🏃Exercise Physiology Unit 2 – Bioenergetics and Metabolism

Key Concepts and Definitions

• Bioenergetics studies energy transformations and energy exchanges within and between living systems and their environments
• Metabolism encompasses all chemical reactions involved in maintaining the living state of cells and organisms
• Catabolism breaks down organic matter and harvests energy by breaking down complex molecules into simpler ones (cellular respiration)
• Anabolism constructs molecules from smaller units, consuming energy for synthesis of compounds (protein synthesis)
• ATP (adenosine triphosphate) is the primary energy currency in living organisms, providing energy for cellular processes
  • Consists of an adenosine molecule bonded to three phosphate groups
  • Hydrolysis of ATP to ADP (adenosine diphosphate) and inorganic phosphate (Pi) releases energy
• Substrate is a molecule upon which an enzyme acts, forming a product (glucose is the substrate for glycolysis)
• Metabolic pathways are series of enzymatic reactions that convert a starting molecule into an end product

Energy Systems Overview

• Three energy systems work together to supply ATP for cellular processes and physical activity
  • Phosphagen system provides immediate energy using creatine phosphate to regenerate ATP
  • Glycolytic system breaks down carbohydrates anaerobically to produce ATP
  • Oxidative system aerobically metabolizes carbohydrates, fats, and proteins in the mitochondria
• Energy systems differ in their capacity and rate of ATP production
  • Phosphagen system has the highest rate but lowest capacity
  • Oxidative system has the lowest rate but highest capacity
• Intensity and duration of physical activity determine the relative contribution of each energy system
  • High-intensity, short-duration activities rely more on the phosphagen and glycolytic systems (sprinting)
  • Low-intensity, long-duration activities predominantly use the oxidative system (marathon running)
• Energy systems work simultaneously, with the relative contribution shifting depending on the activity and individual's fitness level

Carbohydrate Metabolism

• Carbohydrates are the primary and preferred energy source for the human body
• Glucose, a monosaccharide, is the main carbohydrate used for energy production
  • Stored as glycogen in the liver and skeletal muscles
• Glycolysis is the anaerobic breakdown of glucose to pyruvate, producing ATP and NADH
  • Occurs in the cytoplasm of cells
  • Net yield of 2 ATP per glucose molecule
• Pyruvate has three possible fates depending on oxygen availability and cellular conditions
  • Conversion to lactate under anaerobic conditions
  • Entry into the Krebs cycle for aerobic metabolism
  • Conversion to acetyl-CoA for fatty acid synthesis
• Krebs cycle (citric acid cycle) is a series of aerobic reactions that generate high-energy molecules (NADH, FADH2) for ATP production
  • Occurs in the mitochondrial matrix
  • Carbon dioxide is released as a byproduct
• Electron transport chain is the final stage of aerobic respiration, producing the majority of ATP
  • NADH and FADH2 from glycolysis and the Krebs cycle donate electrons to the electron transport chain
  • Oxygen serves as the final electron acceptor, forming water

Lipid Metabolism

• Lipids, primarily in the form of triglycerides, are an important energy reserve in the human body
• Triglycerides are stored in adipose tissue and can be mobilized for energy production during prolonged exercise or fasting
• Lipolysis is the breakdown of triglycerides into glycerol and free fatty acids
  • Hormone-sensitive lipase catalyzes the process, which is stimulated by epinephrine, norepinephrine, and glucagon
• Beta-oxidation is the primary pathway for fatty acid catabolism, occurring in the mitochondrial matrix
  • Fatty acids are converted into acetyl-CoA, which enters the Krebs cycle
  • Produces NADH and FADH2 for ATP generation via the electron transport chain
• Ketogenesis occurs when acetyl-CoA accumulates faster than it can be oxidized by the Krebs cycle (during prolonged fasting or low-carbohydrate diets)
  • Acetyl-CoA is converted into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone)
  • Ketone bodies serve as an alternative fuel source for the brain, heart, and skeletal muscle
• Lipid metabolism is regulated by hormones and the availability of other energy substrates (glucose)

