Types Of Respiration

Types Of Respirations and Its classifications 

Respiration is of two types aerobics and anerobic.

Some organisms like bacteria are obligate aerobe (necessarily use oxygen) or obligate anaerobes (necessarily in absence of oxygen)

Facultative aerobes are usually in absence of O₂ and Facultative anaerobes are usually in presence of O₂.

Aerobic respiration : results in complete breakdown of glucose in presence of O₂ releasing 686 Kcal energy.  It has two pathways; common and PPP (Pentose Phosphate Pathway).

Anaerobic respiration : It is an energy releasing, catabolic, enzymatic stepwise incomplete breakdown of  food without employing oxygen.  It is a less efficient mode of respiration as it produces little amount of energy.  Its end products contain atleast one organic component which can be further oxidised on getting O₂.  Since no oxygen is used, therefore, H2O is not produced as end product.

Kostychev coined the term anaerobic respiration.

Pfluger called it as intramolecular respiration.

Anaerobic respiration by microorganisms or their enzymes outside the cell in a liquid medium is usually called fermentation.

Fermentation : It is a type of anaerobic, incomplete break down of organic food carried out primarily by fungi and bacteria or their enzymes in liquid medium.  It is independent of presence or absence of oxygen but is more rapid in the absence of oxygen.

Gay Lussac was the first to give a reaction for the fermentation of sugar.

Cruick shank (1997) studied fermentation.  Buchner (1897) found extract of yeast that contains an enzyme complex zymase could bring about fermentation.

Mechanism of Anaerobic Respiration

It involves two steps

• Glycolysis : It is similar to glycolysis of aerobic respiration except that aerobic glycolysis produces 2 ATP and 2 NADH2. (Aerobic glycolysis is 4 times more efficient than anaerobic glycolysis.)
• Anaerobic breakdown of pyruvic acid : Depending upon the organism, type of tissue and nature of end product, anaerobic respiration or fermentation is of following types :

(i) Alcoholic fermentation : It is that fermentation where product is ethyl alcohol and CO2.

The acetaldehyde is the intermediate product of alcoholic fermentation.

In the second step acetaldehyde is reduced to alcohol by using NADH2. NADH2 is re-oxidised and re-cycled back to glycolysis.

Thus in this reductive dacarboxylative process of alcohol fermentation; both molecules of NADH2 formed in glycolysis are used up and thus there is a loss of 6 ATP per molecule of glucose and net ATP production in anaerobic respiration remains to be 2 only.

(ii) Lactic acid fermentation : It is that fermentation where sugar is converted into lactic acid.

In dairy industry, lactose sugar of milk is converted into lactic acid by bacteria Lactobacillus acidi, making milk sour in taste.  This lactic acid causes curdling.

During strenuous exercise, lactic acid is produced in voluntary muscles of humans.  This accumulation of lactic acid in muscles causes fatigue and cramps.  During rest when oxygen supply is restored latic acid is oxidised into glycogen or glucose and finally to pyruvic acid in liver by reverse anaerobic glycolysis or Cori cycle.

Cancer cells deep within tumour survive by lactic acid fermentation.

(iii) Butyric acid fermentation : It occurs in butter by acid bacteria (Bacillus butyricus and Clostridium butyricum and makes it sour and fowl smelled.  These bacteria convert hexose sugar and lactic acid into butyric acid.

Pyruvic acid + NADH₂ C₄H₈O₂ + CO₂ + NAD⁺ + H₂

(iv) Acetic acid fermentation : It requires oxygen and is carried out by bacteria like Acetobacter aceti. It is completed in two steps.  In the first step, glucose in converted into alcohol anaerobically and in second step, alcohol is oxidised into acetic acid using O₂

C₆H₁₂O₆  2C₂H₅OH + 2CO₂

Baking and brewery industries are totally dependent upon fermentation by yeast. (Saccharomyces).