The origin of aerobic respiration
Energy From Dietary Molecules
Glycolysis converts glucose to pyruvate, and electrons are transferred from pyruvate to coenzymes during aerobic respiration’s second stage. In other words, glucose becomes oxidized (it gives up electrons) and coenzymes become reduced (they accept electrons). Oxidizing an organic molecule can break the covalent bonds of its carbon backbone. Aerobic respiration produces a lot of ATP by fully oxidizing glucose, completely dismantling it carbon by carbon. Cells also dismantle other organic molecules by oxidizing them. Complex carbohydrates, fats, and proteins in food can be converted to molecules that enter glycolysis or the Krebs cycle. As in glucose metabolism, many coenzymes are reduced, and the energy of the electrons they carry ultimately drives the synthesis of ATP in electron transfer phosphorylation.
Complex Carbohydrates
In humans and mammals, the digestive system breaks down starch and other complex carbohydrates to monosaccharides. The sugars are taken up by cells and converted to glucose-6-phosphate for glycolysis. When a cell produces more ATP than it uses, ATP accumulates in the cytoplasm. The high concentration of ATP causes glucose-6-phosphate to be diverted away from glycolysis and into a pathway that builds glycogen
In nearly all eukaryotes and some bacteria, the main carbohydrate-breakdown pathway is aerobic respiration. The bonds of organic molecules
hold a lot of energy that can be released in a reaction with oxygen. Aerobic respiration harvests energy from sugars by completely breaking apart their carbon backbones, bond by bond, and it uses oxygen to do this. The pathway’s products—carbon dioxide and water—are the raw materials used by the vast majority of photosynthetic organisms to build the sugars in the first place. With this connection, the cycling of carbon, hydrogen, and oxygen through living things comes full circle through the biosphere. The reactions in the pathway occur in three stages.
Glycolysis is the first stage of sugar breakdown in aerobic respiration.
The first stage, glycolysis, is a linear pathway that takes place in cytoplasm. Glycolysis begins the breakdown of one sugar molecule for a net
yield of 2 ATP. In eukaryotes, the next two stages take place in mitochondria.
In the second stage, the Krebs cycle completes the breakdown of the sugar molecule to CO2.
The second stage of aerobic respiration occurs in mitochondria. It includes two sets of reactions, acetyl–CoA formation and the Krebs cycle, that break down pyruvate, the product of glycolysis. All of the carbon atoms that were once part of glucose end up in CO2, which departs the cell. Only two ATP form, but the reactions reduce many coenzymes. The energy of electrons carried by these coenzymes will drive the reactions of aerobic respiration’s third stage. This cyclic pathway produces 2 ATP and reduces ( or charges energetically ) many coenzymes.
In the third stage, electron transfer phosphorylation, the coenzymes reduced during glycolysis and the Krebs cycle deliver electrons and hydrogen ions to electron transfer chains. Energy released by electrons as they move through the chains drives the formation of as many as 32 ATP. At the end of the electron transfer chains, water forms when oxygen accepts hydrogen ions and electrons.
In exactly the reverse of the photolysis reaction that splits water during the noncyclic, light-dependent reactions of photosynthesis, oxygen combines with electrons and hydrogen ions to form water. Aerobic respiration, which means “taking a breath of air,” refers to this pathway’s requirement for oxygen as the final acceptor of electrons.
Acetyl–CoA Formation
Aerobic respiration’s second stage begins when the two pyruvate molecules that formed during glycolysis enter a mitochondrion. Pyruvate is transported across the mitochondrion’s two membranes and into the inner compartment, which is called the mitochondrial matrix. There, an enzyme immediately splits each pyruvate into one molecule of CO2 and a two-carbon acetyl group The CO2 diffuses out of the cell, and the acetyl group combines with a coenzyme rather unimaginatively named coenzyme A (abbreviated CoA). The product of this reaction is acetyl–CoA. Electrons and hydrogen ions released by the reaction combine with NAD+, so NADH also forms.
The second-stage reactions convert the two pyruvate that formed in glycolysis to six CO2. Two ATP form, and ten coenzymes (eight NAD+ and two FAD) are reduced.
What happens during the third stage of aerobic respiration?
