Aerobic Metabolism Overview

Aerobic Metabolism Overview

• Initial exercise (first 30 seconds) relies heavily on the ATP-PC system.
• This system is quickly depleted.
• Anaerobic glycolysis provides the majority of energy for the next 2-3 minutes.
• Other systems become involved when the exercise demands exceed the ATP-PC system's capacity.

Glycolysis

• Glycolysis is the breakdown of glucose or glycogen to produce ATP.
• It involves 10/11 steps.
• It produces pyruvate, a 3-carbon molecule.
• Anaerobic glycolysis is fast, oxygen-independent, and produces little ATP.
• Aerobic glycolysis is slower, oxygen-dependent, and produces more ATP.

Aerobic Metabolism Substrates and Byproducts

• Substrates: Carbohydrates (glucose/glycogen), fats (fatty acids/triglycerides), proteins (amino acids), and lactate.
• Byproducts: Carbon dioxide (expired by the lungs), water.

Aerobic Metabolism Processes

• Glycolysis: Occurs in the sarcoplasm.
• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix.
• Electron Transport Chain (ETC): Occurs in the inner mitochondrial membrane.

Glucose Metabolism: Transport and Breakdown

• Transport: Glucose moves from the blood into muscle cells via facilitated diffusion. At rest, insulin stimulates glucose transporters; during exercise, muscle contraction stimulates transporters.
• Glycogen Breakdown: Glycogen phosphorylase cleaves glucose from glycogen for glycolysis.

Fat Metabolism: Beta-Oxidation

• Beta-oxidation: Breaks down long-chain fatty acids into two-carbon segments forming acetyl CoA.
• This process occurs in the mitochondria.
• Fatty acids and glycerol can be synthesized from glucose and acetyl CoA.

Protein Metabolism

• Amino acids undergo deamination and transamination.
• They enter the metabolic pathways at pyruvate, acetyl CoA, and the Krebs cycle.

Krebs Cycle Function

• Removes hydrogen ions (H+), carried to the ETC by NADH and FADH2.
• Produces a small amount of ATP (1 ATP per acetyl CoA).
• Starts and ends with oxaloacetate.
• Expels carbon dioxide.
• Per acetyl CoA: 2 CO₂, 3 NADH, 1 FADH₂, 1 GTP/ATP.

Electron Transport Chain (ETC) Function

• Generates most ATP during aerobic metabolism (oxidative phosphorylation).
• Regenerates NAD+ for glycolysis and the Krebs cycle.
• Oxygen (O₂) acts as the final H+ and electron acceptor.
• Electrons move between acceptors, creating energy.
• Energy pumps H+ from the inner to outer mitochondrial compartment creating a gradient.
• H+ moves through ATP synthase, generating ATP.
• Without oxygen, energy cannot be created.

Total ATP Production

• Total ATP varies based on substrate (carbohydrate vs. fat).
• Fatty acids often create more acetyl CoA but require oxygen.

Lactate Role

• Produced during high-intensity exercise.
• The Cori cycle converts lactate into glucose.
• Other tissues use lactate to synthesize glycogen or convert to pyruvate.

Substrate Utilization

• Substrate use depends on availability, intensity, duration, and diet.
• Anaerobic/ATP-PC: Glucose only.
• Aerobic: Primarily a mixture of carbohydrates and fats, preference varies by intensity and duration.

Exercise Intensity and Duration Influence

• Intensity: Higher intensity leads to a greater reliance on carbohydrates.
• Duration: Exercise lasting more than the body’s fuel store leads to a switch to fat metabolism. Glycogen stores deplete in 1-2 hours.

Dietary Recommendations

• Emphasize carbohydrates.
• Carb loading maximizes glycogen stores.

Carbohydrate Availability and Performance

• Depleted carbohydrate stores lead to reduced energy supply.
• High-carbohydrate diets enhance athletic performance.
• Carbohydrate loading beneficial for events over 90 minutes.

Lactate Threshold

• Exercise intensity at which blood lactate exceeds resting concentration.
• Untrained: 50-60% VO2 max
• Trained: 65-80% VO2 max
• Above lactate threshold: H+ accumulation hinders muscle contraction and enzyme function.

Lactate Threshold vs. OBLA

• OBLA (Onset of Blood Lactate Accumulation): Intensity when blood lactate exceeds 4 mmol/L.

Metabolic Recovery

• Post-exercise: PC resynthesis, reduced acidity.
• Elevated HR, breathing, and metabolic rate used for recovery.
• Oxygen deficit: Difference between O₂ consumed and the amount needed if aerobic metabolism could meet demands.
• Steady-state O₂ consumption: When all energy demands are met aerobically.
• EPOC (Excess Post-exercise Oxygen Consumption): Oxygen intake above resting levels after exercise. Higher intensity = higher EPOC.

Active vs. Passive Recovery (Partial)

• Active recovery lowers lactate by using lactate for ATP production via aerobic metabolism and promotes blood flow (incomplete topic)

Aerobic Metabolic Adaptations to Exercise

• Increased mitochondrial density and enzyme activity enhance ATP production.
• Trained individuals can better utilize fat at higher workloads, sparing glycogen.
• Adaptations increase the intensity at which lactate increases, benefiting high-intensity endurance events.

Energy System Usage During Sports

• Varying durations and intensities result in usage of different energy systems.
• ATP-PC, anaerobic glycolysis and aerobic metabolism are used for all events.
• Sprinters' limitations arise from the short duration of ATP-PC and anaerobic glycolysis.
• Marathoners rely more on aerobic metabolism for longer duration.

Measuring Aerobic Metabolism

• Oxygen Consumption (VO₂):
• Absolute (L/min): Doesn't account for body weight.
• Relative (mL/kg/min): Accounts for body weight; measures aerobic ATP production. Uses open circuit spirometry.
• Respiratory Exchange Ratio (RER): RER = VCO₂/VO₂.
  • Lower RER indicates higher fat use.
  • Greater CHO/TG use when RER is between 0.7-1.0.
  • RER > 1.0 occurs at higher intensities with some anaerobic contribution and bicarbonate buffering.

Endurance Event Metabolic Interactions

• Short sprints initially rely on anaerobic sources, but transition to aerobic metabolism.
• Longer distance events primarily use aerobic sources but may also utilise anaerobic sources at higher intensity periods.