24.2 Carbohydrate Metabolism
3 min read•june 18, 2024
Glucose breakdown and energy production are vital cellular processes. splits glucose into , while the further breaks down . These steps generate ATP and electron carriers for the .
The electron transport chain uses electron carriers to create a proton gradient. This gradient powers , producing most of the cell's ATP. reverses this process, making glucose from non-carbohydrate sources when needed.
Glycolysis and the Krebs Cycle
Steps and outcomes of glycolysis
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- breaks down glucose into two pyruvate molecules through a 10-step process
- ATP phosphorylates glucose forming (catalyzed by hexokinase)
- ATP phosphorylates creating (catalyzed by )
- Fructose-1,6-bisphosphate splits into (G3P) and (DHAP), two 3-carbon molecules
- Oxidation and phosphorylation of G3P yields
- Conversion of 1,3-bisphosphoglycerate to generates 2 ATP
- 3-phosphoglycerate converts to
- 2-phosphoglycerate converts to (PEP)
- PEP converts to pyruvate generating 2 ATP
- Glycolysis yields a net energy outcome of 2 ATP and 2 per glucose molecule (glucose to 2 pyruvate)
Pyruvate in the Krebs cycle
- Pyruvate converts to releasing CO2 and generating 1 NADH (catalyzed by )
- Acetyl-CoA combines with forming
- Citrate converts to
- Oxidation of isocitrate to releases CO2 and generates 1 NADH
- Oxidation of α-ketoglutarate to releases CO2 and generates 1 NADH
- Conversion of succinyl-CoA to generates 1 (or ATP)
- Oxidation of succinate to generates 1
- Fumarate undergoes hydration to
- Oxidation of malate to oxaloacetate generates 1 NADH
- Each pyruvate molecule yields 3 NADH, 1 FADH2, 1 GTP (or ATP), and 2 CO2 in the ()
Electron Transport Chain and ATP Synthesis
Electron flow in cellular respiration
- The consists of protein complexes in the inner mitochondrial membrane
- NADH and FADH2 from glycolysis and the Krebs cycle donate electrons to the ETC
- Complex I receives electrons from NADH
- Complex II receives electrons from FADH2
- Electrons undergo redox reactions as they pass through ETC complexes (I, III, and IV)
- Protons (H+) are pumped from the mitochondrial matrix into the intermembrane space during electron flow
- The ETC-generated proton gradient drives ATP synthesis via oxidative phosphorylation
- Oxygen acts as the final electron acceptor combining with protons to form water (cellular respiration)
Mechanism of oxidative phosphorylation
- ATP synthase, an enzyme complex in the inner mitochondrial membrane, synthesizes ATP
- The ETC-generated proton gradient drives protons back into the mitochondrial matrix through ATP synthase
- Proton flow through ATP synthase causes conformational changes that catalyze ADP phosphorylation to ATP
- links the proton gradient's chemical energy to ATP synthesis
- The number of ATP molecules generated per NADH and FADH2 depends on the specific ETC complexes involved
- NADH typically yields 2.5-3 ATP
- FADH2 typically yields 1.5-2 ATP
Gluconeogenesis
Glucose production via gluconeogenesis
- synthesizes new glucose molecules from non-carbohydrate precursors
- Main gluconeogenic substrates include:
- Amino acids (from protein catabolism)
- Glycerol (from triglyceride breakdown)
- Lactate (from anaerobic glycolysis in skeletal muscle)
- The liver is the primary site of gluconeogenesis with minor contributions from the kidneys
- Key gluconeogenic steps:
- carboxylates pyruvate to oxaloacetate
- decarboxylates and phosphorylates oxaloacetate forming phosphoenolpyruvate (PEP)
- converts fructose-1,6-bisphosphate to fructose-6-phosphate
- removes the phosphate from yielding free glucose
- Hormones like and cortisol regulate gluconeogenesis promoting the process during fasting or stress (low blood glucose)
Additional Carbohydrate Metabolism Pathways
Glycogen metabolism
- Glycogenesis: synthesis of glycogen from glucose for storage
- : breakdown of glycogen to glucose-1-phosphate for energy production
Alternative glucose metabolism
- : generates NADPH and ribose-5-phosphate for biosynthetic processes
Metabolic regulation
- Allosteric regulation of key enzymes (e.g., phosphofructokinase) controls flux through metabolic pathways
- Hormonal control (e.g., , ) coordinates carbohydrate metabolism with whole-body energy needs
Key Terms to Review (55)
1,3-bisphosphoglycerate: 1,3-bisphosphoglycerate (1,3-BPG) is a high-energy intermediate in the glycolysis pathway, formed from the conversion of glyceraldehyde-3-phosphate. This compound plays a crucial role in the metabolic process by storing energy that can later be transferred to ATP production, highlighting its importance in cellular respiration and energy balance.
