The yeast mixture in test tube A turned blue at first but slowly changed back to its original colour and stayed that way. The yeast mixture in test tube B turned blue and remained unchanged. The yeast mixture in test tube A are alive, whereas the yeast mixture in test tube B are dead. Reactions:
Methylene blue when reduced turns changes to a colourless methylene blue when it gains electrons, and can be oxidised back to blue when it loses electrons (oxidation). Therefore, it can be deducted that reactions in the yeast cells during respiration reduces blue methylene.
Methylene blue can be used as an indicator to determine if a cell, in this case yeast, is alive or not. The blue indicator turns colourless in the presence of active enzymes, thus indicating living cells. However, if it stays blue it doesn't mean that the cell is dead - the enzymes could be inactive/denatured. Methylene blue can inhibit the respiration of the yeast as it picks up hydrogen ions made during the process. The yeast cell cannot then use those ions to release energy.