Aerobic Respiration- Definition, Steps, ATP Yield, Diagram, Uses

Aerobic respiration is a biological process that occurs in the presence of oxygen and converts food molecules, such as glucose, into chemical energy that can be used by cells. Aerobic respiration is the main way that most living organisms, including plants, animals and humans, obtain energy from their food sources. Aerobic respiration involves a series of chemical reactions that take place in different parts of the cell, such as the cytoplasm and the mitochondria. These reactions break down glucose into carbon dioxide and water, and release energy in the form of adenosine triphosphate (ATP). ATP is a molecule that stores and transfers energy for various cellular functions.

The general equation for aerobic respiration is:

C6H12O6 (Glucose) + 6O2 (Oxygen) → 6CO2 (Carbon dioxide) + 6H2O (Water) + ATP (Energy)

Aerobic respiration can be divided into four main steps: glycolysis, pyruvate oxidation, Krebs cycle and oxidative phosphorylation. Each step involves different enzymes and substrates, and produces different products and intermediates. The overall process of aerobic respiration can be summarized in the following diagram:

Aerobic respiration is much more efficient than anaerobic respiration, which occurs without oxygen and produces less energy but more quickly. Aerobic respiration can produce up to 38 ATP molecules per glucose molecule, while anaerobic respiration can produce only 2 ATP molecules per glucose molecule. However, aerobic respiration also requires more oxygen and takes longer than anaerobic respiration.

Aerobic respiration has many applications in living organisms and in the environment. It provides energy for growth, movement, metabolism and other cellular activities. It also produces carbon dioxide and water, which are essential for photosynthesis and the carbon cycle. Furthermore, aerobic respiration can be used for aerobic composting and biodegradation of organic matter by microorganisms.

In this article, we will explore the details of each step of aerobic respiration, how ATP is generated in this process, and some examples of aerobic respiration in different organisms.

Aerobic respiration is the process by which organisms use oxygen to turn fuel, such as fats and sugars, into chemical energy. The product of respiration is a molecule called adenosine triphosphate (ATP), which uses the energy stored in its phosphate bonds to power cellular processes. It is often referred to as the “currency” of the cell.

Aerobic respiration can be summarized by the following equation:

$$C6H{12}O_6 (Glucose) + O_2 (Oxygen) \rightarrow 6CO_2 (Carbondioxide) + 6H_2O (water) + ATP (Energy)$$

One molecule of glucose is oxidized to 6 molecules of carbon dioxide, 6 molecules of water, and 32 molecules of ATP. It occurs in most living organisms, including every higher plant and animal, and most microorganisms in the aerobic and facultative mode of respiration. It occurs in the mitochondria of eukaryotes and the cytoplasm of prokaryotes’ cells. It is slower compared to the anaerobic type but has a higher yield of ATP molecules.

Aerobic respiration consists of four main steps: glycolysis, pyruvate oxidation, Krebs cycle, and oxidative phosphorylation. Each step is a complex multistep enzymatic reaction process that involves different substrates, intermediates, products, enzymes, and coenzymes.

• Glycolysis is the first step that occurs in the cytoplasm of every cell. It involves breaking down a glucose molecule into two molecules of pyruvate, two molecules of NADH, and two molecules of ATP. It does not require oxygen and can occur in both aerobic and anaerobic conditions .
• Pyruvate oxidation is the second step that occurs in the mitochondrial matrix of eukaryotes or the cytoplasm of prokaryotes. It involves converting pyruvate into acetyl-CoA, which can then enter the Krebs cycle. It also produces carbon dioxide and NADH. It requires oxygen and can only occur in aerobic conditions.
• Krebs cycle is the third step that occurs in the mitochondrial matrix of eukaryotes or the cytoplasm of prokaryotes. It involves oxidizing acetyl-CoA into carbon dioxide and water, and generating ATP, NADH, and FADH2. The Krebs cycle occurs twice per glucose molecule. It requires oxygen and can only occur in aerobic conditions .
• Oxidative phosphorylation is the fourth and final step that occurs in the inner mitochondrial membrane of eukaryotes or the plasma membrane of prokaryotes. It involves transferring electrons from NADH and FADH2 to oxygen through a series of protein complexes called the electron transport chain. This creates a proton gradient that drives the synthesis of ATP by an enzyme called ATP synthase. This is where most of the ATP is produced in aerobic respiration. It requires oxygen and can only occur in aerobic conditions.