2-phosphoglycerate: 2-phosphoglycerate is an important intermediate in the glycolysis pathway, which is the metabolic process that breaks down glucose to produce energy for the body. It is the sixth step in the 10-step glycolysis process and plays a crucial role in the conversion of 3-phosphoglycerate to phosphoenolpyruvate, the penultimate step before the formation of pyruvate, the final product of glycolysis.
3-phosphoglycerate: 3-phosphoglycerate is an important intermediate in the glycolysis pathway, which is the metabolic process that converts glucose into energy for the body. It is produced during the third step of glycolysis and plays a crucial role in carbohydrate metabolism.
Acetyl coenzyme A (acetyl CoA): Acetyl Coenzyme A, commonly known as Acetyl CoA, is a central molecule in metabolism that carries acetyl groups for chemical reactions in the cell. It plays a pivotal role in the breakdown of carbohydrates, fats, and proteins into energy.
Acetyl-CoA: Acetyl-CoA is a key molecule in cellular metabolism, serving as a central hub that connects the breakdown of carbohydrates, fats, and some amino acids to the production of energy through the citric acid cycle. It is the entry point for these nutrients into the final common pathway of cellular respiration.
ATP Synthase: ATP synthase is a crucial enzyme complex responsible for the final step of cellular respiration, the production of adenosine triphosphate (ATP), the primary energy currency of cells. It is embedded in the inner mitochondrial membrane and utilizes the proton gradient established by the electron transport chain to drive the synthesis of ATP from ADP and inorganic phosphate.
Chemiosmotic coupling: Chemiosmotic coupling is the process that links the flow of ions across a membrane to the synthesis of ATP, a key energy currency in cells. This process occurs primarily in mitochondria during cellular respiration, where a proton gradient is established, allowing protons to flow back into the mitochondrial matrix and drive the conversion of ADP and inorganic phosphate into ATP through ATP synthase.
Citrate: Citrate is a key intermediate in the citric acid cycle, also known as the Krebs cycle, which is a central metabolic pathway in cellular respiration. It plays a crucial role in the conversion of carbohydrates, fats, and proteins into energy in the form of ATP within the mitochondria of cells.
Citric acid cycle: The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide. It is a crucial part of cellular respiration in the metabolism and nutrition chapter, under the topic of carbohydrate metabolism.
Citric Acid Cycle: The citric acid cycle, also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle, is a series of chemical reactions that occur in the mitochondria of cells. It is a key metabolic pathway that plays a central role in the aerobic cellular respiration process, generating energy in the form of ATP for the body's cells.
Dihydroxyacetone Phosphate: Dihydroxyacetone phosphate (DHAP) is an important intermediate in the glycolytic pathway, the metabolic process that converts glucose into energy. DHAP is a three-carbon sugar phosphate that serves as a key branch point in carbohydrate metabolism, connecting glycolysis to other metabolic pathways.
Electron Transport Chain: The electron transport chain is a series of protein complexes and electron carriers embedded in the inner membrane of mitochondria that facilitate the final steps of cellular respiration. It is a crucial component of the metabolic pathways that convert the energy stored in nutrients into usable forms of energy for the cell.
Electron transport chain (ETC): The electron transport chain is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions, and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. This process generates ATP, the main energy currency in cells, through oxidative phosphorylation.
