The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen used in respiration, in the same time:RQ = CO2 produced ÷ O2 consumed RQ has no units, because it is a ratio of two volumes (or numbers of molecules). Equal volumes of gases contain equal numbers of molecules, so RQ can be worked out from the balanced equation for the respiration of a substrate.Each substrate has a typical RQ in aerobic respiration:• Carbohydrate: 1.0
• Protein: about 0.9
• Lipid: about 0.7Lipids have a low RQ because they contain many hydrogen atoms and little oxygen. Extra oxygen is needed to oxidise the hydrogen to water, so more oxygen is used for each carbon dioxide produced.Interpreting RQ values. An RQ of 1.0 suggests carbohydrate is being respired, and 0.7 suggests lipid. A value between, such as 0.85, suggests a mixture of substrates. Resting humans usually have an RQ of about 0.8 to 0.85. An RQ greater than 1 means some anaerobic respiration is happening, because carbon dioxide is released (for example by yeast producing ethanol) without oxygen being used.Worked example: a seed produces 40 cm3 of CO2 while using 56 cm3 of O2. RQ = 40 ÷ 56 = 0.71, so the seed is respiring mainly lipid.
• Protein: about 0.9
• Lipid: about 0.7Lipids have a low RQ because they contain many hydrogen atoms and little oxygen. Extra oxygen is needed to oxidise the hydrogen to water, so more oxygen is used for each carbon dioxide produced.Interpreting RQ values. An RQ of 1.0 suggests carbohydrate is being respired, and 0.7 suggests lipid. A value between, such as 0.85, suggests a mixture of substrates. Resting humans usually have an RQ of about 0.8 to 0.85. An RQ greater than 1 means some anaerobic respiration is happening, because carbon dioxide is released (for example by yeast producing ethanol) without oxygen being used.Worked example: a seed produces 40 cm3 of CO2 while using 56 cm3 of O2. RQ = 40 ÷ 56 = 0.71, so the seed is respiring mainly lipid.