Aerobic respiration is a vital process in most organisms' cells where oxygen is used to break down glucose to generate energy in the form of ATP. This comprehensive guide includes specific details about the conversion of pyruvate to acetyl CoA, the Krebs cycle, the electron transport chain, oxidative phosphorylation, chemiosmosis, ATP yield, and the importance of NADH and FADH2.
Conversion of Pyruvate to Acetyl CoA
In the mitochondria, each pyruvate undergoes conversion to Acetyl CoA:
1. Pyruvate Decarboxylation
- Pyruvate loses a carboxyl group as CO2.
- Enzyme: Pyruvate dehydrogenase.
- This process is irreversible.
2. Formation of Acetyl CoA
- Remaining 2-carbon compound (acetyl group) joins with Coenzyme A.
- NAD+ is reduced to NADH.
- Importance: It acts as a link between glycolysis and the Krebs cycle.
The Krebs Cycle (Citric Acid Cycle)
The Krebs cycle is central to aerobic respiration:
1. Formation of Citrate
- Acetyl CoA and oxaloacetate combine.
- Enzyme: Citrate synthase.
2. Isomerisation of Citrate
- Citrate rearranges to form isocitrate.
- Enzyme: Aconitase.
3. Oxidation to α-Ketoglutarate
- Isocitrate is oxidised, releasing CO2.
- NAD+ is reduced to NADH.
- Enzyme: Isocitrate dehydrogenase.
4. Formation of Succinyl CoA
- α-Ketoglutarate is oxidised to Succinyl CoA, releasing CO2.
- Enzyme: α-Ketoglutarate dehydrogenase.
5. Conversion to Succinate
- GTP is produced.
- Enzyme: Succinyl CoA synthetase.
6. Formation of Fumarate
- Succinate is oxidised to fumarate.
- FAD is reduced to FADH2.
- Enzyme: Succinate dehydrogenase.
7. Formation of Malate
- Fumarate gains a water molecule to form malate.
- Enzyme: Fumarase.
8. Regeneration of Oxaloacetate
- Malate is oxidised to oxaloacetate.
- NAD+ is reduced to NADH.
- Enzyme: Malate dehydrogenase.
9. Total Yield per Glucose
- ATP: 2
- NADH: 6
- FADH2: 2
- CO2: 4
Electron Transport Chain (ETC) and Oxidative Phosphorylation
The ETC is in the inner mitochondrial membrane and drives ATP synthesis:
1. Electron Donation
- NADH and FADH2 release electrons into the chain.
- Electrons move through complexes I-IV.
2. Proton Pumping
- Energy from electrons pumps protons into the intermembrane space.
- Creates an electrochemical gradient.
3. Oxygen's Role
- Oxygen accepts electrons, forming water.
- Essential to prevent electron backup.
4. Chemiosmosis
- Protons flow through ATP synthase, driving ATP synthesis.
- 10 NADH produce 30 ATP (3 per NADH).
- 2 FADH2 produce 4 ATP (2 per FADH2).
5. ATP Yield
- Total from aerobic respiration: 36-38 ATP per glucose.
Significance in Energy Provision
- Efficient ATP Production: 18 times more ATP per glucose than anaerobic respiration.
- Supports Energy-Intensive Processes: E.g., movement, active transport.
- Regulation: Feedback inhibition controls the rate of respiration.
Importance of NADH and FADH2
- Energy Carriers: Store energy as high-energy electrons.
- Bridge between Stages: Link different stages of respiration.
- Drive ATP Synthesis: Their oxidation in the ETC leads to ATP production.
FAQ
The electron transport chain (ETC) consists of a series of complexes to allow for a stepwise release of energy from the electrons as they are passed down the chain. This controlled, gradual release of energy is harnessed to pump protons across the inner mitochondrial membrane, creating a proton gradient. A single complex would not provide the same controlled energy release, which might lead to energy loss in the form of heat or other non-useful forms, thereby reducing the efficiency of ATP production.
The inner mitochondrial membrane is highly folded into structures called cristae to increase its surface area. This increased surface area allows for a greater number of electron transport chain complexes and ATP synthase enzymes to be embedded in the membrane. Consequently, the higher number of these proteins enhances the capacity of the mitochondria to produce ATP. Thus, the folding of the inner membrane is essential for maximizing energy production in the cell.
The Krebs cycle is regulated through feedback inhibition by key enzymes. When the energy demand is low, and ATP levels are high, ATP inhibits enzymes like citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. Conversely, high levels of ADP and NAD+ activate these enzymes, allowing the cycle to proceed when more energy is required. This regulation ensures that the Krebs cycle operates efficiently, matching the cell's energy needs without wasting resources.
The range in ATP yield (36-38) is due to differences in the efficiency of the transport of NADH and FADH2 produced in glycolysis into the mitochondria. Depending on the shuttle system used to transport these molecules, the energy yield may vary. Different tissues and organisms might use different mechanisms, leading to variation in the total ATP produced from one molecule of glucose during aerobic respiration.
A mutation in a gene coding for a protein like citrate synthase in the Krebs cycle could lead to the enzyme's malfunction or absence. This could disrupt the cycle, possibly inhibiting the conversion of acetyl CoA to citrate and stalling the entire cycle. Consequently, NADH and FADH2 production would decrease, reducing the ETC's activity and ATP synthesis. Depending on the nature and location of the mutation, the effects might vary in severity, potentially leading to metabolic disorders or diseases due to impaired energy production.
Practice Questions
Oxygen acts as the terminal electron acceptor in the electron transport chain (ETC) of aerobic respiration. In the ETC, high-energy electrons are passed along a series of protein complexes embedded in the inner mitochondrial membrane. Oxygen's role is to accept these electrons, combining with free protons to form water. This step is essential because it prevents the backup of electrons in the chain, ensuring that the flow of electrons continues smoothly. Without oxygen to accept the electrons, the ETC would become blocked, halting the process of oxidative phosphorylation and, ultimately, the production of ATP, the cell's energy currency.
The conversion of pyruvate to acetyl CoA occurs in the mitochondria and involves two main steps. First, pyruvate undergoes decarboxylation, where it loses a carboxyl group in the form of CO2, catalysed by the enzyme pyruvate dehydrogenase. The remaining 2-carbon compound then joins with Coenzyme A, and NAD+ is reduced to NADH. This formation of acetyl CoA links glycolysis with the Krebs cycle. The acetyl CoA combines with oxaloacetate to form citrate, initiating the Krebs cycle. The conversion ensures continuity in energy metabolism and facilitates the complete oxidation of glucose, allowing for more efficient energy extraction.
