24 August 2015

#89 The Krebs cycle

The Krebs cycle occurs in the matrix of the mitochondrion and is the aerobic phase and requires oxygen.

This is also known as the citric acid cycle or the tricarboxylic acid cycle.

The Krebs cycle is a series of steps catalysed by enzymes in the matrix:

Photo credit: BBC.

• A 2-carbon atoms Acetyl CoA enters the cycle and combines with a 4-carbon compound (oxaloacetate) to give a 6-carbon compound (citrate/citric acid). Coenzyme A is reformed. Cycle turns twice for each original glucose molecule.

• The citrate is then gradually converted back to the 4-carbon oxaloacetate again in a series of small enzyme-controlled steps involving decarboxylation and dehydrogenation. 2 C atoms are released in 2 CO2 molecules and 4 pairs of H atoms are removed.

• The CO2 removed is given off as a waste product. It diffuses rut of the mitochondrion and out of the cell.

• The hydrogens removed are picked up by NAD and another coenzyme called FAD (flavin adenine dinucleotide). 1 FAD and 3 NAD molecules are reduced during each turn of the cycle. H in reduced NAD/FAD will be released in oxidative phosphorylation. The main role of the Krebs cycle in respiration is to generate a pool of reduced hydrogen carriers to pass on to the next stage.

• The regenerated oxaloacetate can combine with another ACoA.

• 1 ATP is produced directly by substrate-level phosphorylation for each ACoA entering the cycle.

• Amino acids and fatty acids can be broken down and fed into cycle.

Syllabus 2016-2018  

12.1  Energy

 c) explain that ATP is synthesised in Krebs cycle.

12.2 Respiration

d)   outline  the Krebs cycle, explaining that  oxaloacetate (a 4C compound) acts  as an acceptor of the 2C fragment from acetyl coenzyme A to form citrate  (a 6C compound), which is reconverted to oxaloacetate in a series of small steps

e)   explain that  reactions in the Krebs cycle involve decarboxylation and dehydrogenation and the reduction of NAD and FAD

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