The bubbles of carbon (IV) oxide produced by yeast cells in a glucose solution containing pyrogallol indicate that the yeast cells are performing anaerobic respiration (fermentation), breaking down glucose to produce carbon dioxide and ethanol. 

Yeast, like many microorganisms, can survive and reproduce in the absence of oxygen through a process called fermentation.During fermentation, yeast breaks down glucose (a type of sugar) into simpler molecules, such as ethanol and carbon dioxide, releasing energy in the process.