# 12.1.7 Investigating RQs

### Investigating RQs

• Respirometers are used to measure and investigate the rate of oxygen consumption during respiration in organisms
• They can also be used to calculate respiratory quotients
• The experiments usually involve organisms such as seeds or invertebrates

#### Equation for calculating change in gas volume

• The volume of oxygen consumed (cm3 min-1) can be worked out using the diameter of the capillary tube r (cm) and the distance moved by the manometer fluid h (cm) in a minute using the formula:

πr2h

#### Using a respirometer to determine the RQ

Method

• Measure oxygen consumption: set up the respirometer and run the experiment with soda-lime present in both tubes. Use the manometer reading to calculate the change in gas volume within a given time, x cm3 min-1
• Reset the apparatus: allow air to re-enter the tubes via the screw cap and reset the manometer fluid using the syringe
• Run the experiment again: remove the soda-lime from both tubes and use the manometer reading to calculate the change in gas volume in a given time, y cm3 min-1

Calculations

• x tells us the volume of oxygen consumed by respiration within a given time
• y tells us the volume of oxygen consumed by respiration within a given time minus the volume of carbon dioxide produced within a given time
• y may be a positive or negative value depending on the direction that the manometer fluid moves (up = positive value, down = negative value)
• The two measurements x and y can be used to calculate the RQ

RQ = CO2 / O2

RQ = (x + y)  / x

#### Worked example: Calculating RQ from a respirometer experiment

x = 2.9 cm3 min-1

y = -0.8 cm3 min-1

(x + y) / x = RQ

(2.9 – 0.8) / 2.9 = 0.724

When equal volumes of oxygen are consumed and carbon dioxide produced (as seen with glucose) the manometer fluid will not move and y will be 0, making the RQ 1.

Analysis

• Respirometers can be used in experiments to investigate how different factors affect the RQ of organisms over time
• Eg. temperature – using a series of water baths
• When an RQ value changes it means the substrate being respired has changed
• Some cells may also be using a mixture of substrates in respiration an RQ value of 0.85 suggests both carbohydrates and lipids are being used
• This is because the RQ of glucose is 1 and the RQ of lipids is 0.7
• Under normal cell conditions the order substrates are used in respiration: carbohydrates, lipids then proteins
• The RQ can also give an indication of under or overfeeding:
• An RQ value of more than 1 suggests excessive carbohydrate/calorie intake
• An RQ value of less than 0.7 suggests underfeeding

#### Exam Tip

There are several ways you can manage variables and increase the reliability of results in respirometer experiments:

• Use a controlled water bath to keep the temperature constant
• Have a control tube with an equal volume of inert material to the volume of the organisms to compensate for changes in atmospheric pressure
• Repeat the experiment multiple times and use an average ### Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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