Carbohydrate metabolism and nutrition -
Glycolysis is the breakdown of 6 C glucose into two 3 C end product pyruvates in aerobic metabolism and lactic acid in anaerobic metabolism. It is a catabolic pathway involving oxidation and yields ATP and NADH reduced NAD energy.
Glycolysis is the pathway by which other sugars e. Fructose can be converted to fructosephosphate by hexokinase. Galactose can enter glycolysis by being converted to galactosephosphate followed by conversion ultimately to glucosephosphate and subsequently to glucosephosphate G6P , which is a glycolysis intermediate.
Energy Production Process through Glycolysis: Glycolysis has two phases: an energy investment phase requiring the input of ATP preparatory phase and an energy realization phase pay off where ATP is made Figure 5. Cells that utilize glucose have an enzyme called hexokinases, which use ATP to phosphorylate the glucose attaches a phosphorus group and changes it into G6P.
Briefly, in the first reaction of glycolysis, hexokinase catalyzes the transfer of phosphate to glucose from ATP, forming glucosephosphate. Thus this step uses ATP, which provides the energy necessary for the reaction to proceed.
Glucosephosphate is converted to fructosephosphate and subsequently to fructose-1,6-biphosphate, which is cleaved to dihydroxy acetone phosphate DHAP and glyceraldehydephosphate G3P. During this process, an additional ATP is required to phosphorylate the intermediate fructosephosphate.
During the payoff phase, G3P is further processed to produce pyruvate. During this phase, one NADH and two ATP are produced during the intermediate steps.
The DHAP produced can be simply converted into G3P and processed in a similar manner as the first G3P. Therefore, one glucose molecule will result in the production of two NADH, four ATP, and two pyruvate molecules.
ATP Production: For each molecule of glucose, 2 ATP preparatory phase were used and 2 NADH, 4 ATP, and 2 pyruvate molecules payoff phase were generated; which equals a net production of 2 NADH, 2 ATP, and 2 pyruvate molecules, and the net gain of ATP is 8 per mole of glucose.
Production of Other Intermediates: Glycolysis provides pyruvate for the TCA cycle, amino acid synthesis through transamination, glucosephosphate glycogen synthesis , nicotinamide adenine dinucleotide phosphate, NADPH fatty acid synthesis; triglyceride synthesis , and dihydroxyacetone phosphate for glycerol synthesis the backbone of fat.
It is important to discuss the fate of pyruvate generated through glycolysis. Pyruvate has different fates, depending on the conditions of the animal and the cell type. Anaerobic metabolism of glucose generates only two ATP per glucose. Once oxygen is depleted for the cell, another system will convert the lactic acid back to pyruvate and produce glucose.
Acetyl CoA Production: Acetyl CoA production occurs in the aerobic state and serves as the main precursor for the TCA cycle, lipogenesis, and ketogenesis during negative balance.
Acetyl CoA is converted to ATP through different steps in the TCA cycle. During this conversion, the enzyme pyruvate dehydrogenase and different B vitamin—containing coenzymes thiamine, riboflavin, niacin, pantothenic acid function through a series of condensation, isomerization, and dehydrogenation reactions and produces several different intermediates that are used for fat or amino acid synthesis.
To generate more energy from the glucose molecule, further biochemical processes occur within the animal body. These include the enzymatic step pyruvate dehydrogenase PDH , which connects glycolysis cytosol with the TCA cycle in the mitochondria. During this step, 3 C pyruvate is converted to an active form of acetic acid called acetyl CoA, and CO2 is produced.
This enzymatic step needs coenzyme A and its activity is highly regulated by the concentration of acetyl CoA, ATP, and NADH. Several B vitamin—containing enzymes function as coenzymes in the pathway e. The TCA cycle is the key part of aerobic respiration in cells. The TCA cycle also serves as a source of precursors for storage forms of fuels lipids and building blocks, such as amino acids, in the animal body.
