Question

You are growing yeast cells in a culture. After a few days of consistent growth, you notice that cell growth has slowed considerably. After some testing you note gas bubbles in the culture fluid along with the presence of ethanol. What is the most likely cause of the decrease in cell growth? A) There is too much oxygen in the culture medium. B) There is too much glucose in the culture medium. C) There is not enough oxygen in the culture medium. D) There is not enough glucose in the culture medium.

Answer 1

The correct answer is C) There is not enough oxygen in the culture medium. This is because of alcoholic fermentation, and anaerobic process where the yeast transform sugar (glucose) in ethylic alcohol (ethanol) and carbon dioxide. Glucose is decomposed into pyruvic acid which then after turns into CO2 and ethanol. The bubbles described, are produced by the carbon dioxide.

The yeast, as well as some bacteria, use the glucose molecule through "glycolysis"  to obtain a 3-carbon molecule called pyruvates. Glycolysis consists of 10 coupled reactions, in the end, from one glucose (6 carbons) the yeast will obtain two pyruvates (3 carbons each).

Pyruvate can follow three main routes to obtain ATP, end up as lactate, as carbon dioxide (CO2) and water or as ethanol (alcohol) and CO2. Regarding yeast, it can only be used to obtain Ethanol plus CO2 or to obtain CO2 plus water.

The path that follows from here depends on the reaction medium. The cell gets much more energy (38 molecules of ATP) by converting pyruvate into water + CO2 than by turning it into ethanol + CO2 (2 molecules of ATP). Then, whenever possible, the yeast will follow the CO2 + water path. To support this route the cell needs oxygen.  In this case, the cell obtains its energy by breathing when there is no oxygen available, the yeast has a way that allows it to gain much less energy but allows it to survive, the alcoholic fermentation, previously mentioned.

Therefore, A, B, and D answers are wrong for the reasons mentioned above.