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?

The possible reason for the decrease in the cell growth would be the lack of glucose (food) in the culture.

Yeast is a eukaryotic unicellular organism capable of respiring aerobically as well as anaerobically.

In anaerobic respiration, one molecule of glucose produces a net gain of 2 ATP (glycolysis) only as compared to 36-38 ATP produced in aerobic respiration.

In addition, the pyruvate produced  during glycolysis is converted into ethanol and CO₂ in order to produce the lost NAD⁺.

Hence, by observation like production of ethanol and gas bubbles (caused by release of carbon dioxide) we can conclude that yeast were respiring anaerobically.

This increased the consumption rate of glucose or food by the yeast which resulted in the depletion of sugar in the culture.

Lastly, it caused considerable delay in the cell growth of the culture.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.