Aerobic respiration is much more efficient than anaerobic respiration because it produces more ATP per glucose molecule and uses oxygen as the final electron acceptor, which has a high affinity for electrons and allows more energy to be released. Aerobic respiration also produces less toxic byproducts than anaerobic respiration, such as lactic acid or ethanol. Aerobic respiration is essential for most living organisms to survive and thrive.

Aerobic respiration is a complex multistep process that involves four main stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. Each stage consists of a series of biochemical reactions that occur in different locations within the cell. The following diagram summarizes the main steps and products of aerobic respiration.

Glycolysis

Glycolysis is the first stage of aerobic respiration and takes place in the cytoplasm of the cell. It is a sequence of 10 reactions that convert one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). In the process, two molecules of ATP (the energy currency of the cell) and two molecules of NADH (an electron carrier) are produced. Glycolysis does not require oxygen and can also occur in anaerobic conditions. However, in aerobic respiration, pyruvate is further oxidized to generate more ATP .

Pyruvate Oxidation

Pyruvate oxidation is the second stage of aerobic respiration and takes place in the mitochondrial matrix (the inner compartment of the mitochondria). It is a single reaction that converts each pyruvate molecule into acetyl-CoA (a two-carbon compound attached to coenzyme A) and releases one molecule of CO2 (carbon dioxide) and one molecule of NADH. Acetyl-CoA is then ready to enter the next stage, the citric acid cycle .

The Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid cycle, is the third stage of aerobic respiration and takes place in the mitochondrial matrix. It is a circular pathway that consists of eight reactions that break down acetyl-CoA into two molecules of CO2, one molecule of ATP, three molecules of NADH, and one molecule of FADH2 (another electron carrier). The cycle also produces oxaloacetate, a four-carbon compound that combines with another acetyl-CoA to start the cycle again. The citric acid cycle completes the oxidation of glucose and generates most of the electron carriers for the final stage .

Oxidative Phosphorylation

Oxidative phosphorylation is the fourth and final stage of aerobic respiration and takes place in the inner membrane of the mitochondria. It consists of two components: the electron transport chain and chemiosmosis. The electron transport chain is a series of protein complexes that transfer electrons from NADH and FADH2 to oxygen (O2), forming water (H2O) as a byproduct. The energy released by this electron flow is used to pump protons (H+) across the membrane, creating an electrochemical gradient. Chemiosmosis is the process by which protons flow back through a protein called ATP synthase, driving the synthesis of ATP from ADP and phosphate. Oxidative phosphorylation produces most of the ATP in aerobic respiration .

Glycolysis is the first step of aerobic respiration, where glucose is split into two molecules of pyruvate. Glycolysis occurs in the cytoplasm of both prokaryotic and eukaryotic cells. It does not require oxygen and can also occur under anaerobic conditions. Glycolysis consists of ten enzyme-catalyzed reactions that can be divided into two phases: an energy-investing phase and an energy-releasing phase.

Energy-investing phase

In this phase, two molecules of ATP are used to phosphorylate glucose and its derivatives, making them more reactive and unstable. The reactions are:

• Glucose + ATP → Glucose-6-phosphate + ADP (hexokinase)
• Glucose-6-phosphate → Fructose-6-phosphate (phosphoglucose isomerase)
• Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP (phosphofructokinase)

Fructose-1,6-bisphosphate is then cleaved into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). DHAP can be converted into G3P by the enzyme triose phosphate isomerase, so that both molecules can enter the next phase.

