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Respiration is a complex process that includes the combination of breathing and cellular respiration.


Breathing is exchange of gases i.e intake of oxygen from the atmosphere and release of carbon dioxide.

Cellular respiration:

It is the complex catabolic process, in which the biochemical oxidation of nutrients takes place for the production of energy in the form of ATP.

The food we intake is digested as simple molecules, they enter the blood stream and then assimilated into the cell. The assimilated food can be oxidised for the purpose of energy otherwise utilized for the growth purpose.

Based upon the availability of the oxygen respiration can be divided into two types:

(a) Aerobic respiration

(b) Anaerobic respiration


Breaking down of glucose molecule in the presence of oxygen is called as aerobic respiration. This results in the liberation of carbon dioxide, water and energy. This process includes 2 phases – glycolysis, Kerb’s cycle.

(a) Glycolysis: Glucose molecule is broken down into 2 pyruvate molecules. It is known as glycolysis or EMP pathway, takes place in the cytoplasm of the cell.

(b) Kreb’s cycle: In the presence of oxygen, pyruvate molecules enters into the mitochondria. In mitochondria it is completely broken down to release carbon dioxide, water and energy in the form of ATP.


A glucose molecule subjected for complete oxidation liberated about 38 ATP molecules.


Breakdown of glucose molecule in the absence of oxygen is known as anaerobic respiration. It involves the incomplete breakdown of glucose molecule into ethanol or lactic acid, CO2 and energy.

Glucose is broken down into pyruvic acid in the cytoplasm. Then this pyruvate in the absence of O2 produces ethanol or lactic acid. It occurs in certain microorganisms such as bacteria and yeast.


In muscle cells in the absence of oxygen (during vigorous muscle activities sometimes O2 may be insufficient), the glucose molecule is converted as pyruvate and pyruvate is converted into lactic acid, produces energy. Production of lactic acid in muscles causes muscle cramps.