Yeast And Anaerobic Respiration

Yeasts (Fig. 1 & 2) are a type of unicellular fungi often used to ferment alcohol. They are a heterotrophic organism, which means that the cells utilise the energy produced from processing other organic material. Heterotrophic organisms use cellular respiration to acquire this energy. This process is vital: it converts large, unusable energy molecules such as glucose into the more useful energy form of Adenosine Triphosphate (ATP), which allows essential cell activity to occur.
Most species of yeast are readily able to use aerobic respiration to gain energy when oxygen is available. This method produces carbon dioxide, water and energy. It is also very efficient, yielding 36 ATP molecules for each glucose molecule:
C6H12O6 + 6O2 6CO2 + 6H2O + 36ATP
If oxygen is not readily available, yeasts will use an alternative pathway (Fig. 3), from which ethanol, carbon dioxide and energy are produced. This is known as anaerobic fermentation. It is carried out around one hundred times faster than aerobic respiration; it is only able to yield two ATP molecules for every six glucose molecules:
C6H12O6  2C2H5OH + 2CO2 + 2ATP

Glycolysis initially occurs through a sequence of enzyme-catalysed reactions in the cytosol of a yeast cell. High-energy carbon bonds in glucose are broken and the lower-energy molecule Pyruvate is formed. This produces enough energy to form ATP from ADP and Phosphate (Pi). NAD+ is reduced to form the coenzyme NADH:
C6H12O6 + 2NAD+ + 2ADP + 2Pi →
2CH3COCOO− + 2NADH + 2ATP + 2H2O + 2H+
NAD+ is then regenerated from NADH so that glycolysis can continue to occur. The electrons lost reduce Pyruvate, which is then converted to Acetaldehyde. Two CO2 molecules are given off as waste. Acetaldehyde is further converted to e...

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...ose, lactose) to observe which produces the greatest and least amount of energy. Different concentrations of glucose or different temperatures of the glucose solution could also be used as the independent variable in order to establish the most efficient concentration or temperature. It could also be relevant to measure the thermal energy produced from yeast aerobic respiration by constantly bubbling oxygen gas through a glucose solution. Similar calculations could be carried out to compare the theoretical and practical energy produced.

Conclusion
The hypothesis was accepted: the temperature of the yeast solution containing glucose did rise in temperature due to the fermentation of glucose by the yeast. The theoretical energy produced from the glucose was less than the experimental result; this is because not all chemical energy was converted to thermal energy.