residue where it is linked by the alpha-1,6 bond. Digestion is the breakdown of carbohydrates to yield an energy rich compound called ATP. A single glucose molecule is cleaved from a branch of glycogen, and is transformed into glucose-1-phosphate during this process. We explain the process of carbohydrate digestion and how many carbs you should aim to eat daily. Blood sugar concentrations are controlled by three hormones: insulin, glucagon, and epinephrine. Test. Monosaccharide Metabolism. 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. Under anaerobic conditions, ATP production is limited to those generated by glycolysis. It produces products that are used in other cell processes, while reducing NADP to NADPH. This enzyme transfers a small fragment of five to eight glucose residues from the non-reducing end of the glycogen chain. The energy released is used to power the cells and systems that make up your body. Carbohydrates give your body energy to do everyday tasks. SUMMARY OF CARBOHYDRATE METABOLISM 89 90. Loss of muscle mass is the equivalent of reduced strength, which tends to inhibit seniors from engaging in sufficient physical activity. energy-consuming phase into 1,3-bisphosphoglycerate. The second phase of glycolysis, the energy-yielding phase, creates the energy that is the product of glycolysis. of energy, which is used to pump H+ ions across the inner membrane. Upon entry into the cell, hexokinase or glucokinase phosphorylates glucose, converting it into glucose-6-phosphate. The disaccharide lactose, for instance, requires the enzyme lactase to be broken into its monosaccharide components, glucose and galactose. The reverse of Glycolysis step 10 takes two steps. At this point, a second ATP donates its phosphate group, forming fructose-1,6-bisphosphate. Although humans consume a variety of carbohydrates, digestion breaks down complex carbohydrates into a few simple monomers (monosaccharides) for metabolism: glucose, fructose, and galactose. Glycosaminoglycan metabolism and xylulose-5-phosphate synthesis from glucuronate are also annotated as parts of carbohydrate metabolism. Hexokinase has a higher affinity for glucose than glucokinase and therefore is able to convert glucose at a faster rate than glucokinase. Hormones released from the pancreas regulate the overall metabolism of glucose. 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. Respond using the list on the right. This pathway occurs in the muscles, adipose tissue, and kidney. Like glucose, fructose is also a six carbon-containing sugar. DHAP can either enter the glycolytic pathway or be used by the liver as a substrate for gluconeogenesis. 20-10C Alternative Routes in Carbohydrate Metabolism. 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. Without oxygen, electron flow through the ETC ceases. In addition to ATP, the Krebs cycle produces high-energy FADH2 and NADH molecules, which provide electrons to the oxidative phosphorylation process that generates more high-energy ATP molecules. Gluconeogenesis is the synthesis of glucose from pyruvate, lactate, glycerol, alanine, or glutamate. Insulin and glucagon are the primary hormones involved in maintaining a steady level of glucose in the blood, and the release of each is controlled by the amount of nutrients currently available. The triosephosphate isomerase enzyme then converts dihydroxyacetone phosphate into a second glyceraldehyde-3-phosphate molecule. Polysaccharides serve as energy storage (e.g., starch and glycogen) and as structural components (e.g., chitin in insects and cellulose in plants). Its functions are: 1. The electron transport chain (ETC) uses the NADH and FADH2 produced by the Krebs cycle to generate ATP. Glycogenesis refers to the process of synthesizing glycogen. This conversion step requires one ATP and essentially traps the glucose in the cell, preventing it from passing back through the plasma membrane, thus allowing glycolysis to proceed. The three-carbon pyruvate molecule generated during glycolysis moves from the cytoplasm into the mitochondrial matrix, where it is converted by the enzyme pyruvate dehydrogenase into a two-carbon acetyl coenzyme A (acetyl CoA) molecule. Carbohydrates are central to many essential metabolic pathways. Importance of the glycolysis pathway: zIt is the only pathway that is taking place in all the cells of the body. As the terminal step in the electron transport chain, oxygen is the terminal electron acceptor and creates water inside the mitochondria. Insulin It is secreted by the beta-cells of the pancreas in response to a high blood glucose. Overview of Carbohydrate Metabolism. PLAY. Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms. Discuss the mechanism cells employ to create a concentration gradient to ensure continual uptake of glucose from the bloodstream. 