Energy-consuming phase: The energy-consuming phase is a stage in carbohydrate metabolism where the body uses energy, typically ATP, to initiate reactions that ultimately lead to the synthesis of complex molecules from simpler ones. This phase is crucial for building up tissues and storing energy for future needs.
Energy-yielding phase: The energy-yielding phase is a stage in carbohydrate metabolism where cells break down glucose to produce ATP, which serves as the primary energy currency of the cell. This process mainly occurs during glycolysis and the citric acid cycle, efficiently converting nutrients into usable cellular energy.
FADH2: FADH2, or flavin adenine dinucleotide, is a coenzyme that plays a crucial role in cellular respiration and energy production within the body. It is a key component of the electron transport chain, a series of reactions that generate the majority of the cell's energy in the form of ATP.
Fructose-1,6-bisphosphatase: Fructose-1,6-bisphosphatase is a key regulatory enzyme in gluconeogenesis, the process of synthesizing glucose from non-carbohydrate precursors. It catalyzes the hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate, an essential step in the conversion of three-carbon molecules like lactate and amino acids into glucose.
Fructose-1,6-bisphosphate: Fructose-1,6-bisphosphate, also known as F-1,6-BP or fructose bisphosphate, is a key intermediate in the glycolytic pathway of carbohydrate metabolism. It is an important regulatory molecule that links the breakdown of glucose to the production of energy-rich ATP molecules within the cell.
Fructose-6-phosphate: Fructose-6-phosphate is a key intermediate in the glycolytic pathway and the pentose phosphate pathway, formed from fructose-1,6-bisphosphate. This molecule plays a significant role in carbohydrate metabolism, as it is involved in various enzymatic reactions that contribute to energy production and the synthesis of important biomolecules.
Fumarate: Fumarate is a key intermediate in the citric acid cycle, also known as the Krebs cycle, which is a central metabolic pathway in cellular respiration. It is formed by the oxidation of succinate and serves as a crucial link between carbohydrate, lipid, and amino acid metabolism, playing a vital role in energy production within the body.
Glucagon: Glucagon is a hormone produced by the alpha cells in the pancreas that raises blood glucose levels by promoting the conversion of stored glycogen to glucose in the liver. It plays a critical role in glucose homeostasis, especially during periods of fasting or low blood sugar.
Glucagon: Glucagon is a hormone produced by the alpha cells of the pancreatic islets. As a key regulator of glucose metabolism, glucagon plays a crucial role in the endocrine system, carbohydrate metabolism, and overall metabolic states of the body.
Gluconeogenesis: Gluconeogenesis is a metabolic process by which the body produces glucose from non-carbohydrate sources, such as amino acids, lactate, and glycerol. This pathway is crucial for maintaining blood glucose levels during periods of fasting or intense exercise.
Gluconeogenesis: Gluconeogenesis is the metabolic process by which the body synthesizes glucose from non-carbohydrate precursors, such as amino acids, lactate, and glycerol. This process is crucial for maintaining blood glucose levels, especially during periods of fasting or prolonged exercise when carbohydrate stores are depleted.
Glucose-6-phosphatase: Glucose-6-phosphatase is an enzyme that plays a crucial role in carbohydrate metabolism by catalyzing the hydrolysis of glucose-6-phosphate to glucose and inorganic phosphate. This reaction is essential for maintaining blood glucose levels, especially during fasting, as it allows for the release of free glucose into the bloodstream from the liver. The enzyme is primarily found in the liver and kidneys, where it contributes to gluconeogenesis and glycogenolysis, processes critical for energy production and glucose homeostasis.
Glucose-6-Phosphate: Glucose-6-phosphate is a key intermediate in carbohydrate metabolism, formed during the first step of glycolysis when glucose is phosphorylated by the enzyme hexokinase. It serves as a central hub, directing glucose towards various metabolic pathways depending on the body's energy needs and cellular conditions.
Glucose-6-phosphate : Glucose-6-phosphate is a glucose sugar molecule that has been phosphorylated at the sixth carbon. It is a critical intermediate in the pathways of glucose metabolism, including glycolysis and gluconeogenesis.
