Metabolism and Energy Production

Glycogen Storage and Utilization

• Where is glycogen stored?

  • Glycogen is stored primarily in two places in the body:

  • Muscles

  • Liver

• How does glucose enter the blood?

  • Glucose enters the bloodstream and is delivered through the blood.

• How does glucose from the blood enter the muscle?

  • Mechanism of Transport:

  • The transfer of glucose from the blood into muscle cells occurs via facilitated diffusion using glucose transporter proteins (GLUT).

Glycogen Breakdown and Conversion

• Source of Glucose for Muscles:

  • Glycogen stored in the muscles is utilized directly within the muscle cells.

  • Glycogen stored in the liver needs to be converted into glucose before it can be transported to the muscles.

• Energy Conversion Process:

  • Conversion pathway:

  • Glycogen is converted into glucose, which then enters the muscle cells by facilitated diffusion through GLUT transporters.

  • This process utilizes 1 ATP to convert glucose to a common metabolic pathway.

• End Products of Glycolysis:

  • The glycolytic process results in:

    • Pyruvate

    • NADH

    • ATP (Three molecules of ATP produced)

Krebs Cycle

• Key Processes and Outcomes:

  • The Krebs cycle takes place in the mitochondria. As pyruvate enters the Krebs cycle, it undergoes further conversions leading to:

  • Production of Carbon Dioxide

  • Synthesis of ATP

  • Generation of NADH and FADH in the cycle.

• Important Enzymes:

  • Succinate Dehydrogenase and Citrate Synthase are key enzymes involved in the Krebs cycle and serve as markers for monitoring Krebs cycle activity.

Electron Transport Chain (ETC)

• ETC Functionality:

  • The ETC is housed in the inner mitochondrial membrane, and its key function is to utilize NADH and FADH to produce:

  • ATP (Primary output)

  • Water (Requires oxygen to generate)

• ATP Yield from Electron Transport:

  • The total ATP produced through this entire process can be quantified as approximately 32 or 33 molecules of ATP per glucose molecule.

Lactic Acid Production and Clearance

• Role of Lactate:

  • Lactate forms as a byproduct during glycolysis under anaerobic conditions and is impacted by:

  • The speed of glucose/glycogen processing.

  • Muscle fiber type, with Type II muscle fibers having a higher propensity to produce lactate.

  • Oxygen availability:

    • Oxygen availability determines whether pyruvate enters anaerobic pathways (to form lactate) or aerobic pathways (to form acetyl-CoA).

• Clearance Mechanisms of Lactate:

  • Three main pathways exist for lactate clearance:

  • Utilization in the same muscle cell.

  • Transport to different muscle cells.

  • Conversion back to glucose in the liver (Cori cycle).

Fat Energy Utilization

• Storage and Breakdown of Fat:

  • Fats are stored as Triglycerides in adipose tissue.

  • Fat is broken down into Free Fatty Acids (FFA) to enter metabolism through:

  • Beta-Oxidation leading to acetyl-CoA, which enters the Krebs cycle.

• ATP Yield from Fat vs. Carbohydrates:

  • Fat metabolism yields approximately 100 ATP per fat molecule, in contrast to the 32 or 33 ATP from glucose.

• When is Fat Utilized for Energy?

  • Fats are predominantly used during low-intensity exercise or at rest, where up to 50% of energy may derive from fat stores.

Exercise Intensity and Energy Source Dynamics

• Energy Source Utilization in Exercise:

  • At the beginning (first 10 seconds) of exercise, ATP and phosphocreatine (PCr) are utilized before transitioning into glycolysis.

  • As exercise intensity increases:

  • High Intensity, Short Duration:

    • ATP production shifts rapidly from glycogenolysis towards anaerobic pathways.

  • Low Intensity, Longer Duration:

    • Oxidative metabolism predominates, employing fats extensively after initial glycogen stores deplete.

• Endpoint of Energy Pathways:

  • Oxidative metabolism eventually plateaus at steady state conditions when adequate oxygen is available for ATP generation via aerobic pathways.

The Concept of VO2 and Exercise Metabolism

• VO2 Dynamics:

  • VO2 (oxygen consumption) increases with exercise intensity until it plateaus at a steady state.

  • When exercise ceases or reduces, VO2 remains elevated temporarily, indicating recovery processes.

  Calorimetry and Energy Measurement

• Types of Calorimetry:

  • Direct Calorimetry:

  • Involves measuring heat release to determine energy expenditure.

  • Indirect Calorimetry:

  • Measures oxygen consumption and carbon dioxide production to infer substrate utilization and caloric expenditure.

• Respiratory Exchange Ratio (RER):

  • Determines substrate utilization (fats vs. carbohydrates) based on the ratio of CO2 produced to O2 consumed.

  • Typical values:

  • Pure carbohydrate metabolism yields an RER of 1.0.

  • Pure fat metabolism yields an RER of 0.7.

Basal vs. Resting Metabolic Rate

• Distinction between BMR and RMR:

  • Basal Metabolic Rate (BMR):

  • Requires 8 hours of sleep, 12 hours of fasting, a supine position, and a thermally neutral environment to stabilize.

  • Resting Metabolic Rate (RMR):

  • Can be measured under less strict conditions and represents the rate at which energy is expended while at rest.

• Summary on Metabolism Measurement:

  • Various factors including exercise intensity, duration, and recovery need to be considered when assessing metabolic rates and energy substrates being utilized by the body during different states of activity.