RP - 9 Investigation into the effect of a named variable on the rate of respiration of cultures of single-celled organisms

Question 1

Describe how the rate of respiration can be calculated

Answer 1

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

Question 2

Describe how a respirometer can be used to measure the rate of anaerobic
respiration

Answer 2

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

Question 3

Explain why the liquid moves.

Answer 3

● 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

Question 4

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

Answer 4

● Allow time for oxygen to be used / respired

Question 5

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

Answer 5

● 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

Question 6

Give examples of variables that
could be controlled.

Answer 6

● 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 7

Why leave tubes in the water
bath for 5 minutes?

Answer 7

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

Question 8

Describe a control experiment
and why it would be done.

Answer 8

● 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 9

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

Answer 9

● 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 10

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

Answer 10

● 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