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 $<br>ightarrow$ decreases isocitrate dehydrogenase activity.

  • Decreased ATP concentration $<br>ightarrow$ 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$<em>2$: Each FADH$</em>2$ 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$<em>2$: $2$ (Krebs) = $2 ext{ FADH}</em>2$.

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

• 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