Measuring the rate of respiration

Overview

Recall questions on using a respirometer with soda lime and a control tube, calculating oxygen consumption and respiratory quotients from manometer readings, following yeast respiration with methylene blue, investigating temperature and sugar type, creating anaerobic conditions, and sources of error.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Describe how a simple respirometer is used to measure the rate of oxygen consumption of a small organism.

   Model answer

   The organism is sealed in a tube above a chemical that absorbs carbon dioxide, so any fall in gas volume is due only to oxygen uptake, and this is measured from the distance the manometer fluid moves along a capillary tube in a known time.

2. 2

   Name a substance used in a respirometer to absorb carbon dioxide, and explain why it is needed.

   Model answer

   Soda lime or potassium hydroxide solution is used, so that the carbon dioxide released by the organism does not replace the oxygen removed and mask the fall in volume.

3. 3

   Explain the purpose of the control tube in a respirometer, and state what it contains.

   Model answer

   It contains glass beads of the same mass as the organism, so that any movement of the fluid caused by changes in room temperature or atmospheric pressure affects both tubes equally and is cancelled out.

4. 4

   Why is a respirometer placed in a water bath during an experiment?

   Model answer

   A constant temperature keeps the metabolic rate steady and stops the gas in the apparatus expanding or contracting, which would move the fluid for reasons unconnected with respiration.

5. 5

   The manometer fluid in a respirometer moved 60 mm in 10 minutes. The capillary tube has an internal radius of 0.50 mm and the seeds have a mass of 1.5 g. Calculate the rate of oxygen consumption in mm³ g⁻¹ min⁻¹.

   Model answer

   The volume of oxygen used is π × 0.50² × 60 = 47 mm³, so the rate is 47 ÷ (1.5 × 10) = 3.1 mm³ g⁻¹ min⁻¹.

6. 6

   Explain how a respirometer can be used to find the respiratory quotient of an organism.

   Model answer

   The organism is first run with the carbon dioxide absorbent present, which gives the volume of oxygen used, and then run again with the absorbent removed, when the smaller change in volume is the difference between the oxygen used and the carbon dioxide produced; the two values give the quotient.

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