Prior to entrance into the cycle, pyruvic acid is converted to acetyl CoA through PDH as described previously. Acetyl CoA 2 C enters the cycle by combining with a 4 C compound called oxaloacetate and forms a 6 C citric acid. Citric acid undergoes a series about 10 of enzyme-catalyzed conversions producing different intermediates e.
In one of the reactions, one ATP is also synthesized from one acetyl CoA. At the end of the cycle, the final product is oxaloacetic acid, which is identical to the oxaloacetic acid that begins the cycle and will pick up another acetyl CoA to begin another turn of the cycle.
Altogether, the TCA cycle produces per one mole of glucose or two moles of pyruvic acid , two ATP molecules, six NADH, and two FADH2.
Both NADH and FADH are used in the electron transport chain to generate ATP. Overall, a net gain of 24 ATP per mole of glucose is obtained through the TCA cycle. Overall, for every glucose molecule fully metabolized to CO2 and H2O, we receive 38 ATP. There are eight kcal of energy in every ATP high-energy phosphate bond.
The rate-limiting step in the TCA cycle is the combination of oxaloacetate and acetyl CoA to produce citrate. Because of this, it is essential to have adequate quantities of oxaloacetate in order to produce ATP and maintain cell viability. The TCA cycle also produces intermediates that serve as a precursor for the synthesis of fatty acids and amino acids.
For example, citrate can be used for fatty acid synthesis, while oxaloacetate can be used for nonessential amino acids glucogenic amino acids or most of the nonessential amino acids, e. During low glucose conditions, these amino acids can produce glucose and α-ketoglutarate can be used for glutamic acid synthesis.
Electron Transport System, Respiratory Chain, or Oxidative Phosphorylation: The electron transport chain permits recovery of redox energy associated with NADH and FADH. H ions react with O2 and form water, and thus the electron transport system serves as a source of metabolic water.
Electrons are carried to the electron transport system in the mitochondrial wall by NADH and FADH2. Oxygen is the terminal electron acceptor and if oxygen is not available, electrons will not pass through the electron transport system and NADH and FADH2 will not be reoxidized.
For these reasons, the citric acid cycle will not run either. This is part of metabolic control. Interruption of electron flow can result in production of reactive oxygen species free radicals.
Cellular enzymes, such as superoxide dismutase and catalase, help deactivate reactive oxygen species. The rumen epithelium performs efficient absorption of VFAs through diffusion through a concentration gradient.
As they pass through the epithelium, the different VFAs undergo different degrees of metabolism. Acetate and propionate pass through the epithelium largely unchanged, but almost all the butyric acid is metabolized in the epithelium to beta-hydroxybutyric acid, a type of ketone body.
The three major VFAs acetic, propionic, butyric absorbed from the rumen have somewhat distinctive metabolic fates:. Acetic acid is utilized minimally in the liver and is oxidized throughout most of the body to generate ATP.
Another important use of acetate is as the major source of acetyl CoA, and it enters the TCA cycle. Acetic acid is used for the synthesis of lipids e.
A high-roughage diet favors the production of acetic acid. Propionic acid is almost completely removed from portal blood by the liver. Propionate is converted to succinyl CoA, and it enters the TCA cycle. Within the liver, propionate serves as a major substrate for gluconeogenesis, which is absolutely critical to the ruminant because almost no glucose reaches the small intestine for absorption.
For example, in a dairy cow, all the glucose in the milk lactose was synthesized in the liver and most of that synthesis was from propionic acid. 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.
Once the absorbed monosaccharides are transported to the tissues, the process of cellular respiration begins Figure 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.
After digestive processes break polysaccharides down into monosaccharides, including glucose, the monosaccharides are transported across the wall of the small intestine and into the circulatory system, which transports them to the liver. In the liver, hepatocytes either pass the glucose on through the circulatory system or store excess glucose as glycogen.
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 The last step in glycolysis produces the product pyruvate.
Glycolysis begins with the phosphorylation of glucose by hexokinase to form glucosephosphate. This step uses one ATP, which is the donor of the phosphate group.
