Required Practical 1-6 & How Science Works Flashcards
RP2 (Mitosis)
Mitotic index equation
RP2 (Mitosis)
Describe how you would determine a reliable mitotic index from tissue observed with an optical microscope (3 marks).
1. Count cells in mitosis in field of view;
2. Divide this by total number of cells in field of view;
3. Repeat many / at least 5 times to calculate a reliable mean mitotic index
OR select fields of view at random to reduce bias;
RP2 (Mitosis)
A student prepared a plant root to observe cells undergoing mitosis.
He put the root in a small bottle of hydrochloric acid in a 40 °C water bath.
Why did he put the plant root in acid? (2 marks)
To stop mitosis;
To break down links between cells / cell walls;
To separate cells;
To break down / hydrolyse cellulose/cell wall;
Allowing the stain to pass/diffuse into the cells;
RP2 (Mitosis)
State two precautions required when working with hydrochloric acid.
1. Eye protection;
2. Gloves;
3. Add water to spills (immediately);
4. Do not pour away down sink;
RP2 (Mitosis)
Pressing the coverslip downwards enabled the student to observe the stages of mitosis clearly.
Explain why (2 marks).
1. To create a single/thin layer of cells;
2. So that light could pass through;
RP2 (Mitosis)
Where dividing cells are found / mitosis occurs;
OR
No dividing cells / mitosis in tissue further away / more than 5 mm from tip;
RP2 (Mitosis)
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 (2 marks).
1. Examine large number of fields of view;
2. To ensure representative / reliable sample;
OR
3. Method to deal with part cells shown at edge / count only whole cells;
4. To standardise counting;
RP2 (Mitosis)
1. Stops anaphase / cell division / mitosis;
Accept prevents telophase / cytokinesis
2. (By) stopping / disrupting the spindle fibres forming / attaching / pulling;
3. Preventing separation / splitting of (sister) chromatids;
4. So no new cells added (to root tip);
RP2 (Mitosis)
The student counted the number of cells she observed in each stage of mitosis.
Of the 200 cells she counted, only six were in anaphase.
One cell cycle of onion root tissue takes 16 hours.
Calculate how many minutes these cells spend in anaphase.
28.8 minutes
Working out:
6 / 200 = proportion in anaphase
Multiply this answer by 16 x 60
(convert hours into minutes).
RP2 (Mitosis)
When comparing the mitotic index in the roots of two different species.
Give two considerations to ensure this comparison is valid.
1. Roots/plant of the same age;
2. Same growing conditions (for all roots);
3. Same distance from root tip;
4. Same time in acid
OR Same temperature of acid;
5.Same concentration of acid;
6. Several fields of view (for each species) to calculate a mean / reliable mitotic index;
RP2 (Mitosis)
The dark stain used on the chromosomes binds more to some areas of the chromosomes than others, giving the chromosomes a striped appearance.
Suggest one way the structure of the chromosome could differ along its length to result in the stain binding more in some areas.
Differences in base sequences
OR
Differences in histones / interactions with histones
OR
Differences in condensation / super coiling;
RP3 (Osmosis)
Equation for making up a dilution
C1 x V1 = C2 x V2
RP3 (Osmosis)
In equation C1 x V1 = C2 x V2 what does C1 represent?
C1 = stock concentration
This was always be the highest concentration available
RP3 (Osmosis)
Describe how you would use a 1.0 mol dm^−3 solution of sucrose to produce 30cm^3 of a 0.15 mol dm^−3 solution of sucrose.
Clue: C1 x V1 = C2 x V2
Answer = Add 4.5 cm3 of (1.0 mol dm^–3) solution to 25.5 cm^3 (distilled) water.
Step-by-step working:
C1 x V1 = C2 x V2
1 x V1 = 0.15 x 30
V1 = (0.15 x 30) / 1
V1 = 4.5cm^3
How Science Works (Maths)
Hint: C1 x V1 = C2 x V2
RP3 (Osmosis)
Calculations made (from raw data)
OR raw data would have recorded initial and final masses.
RP3 (Osmosis)
1. Water potential of solution is less than that of potato tissue;
2. Tissue loses water by osmosis so masses decreases;
RP3 (Osmosis)
1. Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis;
2. Find concentration where curve crosses the x-axis / where percentage change is zero;
3. Use another resource to find water potential of sucrose concentration (where curve crosses x-axis);
RP3 (Osmosis)
1. Method to ensure all cut surfaces of the eight cubes are exposed to the sucrose solution;
2. Method of controlling temperature / at room temperature;
3. Method of drying cubes before measuring;
4. Measure mass of cubes at stated time intervals e.g. every 5 / 10 minutes;
RP3 (Osmosis)
1. Name of solution / independent variable in first column;
2. Same number of decimal places in final / column on right;
How science works (AO3)
Blood cells can be counted using a haemocytometer (see below). When counting, cells that touch top or left lines are counted but cells that touch right or bottom lines are not counted.
Suggest two reasons for this rule.
1. To avoid dealing with parts of cells;
2. To avoid counting same cells twice / more than once;
3. To be consistent / get comparable results;
OR more accuracy
How science works (AO3)
A doctor used a haemocytometer (see below) to count white blood cells per mm3.
When counting white blood cells, the doctor only diluted the blood sample by a factor of 20 times, instead of 200 times when counting red blood cells.
Suggest why he only diluted the sample by a factor of 20 times.
There are fewer white cells, so no need to dilute (further to see enough);
OR accept converse i.e. too few to see if greater dilution / at 200 times;
