Glycolysis and Krebs Cycle Notes

Glycolysis

• Substrate and initial step: glucose, a 6-carbon sugar, is broken down into two molecules of pyruvate (each 3 carbons).
• End products and energy yield (per glucose):
  • Pyruvate: two molecules (each 3 carbons)
  • Net ATP: +2 ATP
  • NADH: +2 NADH
• Key phrasing from transcript:
  • "In glycolysis, the 6-carbon sugar, glucose, is broken down into two molecules".
  • "Glycolysis … a net gain of 2 ATP molecules and 2 NADH molecules."
• Conceptual takeaway: glycolysis converts glucose into pyruvate while harvesting a small amount of usable energy in the form of ATP and high-energy electrons carried by NADH.
• Simple stoichiometric summary (LaTeX):
  $\text{Glucose} \rightarrow 2\,\text{Pyruvate} \quad +2\,\text{NADH} \quad +2\,\text{ATP (net)}$

Pyruvate entry into mitochondria and formation of Acetyl-CoA

• Pyruvate transport: Pyruvate is transported into the mitochondria.
• Decarboxylation and activation: Pyruvate loses carbon dioxide (CO₂) to form acetyl-CoA, a 2-carbon molecule.
• Transcript detail: "Pyruvate is transported into the mitochondria and loses carbon dioxide to form acetyl-CoA, a 2-carbon molecule."
• Key reaction (mitochondrial bridge step):
  $\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl-CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+$
• Significance: Acetyl-CoA enters the Krebs cycle as the substrate for oxidative decarboxylation and energy extraction; CO₂ is a waste product from this step, and NADH is an energy carrier produced before the Krebs cycle proper.

Krebs Cycle

• Location: mitochondrial matrix.
• Purpose: Generates chemical energy (ATP, NADH, and FADH₂) from the oxidation of acetyl-CoA, the end product of glycolysis.
• Key statement from transcript: "The Krebs Cycle occurs in the mitochondrial matrix and generates a pool of chemical energy (ATP, NADH, and FADH2) from the oxidation of pyruvate, the end product of glycolysis."
• Per acetyl-CoA (cycle turn):
  • NADH: 3 molecules
  • FADH₂: 1 molecule
  • ATP (substrate-level): 1 molecule
  • CO₂: 2 molecules released
  • CoA is regenerated for reuse
• Transcript fragments: references to NAD+, FADH, FAD, CO₂, COA, NADH + H+, COA, and notes like "3 NAD" and "2 CO₂" consistent with the NADH and CO₂ outputs and the cycling of CoA.
• Overall per glucose (two acetyl-CoA produced from the two pyruvates):
  • CO₂: 4 molecules
  • NADH: 6 molecules
  • FADH₂: 2 molecules
  • ATP: 2 molecules (substrate-level)
  • CoA cycles back for another turn
• Per-acetyl-CoA stoichiometry (LaTeX):
  $\text{Acetyl-CoA} + 3\,\text{NAD}^+ + \text{FAD} + \text{ADP} + P<em>i \rightarrow 2\,\text{CO}</em>2 + 3\,\text{NADH} + \text{FADH}_2 + \text{ATP} + \text{CoA}$
• Per glucose stoichiometry (LaTeX):
  $2\,\text{Acetyl-CoA} \rightarrow 4\,\text{CO}<em>2 + 6\,\text{NADH} + 2\,\text{FADH}</em>2 + 2\,\text{ATP}$
• Total substrate-level energy yield and electron carriers (per glucose):
  • ATP (substrate-level): 4 ATP (2 from glycolysis net + 2 from Krebs cycle)
  • NADH: 10 NADH (2 from glycolysis, 2 from pyruvate to acetyl-CoA, 6 from Krebs cycle)
  • FADH₂: 2 FADH₂ (from Krebs cycle)
  • CO₂: 6 CO₂ (2 released in PDH step + 4 released in Krebs cycle)
• Real-world relevance of the energy carriers:
  • NADH and FADH₂ carry electrons to the electron transport chain (not detailed in transcript) to generate large amounts of ATP; Krebs cycle’s role is to maximize NADH/FADH₂ production from acetyl-CoA and to release CO₂ as waste.
• Conceptual analogy: glycolysis is like breaking a glucose molecule into two energy-rich halves (pyruvate), while the Krebs cycle is the main energy-extraction furnace that converts acetyl-CoA into CO₂ and energy carriers (NADH, FADH₂, ATP).

Key molecules referenced in the transcript

• Glucose (C₆H₁₂O₆)
• Pyruvate (C₃H₃O₃⁻; 3-carbon molecule)
• Acetyl-CoA (2-carbon unit)
• NAD⁺ / NADH
• FAD / FADH₂
• CO₂ (carbon dioxide)
• CoA (Coenzyme A)
• ADP / ATP
• Pi (inorganic phosphate)
• H⁺ (protons)

Summary connections

• Glycolysis produces pyruvate, which feeds into the mitochondria for further energy extraction via the pyruvate dehydrogenase step to form acetyl-CoA.
• Acetyl-CoA enters the Krebs cycle, where it is oxidized to CO₂, yielding NADH, FADH₂, and ATP that fuel subsequent energy production via the electron transport chain (not detailed in transcript).