Animals require a constant supply of energy. This energy performs biological work. The ATP provides this energy. All animals can generate ATP by breaking down organic nutrients. These nutrients are carbohydrates, fats, and proteins. Energy is released during respiration. This energy is used to join ADP and phosphate (Pi) to form ATP.

The breakdown of organic nutrients begins in a step by step series of chemical ‘reactions called glycolysis. The end product of glycolysis is pyruvate. The pyruvate is then further broken down. This breakdown occurs in two processes:

(a)     Aerobic respiration: It takes place in the presence of free oxygen ‘

(b)     Anaerobic respiration: It takes place in the absence of free oxygen. This process is called fermentation.

G ycolysis•and fermentation (anaerobic processes) take place in the cytoplasm of a cell. But aerobic respiration takes place in the mitochondrion.

Evolution of glycolysis and fermentation

It is believed that evolution of glycolysis and fermentation occurred in earliest oiganism. Two factors support this view:

1 The enzymes remained dissolved in the cytoplasm during eukaryotic cell evolution. These enzymes were not enclosed in membranous organelles.

Therefore, glycolysis and fermentation occur in the cytoplasm. Thus the origin of glycolysis and fermentation took place before evolution of complex organelles.

2. Similarly these reactions have occurred in the earliest primitive environment of earth. This atmosphere did not contain free oxygen. So these reactions are

also older than aerobic respiration.





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