A Level Biology: Common practical
questions with mark schemed
answers(paper 1 ,2 and 3 )
, Describe how you would test a liquid sample for the presence of lipid and how
you would recognise a positive result. - ANSWER >>>>1. (Mix / shake sample)
with ethanol, then water;
Sequence is important
2. White / milky (emulsion);
Explain why the student:
1. used only the first 5 mm from the tip of an onion root.
2. pressed down firmly on the cover slip. - ANSWER >>>>1. Where dividing
cells are found / mitosis occurs
2. Single / thin layer of cells / spread out cells so light passes through (making
cells / nuclei visible);
Describe and explain what the student should have done when counting cells
to make sure that the mitotic index he obtained for this root tip was accurate. -
ANSWER >>>>1. Examine large number of fields of view / many cells;
Mark as pairs only
Accept large number / 20 or more for many
2. To ensure representative sample;
Accept typical / reliable
OR
3. Repeat count;
4. To ensure figures are correct;
OR
5. Method to deal with part cells shown at edge /count only whole cells;
6. To standardise counting;
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. - ANSWER >>>>1. Add 1
part (bacteria) culture to 9 parts (sterile) liquid (to make 10-1 dilution);
Accept water / nutrient / broth for liquid
2. Mix (well);
Accept stir
questions with mark schemed
answers(paper 1 ,2 and 3 )
, Describe how you would test a liquid sample for the presence of lipid and how
you would recognise a positive result. - ANSWER >>>>1. (Mix / shake sample)
with ethanol, then water;
Sequence is important
2. White / milky (emulsion);
Explain why the student:
1. used only the first 5 mm from the tip of an onion root.
2. pressed down firmly on the cover slip. - ANSWER >>>>1. Where dividing
cells are found / mitosis occurs
2. Single / thin layer of cells / spread out cells so light passes through (making
cells / nuclei visible);
Describe and explain what the student should have done when counting cells
to make sure that the mitotic index he obtained for this root tip was accurate. -
ANSWER >>>>1. Examine large number of fields of view / many cells;
Mark as pairs only
Accept large number / 20 or more for many
2. To ensure representative sample;
Accept typical / reliable
OR
3. Repeat count;
4. To ensure figures are correct;
OR
5. Method to deal with part cells shown at edge /count only whole cells;
6. To standardise counting;
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. - ANSWER >>>>1. Add 1
part (bacteria) culture to 9 parts (sterile) liquid (to make 10-1 dilution);
Accept water / nutrient / broth for liquid
2. Mix (well);
Accept stir