In aerobic respiration’s third stage, electron transfer phosphorylation, energy released by electrons moving through electron transfer chains is ultimately captured in the attachment of phosphate to ADP.
Coenzymes that were reduced in the first and second stages deliver electrons and hydrogen ions to electron transfer chains in the inner
mitochondrial membrane.
Energy released by electrons as they pass through electron transfer chains is used to pump H+ from the mitochondrial matrix to the
intermembrane space. The H+ gradient that forms across the inner mitochondrial membrane drives the flow of hydrogen ions through ATP
synthases, which results in ATP formation.
About thirty-two ATP form during the third-stage reactions, so a typical net yield of all three stages is thirty-six ATP per glucose.
In eukaryotes, the third stage of aerobic respiration, electron transfer phosphorylation, occurs at the inner mitochondrial membrane. The reactions of electron transfer phosphorylation begin with the coenzymes NADH and FADH+, which became reduced in the first two stages of aerobic respiration. These coenzymes donate their cargo of electrons and hydrogen ions to electron transfer chains embedded in the inner mitochondrial membrane As the electrons move through the chains, they give up energy little by little. Some molecules of the electron transfer chains harness that energy to actively transport hydrogen ions across the inner membrane, from the matrix to the intermembrane space . The accumulating hydrogen ions form a gradient across the inner mitochondrial membrane. This gradient attracts the ions back toward the matrix, but ions cannot diffuse through a lipid bilayer on their own. Hydrogen ions cross the inner mitochondrial membrane only by flowing through ATP synthases embedded in the membrane. The flow of hydrogen ions through ATP synthases causes these proteins to attach phosphate groups to ADP, so ATP forms.
Oxygen accepts electrons at the end of the mitochondrial electron transfer chains. When oxygen accepts electrons, it combines with H+ to form water, which is a product of the third-stage reactions. For each glucose molecule that enters aerobic respiration, four ATP form in the first- and second-stage reactions. The twelve coenzymes reduced in these two stages deliver enough H+ and electrons to fuel the synthesis of about thirty-two additional ATP in the third stage. Thus, the breakdown of one glucose molecule typically yields thirty-six ATP.
Enzymes that require metal cofactors are a critical part of metabolism. Oxygen reacts with metal cofactors, and free radicals form during those reactions. Free radicals damage biological molecules, so they are dangerous to life. ATP participates in almost all cellular reactions, so a cell benefits from making a lot of it. However, aerobic respiration is a dangerous occupation. When an oxygen molecule (O2) accepts electrons from an electron transfer chain, it dissociates into oxygen atoms. Most of the atoms immediately combine with hydrogen ions and end up in water molecules. Occasionally, however, an oxygen atom escapes this final reaction. The atom has an unpaired electron, so it is a free radical.
Mitochondria cannot detoxify free radicals, so they rely on antioxidant enzymes and vitamins in the cell’s cytoplasm to do it for them. The system works well, at least most of the time. However, a genetic disorder or an encounter with a toxin or pathogen can tip the normal cellular balance of aerobic respiration and free radical formation. Free radicals accumulate and destroy first the function of mitochondria, then the cell. The resulting tissue damage is called oxidative stress.
At least 83 proteins are directly involved in mitochondrial electron transfer chains. A defect in any one of them—or in any of the thousands of other proteins used by mitochondria—can wreak havoc in the body. New research is showing that oxidative stress caused by mitochondrial malfunction is also involved in many other illnesses, including cancer, hypertension, Alzheimer’s and Parkinson’s diseases, and even aging. Hundreds of incurable genetic disorders are associated with mitochondrial defects, and more are being discovered all the time. Nerve cells, which require a lot of ATP, are particularly affected. Symptoms of these disorders range from mild to major progressive neurological deficits, blindness, deafness, diabetes, strokes, seizures, gastrointestinal malfunction, and disabling muscle weakness. Photosynthetic organisms capture energy from the sun and store it in the form of sugars.
They and most other organisms use energy stored in sugars to run various endergonic reactions of metabolism that sustain life. However, sugars rarely participate in such reactions, so how do cells harness their energy? In order to use the energy stored in sugars, cells must first transfer it to molecules—ATP in particular— that do participate in energy-requiring reactions. Cells break the bonds between carbon atoms of a sugar molecule, and use energy released as these bonds break to drive ATP synthesis.
