What is respiratory quotient?

Explanation

The ratio of volume of carbon dioxide given out and volume of oxygen taken in during respiration is called Respiratory Quotient or Respiratory ratio. RQ value depends upon respiratory substrates and their oxidation.
`RQ =("Volume of "CO_(2) " libereated ")/("Volume of " O_(2) "Consummer ")`
(i) The respiratory substrate is a carbohydrate, it will be completely oxidised in aerobic respiration and the value of the RQ will be equal to unity.
` underset("glucose")(C_(6)H_(12)O_(6))+6O_(2) to 6CO_(2) uarr + 6H_(2) O + "energy "`
` RQ "of glucose " =(6" molecules of"CO_(2))/(6 "molecules of "O_(2))= 1 ("unity ")`
(ii) If the respiratory substrate is a carbohydrate it will be incompletely oxidised when it goes through anaerobic respiration and the RQ value will be infinity.
` "RQ of glucose Anaerobically "} = ( 2 " molecules of "CO_(2) )/( " zero molecules of " O_(2))= oo ("infinity") `
(iii) In some succulent plants like Opuntia, Bryophyllum carbohydrates are partially oxidised to organic acid, particularly malic acid without corresponding release of `CO_(2)` but `O_(2)` is consumed hence the RQ value will be zero.
` underset( " glucose ") ( 2 C_(6 ) H_(12) O_(6) )+ 3O_(2) to 3C_(4) H_(6) O_(5) + underset( "Malic acid ") (3H_(2) O ) `+ Energy
RQ of glucose in succulents `= ( "zero molecules of " CO_(2))/( 3 "molecules of " O_(2))=0 ` ( zero)
(iv) When respiratory substrate is protein or fat, then RQ will be less than unity.
` underset("Tripalmin ( Fat )") (2(C_(15 ) H_(98) O_(6)) )+ 145 O_(2) to 102 CO_(2) uarr + 98 H_(2) O + " Energy " `
` "RQ of Tripalmin " =( 102 molecules of "CO_(2)) /( 145 "molecules of " O_(2)) `
=0.7 ( less than unity )
(v) When respiratory substrate is an organic acid the value of RQ will be more than unity.
` underset( " malic acid ")(C_(4)H_(6 ) O_(5) ) +3O_(2) to 4 CO_(2) uarr + 3 H_(2) O+ " energy " `
` "RQ of malic acid " = ( 4 " molecules of "CO_(2) )/( 3 molecules of ""O_(2)) `.
`= 1.33 ` ( more than unity )