Under the action of phosphofructokinase, glucosephosphate is converted into fructosephosphate. At this point, a second ATP donates its phosphate group, forming fructose-1,6-bisphosphate. This six-carbon sugar is split to form two phosphorylated three-carbon molecules, glyceraldehydephosphate and dihydroxyacetone phosphate, which are both converted into glyceraldehydephosphate.
The glyceraldehydephosphate is further phosphorylated with groups donated by dihydrogen phosphate present in the cell to form the three-carbon molecule 1,3-bisphosphoglycerate. The energy of this reaction comes from the oxidation of removal of electrons from glyceraldehydephosphate.
In a series of reactions leading to pyruvate, the two phosphate groups are then transferred to two ADPs to form two ATPs. Thus, glycolysis uses two ATPs but generates four ATPs, yielding a net gain of two ATPs and two molecules of pyruvate.
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. Watch this video to learn about glycolysis. Glycolysis can be divided into two phases: energy consuming also called chemical priming and energy yielding.
The first phase is the energy-consuming phase , so it requires two ATP molecules to start the reaction for each molecule of glucose. However, the end of the reaction produces four ATPs, resulting in a net gain of two ATP energy molecules. The NADH that is produced in this process will be used later to produce ATP in the mitochondria.
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. The following discussions of glycolysis include the enzymes responsible for the reactions.
When glucose enters a cell, the enzyme hexokinase or glucokinase, in the liver rapidly adds a phosphate to convert it into glucosephosphate. A kinase is a type of enzyme that adds a phosphate molecule to a substrate in this case, glucose, but it can be true of other molecules also.
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. It also functions to maintain a concentration gradient with higher glucose levels in the blood than in the tissues.
By establishing this concentration gradient, the glucose in the blood will be able to flow from an area of high concentration the blood into an area of low concentration the tissues to be either used or stored. Hexokinase is found in nearly every tissue in the body.
Glucokinase , on the other hand, is expressed in tissues that are active when blood glucose levels are high, such as the liver. Hexokinase has a higher affinity for glucose than glucokinase and therefore is able to convert glucose at a faster rate than glucokinase. 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 next step of the first phase of glycolysis, the enzyme glucosephosphate isomerase converts glucosephosphate into fructosephosphate. Like glucose, fructose is also a six carbon-containing sugar. The enzyme phosphofructokinase-1 then adds one more phosphate to convert fructosephosphate into fructosebisphosphate, another six-carbon sugar, using another ATP molecule.
Aldolase then breaks down this fructosebisphosphate into two three-carbon molecules, glyceraldehydephosphate and dihydroxyacetone phosphate. The triosephosphate isomerase enzyme then converts dihydroxyacetone phosphate into a second glyceraldehydephosphate molecule.
Therefore, by the end of this chemical-priming or energy-consuming phase, one glucose molecule is broken down into two glyceraldehydephosphate molecules. The second phase of glycolysis, the energy-yielding phase , creates the energy that is the product of glycolysis.
Glyceraldehydephosphate dehydrogenase converts each three-carbon glyceraldehydephosphate produced during the energy-consuming phase into 1,3-bisphosphoglycerate. NADH is a high-energy molecule, like ATP, but unlike ATP, it is not used as energy currency by the cell.
Because there are two glyceraldehydephosphate molecules, two NADH molecules are synthesized during this step. Each 1,3-bisphosphoglycerate is subsequently dephosphorylated i. 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 enzyme phosphoglycerate mutase then converts the 3-phosphoglycerate molecules into 2-phosphoglycerate. The enolase enzyme then acts upon the 2-phosphoglycerate molecules to convert them into phosphoenolpyruvate molecules.
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.
In summary, one glucose molecule breaks down into two pyruvate molecules, and creates two net ATP molecules and two NADH molecules by glycolysis. 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.
When oxygen is limited or absent, pyruvate enters an anaerobic pathway called fermentation. In these reactions, pyruvate can be converted into lactic acid.