Energy From Dietary Molecules
Glycolysis converts glucose to pyruvate, and electrons are transferred from pyruvate to coenzymes during aerobic respiration’s second stage. In other words, glucose becomes oxidized (it gives up electrons) and coenzymes become reduced (they accept electrons). Oxidizing an organic molecule can break the covalent bonds of its carbon backbone. Aerobic respiration produces a lot of ATP by fully oxidizing glucose, completely dismantling it carbon by carbon. Cells also dismantle other organic molecules by oxidizing them. Complex carbohydrates, fats, and proteins in food can be converted to molecules that enter glycolysis or the Krebs cycle. As in glucose metabolism, many coenzymes are reduced, and the energy of the electrons they carry ultimately drives the synthesis of ATP in electron transfer phosphorylation.
Complex Carbohydrates
In humans and mammals, the digestive system breaks down starch and other complex carbohydrates to monosaccharides. The sugars are taken up by cells and converted to glucose-6-phosphate for glycolysis. When a cell produces more ATP than it uses, ATP accumulates in the cytoplasm. The high concentration of ATP causes glucose-6-phosphate to be diverted away from glycolysis and into a pathway that builds glycogen
In nearly all eukaryotes and some bacteria, the main carbohydrate-breakdown pathway is aerobic respiration. The bonds of organic molecules
hold a lot of energy that can be released in a reaction with oxygen. Aerobic respiration harvests energy from sugars by completely breaking apart their carbon backbones, bond by bond, and it uses oxygen to do this. The pathway’s products—carbon dioxide and water—are the raw materials used by the vast majority of photosynthetic organisms to build the sugars in the first place. With this connection, the cycling of carbon, hydrogen, and oxygen through living things comes full circle through the biosphere. The reactions in the pathway occur in three stages.
Glycolysis is the first stage of sugar breakdown in aerobic respiration.
The first stage, glycolysis, is a linear pathway that takes place in cytoplasm. Glycolysis begins the breakdown of one sugar molecule for a net
yield of 2 ATP. In eukaryotes, the next two stages take place in mitochondria.
In the second stage, the Krebs cycle completes the breakdown of the sugar molecule to CO2.
The second stage of aerobic respiration occurs in mitochondria. It includes two sets of reactions, acetyl–CoA formation and the Krebs cycle, that break down pyruvate, the product of glycolysis. All of the carbon atoms that were once part of glucose end up in CO2, which departs the cell. Only two ATP form, but the reactions reduce many coenzymes. The energy of electrons carried by these coenzymes will drive the reactions of aerobic respiration’s third stage. This cyclic pathway produces 2 ATP and reduces ( or charges energetically ) many coenzymes.
In the third stage, electron transfer phosphorylation, the coenzymes reduced during glycolysis and the Krebs cycle deliver electrons and hydrogen ions to electron transfer chains. Energy released by electrons as they move through the chains drives the formation of as many as 32 ATP. At the end of the electron transfer chains, water forms when oxygen accepts hydrogen ions and electrons.
In exactly the reverse of the photolysis reaction that splits water during the noncyclic, light-dependent reactions of photosynthesis, oxygen combines with electrons and hydrogen ions to form water. Aerobic respiration, which means “taking a breath of air,” refers to this pathway’s requirement for oxygen as the final acceptor of electrons.
Acetyl–CoA Formation
Aerobic respiration’s second stage begins when the two pyruvate molecules that formed during glycolysis enter a mitochondrion. Pyruvate is transported across the mitochondrion’s two membranes and into the inner compartment, which is called the mitochondrial matrix. There, an enzyme immediately splits each pyruvate into one molecule of CO2 and a two-carbon acetyl group The CO2 diffuses out of the cell, and the acetyl group combines with a coenzyme rather unimaginatively named coenzyme A (abbreviated CoA). The product of this reaction is acetyl–CoA. Electrons and hydrogen ions released by the reaction combine with NAD+, so NADH also forms.
The second-stage reactions convert the two pyruvate that formed in glycolysis to six CO2. Two ATP form, and ten coenzymes (eight NAD+ and two FAD) are reduced.
What happens during the third stage of aerobic respiration?
In aerobic respiration’s third stage, electron transfer phosphorylation, energy released by electrons moving through electron transfer chains is ultimately captured in the attachment of phosphate to ADP.