Glyceraldehyde-3-Phosphate: Glyceraldehyde-3-phosphate (G3P or GAP) is a key intermediate in the glycolysis pathway, which is the metabolic process that converts glucose into energy for the cell. It is an important molecule that links the earlier and later stages of carbohydrate metabolism.
Glycogenolysis: Glycogenolysis is the metabolic process by which glycogen, the storage form of glucose in the body, is broken down into glucose. This process occurs primarily in the liver and skeletal muscles, providing a readily available source of glucose for energy production when needed by the body.
Glycolysis: Glycolysis is a metabolic pathway that breaks down glucose into pyruvate, releasing energy and producing ATP (adenosine triphosphate) and NADH. It occurs in the cytoplasm of cells and does not require oxygen, making it an anaerobic process.
Glycolysis: Glycolysis is the metabolic pathway that converts glucose, the primary fuel for cellular respiration, into two molecules of pyruvate. This process is the first step in the breakdown of glucose to produce energy in the form of ATP for the body's cells. Glycolysis is a fundamental metabolic process that is central to the functions of human life, exercise and muscle performance, and overall energy metabolism.
GTP: GTP, or Guanosine Triphosphate, is a high-energy nucleotide that plays crucial roles in both protein synthesis and carbohydrate metabolism. It serves as an energy-carrying molecule, providing the necessary energy for various cellular processes, including the translation of mRNA into proteins and the regulation of metabolic pathways involving carbohydrates.
Insulin: Insulin is a hormone produced by the pancreas that plays a crucial role in regulating blood glucose levels and facilitating the metabolism of carbohydrates, fats, and proteins in the body. It is essential for maintaining homeostasis, supporting the functions of human life, and ensuring the proper utilization of organic compounds necessary for human functioning.
Insulin-like growth factors (IGFs): Insulin-like Growth Factors are proteins with a high similarity to insulin that play a crucial role in childhood growth and continue to have anabolic effects in adults. They are produced by the liver upon stimulation by growth hormone (GH) and act on various tissues, contributing to growth and development.
Isocitrate: Isocitrate is a key intermediate in the citric acid cycle, also known as the Krebs cycle, which is a central metabolic pathway in cellular respiration. It is an important molecule involved in the oxidation of acetyl-CoA to produce energy in the form of ATP within the mitochondria of cells.
Krebs cycle: The Krebs cycle is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide and water. It is a key component of the cellular respiration process that occurs in the mitochondria, producing ATP, NADH, and FADH2 as energy carriers.
Krebs Cycle: The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a series of chemical reactions that occur in the mitochondria of cells. It is a key metabolic pathway that generates energy in the form of ATP during cellular respiration, and it is a central component of carbohydrate, fat, and protein metabolism.
Malate: Malate is a four-carbon dicarboxylic acid that plays a crucial role in the citric acid cycle (Krebs cycle) and in various metabolic pathways related to carbohydrate metabolism. It is formed from fumarate through the action of the enzyme fumarase and can be converted into oxaloacetate, making it essential for energy production in cells and facilitating the process of gluconeogenesis, where glucose is synthesized from non-carbohydrate sources.
Metabolic Regulation: Metabolic regulation is the process by which the body controls and modulates its various metabolic pathways to maintain homeostasis and ensure efficient energy production, storage, and utilization. It involves the coordination of catabolic and anabolic processes to meet the body's energy demands and support cellular function.
Mitochondria: Mitochondria are double-membrane-bound organelles found in the cytoplasm of eukaryotic cells, often referred to as the powerhouse of the cell due to their role in producing adenosine triphosphate (ATP) through cellular respiration. These organelles play a crucial role in energy metabolism, converting nutrients into usable energy while also being involved in other important cellular functions such as apoptosis and calcium homeostasis.
NADH: NADH, or nicotinamide adenine dinucleotide (reduced), is a coenzyme that plays a crucial role in cellular metabolism. It is the reduced form of NAD+, which is an essential cofactor in numerous metabolic reactions, including those involved in energy production, carbohydrate and lipid metabolism, and cellular signaling.
