Cellular-Respiration-Aerobic-Respiration

Major Steps in Aerobic Respiration

  • Pyruvate Oxidation

  • Krebs Cycle/Tricarboxylic Acid Cycle/Citric Acid Cycle

  • Oxidative Phosphorylation (Electron Transport Chain & Chemiosmosis)

Pyruvate Oxidation

  • Converts each pyruvate molecule from glycolysis to a 2-carbon molecule (acetyl CoA) in the mitochondrial matrix.

  • Known as a “link reaction,” it connects glycolysis and the citric acid cycle.

NADH Management

  • Importance of NAD+: Essential for glycolysis; without it, glycolysis halts.

  • Regeneration of NAD+:

    • When oxygen is present, NADH donates electrons to the electron transport chain, regenerating NAD+.

    • In absence of oxygen, cells use fermentation, a simpler pathway where NADH donates its electrons to another molecule, regenerating NAD+ without producing ATP.

Krebs Cycle/Citric Acid Cycle/Tricarboxylic Acid Cycle

  • Takes place in the mitochondrial fluid matrix.

  • Pyruvic acid is restructured to acetyl CoA, which enters the cycle.

  • Produces NADH, CO2, and hydrogen through various enzymatic reactions.

  • Discovered by biochemist Hans Adolf Krebs.

Steps in Krebs Cycle

Step 1

  • Acetyl CoA combines with oxaloacetate (4-carbon) to form citrate (6-carbon); catalyzed by Citrate synthase.

Step 2

  • Citrate is converted to isocitrate via two steps:

    • Dehydration to form cis-aconitate

    • Hydration of cis-aconitate to form isocitrate, catalyzed by aconitase.

Step 3

  • Isocitrate oxidizes, releasing CO2, forming α-ketoglutarate (5-carbon); NAD+ is reduced to NADH, catalyzed by isocitrate dehydrogenase.

Step 4

  • α-ketoglutarate oxidizes, releasing CO2 and reducing NAD+ to NADH; forms succinyl CoA, catalyzed by α-ketoglutarate dehydrogenase.

Step 5

  • In succinyl-CoA, CoA is replaced by a phosphate, transferring to GDP to form GTP, resulting in succinate (4-carbon); catalyzed by succinyl-CoA synthetase.

Step 6

  • Succinate oxidized to fumarate (4-carbon); forms FADH2; catalyzed by succinate dehydrogenase.

Step 7

  • Water is added to fumarate to form malate (4-carbon); catalyzed by fumarase.

Step 8

  • Malate oxidizes to regenerate oxaloacetate; reduces NAD+ to NADH, catalyzed by malate dehydrogenase.

Overview of Reactions in Krebs Cycle

  1. Condensation

  2. Dehydration & Hydration

  3. Oxidative Decarboxylation

  4. Substrate-level Phosphorylation

  5. Dehydrogenation

  6. Hydration

  7. Dehydrogenation

Products of the Krebs Cycle

  • For one turn:

    • 2 carbons from acetyl CoA enter

    • 2 CO2 released

    • 3 NADH and 1 FADH2 produced

    • 1 ATP or GTP produced

  • Multiply by 2 for glucose breakdown.

Electron Transport Chain + Chemiosmosis

  • Composed of protein complexes that couple redox reactions to proton pumping, generating an electrochemical gradient.

Purpose

  • ATP production; located in the inner mitochondrial membrane.

Characteristics

  • Electrons from metabolism are transported to O2, the final electron acceptor.

  • Maximally utilizes body’s oxygen and functions as a final common pathway of metabolism.

Components of the Electron Transport Chain

Complexes Overview

Complex I - NADH Dehydrogenase

  • Collects electrons from NADH; releases 4 H+ ions.

Complex II - Succinate Dehydrogenase

  • Receives FADH2; transfers its electrons to CoQ; does not release protons.

Coenzyme Q (CoQ)

  • Lipid soluble, non-protein component of ETC; mobile.

Complex III - Cytochrome Reductase

  • Releases 4 H+ and transfers electrons to cytochrome c.

Complex IV - Cytochrome Oxidase

  • Releases 2 H+; final electron acceptor oxygen splits into water.

ATP Synthase

  • Uses the proton gradient generated by the electron transport chain to synthesize ATP.

  • Consists of two domains: F0 (membrane spanning) and F1 (extramembranous).

Inhibitors of ETC

  • Compounds that block electron passage, inhibiting ATP synthesis.

Total ATP Yield per Glucose

  • Overall yield from glycolysis to aerobic respiration:

    • Glycolysis: 4 ATP (2 net) + 2 NADH (yielding 4 ATP)

    • Pyruvate Conversion: 2 NADH (yielding 6 ATP)

    • Krebs Cycle: 2 ATP + 2 FADH2 (yielding 3-4 ATP) + 6 NADH (yielding 15-18 ATP)

  • Total: 38 ATP per glucose (32-38 depending on NADH/FADH2 conversion rates).