Respiratory Quotient
சுவாச ஈவு
Most students fail to realize that Respiratory Quotient (RQ) is not a constant. It is a diagnostic tool for the cell's metabolic state. If you assume RQ is always 1.0, you are losing marks on the substrate identification question.
The idea
Respiratory Quotient is the ratio of the volume of CO2 evolved to the volume of O2 consumed. The formula is RQ = Volume of CO2 evolved / Volume of O2 consumed. The value depends entirely on the respiratory substrate being oxidized. Carbohydrates yield 1.0, fats yield less than 1.0, and proteins yield approximately 0.9.
Why it matters
It exists as a diagnostic tool to determine the nature of the substrate (carbohydrate, fat, or protein) being utilized by the organism for respiration.
NEET lens
NEET expects you to memorize the specific RQ values for different substrates. Expect a matching-type question or a calculation based on the chemical equation of respiration. Remember: anaerobic respiration has an infinite RQ because no O2 is consumed. Do not confuse this with aerobic values.
Clinical significance
In clinical nutrition, RQ helps determine the metabolic fuel mix of a patient. A shift in RQ indicates whether the body is burning glycogen stores or utilizing adipose tissue. Monitoring this is critical in managing patients with severe respiratory failure or metabolic disorders.
Introduction
Respiratory quotient (RQ) is the ratio of volume of CO₂ released to volume of O₂ consumed during respiration in a given time: RQ = CO₂ released / O₂ consumed.
Core concept
RQ values by substrate: Carbohydrates: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O → RQ = 6/6 = 1.0. Fats (palmitic acid): C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O → RQ = 16/23 ≈ 0.7. Proteins: ~0.8. Organic acids (succinic acid): RQ > 1 (anaerobic conditions / less O₂ needed). When RQ 1: anaerobic respiration or organic acids being oxidized.
Go deeper
RQ > 1 occurs during anaerobic conditions when CO₂ is produced without O₂ consumption, or when organic acids (like malic acid) are used as substrates (they already contain oxygen atoms and need less O₂). Succulents in CAM metabolism often show RQ >1 at night.
From Class 10
The type of food a cell is burning affects how much O₂ it consumes and CO₂ it produces. The ratio of CO₂ produced to O₂ consumed tells us what's being used — this ratio is the respiratory quotient.