Measuring the rate of oxygen uptake

Question 1

Other variables to be controlled (4)

Answer 1

• No of organisms
• Temperature
• Time
• Volume of soda lime

Question 2

Equipment (7)

Answer 2

• Respirometer
• Soda lime
• Coloured liquid 5g
• Organisms e.g. maggots, germinating peas, woodlice
• Cotton wool
• Stop clock
• Marker pen

Question 3

Outline the method

Answer 3

Place 5g of organism (maggots) into the tube and replace the bung. Introduce a drop of dye into the glass tube. Open the connection (three-way tap) to the syringe and move the fluid to a convenient place on the pipette (i.e. towards the end of the scale that is furthest from the test tube). Mark the starting position of the fluid on the pipette tube with a permanent OHT pen. Isolate the respirometer by closing the connection to the syringe and the atmosphere and immediately start the stop clock. Mark the position of the fluid on the pipette at 1 minute intervals for 5 minutes. 6. At the end of 5 minutes open the connection to the outside air. Measure the distance travelled by the liquid during each minute (the distance from one mark to the next on your pipette).If your tube does not have volumes marked onto it you will need to convert the distance moved into volume of oxygen used. (Remember the volume used = πr2 × distance moved, where r = the radius of the hole in the pipette.) Record your results in a suitable table. Calculate the mean rate of oxygen uptake during the 5 minutes.

Question 4

Possible evaluation issues (2)

Answer 4

• Simple respirometer – disadv. = does not allow you to reset; it needs a control tube used alongside it; no scale so measurements likely to be less accurate. Adv = very simple to set up; minimal number of connections makes a good seal easier to obtain.
• U-tube respirometer – disadv. = tendency for the connections to leak in elderly school/college models (making the equipment useless); expense. Adv. = does not need to have an additional control as the second tube balances out the effects of changes in temperature or atmospheric pressure; the syringe allows you to move the liquid in the U to reset the apparatus.