Protein Metabolism

• Proteins are essential for tissue growth, repair, and maintenance, but can also serve as an energy source
• Amino acids, the building blocks of proteins, are obtained from the diet or the breakdown of body proteins
• Transamination is the process of transferring an amino group from an amino acid to an alpha-keto acid
  • Produces a new amino acid and a new alpha-keto acid
  • Important for the synthesis of non-essential amino acids and the removal of excess amino groups
• Deamination removes the amino group from an amino acid, converting it into ammonia and the corresponding alpha-keto acid
  • Occurs in the liver
  • Ammonia is converted into urea and excreted by the kidneys
• Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors, such as amino acids
  • Occurs during prolonged fasting, low-carbohydrate diets, or intense exercise
  • Helps maintain blood glucose levels when glycogen stores are depleted
• Protein catabolism for energy production increases during prolonged exercise or energy deficit states (starvation)

Metabolic Pathways and ATP Production

• Metabolic pathways are interconnected series of enzymatic reactions that convert starting molecules into end products
• Glycolysis, the Krebs cycle, and the electron transport chain are the main pathways for ATP production
  • Glycolysis (anaerobic) produces 2 ATP per glucose molecule
  • Krebs cycle (aerobic) generates high-energy molecules (NADH, FADH2) for the electron transport chain
  • Electron transport chain (aerobic) produces the majority of ATP through oxidative phosphorylation
• Oxidative phosphorylation couples the transfer of electrons through the electron transport chain with the synthesis of ATP
  • Electron transport chain creates a proton gradient across the inner mitochondrial membrane
  • ATP synthase uses the proton gradient to drive ATP synthesis
• Substrate-level phosphorylation directly produces ATP through the transfer of a phosphate group from a high-energy intermediate to ADP
  • Occurs in glycolysis (phosphoenolpyruvate to ATP) and the Krebs cycle (succinyl-CoA to ATP)
• ATP yield per molecule of substrate varies depending on the metabolic pathway
  • Glucose: 30-32 ATP (aerobic), 2 ATP (anaerobic)
  • Palmitate (fatty acid): 106 ATP (aerobic)
  • Amino acids: variable ATP yield depending on the specific amino acid and metabolic fate

Energy Substrate Utilization During Exercise

• The relative contribution of carbohydrates, lipids, and proteins to energy production during exercise depends on factors such as exercise intensity, duration, and individual fitness level
• Carbohydrates are the primary fuel source for high-intensity exercise
  • Glycogen stores in the liver and skeletal muscle are rapidly depleted during intense activity
  • Blood glucose, derived from liver glycogenolysis or gluconeogenesis, also contributes to energy production
• Lipids are the main fuel source for low-to-moderate intensity exercise and prolonged activities
  • Triglycerides stored in adipose tissue are broken down into free fatty acids and released into the bloodstream
  • Intramuscular triglycerides also contribute to energy production during exercise
• Proteins contribute minimally to energy production during exercise (usually less than 5%)
  • Amino acid oxidation increases during prolonged exercise or when carbohydrate and lipid availability is limited
• The "crossover concept" describes the shift in substrate utilization from lipids to carbohydrates as exercise intensity increases
  • At low intensities, lipids are the primary fuel source
  • As intensity increases, the contribution of carbohydrates progressively increases while that of lipids decreases
• Trained individuals exhibit a greater reliance on lipids during submaximal exercise compared to untrained individuals

Metabolic Adaptations to Training

• Regular exercise training induces metabolic adaptations that enhance the body's ability to produce and utilize energy
• Aerobic training (endurance exercise) adaptations:
  • Increased mitochondrial density and size in skeletal muscle
  • Enhanced activity of oxidative enzymes (citrate synthase, succinate dehydrogenase)
  • Improved capillarization of skeletal muscle, facilitating oxygen and nutrient delivery
  • Greater reliance on lipids as a fuel source during submaximal exercise (glycogen sparing)
• Anaerobic training (resistance exercise) adaptations:
  • Increased glycolytic enzyme activity (phosphofructokinase, lactate dehydrogenase)
  • Enhanced creatine phosphate storage and utilization
  • Hypertrophy of fast-twitch (type II) muscle fibers
• Metabolic flexibility, the ability to switch between fuel sources depending on availability and demand, improves with training
  • Trained individuals can more efficiently utilize lipids during exercise and spare glycogen stores
• High-intensity interval training (HIIT) has been shown to induce rapid metabolic adaptations
  • Increases mitochondrial biogenesis and oxidative capacity
  • Enhances insulin sensitivity and glucose uptake in skeletal muscle
• Detraining, or the cessation of regular exercise, can reverse many of the metabolic adaptations acquired through training
  • Mitochondrial density and enzyme activity decrease
  • Glycogen storage capacity and insulin sensitivity may decline