Topic 5: Energy Transfers: Krebs cycle

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7 Terms

1
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Also known as the citric acid cycle

Stage of aerobic respiration that follows glycolysis and link reaction

2
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Occurs in the mitochondrial matrix

Processes acetyl coenzyme A to prepare substrates for the final stage of aerobic respiration (oxidative phosphorylation)

3
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For each molecule of acetyl coenzyme A, the Krebs cycle produces:

  • two molecules of carbon dioxide as a by product

  • the electron carriers 3 reduced NAD and 1 reduced FAD, which are needed for oxidative phosphorylation


4
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  1. Acetyl coenzyme A (which contains 2C acetate) merges with a 4C molecule to create a 6C molecule

  2. The 6C molecule is decarboxylated, releasing two molecules of carbon dioxide

  3. The 6C molecule is also dehydrogenated (oxidised), releasing hydrogen that reduce NAD and FAD


  1. For each acetyl coenzyme A that enters the cycle, one ATP (or GTP in some organisms) is synthesised directly via substrate level phosphorylation

  2. The 4C molecule is regenerated for the next turn of the cycle


5
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NAD and FAD differ in how they are reduced:

  • NAD acts as an oxidising agent, accepting an electron and a proton to form reduced NAD

  • FAD acts as an oxidising agent, accepting two protons and two electrons to form reduced FAD


6
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These reduced coenzymes later donate these gained electrons to the electron transport chain, facilitating the transfer of electrons that is crucial for the synthesis of ATP

While NAD participates in all stages of respiration, FAD only accepts protons and electrons in the Krebs cycle

7
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Krebs cycle is essential for several reasons:

  • it oxidises and breaks down large nutrients into smaller ones, like carbon dioxide, which can be removed as a waste product

  • it generates reduced NAD and reduced FAD, which carry protons and electrons into oxidative phosphorylation

  • it continually regenerates the 4C molecule to combine with acetyl coenzyme A molecules

  • it provides a variety of intermediate compounds required for the biosynthesis of essential cellular components such as fatty acids, amino acids and chlorophyll