Energy-releasing phase

In this phase, each molecule of G3P is oxidized and phosphorylated, producing two molecules of NADH and four molecules of ATP. The reactions are:

• G3P + NAD+ + Pi → 1,3-bisphosphoglycerate + NADH + H+ (glyceraldehyde-3-phosphate dehydrogenase)
• 1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP (phosphoglycerate kinase)
• 3-phosphoglycerate → 2-phosphoglycerate (phosphoglycerate mutase)
• 2-phosphoglycerate → phosphoenolpyruvate + H2O (enolase)
• Phosphoenolpyruvate + ADP → Pyruvate + ATP (pyruvate kinase)

The net result of glycolysis is:

Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O

Glycolysis produces a small amount of ATP (two molecules per glucose) and some reduced coenzymes (NADH) that can be used in other metabolic pathways. Pyruvate, the end product of glycolysis, can be further oxidized in the presence of oxygen or converted into other products in the absence of oxygen.

Pyruvate oxidation is the second step of aerobic respiration, where pyruvate is converted to acetyl-CoA, which can then enter the Krebs cycle. This step results in the oxidative decarboxylation of pyruvate produced by glycolysis.

First, the pyruvate is transferred to the mitochondrial matrix by the “pyruvate translocase” enzyme. The enzyme “pyruvate dehydrogenase complex” irreversibly catalyzes the conversion of pyruvate to acetyl-CoA. There is a loss of an atom of carbon from pyruvate, forming a molecule of CO2. Additionally, NAD+ is reduced to NADH in the process.

The overall reaction can be summarized as:

Pyruvate + NAD+ + CoA → Acetyl CoA + CO2 + NADH + H+

Pyruvate oxidation occurs twice for each glucose molecule that enters glycolysis, producing a total of 2 acetyl-CoA molecules, 2 CO2 molecules, and 2 NADH molecules. Pyruvate oxidation is a key connector that links glycolysis to the Krebs cycle and provides substrates for energy production and biosynthesis.

The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid cycle, is the third step of aerobic respiration. It is a series of chemical reactions that take place in the mitochondrial matrix of eukaryotic cells or the cytoplasm of prokaryotic cells. The Krebs cycle oxidizes acetyl-CoA, which is derived from carbohydrates, fats, and proteins, and produces carbon dioxide, water, and energy in the form of ATP and reduced coenzymes NADH and FADH2. The Krebs cycle is named after its discoverer, Hans Adolf Krebs, who received the Nobel Prize in Physiology or Medicine in 1953 for his work on this pathway.

The Krebs cycle consists of eight major steps, each catalyzed by a specific enzyme. The cycle starts with the condensation of acetyl-CoA and oxaloacetate to form citrate, a six-carbon molecule. Citrate then undergoes a series of transformations that release two molecules of carbon dioxide and regenerate oxaloacetate, completing the cycle. Along the way, three molecules of NADH, one molecule of FADH2, and one molecule of ATP (or GTP) are produced for each acetyl-CoA that enters the cycle. The NADH and FADH2 molecules carry electrons to the electron transport chain, where they generate more ATP through oxidative phosphorylation. The ATP molecules provide energy for various cellular processes. The carbon dioxide molecules are released as waste products or used in other metabolic pathways such as photosynthesis. The water molecules are formed as a result of oxygen reduction in the electron transport chain.

The following is a summary of the steps of the Krebs cycle:

1. Formation of citrate: Acetyl-CoA (a two-carbon molecule) combines with oxaloacetate (a four-carbon molecule) to form citrate (a six-carbon molecule) and release CoA. This reaction is catalyzed by the enzyme citrate synthase.
2. Isomerization of citrate: Citrate is converted into its isomer, isocitrate, by the enzyme aconitase. This reaction involves a dehydration and a rehydration step.
3. Oxidation and decarboxylation of isocitrate: Isocitrate is oxidized and decarboxylated to form alpha-ketoglutarate (a five-carbon molecule) and carbon dioxide. This reaction is catalyzed by the enzyme isocitrate dehydrogenase and produces one molecule of NADH.
4. Oxidation and decarboxylation of alpha-ketoglutarate: Alpha-ketoglutarate is oxidized and decarboxylated to form succinyl-CoA (a four-carbon molecule) and carbon dioxide. This reaction is catalyzed by the enzyme alpha-ketoglutarate dehydrogenase complex and produces one molecule of NADH.
5. Formation of succinate: Succinyl-CoA transfers its CoA group to GDP (or ADP) to form GTP (or ATP) and succinate. This reaction is catalyzed by the enzyme succinyl-CoA synthetase.
6. Oxidation of succinate: Succinate is oxidized to form fumarate by the enzyme succinate dehydrogenase. This reaction produces one molecule of FADH2.
7. Hydration of fumarate: Fumarate is hydrated to form malate by the enzyme fumarase.
8. Oxidation of malate: Malate is oxidized to form oxaloacetate by the enzyme malate dehydrogenase. This reaction produces one molecule of NADH.