Oxaloacetate: Oxaloacetate is a key intermediate in the citric acid cycle, also known as the Krebs cycle, which is a central metabolic pathway in cellular respiration. It is a four-carbon dicarboxylic acid that plays a crucial role in the conversion of carbohydrates, fats, and proteins into energy in the form of ATP within the mitochondria of cells.
Pentose Phosphate Pathway: The pentose phosphate pathway, also known as the hexose monophosphate shunt, is an alternative metabolic pathway to glycolysis that generates NADPH and pentose sugars. It is an important process in carbohydrate metabolism, providing reducing power and precursors for biosynthesis.
PEP carboxykinase: PEP carboxykinase, also known as phosphoenolpyruvate carboxykinase, is a key enzyme involved in gluconeogenesis, the process of synthesizing glucose from non-carbohydrate precursors. It catalyzes the conversion of oxaloacetate to phosphoenolpyruvate, a crucial step in the gluconeogenic pathway.
Phosphoenolpyruvate: Phosphoenolpyruvate (PEP) is a key intermediate in the metabolic pathways of carbohydrates, serving as an important precursor in both glycolysis and gluconeogenesis. It is a high-energy phosphate compound that plays a crucial role in the regulation and coordination of cellular energy production and utilization.
Phosphofructokinase: Phosphofructokinase is a critical enzyme that catalyzes the irreversible, rate-limiting step in glycolysis - the metabolic pathway that converts glucose into usable energy for the body. It plays a pivotal role in regulating carbohydrate metabolism by controlling the flow of glucose through this essential metabolic process.
Pyruvate: Pyruvate is a key end product of glycolysis, which is the process of breaking down glucose for energy in cells. It serves as a crucial intersection in several metabolic pathways, especially in carbohydrate metabolism within the context of anatomy and physiology.
Pyruvate: Pyruvate is a key intermediate in carbohydrate metabolism, serving as the final product of glycolysis and the entry point for the citric acid cycle. It is a versatile molecule that plays a central role in energy production within cells.
Pyruvate Carboxylase: Pyruvate carboxylase is a crucial enzyme involved in carbohydrate metabolism, catalyzing the conversion of pyruvate to oxaloacetate. This reaction is an important anaplerotic process that replenishes intermediates in the tricarboxylic acid (TCA) cycle, allowing for the continued oxidation of acetyl-CoA and the production of cellular energy in the form of ATP.
Pyruvate Dehydrogenase Complex: The pyruvate dehydrogenase complex is a large, multienzyme complex that catalyzes the conversion of pyruvate, the end product of glycolysis, into acetyl-CoA, which can then enter the citric acid cycle for further energy production. This complex plays a crucial role in the overall process of carbohydrate metabolism.
Succinate: Succinate is an important intermediate in the citric acid cycle, also known as the Krebs cycle, which is a central metabolic pathway in cellular respiration. It is a dicarboxylic acid that plays a crucial role in the conversion of nutrients into usable energy for the body.
Succinyl-CoA: Succinyl-CoA is a key intermediate in the citric acid cycle, also known as the Krebs cycle, which is a central metabolic pathway in cellular respiration. It is formed from the oxidation of pyruvate and serves as an important entry point for the breakdown of fatty acids and amino acids into the cycle, ultimately leading to the production of ATP, the primary energy currency of the cell.
Terminal electron acceptor: A terminal electron acceptor is a compound that receives or accepts electrons at the end of an electron transport chain during cellular respiration or photosynthesis. This process is crucial for the synthesis of ATP, the energy currency of cells.
Tricarboxylic acid cycle (TCA): The Tricarboxylic Acid Cycle, commonly known as the citric acid cycle or Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide. In addition to ATP production, it provides precursors for many compounds including some amino acids and is crucial for cellular respiration in the mitochondria.
α-ketoglutarate: α-ketoglutarate is a key intermediate in the citric acid cycle (Krebs cycle) that plays a significant role in energy production and amino acid metabolism. It serves as a crucial link between carbohydrate metabolism and the synthesis of amino acids, particularly glutamate, and is also involved in the regulation of cellular energy status through its conversion to succinyl-CoA.