Respiratory Quotient (RQ)

The Respiratory Quotient (RQ), or respiratory ratio, is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration, and because different respiratory substrates require different amounts of oxygen to oxidise fully, RQ acts as a chemical fingerprint of which substrate a plant is actually burning. When the substrate is a carbohydrate and it is completely oxidised aerobically, exactly as many CO2 molecules are released as O2 molecules consumed, so RQ = 1 (unity) -- worked out from $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$ as $6\,CO_2 / 6\,O_2 = 1$. If a carbohydrate is instead broken down anaerobically (incompletely, as in alcoholic fermentation: $C_6H_{12}O_6 \rightarrow 2CO_2 + 2C_2H_5OH + \text{Energy}$), CO2 is released but no O2 is consumed at all, so RQ becomes infinite. In a few succulent plants such as Opuntia and Bryophyllum, carbohydrates are only partially oxidised into an organic acid (typically malic acid) with oxygen being consumed but essentially no CO2 released in the process ($2C_6H_{12}O_6 + 3O_2 \rightarrow 3C_4H_6O_5 + 3H_2O + \text{Energy}$), so RQ works out to zero. When the substrate is a protein or a fat, RQ is less than unity, because these molecules are relatively hydrogen- and carbon-rich and oxygen-poor compared with carbohydrate, so proportionally more oxygen is needed per CO2 released -- for example, oxidising the fat tripalmitin ($2\,C_{51}H_{98}O_6 + 145\,O_2 \rightarrow 102\,CO_2 + 98\,H_2O + \text{Energy}$) gives $RQ = 102/145 \approx 0.7$. Conversely, when the substrate is an organic acid, RQ is greater than unity because organic acids are already partly oxidised and therefore need comparatively little additional oxygen -- oxidising malic acid ($C_4H_6O_5 + 3O_2 \rightarrow 4CO_2 + 3H_2O + \text{Energy}$) gives $RQ = 4/3 \approx 1.33$. Typical RQ values for other substances include proteins (0.8-0.9), oleic acid (0.71), palmitic acid (0.36), tartaric acid (1.6) and oxalic acid (4.0). RQ is measured using an apparatus called Ganong's Respirometer, and can also be demonstrated with a simple seed-germination set-up in which the CO2 released by germinating seeds is absorbed by KOH solution, causing water to rise inside a connecting gl …