Muscle Bio Week 1: fuelling muscle contraction

Energy Requirements for Skeletal Muscle

• Skeletal muscle relies on stored energy and fuel from external sources (e.g., adipose tissue, liver) for contraction and cellular processes.
• ATP is crucial as the primary energy source for muscle contraction and all cellular functions.

Phosphagen System

• Provides energy for only a few seconds of high-intensity work.
• Utilizes two buffering systems to manage ATP recycling and energy supply.
• Creatine: Stored in muscle, buffers ATP regeneration quickly. Higher creatine content leads to increased muscle water content, enhancing muscle fullness.
• Adenylate Kinase (ADK): Maintains higher ATP concentrations relative to ADP, crucial for energy production. Facilitates rapid glycolytic rate increase in response to energy demands.

Introduction to Bioenergetics

• ATP consists of an adenine nucleotide, ribose, and three phosphate groups, with energy stored in the bonds connecting phosphate groups.
• Hydrolysis of ATP (ATP + H2O = ADP + Pi + energy) releases energy for muscle contraction and other cellular activities.
• Stored ATP in muscle fibers is minimal (approximately 4g), supplying energy only for a few seconds of activity.

Energy Systems for ATP Resupply

• ATP-PC System:

  • Short-term energy reserve using phosphocreatine to rapidly replenish ATP.
  • Creatine rapidly depletes, limiting duration of effectiveness.

• Anaerobic Glycolysis:

  • Converts glucose to lactate, producing 2 ATP and 2 NADH molecules.
  • Functional for 30 seconds to 2 minutes of maximal exercise without oxygen.

• Aerobic Respiration:

  • Occurs in mitochondria using oxygen, efficient for prolonged activities over 30 seconds.
  • Breakdown of glucose yields up to 38 ATPs through the Krebs Cycle and electron transport chain.
  • Can also metabolize fats, producing up to 129 ATPs per fat molecule, albeit at a slower rate.

Fuel Storage and Usage

• Body has internal fuel stores for energy during periods without food intake.
• Carb stores can sustain energy for approximately 90 minutes of intense exercise.
• Fat tissue is the largest energy reserve, potentially supporting activity for up to 120 hours at marathon pace.
• Differences in fat and carbohydrate storage and usage rates impact energy availability during extended exercise.

ATP Production and Energy Systems

• Creatine phosphate rapidly regenerates ATP from ADP during high-demand muscle contractions.
• Anaerobic glycolysis converts glucose to 2 lactate, producing 2 ATP and 2 NADH, without needing oxygen; energy lasts 30 seconds to 2 minutes.
• Aerobic respiration occurs in mitochondria, requiring oxygen, suitable for activity over 30 seconds, yields up to 38 ATP from one glucose molecule.
• Aerobic respiration can utilize fats, generating more ATP (up to 129 ATP) but takes longer and requires more oxygen.
• Carbohydrate stores provide energy for about 90 minutes of intense activity; fat stores can sustain energy for up to 120 hours at a marathon pace.

Fuel Storage and Usage

• Body stores fuel for instant energy when food intake is unavailable, utilizing muscle and fat stores.
• Muscle releases glucose to the brain by converting amino acids in the liver, regulated by FOXO transcription factors.
• FOXO factors also promote muscle protein breakdown for glucose production in the liver.

Fats as an Energy Source

• Fats are stored abundantly in adipose tissue and in smaller amounts (up to 300g) in muscle fibers near mitochondria.
• Fats have a high energy density and store more energy per molecule than carbohydrates, but require aerobic metabolism.
• They are primarily used during low-intensity, long-duration exercise or at rest, with limited utilization during high-intensity exercise.

Proteins as Metabolic Fuel

• Skeletal muscle holds the most significant protein reserves, which can be broken down for energy, mainly during prolonged fasting.
• Proteins lack direct storage; instead, skeletal muscle acts as a reservoir when needed.
• Proteins function in various roles, including transport, enzyme activity, and structural support.

Energy Density and Metabolism

• Fats provide 9 kcal/g, while carbohydrates and proteins provide 4 kcal/g; fats yield more energy per gram due to lower associated water weight.
• Glycogen stored in muscle is less energy-dense due to hydration, effectively yielding ~1 kcal/g after accounting for water.
• Excess carbohydrates are converted into fat via de novo lipogenesis, a preferred efficient storage method.

Ketogenesis and Survival Mechanisms

• During starvation, the brain's glucose needs can be met by ketones produced from fatty acids when carbohydrate stores are depleted.
• The liver can convert fatty acids and glycerol from triglycerides into glucose through gluconeogenesis.
• Muscle protein breakdown during prolonged exercise or starvation provides gluconeogenic amino acids for glucose production, illustrating metabolic integration for survival.

ATP Production and Energy Systems

• Creatine phosphate rapidly regenerates ATP from ADP during high-demand muscle contractions.
• Anaerobic glycolysis converts glucose to 2 lactate, producing 2 ATP and 2 NADH, without needing oxygen; energy lasts 30 seconds to 2 minutes.
• Aerobic respiration occurs in mitochondria, requiring oxygen, suitable for activity over 30 seconds, yields up to 38 ATP from one glucose molecule.
• Aerobic respiration can utilize fats, generating more ATP (up to 129 ATP) but takes longer and requires more oxygen.
• Carbohydrate stores provide energy for about 90 minutes of intense activity; fat stores can sustain energy for up to 120 hours at a marathon pace.

Fuel Storage and Usage

• Body stores fuel for instant energy when food intake is unavailable, utilizing muscle and fat stores.
• Muscle releases glucose to the brain by converting amino acids in the liver, regulated by FOXO transcription factors.
• FOXO factors also promote muscle protein breakdown for glucose production in the liver.

Fats as an Energy Source

• Fats are stored abundantly in adipose tissue and in smaller amounts (up to 300g) in muscle fibers near mitochondria.
• Fats have a high energy density and store more energy per molecule than carbohydrates, but require aerobic metabolism.
• They are primarily used during low-intensity, long-duration exercise or at rest, with limited utilization during high-intensity exercise.

Proteins as Metabolic Fuel

• Skeletal muscle holds the most significant protein reserves, which can be broken down for energy, mainly during prolonged fasting.
• Proteins lack direct storage; instead, skeletal muscle acts as a reservoir when needed.
• Proteins function in various roles, including transport, enzyme activity, and structural support.

Energy Density and Metabolism

• Fats provide 9 kcal/g, while carbohydrates and proteins provide 4 kcal/g; fats yield more energy per gram due to lower associated water weight.
• Glycogen stored in muscle is less energy-dense due to hydration, effectively yielding ~1 kcal/g after accounting for water.
• Excess carbohydrates are converted into fat via de novo lipogenesis, a preferred efficient storage method.

Ketogenesis and Survival Mechanisms

• During starvation, the brain's glucose needs can be met by ketones produced from fatty acids when carbohydrate stores are depleted.
• The liver can convert fatty acids and glycerol from triglycerides into glucose through gluconeogenesis.
• Muscle protein breakdown during prolonged exercise or starvation provides gluconeogenic amino acids for glucose production, illustrating metabolic integration for survival.

This quiz explores the different energy systems that power skeletal muscle, including the phosphagen system. It covers how muscle relies on both internal and external fuel sources for energy during contraction and other cellular processes. Test your knowledge on energy supply mechanisms and their buffering capabilities.