RESPIRATORY QUOTIENT (R.Q)
Respiratory quotient is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time.
RQ=Volume of CO2 release/volume of oxygen absorbed
The respiratory quotient besides other factors, depends on the type of respiratory substrate (e.g., glucose) and the completeness of oxidation. It can be measured with the help of an apparatus called, Ganong’s respirometer. The value of respiratory quotient can be 1,zero, more than 1 or less than 1.
RESPIRATORY QUOTIENT EQUAL TO UNITY:
Plant material where carbohydrates serve as the respiratory substrates, (e.g. germinating seeds of cereal, leaves, flowers and fruits rich in carbohydrates) have respiratory quotient equal to unity, i.e.,. 1.
C6H12O6+6O2→6 CO2+6 H2O
RQ=molecule of carbon dioxide released/molecules of oxygen absorbed=6 CO2/6 O2=1
RESPIRATORY QUOTIENT LESS THAN UNITY:
Respiratory quotient can be less than 1 under following two conditions.
Incomplete oxidation of carbohydrates: Under certain circumstances, sugars are incompletely oxidised to organic acid. This takes place in some succulents (e.g., Opuntia) and leaves having anthocyanin pigment. They also change carbohydrates to organic acid which utilize oxygen but do not evolve carbon dioxide. In these reactions, the value of respiratory quotient is less than 1.
2 C6H12O6+3 O2→3 C4H6O5(malic acid)+3H2O
When respiratory substrates are fats and proteins: In germinating fatty seeds (e.g., castor sesame, linseeds etc). The only respiratory substrate is fat. During germination, it is converted into carbohydrates. This process would require oxygen because in the carbohydrates, carbon and oxygen are in equal proportion whereas in fats the proportion of carbon is much more than that of oxygen. The absorption of oxygen is not compensated by any release of CO2. This can be illustrated by the following two examples.
2 (C51H98O6)(tripalmitin)+145 O2→102 CO2+98 H2O
RQ=102 CO2/145 O2=0.7
C57H104O6(triolein)+80 O2→57 CO2+52 H2O
RQ=57 CO2/80 O2=0.71
Respiratory quotient in case of proteins, peptones, etc., is about 0.9. Here the proportion of carbon is more than oxygen.
RESPIRATORY QUOTIENT MORE THAN UNITY: Respiratory quotient can be more than 1 under the following three circumstances.
- When respiratory substrates are organic acids: In fleshy plants such as (e.g., members of Cactaceae and Crassulaceae). The organic acid such as malic acid, oxalic acid etc. produced during night are oxidised during daytime. The organic acid contains high proportion of oxygen as compared to carbohydrates, therefore, less oxygen is absorbed than carbon dioxide liberated.
2 C2H2O4 (oxalic acid)+O2→4 CO2+2 H2O
RQ=4 CO2/+1 O2=4
C4H6O5 (malic acid)+3 O2→4 CO2+3 H2O
RQ=4 CO2/3 O2=1.3
2 C4H6O4 (succinic acid)+7 O2→8 CO2+6 H2O
RQ=8 CO2/7 O2=1.14
- When conditions are anaerobic: In the absence of oxygen, tissues respire anaerobically. There is no consumption of oxygen. Thus, carbon dioxide liberation takes place without any utilization of oxygen. Carbohydrate is the usual substrate.
C6H12O6 (glucose)→2 C2H5OH (alcohol)+2 CO2
RQ=2 CO2/Zero O2 =2 or infinity
An intermediate value is obtained when an organ is undergoing both aerobic and anaerobic modes of respiration.
- When carbohydrates are transferred into fats in mature seeds: In maturing seeds of many plants, carbohydrate gets converted into fats and oxygen is absorbed without any release of carbon dioxide. This is because the fat molecules contain less oxygen in proportion to carbon than in carbohydrates. This free oxygen can serve as an internal supply for respiration and carbon dioxide is produced. In such circumstances, oxygen is not absorbed from outside, hence, respiratory quotient is more than 1.
Knowledge of respiratory quotient helps in determining respiratory substrates. It helps in knowing the type of respiration being performed. It provides some information about major transformation of food materials.
FACTORS AFFECTING RESPIRATION
Factors affecting respiration can be classified into two categories.
- External factors.
- Internal factors.
External Factors: Many external factors like temperature, light, carbon dioxide etc., affect the rate of respiration:
Temperature: Temperature significantly affects the rate of respiration. Usually, the rate of respiration increases with the increase in temperature in the range of 0-45 degree centigrade. Between 0-25 degree centigrade, for every rise of 10 degree centigrade, the rate of respiration increases 2 to 3 times. It means that the Q10 of respiration in the temperature range of 0-25 degrees centigrade is 2 to 3. However, above 30 degree centigrade, there is a decline in rate of respiration. This decrease is due to denaturation and destruction of the respiratory enzyme at high temperature. Respiration is also inhibited at low temperatures because the enzymes become inactive at very low temperature.
Oxygen: Oxygen is essential for aerobic respiration. It is the ultimate acceptor of electrons (hydrogen) in electron transport system (ETS). At low oxygen concentration, both aerobic and anaerobic respiration occur. Under such circumstances, respiration quotient values are greater than 1. As the oxygen concentration approaches zero, the liberation of carbon dioxide is only due to anaerobic respiration, hence, the respiratory quotient becomes infinity.
To conserve substrates, a sufficient supply of oxygen is essential. Louis Pasteur, a French microbiologist, observed that proper supply of oxygen is essential to inhibit fermentation of sugar by yeast cells, i.e., to stop anaerobic respiration. This is known as Pasteur’s effect.
A very high concentration of oxygen inhibits the activity of respiratory enzymes thereby reducing the rate of respiration.
Light: Light affects respiration indirectly in the following ways. Light raises the temperature which in turn increases the rate of respiration. Stomata open in light and thus, facilitate gaseous exchange. In light, sugars are synthesized which are readily utilized as respiratory substrates increasing the rate of respiration.
Carbon dioxide: Increased concentration of carbon dioxide in the atmosphere decreases the rate of respiration. The soil atmosphere in comparison to aerial atmosphere has more carbon dioxide. As a result, root hairs have low rate of respiration. In high concentration of carbon dioxide, stomata close and exchange of gases stop, decreasing the rate of respiration. Many metabolic activities are inhibited in high concentration of carbon dioxide. As a result, the rate of respiration also decreases.
Water: Water is essential for accelerating enzyme activity. In condition of water stress, the rate of respiration decreases. Dry fruits and seeds in which respiratory enzymes are least active, show minimum respiration. Hydration of tissues increases respiration.
Pollutants: High concentration of gaseous air pollutants like SO2, O3 etc., inhibit respiration by damaging cell membranes. Heavy metal pollutants like lead and cadmium inhibit respiration by inactivating respiratory enzymes. Gaseous pollutants increase pH, which in turns affects the ETS, thus, inhibiting respiration.
Internal Factors: Protoplasm and respiratory substrates are two important internal factors which affect respiration.
- Protoplasm: The meristematic cells have more protoplasm than mature cells. Hence, the meristematic cells have higher rate of respiration than the mature cells.
- Respiratory substrates(glucose, fructose, maltose etc): Increase in the amount of substrates increases the rate of respiration.