Respiratory quotient (RQ)

Respiratory Substrates

• glucose is the essential respiratory substrate for some cells such as neurons, red blood cells and lymphocytes
• however other cells can oxidize lipids and amino acids
  • when lipids are respired, carbon atoms are removed in pairs as, acetyl CoA, from fatty acid chains and fed into the Krebs cycle
  • the carbon-hydrogen skeletons of amino acids are converted to pyruvate or acetyl CoA

Energy values of respiratory substrates

• most energy liberated in aerobic respiration comes from oxidation of hydrogen to water when reduced NAD and reduced FAD are passed to the ETC
• hence, the greater the number of hydrogens in the structure of a substrate molecule, the greater the energy value
• fatty acids have more hydrogens per molecule than carbohydrates, so lipids have a greater energy value per unit mass or energy density, than carbohydrates or proteins

The energy value of a substrate is determined by burning a known mass of the substrate in oxygen in a calorimeter.

• the energy liberated by oxidizing the substrate is determined from the rise in temperature of a known mass of water in the calorimeter

Respiratory quotient (RQ)

• The Respiratory Quotient (RQ) is a measure that indicates which substrate is being utilized in cellular respiration and provides insights into the metabolic process.
• It is calculated as the ratio of the volume of carbon dioxide (CO2) produced to the volume of oxygen (O2) consumed during respiration.
• The formula for RQ is RQ = VCO2 produced / VO2 consumed
• it can also show whether or not anaerobic respiration is occurring
  • the RQ will be slightly less than infinity
  • high values of RQ indicate that alcoholic fermentation is occurring
  • No RQ can be calculated for muscle cells under lactate fermentation as no CO2 is produced

Examples of RQ values for different respiratory substrates

• Glucose: RQ = 1 (One molecule of glucose produces one molecule of CO2 and consumes one molecule of O2 in respiration)
• Fatty acids: RQ = 0.7-0.9 (Due to the beta-oxidation process, fatty acid metabolism produces more CO2 and utilizes less O2 compared to glucose)
• Ethanol: RQ = 0.67 (Alcoholic fermentation in yeast produces two molecules of CO2 for each molecule of ethanol formed)
• Lactate: No RQ can be calculated (Muscle cells undergoing lactate fermentation do not produce CO2 as a byproduct)