STPM Biology - chapter 5 (old syllabus) chapter 5 (new syllabus): Revision essay question 8

8. Describe anaerobiosis in yeast and in animal muscle cell.
 Answer

Yeast and animal muscle tissue are one type of the organism which can live in oxygen deficient condition even they need oxygen for a certain length of time. In the absence of the oxygen, yeast undergoes the anaerobic respiration. Anaerobic respiration is defined as a type of respiration is partially oxidized, with the release of chemical energy, in a process not involving atmospheric oxygen. The process is

called fermentation. Yeast is a one type of facultative anaerobes. There are two types of fermentation processes which are alcoholic fermentation and lactate fermentation. Alcoholic fermentation occurs in plant cell whereas lactate fermentation occurs in animal cells. The alcoholic fermentation is carried out by yeast in the absence of oxygen. Yeast is added to allow fermentation to take place. The yeast for example saccharomyces cerevisiae breaks down glucose during fermentation, producing ethanol and carbon dioxide. First, glucose is converted to pyruvate, generates a net gain of 2ATP and NAD⁺ reduced to NADH. Pyruvate is then decarboxylated to become ethanol (acetaldehyde) and carbon dioxide is released. The decarboxylation is catalysed by the enzyme decarboxylase. Ethanal act as a hydrogen acceptor, accepts hydrogen atoms from NADH+H⁺ to be reduced to ethanol. This reaction required the enzyme NADH dehydrogenates to catalyse.

Overall equation for alcoholic fermentation:

 glucose→ 2ethanol + 2CO₂ + 2ATP

Lactate fermentation occurs in muscles during vigorous exercise. When the oxygen level in the muscle is limited, anaerobic respiration occurs. This is due to the rate of consumption exceeding the rate of supply. During glycolysis, glucose is converted to pyruvate. This will generates a net gain of 2ATP and NAD⁺ reduced to NADH. Pyruvate is then reduced to lactate by NADH and NAD⁺ is regenerated.

However, lactate accumulation in the muscle causes fatigue, lowers blood pH and cramping. To break down the lactate, oxygen is required and is known as the oxygen debt. After vigorous exercise, the lactate is oxidized back to pyruvate in the liver. The pyruvate is then oxidized to water and carbon dioxide.

Overall equation for lactate fermentation:

     glucose→2lactate + 2CO₂ + 2ATP

Assignment submitted by S.M., Kang, E.S., Ong, T.W., Tan, K.Y., Ho. X.H., Wong 2011/2012 (IBM)