G

Bioenergetics Overview

  • Summary of Energy Systems' ATP Production

    • ATP-PCR System (Phosphagen System):

    • Low ATP yield: approximately 1 ATP per PCR (Phosphocreatine) molecule.

    • Ratio: 1:1 ATP to PCR.

    • Anaerobic Glycolysis (Moderate Intensity):

    • Low ATP yield: 2 to 3 ATP per glucose or glycogen molecule.

    • Oxidative Phosphorylation (Aerobic System):

    • High ATP yield: approximately 36 ATP for each glucose molecule.

  • Creatine Supplementation versus Natural Production

    • Creatine is a supplement, meaning it enhances what the body already produces naturally.

    • It is not a drug, as drugs introduce substances the body does not naturally produce.

    • Creatine is crucial for various exercises, though its role isn't directly as phosphocreatine or creatine kinase in the acute PCR system but rather supporting broader energy metabolism.

  • Analogy for Energy Systems

    • ATP-PCR System: Emergency battery pack (immediate, short burst).

    • Anaerobic Glycolysis: Gas power generator (moderate, relatively quick).

    • Oxidative Phosphorylation: Power grid (sustained, long-term).

    • Alternative Analogy: Microwave (quick), gas stove (moderate), slow cooker (slow, sustained).

  • Oxidative Phosphorylation: Overview (Aerobic Production of ATP)

    • Uses carbohydrates or fats as fuel sources.

    • Three Phases:

    1. Glycolysis (for carbohydrates) or Beta Oxidation (for fats).

    2. Krebs Cycle (also known as the Citric Acid Cycle).

    3. Electron Transport Chain (ETC).

    • The process begins in the cell's cytosol (glycolysis phase) and ends in the mitochondria.

  • **Carbohydrate Oxidation

    • Phase 1: Glycolysis (with Oxygen)**

    • Occurs with or without oxygen (O_2).

    • The amount of ATP produced during the glycolysis phase itself remains the same (e.g., 2 ext{ net ATP}) regardless of oxygen presence.

    • With Oxygen: Pyruvate, generated in the cytosol, is transported into the mitochondria and converted into Acetyl CoA ($ ext{Acetyl-CoA}$). This is the gateway molecule for the Krebs cycle.

    • Without Oxygen: Pyruvate is converted to lactic acid.

    • Products from glycolysis leading to aerobic metabolism: 2 ext{ Acetyl-CoA}, 2 ext{ NADH}, 2 ext{-} 3 ext{ Gross ATP} (depending on if starting with glucose or glycogen).

    • Phase 2: Krebs Cycle (Citric Acid Cycle)

    • Input: Acetyl CoA.

    • Turns: One molecule of Acetyl CoA generates one turn of the Krebs cycle. Since one glucose molecule yields two Acetyl CoA, the Krebs cycle turns twice per glucose molecule.

    • Net Gain (per two turns from one glucose molecule):

      • 2 ext{ ATP}

      • 6 ext{ NADH}

      • 2 ext{ FADH}_2

    • Rate-Limiting Enzyme: Isocitrate Dehydrogenase.

      • It functions early in the Krebs cycle.

      • Regulation (Negative Feedback):

      • Increased ATP concentration $
        ightarrow$ decreases isocitrate dehydrogenase activity.

      • Decreased ATP concentration $
        ightarrow$ increases isocitrate dehydrogenase activity.

    • Phase 3: Electron Transport Chain (ETC)

    • Location: Inner mitochondrial membrane.

    • Components: A series of protein complexes (Complexes 1, 2, 3, 4).

    • Inputs: Hydrogen protons (H^+) and electrons carried by NADH and FADH$_2$ (which were produced in glycolysis and the Krebs cycle).

    • Process:

      • NADH delivers electrons and H^+ to Complex 1. FADH$_2$ delivers to Complex 2.

      • Electrons travel through the protein complexes, releasing energy.

      • This energy is used to pump H^+ into the intermembrane space, creating a concentration gradient.

      • Specific complexes release H^+:

      • Complex 1: 4 ext{ H}^+

      • Complex 3: 4 ext{ H}^+

      • Complex 4: 2 ext{ H}^+

      • The H^+ then flows back into the mitochondrial matrix through ATP Synthase (an enzyme), which catalyzes the formation of ATP from ADP + Pi.

      • Oxygen's Role: Oxygen acts as the final electron acceptor, combining with H^+ to form water (H_2O), which also helps prevent the environment from becoming too acidic.

    • ATP Yield Calculation in ETC:

      • Average yield: 4 ext{ H}^+ pumped corresponds to 1 ext{ ATP}.

      • From NADH: Each NADH produces approximately 2.5 ext{ ATP}.

      • From FADH$2$: Each FADH$2$ produces approximately 1.5 ext{ ATP}.

      • Total ATP from ETC (One Glucose Molecule):

      • Total NADH: 2 (glycolysis) + 2 (pyruvate to Acetyl-CoA) + 6 (Krebs) = 10 ext{ NADH}.

      • Total FADH$2$: 2 (Krebs) = 2 ext{ FADH}2.

      • ETC ATP: (10 ext{ NADH} imes 2.5 ext{ ATP/NADH}) + (2 ext{ FADH}2 imes 1.5 ext{ ATP/FADH}2) = 25 ext{ ATP} + 3 ext{ ATP} = 28 ext{ ATP}.

    • Overall Net ATP Gain (One Glucose Molecule via Oxidative Phosphorylation):

    • Glycolysis: 2 ext{-} 3 ext{ ATP} (direct)

    • Krebs Cycle: 2 ext{ ATP} (direct)

    • ETC: 28 ext{ ATP}

    • Total: 32 ext{-} 33 ext{ ATP} ext{ per glucose/glycogen molecule}.

  • Fat Oxidation (Aerobic)

    • Fuel Source: Triglycerides, which are broken down into glycerol and three free fatty acid (FFA) chains.

    • FFA Entry into Muscle: The rate is dependent on the FFA concentration gradient. An increase in blood FFA leads to an increased rate of transport into muscle fibers.

    • ATP Yield Comparison: Fat oxidation yields 3 ext{-} 4 times more ATP than glucose oxidation, but at a significantly slower rate.

    • This explains why long-duration, low-to-moderate intensity activities (e.g., long-distance running) rely heavily on fat, while high-intensity, short-duration activities rely on carbohydrates.

    • Phase 1: Beta Oxidation:

    • Process: A series of steps in which 2 ext{ carbon (C) Acyl units} are