A-Level Biology October/November 2024 Q3(b)(i): Explain how the device calculates the RQ value and how this shows whether the cells are…

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  The Respiratory Quotient (RQ) is calculated as the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed.

  $RQ = \frac{\text{Volume of } CO_2 \text{ produced}}{\text{Volume of } O_2 \text{ consumed}}$

  The device calculates this value by making two key measurements:

  1. An air flow meter measures the volume of oxygen inhaled by the person.
  2. A carbon dioxide sensor measures the concentration of CO₂ in the exhaled air, from which the total volume of CO₂ produced can be determined.

  The device then divides the volume of CO₂ produced by the volume of O₂ consumed to give the RQ value.

  This RQ value indicates the primary respiratory substrate being used by the cells. An RQ value of 1.0 indicates that carbohydrates are the main respiratory substrate. An RQ value of approximately 0.7 indicates that lipids are the main substrate. By comparing the calculated RQ to these known values, it is possible to determine which substrate is being predominantly respired.

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  How the marks are awarded

  • B1 — Correctly defining RQ as the ratio of carbon dioxide produced to oxygen consumed.
  • B1 — Stating that an air flow meter is used to measure the volume of oxygen inhaled.
  • B1 — Stating that a carbon dioxide sensor is used to measure the concentration of CO₂ in exhaled air.
  • B1 — Correctly stating the RQ value for carbohydrates is 1.0 and for lipids is 0.7, and linking these to the interpretation of results.

  Common mistakes

  • Inverting the RQ formula, stating it is oxygen consumed divided by carbon dioxide produced.
  • Giving vague descriptions of measurement, such as 'the device measures the gases' without specifying the instruments used (air flow meter, CO₂ sensor).
  • Confusing the RQ values for different substrates, for example stating that the RQ for lipids is 1.0.
  • Failing to state both RQ values for comparison, for instance only mentioning the value for carbohydrates.

  Examiner tip: For questions involving physiological measurements like RQ, always link the theoretical formula to the practical method of data collection and the biological interpretation of the results.

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