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Cellular Respiration: How Do Cell Get Energy?Carbohydrates and Cellular Respiration -
In this paper a summary of the metabolism of carbohydrates is presented in a way that researchers can follow the biochemical processes easily. Keywords: Carbohydrate; Gluconeogenesis; Glycogenesis; Glycogenolysis; Glycolysis; Oxidative pathway; Pentose phosphate pathway; Pyruvate decarboxylation.
Copyright © The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.
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.
Figure 2. During the energy-consuming phase of glycolysis, two ATPs are consumed, transferring two phosphates to the glucose molecule. The glucose molecule then splits into two three-carbon compounds, each containing a phosphate.
During the second phase, an additional phosphate is added to each of the three-carbon compounds. The energy for this endergonic reaction is provided by the removal oxidation of two electrons from each three-carbon compound. During the energy-releasing phase, the phosphates are removed from both three-carbon compounds and used to produce four ATP molecules.
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. 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 FADH2 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.
Figure 3. Click to view a larger image. The process of anaerobic respiration converts glucose into two lactate molecules in the absence of oxygen or within erythrocytes that lack mitochondria.
During aerobic respiration, glucose is oxidized into two pyruvate molecules. 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 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 FADH2, are created.
NADH and FADH2 then pass electrons through the electron transport chain in the mitochondria to generate more ATP molecules. The overall mechanism of cellular respiration involves four processes: glycolysis, in which glucose molecules are broken down to form pyruvic acid molecules; the Krebs cycle, in which pyruvic acid is further broken down and the energy in its molecule is used to form high-energy compounds, such as nicotinamide adenine dinucleotide NADH ; the electron transport system, in which electrons are transported along a series of coenzymes and cytochromes and the energy in the electrons is released; and chemiosmosis, in which the energy given off by electrons pumps protons across a membrane and provides the energy for ATP synthesis.
The general chemical equation for cellular respiration is:. Figure provides an overview of cellular respiration. Glucose is converted to pyruvic acid in the cytoplasm, which is then used to produce acetyl CoA in the mitochondrion. Finally, the Krebs cycle proceeds in the mitochondrion.
Electron transport and chemiosmosis result in energy release; ATP synthesis also occurs in the mitochondrion. Previous Quiz The Process of Photosynthesis. Next Glycolysis. Removing book from your Reading List will also remove any bookmarked pages associated with this title.
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Cllular are one of the major forms Carbohyydrates energy for animals and Carbohydrates and Cellular Respiration. Plants build Carbhoydrates using light energy from the sun during the process of photosynthesiswhile Carbohtdrates eat plants or other animals to obtain Respiratipn. Plants store carbohydrates in Carbohydrates and Cellular Respiration Antioxidant-Rich Haircare Products chains called starch, while animals store carbohydrates as the molecule glycogen. These large polysaccharides contain many chemical bonds and therefore store a lot of chemical energy. When these molecules are broken down during metabolism, the energy in the chemical bonds is released and can be harnessed for cellular processes. The metabolism of any monosaccharide simple sugar can produce energy for the cell to use. Excess carbohydrates are stored as starch in plants and as glycogen in animals, ready for metabolism if the energy demands of the organism suddenly increase.
There are Carbohydrrates primary functions of Carbohydrates and Cellular Respiration in the Carbohydratea body. Carbohydrates and Cellular Respiration are energy production, energy storage, building Glutamine and cell regeneration, sparing protein, and assisting in lipid metabolism. The primary role of carbohydrates is to supply energy to all cells in the body. Many cells prefer glucose as a source of energy versus other compounds like fatty acids. Some cells, such as red blood cells, are only able to produce cellular energy from glucose.
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