Rate of respiration and RQ


Independent variable: The main purpose of this experiment is to understand how to use

a respirometer and how it works. If you only use one species of animal or seed there will

be no independent variable. However, if you use multiple species then the independent

variable is the species of organism being used.


Dependent variable: Distance moved by coloured fluid in the manometer.


Equipment list

  • Sample seeds
  • Sample animals - such as woodlice
  • Soda lime or potassium hydroxide solution
  • Spatula (if using soda lime)
  • Funnel (if using potassium hydroxide solution )
  • Pipette
  • Ruler
  • Marker pen
  • Stopwatch
  • Respirometer - consisting of:

○ Syringe with connecting tap

○ Bung

○ Test tube

○ Capillary tubing

○ Manometer

○ Coloured liquid

○ Scale

○ Gauze


Method

1. Use either the pipette and funnel to add 5 cm3 of potassium hydroxide solution to the test

tube, or the spatula and weighing scales to add 5g of soda lime to the test tube.

2. Secure the gauze in place above the solution / powder in the test tube.

3. Select the first species of organism and place a known mass of them on to the gauze.

4. Set up the remainder of the respirometer as shown in the diagram below. If the manometer

does not already have a drop of coloured fluid in it, one can be added using a dropping

pipette.

5. Use the syringe to move the coloured fluid to the end of the manometer furthest from the

test tube and mark its position with a pen, or record its distance from the end of the

manometer by measuring with a ruler.

6. Close the 3-way tap to allow no more gas exchange to occur between the apparatus set

up and the outside atmosphere and start the stopwatch immediately.

7. Measure the position of the coloured fluid from its starting point every minute for 5 minutes,

recording the distance moved in a suitable table. As the organisms respire they take in

oxygen, causing the pressure in the test tube to decrease which draws in air from the

manometer tube; so the volume of oxygen taken in can be calculated if the diameter of the

tubing is known. Once calculated, the rate of oxygen uptake can be calculated per gram

of organism and then compared between organisms.

8. Open the 3-way tap connection to the outside air and use the syringe to reset the capillary

fluid.

9. Repeat steps 3-8 with any other organisms being investigated.


Respirometer set-up






To calculate the volume of oxygen taken in since the start (cm3) you calculate the volume of the

manometer tube that has been filled with oxygen:


V = 兀r2l


Where r is the radius of the tube (half its diameter)

Where l is the distance moved from the start


E.g. If the fluid moved 1.5 cm in the first minute and the radius of the tube was 0.5mm then the

volume produced would be:


V = 兀 x (0.5)2 x 1.5


= 1.178 cm3


The rate of oxygen uptake per minute can be calculated by dividing the total volume of oxygen

produced by the number of minutes since starting the stopwatch.