The overall equation for one turn of the Krebs cycle can be written as:

Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O → CoA + 3 NADH + 3 H+ + FADH2 + GTP + 2 CO2

During the aerobic respiration process, ATPs are generated in two ways: by substrate-level phosphorylation and by oxidative phosphorylation.

Substrate-level phosphorylation

This is the direct transfer of a phosphate group from a high-energy substrate to ADP, forming ATP. This occurs in the glycolytic pathway and the Krebs cycle.

• In the glycolytic pathway, two ATP molecules are produced by substrate-level phosphorylation. One ATP is generated when 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase. Another ATP is generated when phosphoenolpyruvate is converted to pyruvate by pyruvate kinase.
• In the Krebs cycle, two ATP molecules are produced by substrate-level phosphorylation. One ATP is generated when succinyl-CoA is converted to succinate by succinate thiokinase. Another ATP is generated when GTP is converted to ATP by nucleoside diphosphate kinase.

This is the indirect synthesis of ATP by using the energy released from the electron transport chain and chemiosmosis. This occurs in the inner mitochondrial membrane.

• In the electron transport chain, electrons are transferred from NADH and FADH2 to oxygen, forming water. The electron transfer is coupled with the pumping of protons across the membrane, creating a proton gradient.
• In chemiosmosis, protons flow back through the membrane via ATP synthase, a complex enzyme that uses the proton motive force to catalyze the phosphorylation of ADP to ATP.

The total number of ATP molecules produced by oxidative phosphorylation depends on several factors, such as the efficiency of the electron transport chain, the coupling of proton pumping and ATP synthesis, and the shuttle systems that transport NADH and FADH2 from the cytoplasm to the mitochondria. A common estimate is that each NADH produces about 2.5 ATP and each FADH2 produces about 1.5 ATP.

The total ATP yield per glucose molecule in aerobic respiration can be summarized as:

Aerobic respiration has several important applications for living organisms, both at the cellular and organismal levels. Some of the main applications are:

• Energy production: Aerobic respiration is the primary way of generating ATP, the universal energy currency of cells. ATP is used to power various cellular processes, such as biosynthesis, transport, movement, and signaling. Aerobic respiration produces more ATP per glucose molecule than anaerobic respiration, making it more efficient and sustainable for long-term energy needs.
• Carbon dioxide release: Aerobic respiration releases carbon dioxide as a byproduct of oxidation. Carbon dioxide is essential for photosynthesis, the process by which plants and some microorganisms convert light energy into organic molecules. Photosynthesis also produces oxygen, which is needed for aerobic respiration. Thus, aerobic respiration and photosynthesis form a cycle of carbon and oxygen exchange between different types of organisms.
• Intermediate compounds: Aerobic respiration involves several intermediate compounds, such as pyruvate, acetyl-CoA, and citric acid, that can be used for other metabolic pathways. For example, pyruvate can be converted into amino acids, fatty acids, or glucose through different reactions. Acetyl-CoA can be used for fatty acid synthesis or cholesterol synthesis. Citric acid can be used for amino acid synthesis or as a precursor for other organic acids.
• Aerobic composting and biodegradation: Aerobic respiration can be used by microorganisms to decompose organic matter and recycle nutrients in the environment. Aerobic composting is a process of converting organic waste into humus, a rich soil amendment that improves soil fertility and structure. Aerobic biodegradation is a process of breaking down pollutants or contaminants by microorganisms using oxygen as an electron acceptor. This can help reduce environmental pollution and remediate contaminated sites.

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