Describe how a respirometer can be used to measure the rate of aerobic respiration (by measuring oxygen uptake)
- Add set mass of single-celled organism
- Add a buffer to keep pH constant
- Add a set volume / conc. of a chemical that absorbs CO2 eg. sodium hydroxide
- Place in water bath at a set temperature and allow to equilibrate
- Measure distance moved by coloured liquid in a set time
Explain why the liquid moves in a respirometer
● Organisms aerobically respire so take in O2
● CO2 given out but absorbed by sodium hydroxide solution
● So volume of gas and pressure in container decrease
● Fluid in tube moves down pressure gradient towards orga
Explain why the respirometer apparatus is left open for 10 minutes. (1)
● Allow apparatus to equilibrate
● Allow for overall pressure expansion
Explain why the apparatus must be airtight.
● Prevent air entering or leaving
● Would change volume and pressure, affecting movement of liquid
Suggest a suitable control experiment and explain why it is necessary
- use no organisms or dead organisms and keep all other conditions the same
- to show that respiring organisms are causing liquid to move by taking up oxygen causing a change in volume/pressure
Describe how a respirometer can be used to measure the rate of anaerobic respiration
● Repeat experiment as above but remove chemical that absorbs CO2
● Make conditions anaerobic, for example:
○ Layer of oil / liquid paraffin above yeast to stop O2 diffusing in
○ Add a chemical that absorbs O2
○ Leave for an hour to allow O2 to be respired and used up
Explain why the liquid moves in a respirometer measuring anerobic respiration
● Yeast anaerobically respire so release CO2
● So volume of gas and pressure in container increase
● So fluid in capillary tube moves down a pressure
gradient away from organism
Explain why the apparatus is left for an hour after the culture has reached a constant temperature.
● Allow time for oxygen to be used / respired
Describe how rate of respiration can be calculated
- Calculate volume of O2 / CO2 consumed / released (calculate area of a cylinder)
a. Calculate cross-sectional area of capillary tube using π r2
b. Multiply by distance liquid has moved - Divide by mass of organism and time taken
- Units - unit for volume per unit time per unit mass eg. cm3 min-1 g-1
Describe how redox indicator dyes such as Methylene blue can be used to measure rate of respiration
● Redox indicators (eg. methylene blue) change colour when they accept electrons becoming reduced
● Redox indicators take up hydrogens and get reduced instead of NAD / FAD → modelling their reactions
Method of respiration
- Add a set volume of organism eg. yeast and a set volume of respiratory substrate eg. glucose to tubes
- Add a buffer to keep pH constant
- Place in water bath at a set temperature
and allow to equilibrate for 5 mins - Add a set volume of methylene blue,
shake for a set time (do not shake again) - Record time taken for colour to disappear
in tube - Rate of respiration (s-1) = 1 / time (sec)
Give two examples of variables that could be controlled
● Volume of single-celled organism
● Volume / conc. / type of respiratory substrate
● Temperature (with a water bath)
● pH (with a buffer)
● Volume of redox indicato
Why leave tubes in the water bath for 5 minutes?
● Allow for solutions to equilibrate and reach the same temperature as the water bath
Suggest a suitable control experiment and explain why it is necessary.
● Add methylene blue to boiled / inactive / dead yeast (boiling denatures enzymes)
● All other conditions the same
● To show change is due to respiration in organisms
Suggest and explain why you must not shake tubes containing methylene blue.
● Shaking would mix solution with oxygen
● Which would oxidise methylene blue / cause it to lose its electrons
● So methylene blue would turn back to its original blue colo
Suggest one source of error in using methylene blue. Explain how this can be reduced.
● Subjective as to determination of colour change / end point
● Compare results to a colour standard (one that has already
changed) OR use a colorimeter for quantitative results
