RP9 - Investigation Into The Effect Of A Named Variable On The Rate Of Respiration Of Cultures Of Single Celled Organisms Flashcards
Describe how a respirometer can be used to measure the rate of aerobic respiration (by measuring oxygen uptake) (5)
1) Add a set mass of single-called organism e.g. yeast to set volume / conc of substrate e.g. glucose
2) Add a buffer to keep pH constant
3) Add a set volume / concentration of a chemical that absorbs CO2 e.g. sodium hydroxide
4) Place in water bath at a set temperature and allow to equilibriate
5) Measure distance moved by coloured liquid in a set time
Aerobic respiration
Explain why the liquid moves (4)
1) Organisms aerobically respire so take in O2
2) CO2 given out but absorbed by sodium hydroxide solution
3) So volume of gas and pressure in container decrease
4) Fluid in tube moves down pressure gradient towards organism
Aerobic respiration
Explain why the respirometer apparatus is left open for 10 minutes (3)
1) Allow apparatus to equilibriate
2) Allow for overall pressure expansion / change throughout
3) Allow respiration rate of organisms to stabilise
Aerobic respiration
Explain why the apparatus must be airtight (2)
1) Prevent air entering or leaving
2) Would change volume and pressure, affecting movement of liquid
Aerobic respiration
Describe a more accurate way to measure volume of gas (1)
1) Use a gas syringe
Aerobic respiration
Suggest a suitable control experiment and explain why it is necessary (2)
1) No organisms OR use inert objects OR use dead organisms AND all other conditions / apparatus / equipment the same
2) To show that (respiring) organisms are causing liquid to move / taking up oxygen / causing the change in volume / pressure
Describe how a respirometer can be used to measure the rate of anaerobic respiration (2)
1) Repeat experiment as above but remove chemical that absorbs CO2
2) 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
Anaerobic respiration
Explain why the liquid moves (3)
1) Yeast anaerobically respire so release CO2
2) So volume of gas and pressure in container increase
3) So fluid in capillary tube moves down a pressure gradient away from organism
Anaerobic respiration
Explain why the apparatus is left for an hour after the culture has reached a constant temperature (1)
1) Allow time for oxygen to be used / respired
Describe how rate of respiration can be calculated (3)
1) Calculate volume of O2 / CO2 consumed / released
A. Calculate cross sectional area of capillary using pi x r^2
B. Multiply by distance liquid has moved
2) Divide by mass or organism and time taken
3) Units - unit for volume per unit time per unit mass e.g. cm^3 min^-1 g^-1
Describe the role of redox indicators (2)
1) Redox indicators (e.g. methylene blue) change colour when they accept electrons becoming reduced
2) Redox indicators take up hydrogen and get reduced instead of NAD / FAD -> modelling their reactions
Describe how redox indicator dyes such as Methylene blue can be used to measure rate of respiration (6)
1) Add a set volume of organism e.g yeast and a set volume of respiratory substrate e.g glucose to tubes
2) Add a buffer to keep pH constant
3) Place in water bath at a set temperature and allow to equilibriate for 5 mins
4) Add a set volume of methylene blue, shake for a set time
5) Record time taken for colour to disappear
6) Rate of respiration = 1 / time (sec)
Redox indicators (Methylene blue)
Give 2 examples of variables that could be controlled (5)
1) Volume of single-felled organism
2) Volume / concentration / type of respiratory substrate
3) Temperature
4) pH
5) Volume of redox indicator
Redox indicators (Methylene blue)
Why leave tubes in the water bath for 5 minutes? (1)
1) Allow for solutions to equilibriate and reach the same temperature as the water bath
Redox indicators (Methylene blue)
Suggest a suitable control experiment and explain why it is necessary (3)
1) Add methylene blue to boiled / inactive / dead yeast
2) All other conditions the same
3) To show change is due to respiration in organisms
Redox indicators (Methylene blue)
Suggest and explain why you must not shake tubes containing methylene blue (3)
1) Shaking would mix solution with oxygen
2) Which would oxidise methylene blue / cause it to lose its electrons
3) So methylene blue would turn back to its original blue colour
Redox indicators (Methylene blue)
Suggest one source of error in using methylene blue. Explain how this can be reduced (2)
1) Subjective as to determination of colour change
2) Compare results to a colour standard OR use a colorimeter for quantitative results
