Tag: respiration in plants

Under normal aerobic conditions, the final electron acceptor at the end of the electron transport chain is oxygen. However, fermentation occurs when there is no oxygen available. Therefore, something else must act as the final electron acceptor. In lactic acid fermentation, NADH is the electron carrier that ultimately carries them to pyruvate. Pyruvate is reduced to lactic acid, and thus, acted as the final electron acceptor.

The type of fuel used during physical activity also depends on the efficiency with which oxygen is delivered to muscle cells. The complete breakdown of glucose occurs when $O _2$ is present which is called as aerobic metabolism. When $O _2$ is unavailable, glucose breakdown is incomplete and it results in the formation of lactic acid. Accumulation of lactic acid leads to sore muscles. 
Incomplete breakdown of glucose does not result in the formation of alcohol, water or $CO _2$. 

In fermentation, say by yeast, the incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted to $CO _2$ and ethanol. The enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions. Other organisms like some bacteria produce lactic acid from pyruvic acid. In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase. In both lactic acid and alcohol fermentation not much energy is released; less than seven per cent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP. 

The process of anaerobic respiration takes place in the absence of oxygen. In this type, glucose is oxidized to ethyl alcohol, and it releases carbon dioxide. For example, breathing in yeast is a type of anaerobic respiration.

The breakdown of glucose to form ethanol and carbon dioxide is known as alcoholic fermentation. This takes place in the cytoplasm of cells. The substrate is glucose which undergoes glycolysis to form pyruvate. This pyruvate is then converted into acetaldehyde after the removal of one carbon and then ethanol is formed along with carbon dioxide.

• Lactic acid fermentation does not involve decarboxylation and results in the production of lactic acid and NAD$^{+}$.
  
• Alcoholic fermentation involves decarboxylation and produces CO$ _{2}$ along with ethanol.
  
• Aerobic respiration uses oxygen and glucose to produce CO$ _{2}$ and water as shown below:
        C$ _{6}$H$ _{12}$O$ _{6}$ + 6O$ _{2}$ $\rightarrow$ 6CO$ _{2}$ + 6H$ _{2}$O + energy (heat and ATP)

Fermentation is a type of anaerobic respiration in which the glucose is converted to release gases, acids and alcohols.