Coenzymes that were reduced in the first and second stages deliver electrons and hydrogen ions to electron transfer chains in the inner
mitochondrial membrane.
Energy released by electrons as they pass through electron transfer chains is used to pump H+ from the mitochondrial matrix to the
intermembrane space. The H+ gradient that forms across the inner mitochondrial membrane drives the flow of hydrogen ions through ATP
synthases, which results in ATP formation.
About thirty-two ATP form during the third-stage reactions, so a typical net yield of all three stages is thirty-six ATP per glucose.
In eukaryotes, the third stage of aerobic respiration, electron transfer phosphorylation, occurs at the inner mitochondrial membrane. The reactions of electron transfer phosphorylation begin with the coenzymes NADH and FADH+, which became reduced in the first two stages of aerobic respiration. These coenzymes donate their cargo of electrons and hydrogen ions to electron transfer chains embedded in the inner mitochondrial membrane As the electrons move through the chains, they give up energy little by little. Some molecules of the electron transfer chains harness that energy to actively transport hydrogen ions across the inner membrane, from the matrix to the intermembrane space . The accumulating hydrogen ions form a gradient across the inner mitochondrial membrane. This gradient attracts the ions back toward the matrix, but ions cannot diffuse through a lipid bilayer on their own. Hydrogen ions cross the inner mitochondrial membrane only by flowing through ATP synthases embedded in the membrane. The flow of hydrogen ions through ATP synthases causes these proteins to attach phosphate groups to ADP, so ATP forms.
Oxygen accepts electrons at the end of the mitochondrial electron transfer chains. When oxygen accepts electrons, it combines with H+ to form water, which is a product of the third-stage reactions. For each glucose molecule that enters aerobic respiration, four ATP form in the first- and second-stage reactions. The twelve coenzymes reduced in these two stages deliver enough H+ and electrons to fuel the synthesis of about thirty-two additional ATP in the third stage. Thus, the breakdown of one glucose molecule typically yields thirty-six ATP.
Enzymes that require metal cofactors are a critical part of metabolism. Oxygen reacts with metal cofactors, and free radicals form during those reactions. Free radicals damage biological molecules, so they are dangerous to life. ATP participates in almost all cellular reactions, so a cell benefits from making a lot of it. However, aerobic respiration is a dangerous occupation. When an oxygen molecule (O2) accepts electrons from an electron transfer chain, it dissociates into oxygen atoms. Most of the atoms immediately combine with hydrogen ions and end up in water molecules. Occasionally, however, an oxygen atom escapes this final reaction. The atom has an unpaired electron, so it is a free radical.
Mitochondria cannot detoxify free radicals, so they rely on antioxidant enzymes and vitamins in the cell’s cytoplasm to do it for them. The system works well, at least most of the time. However, a genetic disorder or an encounter with a toxin or pathogen can tip the normal cellular balance of aerobic respiration and free radical formation. Free radicals accumulate and destroy first the function of mitochondria, then the cell. The resulting tissue damage is called oxidative stress.
At least 83 proteins are directly involved in mitochondrial electron transfer chains. A defect in any one of them—or in any of the thousands of other proteins used by mitochondria—can wreak havoc in the body. New research is showing that oxidative stress caused by mitochondrial malfunction is also involved in many other illnesses, including cancer, hypertension, Alzheimer’s and Parkinson’s diseases, and even aging. Hundreds of incurable genetic disorders are associated with mitochondrial defects, and more are being discovered all the time. Nerve cells, which require a lot of ATP, are particularly affected. Symptoms of these disorders range from mild to major progressive neurological deficits, blindness, deafness, diabetes, strokes, seizures, gastrointestinal malfunction, and disabling muscle weakness. Photosynthetic organisms capture energy from the sun and store it in the form of sugars.
They and most other organisms use energy stored in sugars to run various endergonic reactions of metabolism that sustain life. However, sugars rarely participate in such reactions, so how do cells harness their energy? In order to use the energy stored in sugars, cells must first transfer it to molecules—ATP in particular— that do participate in energy-requiring reactions. Cells break the bonds between carbon atoms of a sugar molecule, and use energy released as these bonds break to drive ATP synthesis.
