practical 2 Flashcards
You are required to prepare a microscope slide of the meristem tissue from an onion root. You will add a stain to the material, which allows you to see the chromosomes. You will look at the slide under the microscope to identify any cells showing stages of mitosis. You will then calculate the mitotic index.
1. (a) Describe how temporary mounts of plant tissue are made. 2
Thin slice/section;
2. Put on slide in water / solution / stain;
3. Add cover slip;
Accept: ‘between two slides’
. (b) Describe how the scientist could have used the temporary mounts of leaves to determine the mean number of chloroplasts in mesophyll cells of a leaf.
select large number of cells / select cells at random;
Accept: > 5 for “large number”
Accept: many fields of view for ‘large number of cells’
Accept: all cells in field of view
2. Count number of chloroplasts;
3. Divide number of chloroplasts by number of cells;
Ignore: ‘calculate the mean’
The student cut thin sections of tissue to view with an optical microscope.
Explain why it was important that the sections were thin.
To allow (more) light through;
Accept: transparent
2. A single / few layer(s) of cells to be viewed.
Accept: (thin) for better / easier stain penetration
When the student was making their microscope slides, they were told to ensure they did not move the coverslip sideways. Explain why.
To prevent cells from rolling on top of each other / to not damage the
chromosomes
Mitosis is important in the life of an organism. Give two reasons why.
(b) (i) Explain why the biologist chose to examine 200 cells.
Growth / increase in cell number;
Ignore growth of cells
2. Replace cells / repair tissue / organs / body;
Ignore repair cells
Reject bacteria
3. Genetically identical cells;
‘Produces 2 genetically identical cells’ does not reach MP1
as well as MP3
4. Asexual reproduction / cloning
b(Ensures) representative (sample)
Preparation of stained squashes of cells from plant root tips.
Set-up and use of an optical microscope and preparation of a temporary mount.
a)why do we use root tip of onion cell
b)why do we place root tip in the acid and why heated acid
c)what equipment do we use to cut the root tip
a)tip is growing region of root where cells divide and undergo mitosis
b)acid will soften and loosen the root tissue so later can easily be squashed / make the stain more permeablae to stain inorder to see the chromosome
b) heated = speed up rate of reaction
c)scalpel to cut 5 mm from end
Preparation of stained squashes of cells from plant root tips.
Set-up and use of an optical microscope and preparation of a temporary mount.
a)why do we add distilled water
b)before adding stain we need to remove water how do we do that and why do we add a stain
c)what equipment do we use when lowering coverslip
d) why do we avoid adding excess water/acid
e)how to we squash glass slip onto the root tip what motion and what motion not
a) to remove acid
b) filter paper removes excess water and stain so chromosome visible and to see the stage of mitosis
c)mounted needle prevents air bubbles at 45 degrees
d)may create artefacts(mistaken to be cell but are not)
e)press down not sideways as sideways may break chromosome and not undergo mitosis
Preparation of stained squashes of cells from plant root tips.
Set-up and use of an optical microscope and preparation of a temporary mount.
outline risk assesment
1)hazard;scalpel risk; cut urself contol; cut away from urslef, cut on white tile to prevent slipping keep away form edge of desk
2)hazard: HCL Risk: corrosive, irritant control; eye proetction goggles, avoid skin contact
3)mounted needle risk: sharp stabbing, control; hold wooden end
equation to calculate mitotic index
no; of cells in mitosis / total number of cells x100
student counted number of cells in each stage of mitosis
counted 6 in anaphase out of the 200 examined, one cell cycle takes 16 hours. calc how many minutes cells was in anaphase for, 2
6/200 = 0.03
16x60= 960
0.03 x 960 =
28.8/29
A different set of results was obtained when the count was repeated on another occasion with
a different garlic root tip. Give two reasons for the difference in results.
genetic differences / different types of garlic;
age of root tip;
water availability;
temperature;
nutrient availability;
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.
Examine large number of fields of view / many cells;
Accept large number / 20 or more for many
To ensure representative sample;
Accept typical / reliable
OR
Repeat count;
To ensure figures are correct;
Method to deal with part cells shown at edge /count only whole cells;
6.
To standardise counting;
outline a method after that can be used to Prepare stained squashes of cells from plant root tips. top tip is prepared
place section of root tip into HCl
wash end section of root in water
cut tip from root tip and place into microscope slide
add stain to see the chromosome
break up the tissue with mounted needle
cover with a cover slip and sqaush downwards
