Aerobic Respiration

• Cellular Respiration

  • Cellular respiration (called aerobic respiration when oxygen is available) occurs in the mitochondrion, the cell's powerhouse for eukaryotes.
  • Glucose and oxygen are consumed to produce carbon dioxide and water.
  • The process produces ATP which is used by the cells as its energy currency.

  
• Three stages of cellular respiration:

  • glycolysis
  • citric acid cycle
  • oxidative phosphorylation
• Glycolysis


  literally means "splitting of glucose"

  glucose+2ADP+2NAD+→2ATP+2NADH+2H++pyruvate
• Glycolysis has a net yield of two ATP molecules.
• For every glucose, two pyruvate and two ATP molecules are produced.
• Citric Acid Cycle

  • Also known as Krebs cycle or tricarboxylic acid cycle
  • Completely breaks down the glucose molecule
  • Takes place in the mitochondrial matrix of eukaryotes
  • Proceeds only when pyruvate is converted to acetyl-CoA
• Pyruvate Oxidation

  • Transition reaction: two pyruvate molecules are first converted to acetyl-CoA
  • occurs in the outer mitochondrial membrane
• • Pyruvate from glycolysis does not directly enter the Krebs cycle. It is first converted to acetyl-CoA. During this conversion of pyruvate to acetyl-CoA, carbon dioxide is also produced.
• During the citric acid cycle, one pyruvate molecule produces two CO₂, three NADH, one FADH₂, and one ATP molecule. Since there are two pyruvate molecules from one glucose molecule, four CO₂, six NADH, two FADH₂, and two ATP molecules are produced in a citric acid cycle alone.
• Oxidative Phosphorylation

  • an electron transport chain is coupled with chemiosmosis to generate ATP
  • This stage uses the NADH and FADH₂ produced from the first two stages.
  • Before ATP is produced, electrons are accepted by NADH and FADH₂, which then act as transporters in a series of reactions.
• Electron Transport Chain

  • NADH and FADH₂ lose electron in a stepwise manner.
  • The transfer of electrons releases a huge amount of energy.
  • The energy produced from the series of reactions allows protein complexes in the inner mitochondrial membrane to pump H⁺ from the mitochondrial matrix to the intermembrane space.
  • NADH enters the chain at the NADH-Q reductase complex. FADH₂ enters the chain at the cytochrome reductase complex.
  • It produces a proton gradient across the membrane, which stores energy and drives chemiosmosis.
• Chemiosmosis

  • ATP production is driven by the downhill backflow of H⁺ in the gradient across the mitochondrial membrane.
  • ATP is produced from an enzyme called ATP synthase. The ATP synthase enzyme works like a reverse ion pump for H⁺
• Arrange the following steps in cellular respiration. Write letters A to E, where A is the first step and E is the last step.

  1. Oxaloacetate couples with acetyl-CoA to form citrate. C
  2. Pyruvate undergoes decarboxylation and is converted to acetyl-CoA. B
  3. Succinylis converted into succinate, regenerating CoA and yielding ATP in the process. D
  4. Malate is converted into oxaloacetate, yielding NADH in the process. E
  5. Glucose is split into two molecules of pyruvate. A
• Cellular respiration is the principal mode of harvesting chemical energy from food molecules. It is an example of a catabolic process.
• ATP is a readily usable form of energy for living things.
• Glycolysis

  • Glycolysis literally means "splitting of glucose."
  • Glucose, a six-carbon sugar molecule, is broken down into two molecules of pyruvate, a three-carbon molecule.
  • This process produces ATP.
  • Furthermore, glycolysis takes place in the cytosol (a component of the cytoplasm) of the cell.
• Glycolysis consists of two phases:

  • the energy-investment phase and the energy-harvesting phase
• In the energy-investment phase, two ATP molecules are used to break down glucose.
• In the energy-harvesting phase, further glucose degradation forms four molecules of ATP, two NADH, and two pyruvate molecules. NAD++ (nicotinamide adenine dinucleotide) serves as a coenzyme and as an energy carrier. Its reduced form is NADH. Electrons are usually first transferred to NAD++ during electron transfer. NADH is formed when free electrons and H++ combine with NAD++.
• In glycolysis, ATP is directly produced when an enzyme transfers a phosphate group, from a phosphate-containing compound, to ADP (adenosine diphosphate). This process is called substrate-level phosphorylation.
• Since two molecules of ATP are used and four molecules of ATP are formed, there is a net yield of two ATP molecules in glycolysis. Note that during glycolysis, oxygen is not required.
• Citric Acid Cycle

