Required practical 9

Question 1

Describe how a respirometer can be used to measure the rate of
aerobic respiration (by measuring oxygen uptake)

Answer 1

1. Add set mass of single-celled organism to set volume of substrate eg. glucose
2. Add a buffer to keep pH constant
3. Add a set volume / conc. of a chemical that absorbs CO2 eg. sodium hydroxide
4. Place in water bath at a set temperature and allow to equilibrate
5. Measure distance moved by coloured liquid in a set time

Question 2

Explain why the liquid moves.
(4)

Answer 2

● 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 organism

Question 3

Explain why the respirometer
apparatus is left open for 10
minutes. (1)

Answer 3

Allow apparatus to equilibrate

Question 4

Explain why the apparatus
must be airtight.

Answer 4

• prevent air entering or leaving
• would change volume + pressure, affecting movement of liquid

Question 5

Describe a more accurate way to measure volume of gas. (1)

Answer 5

● Use a gas syringe

Question 6

Describe how a respirometer can be used to measure the rate of anaerobic respiration (by measuring carbon dioxide release)

Answer 6

● 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

Question 6

Suggest a suitable control experiment and explain why it is necessary. (2)

Answer 6

● No organisms OR use inert objects OR use dead organisms AND all other conditions / apparatus / equipment the same ● To show that (respiring) organisms are causing liquid to move / taking up oxygen / causing the change in volume / pressure

Question 7

Explain why the liquid moves. (3) anaerobic

Answer 7

● 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

Question 8

Explain why the apparatus is left for an hour after the culture has reached a constant temperature. (1) anaerobic

Answer 8

● Allow time for oxygen to be used / respired

Question 9

Describe how rate of respiration can be calculated

Answer 9

1. 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
2. Divide by mass of organism and time taken
3. Units - unit for volume per unit time per unit mass eg. cm3min-1g-1

Question 10

Describe how redox indicator dyes such as Methylene blue can be used to measure rate of respiration

Answer 10

● 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

1. Add a set volume of organism eg. yeast
   and a set volume of respiratory substrate
   eg. glucose to tubes
2. Add a buffer to keep pH constant
3. Place in water bath at a set temperature
   and allow to equilibrate for 5 mins
4. Add a set volume of methylene blue,
   shake for a set time (do not shake again)
5. Record time taken for colour to disappear
   in tube
6. Rate of respiration (s-1) = 1 / time (sec)

Question 11

Give two examples of
variables that could be
controlled. (2)

Answer 11

● 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)

Question 12

Why leave tubes in the water bath for 5 minutes?

Answer 12

● Allow for solutions to equilibrate and reach the same temperature as the water bath

Question 13

Suggest a suitable control experiment and explain why it is necessary. (3)

Answer 13

● 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

Question 14

Suggest and explain why you must not shake tubes
containing methylene blue. (3)

Answer 14

● 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

Question 15

Suggest one source of error in using methylene blue. Explain how this can be reduced. (2)

Answer 15

● 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