Overview of the citric acid cycle

Scientific concepts & biological processes presented

• Glycolysis location and products

  • Glycolysis occurs in the cytoplasm.
  • Each glucose yields 2 molecules of pyruvate.

• Link between glycolysis and the citric acid (Krebs/TCA) cycle

  • Pyruvate is transported into the mitochondrial matrix by a carrier protein.
  • In the mitochondrial matrix, pyruvate is oxidized to acetyl-CoA by pyruvate dehydrogenase.
  • During this conversion:
    • CO₂ is released
    • NAD⁺ is reduced to NADH
  • For diagram clarity, acetyl-CoA is treated as a two-carbon acetate group.

• Citric acid cycle overview

  • The cycle has 8 steps.
  • It harvests chemical energy, producing a total of 3 NADH per turn (as stated in the subtitles).

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Methodology / step-by-step outline (8 steps of the citric acid cycle)

1. Citrate formation

   • Citrate synthase combines:
     • 2-carbon acetyl group (from acetyl-CoA)
     • with 4-carbon oxaloacetate
   • Produces 6-carbon citrate.

2. Isomerization

   • Aconitase converts citrate → isocitrate.

3. Oxidation and first CO₂ release

   • Isocitrate dehydrogenase oxidizes isocitrate → α-ketoglutarate.
   • Produces:
     • CO₂
     • NADH (via reduction of NAD⁺).

4. Decarboxylation to succinyl-CoA

   • α-Ketoglutarate dehydrogenase:
     • decarboxylates α-ketoglutarate
     • oxidizes it
   • Produces:
     • CO₂
     • NADH (via reduction of NAD⁺)
     • succinyl-CoA (described as α-ketoglutarate’s product, with coenzyme A involvement).

5. Substrate-level phosphorylation / succinate formation

   • Succinyl-CoA synthetase converts:
     • succinyl-CoA → succinate
   • Produces:
     • GTP (high-energy phosphate; “omitted” in the diagram)
     • releases coenzyme A (as stated).

6. Oxidation to fumarate and connection to respiration

   • Succinate dehydrogenase converts:
     • succinate → fumarate
   • Key features described:
     • Embedded in the inner mitochondrial membrane
     • Part of the electron transport chain as Complex II
     • Uses FAD instead of NAD⁺ to accept electrons
     • Electrons ultimately go to coenzyme Q, then Complex III.

7. Hydration

   • Fumarase (fumarate hydratase) converts:
     • fumarate → malate.

8. Oxidation back to oxaloacetate

   • Malate dehydrogenase converts:
     • malate → oxaloacetate
   • Produces:
     • NADH (via reduction of NAD⁺)
   • Regenerated oxaloacetate combines with another acetyl-CoA in the next cycle.

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List of researchers or sources featured

• No specific researchers or external sources are named in the provided subtitles.