Describe how a respirometer can be used to measure the rate of aerobic respiration. (measures O2 uptake)
- Add a set mass of single-celled organism eg. yeast to a set volume / concentration of substrate eg. glucose
- Add a buffer to keep pH constant
- Add 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.
● Organisms aerobically respire → take in O2
● CO2 given out but absorbed by sodium hydroxide solution
● So volume of gas and pressure in container decrease
● So fluid in capillary tube moves down a pressure gradient towards organism
Explain why the respirometer
apparatus is left open for 10
minutes.
● Allow apparatus to equilibrate
● Allow for overall pressure expansion/change throughout
● Allow respiration rate of organisms to stabilise
Explain why the apparatus
must be airtight.
● Prevent air entering or leaving
● Would change volume and pressure, affecting movement of liquid
Describe a more accurate way
to measure volume of gas.
Use a gas syringe
Describe how the 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. cm3min-1g-1
Describe how a respirometer can be used to measure the rate of anaerobic respiration
Measures CO2 release:
● Repeat experiment as above but remove chemical that absorbs CO2
● Make conditions anaerobic, for example:
○ Layer of oil / liquid paraffin above yeast → 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.
● Yeast anaerobically respire → 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 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 redox indicator dyes such as Methylene
- 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
Give 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 indicator (only control)
Why leave tubes in the water
bath for 5 minutes?
● Allow for solutions to equilibrate and reach the same temperature
as the water bath
Describe a control experiment
and why it would be done.
● 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 colour
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
