Lecture Notes on Respiratory Quotient (RQ)

Definition of Respiratory Quotient (RQ)

• RQ Definition:
  • Ratio of moles of CO2 produced to moles of O2 used during respiration over a period of time.
  • Alternatively, the volume of CO2 produced divided by the volume of O2 used during respiration.

Examples of RQ Calculations

Aerobic Respiration of Glucose

• Chemical Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
• RQ Calculation:
  • Moles of CO₂ produced = 6
  • Moles of O₂ used = 6
  • RQ for Glucose = 6/6 = 1.0

Aerobic Respiration of Fatty Acids

• Example Fatty Acid: C₁₈H₃₄O₂
• Requires 25.5 moles of O₂, produces 18 moles of CO₂.
• RQ Calculation:
  • RQ = 18/25.5 = 0.7

Memorization of RQ Values

• Carbohydrates: Typically around 1.0
• Proteins: Typically around 0.9
• Lipids: Typically around 0.7
• Important for exams, especially carbohydrates and lipids.

Anaerobic Respiration

• Anaerobic Respiration in Yeast:
  • Produces CO₂ but no O₂ is used.
  • RQ cannot be calculated as it involves division by zero.
• RQ is primarily used for aerobic respiration.

Practical Application and Experiments

Experiment Setup for Measuring RQ

• Germinating Seeds:
  • Chamber setup with capillary tube and soda lime.
  • Soda lime absorbs CO₂, leaving only O₂ in the chamber.
  • Measure movement of liquid droplet in capillary tube to calculate O₂ usage.

Calculating Volumes

• With Soda Lime:
  • Measure distance moved by droplet to find volume of O₂ used.
• Without Soda Lime:
  • Repeat experiment to find net movement and calculate CO₂ volume produced.
• Formula: ( \text{Volume} = \pi r^2 L )
  • ( r ) = radius of tube, ( L ) = distance moved.

Example Calculations

Example 1: Seeds Experiment

• With Soda Lime:
  • Droplet moves 5mm, radius = 1mm.
  • Volume of O₂ used = 15.7 mm³.
• Without Soda Lime:
  • Net movement = 0.3 mm, CO₂ volume = 14.8 mm³.
• RQ: 14.8/15.7 = 0.94
  • Indicates usage of proteins/carbohydrates.

Example 2: Insect Experiment

• Temperature: 32°C
• With Soda Lime:
  • Droplet moves 8 mm, radius = 0.3 mm.
  • Volume of O₂ used = 2.3 mm³.
• Without Soda Lime:
  • Net movement = 2 mm, CO₂ volume = 1.7 mm³.
• RQ: 1.7/2.3 = 0.74
  • Indicates usage of lipids.

Conclusion

• RQ is a useful measure for determining the type of substrate being metabolized during respiration.
• Understanding and calculating RQ can reveal metabolic changes under different conditions, like exercise.