Question

You inoculate two species of yeast, A and B, into a lab beaker and let them compete for the only energy source, gluocse, which they respire using O2 (the only terminal electron acceptor they can use). The two yeast species are identical with the exception that species A has 10 c-subunits in the F0 unit of ATP-synthase, while species B has 11 c-subunits. After growing them overnight, (A) which species, A or B, dominates the culture and (B) why? The following day, you repeat the experiment but this time you let the two yeast compete with each other in the absence of O2 (i.e. you grow them anaerobically). Under these conditions, after growing them overnight, (C) which of the two dominates, and (D) why?

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Answer 1

In the first case, the yeast are grown in presence of oxygen which is the only terminal electron acceptor they can use. So two protons will be pumped out as oxygen accepts the electron and becomes reactive to form metabolic water. The two yeast species are identical with the exception that species A has 10 c-subunits in the F0 unit of ATP-synthase, while species B has 11 c-subunits. Therefore, 10 protons can flow from F0 to F1 unit in species A, whereas in species B, 11 protons can flow. So in species B there is an additional flow of 1 proton in F1 via F0 of ATP-synthase. The synthesis of one ATP is driven by the flow of 3 protons through the ATP-synthase. So, here for pumping of 2 protons number of ATP molecules generated= for both species A and B, but when all the protons are available then there will be excess 0.33 moles of ATP produced in species B due to the flow of 1 additional proton in its ATP-synthase. So in the first case, species B will dominate in presence of oxygen.

In the second case, the two yeast species are grown in absence of oxygen, i.e anaerobically. In absence of oxygen the only mechanism to utilise glucose is via glycolysis where the end product, pyruvate is converted to alcohol or lactic acid depending on the mode of fermentation. In both types of fermentation, the net gain is 2 ATP which is produced as a result of glycolysis via substrate level phosphorylation. So when the two yeast species A and B are grown anaerobically, then both of them will grow simultaneously because the intermediates formed in glycolysis cannot enter electron transport system, so the ATP synthase remain non functional as it is not involved in oxidative phosphorylation.