  • The citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle) completely breaks down the glucose molecule. It takes place in the mitochondrial matrix of eukaryotes. In prokaryotes, it occurs in the cytosol.
• Before the citric acid cycle can begin, the two pyruvate molecules from glycolysis are first converted to acetyl-CoA, a two-carbon compound in the outer membrane of the mitochondrion. The following significant transition reactions then take place:

  1. Pyruvate is first converted to acetate.
  2. Acetate combines with coenzyme A (CoA) to form acetyl-CoA.
• These transition reactions link glycolysis and the citric acid cycle. In these processes, NADH and CO₂ molecules are also formed from one pyruvate molecule. Since a molecule of glucose produces two pyruvate molecules, the process will yield two acetyl-CoA, two NADH, and two CO₂ molecules.
• Carbon dioxide is considered a metabolic waste in the overall production of ATP. However, CO₂ also helps maintain blood pH by binding with water to form carbonic acid.
• The net reaction for pyruvate oxidation is presented in the following equation:

  2pyruvate+2CoA+2NAD+→2CO2​+2acetyl-CoA+2NADH
• Pyruvate from glycolysis does not directly enter the Krebs cycle. It is first converted to acetyl-CoA. During this conversion of pyruvate to acetyl-CoA, carbon dioxide is also produced.
• When pyruvate is converted to acetyl-CoA, it is the only time the citric acid cycle takes place in the mitochondrial matrix.
• The following reactions take place in the citric acid cycle:

  1. Acetyl-CoA combines with oxaloacetate to form citrate.
  2. Citrate changes the arrangement of its atoms to form isocitrate.
  3. Isocitrate is converted to α-ketoglutarate. The process yields CO₂ and NADH.
• The following reactions take place in the citric acid cycle: (4-7)

  1. An α-ketoglutarate is converted to succinyl-CoA. This reaction also produces CO₂ and NADH.
  2. Succinyl-CoA is converted to succinate. The process regenerates CoA and yields ATP.
  3. Succinate loses two H⁺ and two electrons to produce fumarate. FADH₂ is also generated.
  4. Fumarate reacts with water to form malate.
  5. Malate is converted to oxaloacetate. Another NADH molecule is produced.
• The citric acid cycle produces two CO₂, three NADH, one FADH₂, and one ATP for every pyruvate molecule.

  Since two pyruvate molecules are produced from one glucose molecule, four CO₂, six NADH, two FADH₂, and two ATP molecules are produced. FADH₂ is the reduced form of FAD (flavin adenine dinucleotide) and serves as an energy carrier similar to NADH.
• The net reaction for the citric acid cycle is given by the following equation:

  • 2acetyl-CoA + 6NAD^+ + 2FADH + 2ADP → 4CO2 + 6NADH + 2FADH2 + 2ATP
• Overall, the transition reaction and the citric acid cycle produce six CO₂, two ATPs, two FADH₂, and eight NADH. Similar to that of glycolysis, ATP formation is achieved through substrate-level phosphorylation.
• Oxidative Phosphorylation

  • During oxidative phosphorylation, an electron transport chain is coupled with chemiosmosis to generate ATP molecules.
  • It occurs in the inner mitochondrial membrane of eukaryotes.
  • In prokaryotes, it occurs in the cell membrane.
  • This stage uses the NADH and FADH₂ produced from the first two stages.
  • Electrons are released by NADH and FADH₂, which act as transporters, in a series of reactions before ATP molecules are produced.
• Steps in Oxidative Phosphorylation

  1. Electron Transport Chain (ETC)
  2. Chemiosmosis
• 1. Chemiosmosis

  • ATP production is driven by the downhill backflow of H⁺ in the gradient across the mitochondrial membrane.
  • ATP is produced with the aid of an enzyme called ATP synthase. The ATP synthase enzyme works like a reverse ion pump for H⁺.
  • When the ATP synthase enzyme rotates, the diffusion of H⁺ to the inner mitochondrial matrix couples with the bonding of ADP and inorganic phosphate to produce ATP.
  • Electrons combine with H⁺ and oxygen molecules to form water molecules. This step is catalyzed by cytochrome oxidase. Lastly, O₂ serves as the final electron acceptor in cellular respiration.
• What is the main function of the Electron Transport Chain (ETC)?

  To transfer electrons and produce energy
• How do NADH and FADH₂ lose electrons in the ETC?

  In a stepwise manner
• What happens when electrons are transferred in the ETC?

  It releases a huge amount of energy
• What is the role of the energy produced in the ETC?

  To pump H⁺ ions across the membrane