Biology Exam Question Help A-LEVEL

Hi, can I have help on this question? I'm confused on the maths required here:

box08.1A student used a dilution series to investigate the number of cells present in a liquid culture of bacteria.
Describe how he made a 1 in 10 dilution and then used this to make a 1 in 1000 dilution of the original liquid culture of bacteria.[3 marks]

Appreciate the help! :smile:

Reply 1

squiddy135

11

Original post by MlightOr

Hi, can I have help on this question? I'm confused on the maths required here:

box08.1A student used a dilution series to investigate the number of cells present in a liquid culture of bacteria.
Describe how he made a 1 in 10 dilution and then used this to make a 1 in 1000 dilution of the original liquid culture of bacteria.[3 marks]

Appreciate the help! :smile:

Hi! I think this is what is required?

1. To make a 1 in 10 dilution, he would accurately measure 10cm3 of the culture, place it in a measuring beaker/volumetric flask and make up the solution to 100cm3.
2. He would then stopper the beaker/flask and slowly invert it several times to thoroughly mix the solution.
3. To make a 1 in 1000 dilution, he would take 1cm3 of the dilution (1 in 10), add this to a volumetric flask and make it up with distilled water to 100cm3.

Reply 2

bigplongus

15

Original post by MlightOr

Hi, can I have help on this question? I'm confused on the maths required here:

box08.1A student used a dilution series to investigate the number of cells present in a liquid culture of bacteria.
Describe how he made a 1 in 10 dilution and then used this to make a 1 in 1000 dilution of the original liquid culture of bacteria.[3 marks]

Appreciate the help! :smile:

1 in 10 dilution mean for every 1 part undiluted culture there is 9 parts of distilled water to dialute it. This is a 1:9 ratio therefore you'll have 1 cm3 of undiluted culture and 9 cm3 distilled water

Now this principle does work for the 1 in 1000 dilution but may not seem to be as simple so its best to break it down
In relation to the undiluted culture the 1 in 1000 dilution is 1 part undiluted culture to 999 part distilled water, but you're using the 1 in 10 dilution.

Since 1000/10 is 100 so you're actually making 1 in 100 dilution series with the 1 in 10 dilution

Therefore you have 1 part 1 in 10 dilution and 99 parts distilled water. So 1cm3 of 1 in 10 dilution and 99cm3 of distilled water or any other multiple of the 1:99 ratio

I'm not sure if that's 3 marks so mixing well might be one of them

(edited 1 year ago)

Reply 3

bigplongus

15

Original post by squiddy135

Hi! I think this is what is required?

1. To make a 1 in 10 dilution, he would accurately measure 10cm3 of the culture, place it in a measuring beaker/volumetric flask and make up the solution to 100cm3.
2. He would then stopper the beaker/flask and slowly invert it several times to thoroughly mix the solution.
3. To make a 1 in 1000 dilution, he would take 1cm3 of the dilution (1 in 10), add this to a volumetric flask and make it up with distilled water to 100cm3.

imo thats too technical for biology this would be an answer for chemistry plus i think you would have to be more specific that youre putting 90cm3 of distilled water to make that 1 in 10 dilution and same for the 1 in 1000 to get the mark cause again A-level biology mark scheme be like

Reply 4

squiddy135

11

Original post by bigplongus

imo thats too technical for biology this would be an answer for chemistry plus i think you would have to be more specific that youre putting 90cm3 of distilled water to make that 1 in 10 dilution and same for the 1 in 1000 to get the mark cause again A-level biology mark scheme be like