Q.Define the respiratory quotient (RQ), stating the formula by which it is calculated.

The RQ (CO2 released / O2 consumed) for protein as a respiratory substrate is approximately 0.9.

Respiratory Quotient (RQ) = Volume of CO2 evolved / Volume of O2 consumed, and its value depends on the chemical nature of the respiratory substrate being oxidised:

• Carbohydrates: RQ = 1 (equal volumes of CO2 released and O2 consumed, since carbohydrates already contain O2 and H in the same ratio as water).

• Fats: RQ is much less than 1 (about 0.7), because fats are relatively oxygen-poor and need more O2 to be fully oxidised relative to the CO2 produced.

• Proteins: RQ is intermediate, approximately 0.9, since proteins have an oxygen content between that of carbohydrates and fats.

Because carbohydrates are fully oxidised with CO2 output equal to O2 input, their RQ = 1.

Respiratory Quotient (RQ) = volume of CO2 evolved / volume of O2 consumed. For a carbohydrate substrate such as glucose, the balanced respiration equation is C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy - exactly six molecules of O2 are consumed for exactly six molecules of CO2 released, so the ratio (RQ) works out to 6/6 = 1. (For comparison, fats have RQ less than 1 since they need more oxygen relative to CO2 released, owing to their lower oxygen content, while some organic acids can have RQ greater than 1.)

Respiratory Quotient (RQ) = volume of CO2 evolved / volume of O2 consumed. For carbohydrates, complete oxidation releases exactly as much CO2 as the O2 consumed, so RQ = 1.

Take glucose (a carbohydrate) as the representative respiratory substrate. Its complete aerobic oxidation is:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

Here, 6 moles of O2 are consumed and exactly 6 moles of CO2 are released. Since carbohydrates already contain O and H in the ratio found in water (CnH2nOn), no extra oxygen is needed beyond what balances the CO2 produced.

RQ = CO2 evolved / O2 consumed = 6/6 = 1

RQ = (volume of CO2 evolved) / (volume of O2 consumed); for carbohydrates this ratio works out to exactly 1.

Consider the complete aerobic oxidation of glucose, a representative carbohydrate:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

Here, 6 molecules of O2 are consumed and exactly 6 molecules of CO2 are released. So:

RQ = 6 CO2 / 6 O2 = 1

This is because carbohydrates already contain oxygen in the same ratio as needed to fully balance the CO2 produced, unlike fats (RQ < 1, since fats need extra O2 relative to the CO2 they yield) or proteins (RQ close to 0.9, intermediate).

The respiratory quotient of protein is about 0.9.

Respiratory Quotient (RQ) = volume of CO2 evolved / volume of O2 consumed during respiration, and its value differs by respiratory substrate: for carbohydrates RQ = 1 (carbohydrates already contain O and H in the same ratio as water, so complete oxidation of glucose releases exactly as much CO2 as O2 consumed); for fats RQ is about 0.7 (fats are relatively oxygen-poor and hydrogen-rich, so more O2 is needed relative to CO2 released); and for proteins RQ is intermediate, approximately 0.9, because protein oxidation is more complex and incomplete (nitrogenous wastes are also produced).

The respiratory quotient (RQ) of carbohydrates is 1, because the complete oxidation of a carbohydrate releases exactly as much CO2 as the O2 it consumes.

Concept. The respiratory quotient (RQ) or respiratory coefficient is defined as the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed in respiration:

RQ = (volume of CO2 evolved) / (volume of O2 consumed)

For carbohydrates (e.g. glucose): complete aerobic oxidation follows

C6H12O6 + 6O2 -> 6CO2 + 6H2O

Here 6 volumes of CO2 are released for every 6 volumes of O2 used, so:

RQ = 6CO2 / 6O2 = 1

Because carbohydrates contain enough oxygen within the molecule and are fully oxidised without a net extra oxygen requirement, their RQ equals unity. (For comparison, fats have RQ less than 1, and organic acids have RQ greater than 1.)

RQ = CO2 evolved / O2 consumed; it indicates the type of respiratory substrate being oxidized.

During aerobic respiration, the ratio of CO2 released to O2 consumed varies with the substrate: RQ = 1 for carbohydrates (complete oxidation, e.g. glucose), RQ < 1 for fats and proteins (since they are less oxidized and consume relatively more O2), and RQ > 1 for organic acids. RQ is therefore used to infer which substrate a tissue is respiring.

Respiratory Quotient (RQ) = volume of CO2 evolved ÷ volume of O2 consumed. For carbohydrates RQ = 1, for fats RQ ≈ 0.7, and for proteins RQ is intermediate, approximately 0.9.

$RQ = \dfrac{\text{Volume of CO}_2 \text{ evolved}}{\text{Volume of O}_2 \text{ consumed}}$

The RQ value depends on which respiratory substrate is being oxidised:

• Carbohydrates (e.g., glucose, $C_6H_{12}O_6$): fully oxidised, RQ = 1, since equal volumes of CO2 are produced for O2 consumed.

• Fats: contain proportionally less oxygen and more hydrogen/carbon, so more O2 is needed for complete oxidation relative to CO2 released — RQ ≈ 0.7.

• Proteins: their oxidation is incomplete (nitrogenous residues are excreted rather than oxidised) and their oxygen content is intermediate between carbohydrates and fats, giving an RQ value between the two, generally cited as about 0.8–0.9.