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. PEP is converted back into 2-phosphoglycerate, which is converted into 3-phosphoglycerate. Match. Gluconeogenesis This process takes place primarily in the liver during periods of low glucose, that is, under conditions of fasting, starvation, and low carbohydrate diets. Step 2: Isomerization glucose 6-phosphate fructose 6-phophate aldose to ketose isomerization reversible, G°´= 1.7 kJ/mole 6 carbon ring 5 carbon ring Enzyme: phosphoglucoisomerase 29. There are various enzymes that are used throughout glycolysis. The first phase is the energy-consuming phase, so it requires two ATP molecules to start the reaction for each molecule 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. Each phosphate released in this reaction can convert one molecule of ADP into one high- energy ATP molecule, resulting in a gain of two ATP molecules. The glycogen in the liver can function as a backup source of glucose between meals. 2.Citric Acid Cycle (Kreb's Cycle) The first step of carbohydrate catabolism is glycolysis, which produces pyruvate, NADH, and ATP. The six-carbon citrate molecule is systematically converted to a five-carbon molecule and then a four-carbon molecule, ending with oxaloacetate, the beginning of the cycle. Carbohydrate metabolism is responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms, It ensures a constant supply of energy to the living cells, The most important carbohydrate is glucose, glucose can be broken down via glycolysis, It enters into the Kreb’s cycle & oxidative phosphorylation to generate ATP. It occurs in the liver, adipose tissue, adrenal cortex, testis, milk glands, phagocyte cells, and red blood cells. 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. 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). In the liver, hepatocytes either pass the glucose on through the circulatory system or store excess glucose as glycogen. Carbohydrate Metabolism. Fructose must undergo certain extra steps in order to enter the glycolysis pathway. Answer the question(s) below to see how well you understand the topics covered in the previous section. • Carbohydrate metabolic disturbances are revealed in the majority of patients with gout and associated with obesity, hypertriglyceridemia, high serum UA levels, chronic disease forms, the high incidence of CHD and arterial hypertension. For each turn of the cycle, three NADH, one ATP (through GTP), and one FADH2 are created. 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. Carbohydrate Metabolism 1. Carbohydrates are central to many essential metabolic pathways. In the cells, glucose, a six-carbon sugar, is processed through a sequence of reactions into smaller sugars, and the energy stored inside the molecule is released. Glycolysis can be expressed as the following equation: $\text{Glucose}+2\text{ATP}+2\text{NAD}^{+}+4\text{ADP}+2\text{P}_{i}\to\text{Pyruvate}+4\text{ATP}+2\text{NADH}+2\text{H}^{+}$. glucose); polysaccharides (e.g. Glucoregulation is the maintenance of steady levels of glucose in the body. Write. This step uses one ATP, which is the donor of the phosphate group. This route achieves the oxidative decarboxylation of glucose to give ribose, as the 5-phosphate ester. When glucose enters a cell, the enzyme hexokinase (or glucokinase, in the liver) rapidly adds a phosphate to convert it into glucose-6-phosphate. Therefore, by the end of this chemical- priming or energy-consuming phase, one glucose molecule is broken down into two glyceraldehyde-3-phosphate molecules. Once the absorbed monosaccharides are transported to the tissues, the process of cellular respiration begins (Figure 1). In humans, insulin is made by beta cells in the pancreas, fat is stored in adipose tissue cells, and glycogen is both stored and released as needed by liver cells. Because all of the glucose has been phosphorylated, new glucose molecules can be transported into the cell according to its concentration gradient. 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. Finally, a series of reactions generates glucose itself. 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). In all phases after glycolysis, the number of ATP, NADH, and FADH, In the ETC, about three ATP are produced for every oxidized NADH. 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. During the energy-consuming phase of glycolysis, two ATPs are consumed, transferring two phosphates to the glucose molecule. Glyceraldehyde-3-phosphate dehydrogenase converts each three-carbon glyceraldehyde-3-phosphate produced during the. The NADH and FADH2 pass electrons on to the electron transport chain, which uses the transferred energy to produce ATP. During the second phase, chemical energy from the intermediates is transferred into ATP and NADH. The process of anaerobic respiration converts glucose into two lactate molecules in the absence of oxygen or within erythrocytes that lack mitochondria. The energy for this endergonic reaction is provided by the removal (oxidation) of two electrons from each three-carbon compound. zIn strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic MLA Citation "Carbohydrate Metabolism." By the end of this section, you will be able to: Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms. 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