Required Practical 9

Question 1

you were told to leave the tubes in the water bath for 5 mins.

why?

Answer 1

allow apparatus to equilibriate

Question 2

you were told to make sure that the water bath temp stayed at 35 degrees.

describe how you monitored the water bath temp and kept it constant at 35 degrees?

Answer 2

• use thermometer - take reg readings
• add hot/cold water if needed - maintain constant temp

Question 3

there was no control experiment in this investigation.

describe the control experiment you would set up.

explain why this control experiment would be important?

Answer 3

• test tube with ONLY glucose and methylene blue

OR

• glucose. methylene blue and boiled yeast
• ensures yeast causing colour change

Question 4

you were told not to shake the tubes again.

shaking the tubes would have caused the methylene blue to go blue again.

why?

Answer 4

• mixing with oxygen
• oxygen oxidises methylene blue

Question 5

respiration by yeast caused methylene blue to go colourless.

how?

Answer 5

• accepts electrons
• from glucose

Question 6

the yeast cultures in the conical flasks were set up some time before you carried out your investigation.

explain why?

Answer 6

allows yeast to be reproduced

Question 7

the conical flasks were stoppered with cotton wool.

explain why?

Answer 7

• prevent contamination by bacteria
• would compete with yeast for O2

Question 8

the flasks were loosely stoppered with cotton wool rather than with rubber bungs.

why?

Answer 8

allow O2 through

Question 9

you were instructed to add 200cm3 sterile water to yeast culture.

why?

Answer 9

• dilutes yeast
• colonies spread out enough to be able to count

Question 10

you were told to stir the mixture in the beaker containing yeast culture and sterile water.

explain why you were told to do this?

Answer 10

distribute yeast evenly

Question 11

more oxygen entered culture in 250cm3 flask than 25cm3 flask.

why?

Answer 11

• larger surface area
• for diffusion of oxygen

Question 12

explain how higher conc of oxygen affects growth of population of yeast cells?

Answer 12

• more oxygen allows more resp
• produces more ATP
• ATP used for growth