The citric acid cycle evolved from preexisting metabolic pathways. Evidence includes organisms that do not use the citric acid cycle, but have genes coding for enzymes used in the citric acid cycle
Which of the statements about the evolution of the citric acid cycle are consistent with existing knowlege? Can be more than one answer!
1) Although some organisms do not use the citric acid cycle under anaerobic conditions, they use a reductive pathway similar to the citric acid cycle to generate some larger molecules.
2) A reductive citric acid cycle can replenish oxidized cofactors such as NADH.
3)Early organisms may have lacked the enzyme ?-ketoglutarate dehydrogenase.
4) The citric acid cycle may have originated from two pathways �a reductive pathway and an oxidative pathway.
5)The citric acid cycle may have evolved from two biosynthetic pathways in early life forms.

1) Although some organisms do not use the citric acid cycle under anaerobic conditions, they use a reductive pathway similar to the citric acid cycle to generate some larger molecules.

3)Early organisms may have lacked the enzyme ketoglutarate dehydrogenase.

4) The citric acid cycle may have originated from two pathways a reductive pathway and an oxidative pathway.

5)The citric acid cycle may have evolved from two biosynthetic pathways in early life forms.

Citric acid cycle evolve when one species of proteobacteria entered into a symbiotic relationship with a primitive eukaryotic cell this happened about two billion years ago. This is the major cause of evolution of mitochondria together with modern versions of the citric acid cycle and electron transport in eukaryotes.

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