respiratory Quotient (RQ)| respiratory Quotient of carbohydrates| RQ of Fat| RQ of proteins| RQ of organic acids|

     RESPIRATORY OTIENT (RQ)    

Respiratory quotient (R.Q.) is the ratio of the volume of carbon dioxide released to the volume of oxygen taken in respiration in the given period of time at standard temperature and pressure.

                              RQ= Volume of CO2 evolved by Volume of O2 absorbed 

Value of RQ varies with the respiratory substrates and their oxidation. Thus, the measurement of RQ may give some guide as to the nature of the substrate being respired by a particular tissue. The value of RQ in the oxidation of different substrates is given below: -

1. RQ of carbohydrates: -. When carbohydrates, such as hexose sugars (i.e., glucose or fructose), Sucrose, or starch, are respired in presence of oxygen (i.e., aerobic respiration), the volume of CO2, evolved during the process is equal to the volume of O2 absorbed. Thus, the value of RQ is equal to one or unity, as shown by the following chemical reaction: - 

C6H12O6 + 6O2 ------> 6CO2 + 6H2O +Energy

RQ = Vol. of CO2 by the Vol. of O2 = 6/6 = 1 or unity 

for example, leaves of many plants, rose petals, germinating seeds of cereal grains (Such as wheat), potato tubers, etc., show their RO value more nearly equal to unity. The germinating seeds of all those plants which contain starch as the main reserve food, also exhibit RQ equal to unity. 

(2) RQ of fats: -. When fats are respiratory substrates, as in case during germination or fatty seeds (e.g. ground nut, mustard, flax, castor, etc.), the fats are hydrolysed to glycerol and fatty acids. 1he complete Oxidation of glycerol results in the RO of 0-86. The fatty acids, being poorer in oxygen, require more O2 for complete oxidation as compared to CO2. Therefore, O2 absorption is more and CO2 absorption is less and the RQ becomes less than unity. For example: -

(i)RQ is 0-7 during oxidative breakdown of glycerol tripalmitate: -

2C51H98O6 (Tripalmitin) + 145O2 ----> 102 CO2 + 98 H2O + ENERGY 

RQ = Vol. of CO2 by the Vol. of O2 =  102/145 = 0.7(less than unity)

(ii) RQ is 0-69 during oxidative breakdown of glycerol trioleate: - 

C57H104O6 (Triolein ) + 83O2 -----> C12H22O11 + 4CO2 + 5H2O + Energy  

RQ= vol. of CO2 by the Vol. of O2 = 57/83 = 0.69(less than unity) 

(iii) RQ is 0.36 during conservation of palmitic acid to sucrose in the endosperm tissue of germinating fatty seeds: -

C16H32O2(Palmitic acid)  + 11O2 ----> C12H22O11 + 4CO2 + 5H2O 

RQ = Vol. of CO2 by the Vol. of O2 = 4/11= 0.36(less than unity)

3). RQ of proteins: -. As in case of fats, the protein also have RQ of less than unity. First of all the proteins are hydrolysed into amino acids. The hydrolytic products of proteins have lower proportion of oxygen as compared to carbohydrates. They required more O, and evolve less CO, during their complete oxidation. Thus, the value of RQ becomes less than unity. Example, germinating seeds of buckwheat (RQ is 0.5). 

(4). RQ of organic acids: -. When organic acids are respired (as in succulents), the RQ is always more than unity. These acids are rich in O2, and, therefore, require less O2, and evolve more CO with the result the value of RQ becomes more. For example: - 

(i) RQ of oxalic acid is 4 as shown in the following reaction: -

2 (COOH)2 + O2 -----> 4CO2 +2 H2O+ 602 K cal. 

RQ =Vol. of CO2 by the Vol. of O2 = 4/1 = 4 (more than unity) 

(ii) RQ of citric acid is 1.33 

2C6H807 + 902 ----> 12 CO2+8 H2O 

RQ = Vol. of CO2 by the Vol. of O2 = 12/9 = 1.33 (more than unity) 

(iii) RQ of tartaric acid is 1.6 

C4H606 + 5O2 ----> 8C02 +6 H20 

RQ = Vol. of CO2 by the Vol. of O2 (more than unity)

(5) Anaerobic respiration: - If glucose molecule is broken down anaerobically into CO, and ethyl alcohol, the value of RQ is always more than unity. Though the numerical value of 2/4 is infinity but practically RQ will be more than unity.

Significance of RQ : -

The significance of determining RQ of plants is that it provides important information regarding the chemical nature of the respiratory substrate being respired. RQ less than unity indicates that respiratory Substrate has low O2 content, such as fats and proteins. The substrates having more O2 content, such as organic acids, show RQ more than unity. The carbohydrates, which have C and O2 in equal proportion, generally show unity (1) RQ.