The Citric Acid Cycle

  • Also called the Krebs cycle

  • Also called the tricarboxylic acid (TCA) cycle

  • Closely intertwined with cellular respiration

  • ATP (adenosine triphosphate) = a nucleotide that serves as the primary energy source for all living cells

  • ADP (adenosine diphosphate) = a molecule that serves as the lower-energy, de-energized form of ATP

  • GTP (guanosine triphosphate) = a high-energy nucleotide that serves as an essential energy source and signaling molecule in a wide range of cellular processes

  • GDP (guanosine diphosphate) = a molecule that acts as the inactive form of guanosine triphosphate

  • Found in all aerobic cells

  • Takes place in the matrix of the mitochondria in eukaryotes

  • Takes place in the cytoplasm in prokaryotes

  • “Engine of life”

  • Very universal, reenforcing the idea that all creatures evolved from others

  • Per cycle of the citric acid cycle:

    • 2 carbons enter and are released

    • 3 molecules of NADH are generated

    • 1 molecule of FADH2 is generated

    • 1 molecule of either ATP or GTP is produced

      • More ATP is produced via the NADH and FADH2 it generates

        • These molecules will connect with the last portion of cellular respiration and deposit their electrons into the electron transport chain so that more ATP can be made with the deposited electrons

Steps

  1. Acetyl COA joins with oxaloacetate to release the COA group & form citrate

    • Oxaloacetate is a 4 carbon molecule

    • Citrate is a 6 carbon molecule

  2. An H2O molecule is added to citrate

  3. An H2O molecule is removed from citrate, converting it to its isomer, isocitrate

  4. Isocitrate is oxidized & releases a molecule of carbon dioxide, turning it into α-ketoglutarate

    • During this step, NAD+ is reduced into NADH

    • Isocitrate dehydrogenase is the enzyme which catalyzes this step

  5. α-ketoglutarate is oxidized, releasing a molecule of carbon dioxide & picking up Coenzyme A, making it succinyl CoA

    • During this step, NAD+ is reduced into NADH

    • α-ketoglutarate dehydrogenase is the enzyme which catalyzes this step

    • Succinyl CoA is unstable

  6. The CoA of succinyl CoA is replaced with a phosphate group, which is then transferred to ADP to make ATP

    • In some cells GDP is used rather than ADP

      • This is to form GTP

    • This step produces succinate

      • Succinate is a 4-carbon molecule

  7. Succinate is oxidized, forming fumarate while 2 hydrogen atoms & their electrons are transferred to FAD, forming FADH2

    • The enzyme which catalyzes this reaction is embedded into the inner membranes of the mitochondrion, so that FADH2 can immediately transfer its electrons into the electron transport chain

    • Fumarate is a 4-carbon molecule

  8. H2O is added to fumarate, converting it to malate

    • Malate is a 4-carbon molecule

  9. Malate is oxidized to form oxaloacetate—the starting molecule

    • A molecule of NAD+ is reduced to NADH in the process

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