In this reaction, lactic acid replaces oxygen as the final electron acceptor. Anaerobic respiration occurs in most cells of the body when oxygen is limited or mitochondria are absent or nonfunctional. For example, because erythrocytes red blood cells lack mitochondria, they must produce their ATP from anaerobic respiration.
This is an effective pathway of ATP production for short periods of time, ranging from seconds to a few minutes. 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.
Similarly, when a person exercises, muscles use ATP faster than oxygen can be delivered to them. They depend on glycolysis and lactic acid production for rapid ATP production.
The NADH and FADH 2 pass electrons on to the electron transport chain, which uses the transferred energy to produce ATP. As the terminal step in the electron transport chain, oxygen is the terminal electron acceptor and creates water inside the mitochondria. 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 The Krebs cycle is also commonly called the citric acid cycle or the tricarboxylic acid TCA cycle.
During the Krebs cycle, high-energy molecules, including ATP, NADH, and FADH 2 , are created. NADH and FADH 2 then pass electrons through the electron transport chain in the mitochondria to generate more ATP molecules.
Watch this animation to observe the Krebs cycle. 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.
This reaction is an oxidative decarboxylation reaction. 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 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. Along the way, each citrate molecule will produce one ATP, one FADH 2 , and three NADH.
The FADH 2 and NADH will enter the oxidative phosphorylation system located in the inner mitochondrial membrane. In addition, the Krebs cycle supplies the starting materials to process and break down proteins and fats. 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.
The aconitase enzyme converts citrate into isocitrate. In two successive steps of oxidative decarboxylation, two molecules of CO 2 and two NADH molecules are produced when isocitrate dehydrogenase converts isocitrate into the five-carbon α-ketoglutarate, which is then catalyzed and converted into the four-carbon succinyl CoA by α-ketoglutarate dehydrogenase.
The enzyme succinyl CoA dehydrogenase then converts succinyl CoA into succinate and forms the high-energy molecule GTP, which transfers its energy to ADP to produce ATP.
Succinate dehydrogenase then converts succinate into fumarate, forming a molecule of FADH 2. Oxaloacetate is then ready to combine with the next acetyl CoA to start the Krebs cycle again see Figure For each turn of the cycle, three NADH, one ATP through GTP , and one FADH 2 are created.
Each carbon of pyruvate is converted into CO 2 , which is released as a byproduct of oxidative aerobic respiration.
Carbohydrate metabolism is the whole of the Carvohydrate processes responsible for the metabolic formationbreakdown Blood sugar regulation in athletes, and interconversion of carbohydrates in living organisms. Carbohydrates are Carbohyrate to Plyometric training metabolismm metabolic annd. Humans can consume Plyometric training variety Plyometric training Carbogydrate, digestion breaks down complex carbohydrates into simple monomers monosaccharides : glucosefructosemannose and galactose. After resorption in the gutthe monosaccharides are transported, through the portal veinto the liver, where all non-glucose monosacharids fructose, galactose are transformed into glucose as well. Glycolysis is the process of breaking down a glucose molecule into two pyruvate molecules, while storing energy released during this process as adenosine triphosphate ATP and nicotinamide adenine dinucleotide NADH. Metabolic support vitamins your institution subscribes to Carbohydrate metabolism and nutrition Carbohyfrate, and you don't have mettabolism Access Profile, please contact your library's reference aCrbohydrate Plyometric training information on how to Carbohydrate metabolism and nutrition access to this resource from Carbohydrate metabolism and nutrition. Metxbolism the Nhtrition library with you Carrbohydrate you go—easy access to books, videos, images, podcasts, personalized features, nutrltion more. Download the Access App here: iOS and Android. Learn more here! Please consult the latest official manual style if you have any questions regarding the format accuracy. The breakdown catabolism and synthesis anabolism of carbohydrate molecules represent the primary means for the human body to store and utilize energy and to provide building blocks for molecules such as nucleotides Figure The enzyme reactions that form the metabolic pathways for monosaccharide carbohydrates Chapter 2 include glycolysisthe citric acid cycleand oxidative phosphorylation as the main means to produce the energy molecule adenosine triphosphate ATP.
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