RP 9 - effect of a named variable on the rate of respiration if culuted of single celled organisms

Question 1

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

Answer 1

1) add set mass of single celled organism to set volume / concentration of substrate
2) Add a buffer to keep pH constant
3) Add a set volume / concentration of a chemical that absorbs CO2
4) Place in water bath at set temperature and allow it to equilibrate
5) Measure distance moved by coloured liquid in a set time

Question 2

EQ : Explain why the liquid moves (4)

Answer 2

• organisms aerobicslly 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

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

Answer 3

• Allow apparatus to equilibrate

Question 4

EQ: Explain why the apparatus must be airtight (2)

Answer 4

• Prevent air entering or leaving
• Would change volume and 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

EQ : suggest a suitable control experiment and explain why it’s necessary (2)

Answer 6

• use dead organisms
• And all other conditions remain the same
• To show that respiring organisms are causing liquid to move causing the change in volume

Question 7

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

Answer 7

• Repeat experiment as above but remove chemical that absorbs CO2
• Make conditions anaerobic for example
  • layer of oil above yeast to stop O2 diffusing in
  • add a chemical test that absorbs O2
  • leave for an hour to allow O2 to be respired and used up

Question 8

EQ : explain why the liquid moves (anaerobic ) (3)

Answer 8

• 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 9

EQ : explain why the apparatus is left for an hour after the culture has reached a constant temperature (1)

Answer 9

Allow time for oxygen to be used / respired

Question 10

Describe how rate of respiration can be calculated

Answer 10

• Calculate volume of O2 consumed ( calculate area of cylinder )
  • Calculate cross sectional area of capillary tube using pi r^2
  • multiply by distance liquid has moved
• Divide by mass of organism and time taken
• Units - unit for volume per unit time per unit mass

Question 11

Describe how redox indicators for example methylene blue can be used to measure rate of respiration

Answer 11

• redox indicators change colour when they accept electrons becoming reduced
• Tske up hydrogens and get reduced instead of NAD / FAD
1) Add a set volume of organism (yeast) and add a set volume of 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
5) Record time taken for colour to disappear in tube
5) Rate of respiration = 1 / time (sec)

Question 12

EQ :Give two examples of variables that could be controlled ( methylene blue experiment ) (2)

Answer 12

• Volume of single celled organism
• Temperature

Question 13

EQ : Why leave tubes in the water bath for 5 minutes ( methylene blue experiment) (1)

Answer 13

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

Question 14

EQ : Suggest a suitable control experiment and explain why it is necessary ( methylene blue experiment ) (3)

Answer 14

• Add methylene blue to dead yeast
• All other conditions the same
• To show change is due to respiration in organisms

Question 15

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

Answer 15

• Shaking would mix solution with oxygen
• Which would oxide methylene blue causing it to lose its electrons
• So methylene blue would turn back to its original blue colour

Question 16

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

Answer 16

• Subjective as to determine if colour change
• Compare results to a colour standard or use a colorimeter