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;
RP3 (Osmosis)
The student wanted to determine the water potential of chicken eggs. She:
* produced a dilution series of sugar solution
* followed the procedure described above.
She calculated the final mass to initial mass ratio of the egg covered in each sugar solution.
How would you advise the student to use her calculated ratios to determine the water potential of the eggs?
In your answer state the independent variable in the student’s investigation (4 marks).
- Independent variable = concentration of sugar solution;
- To determine water potential plot calibration curve
OR graph of ratio (Y axis) against concentration of sugar (X axis);
3. Interpolate from ratio of 1 (where water potential is the SAME);
Accept description of interpolation
4. Change concentration into water potential;
Accept for example, descriptions using a table/graph to find water potential
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;
How science works (AO3)
Explain how a stain allowed doctor to count the white blood cells amongst all the red blood cells in a blood sample.
White blood cells have a nucleus
(that stains but red cells do not);
How science works (AO3)
Suggest why the scientist made repeat measurements.
Increases reliability (of results)
OR
Anomalous results can be identified;
How Science Works (AO3)
Why should sampling be random?
To reduce bias
This also make the results / experiment more reliable
How Science Works (AO3)
Why should a sample be large?
To ensure it is representative;
To calculate a mean so results are more reliable;
Identify anomolus results;
RP4 (Membrane damage)
What is uncertainty?
The amount of error your measurements might contain
RP4 (Membrane damage)
How do you calculate uncertainty?
Is always HALF of the smallest interval you can read with the measuring equipment you are using.
RP4 (Membrane damage)
What is the uncertainty of a measuring cylinder where the smallest interval is 20mls?
10mls
Uncertainty is always HALF of the smallest interval
RP4 (Membrane damage)
What is the uncertainty of a ruler and why?
1mm
This is because you measure twice at both ends with a ruler. So you add the uncertainties together:
0.5mm + 0.5mm = 1.0mm
RP4 (Membrane damage)
What is the equation for percentage error?
RP4 (Membrane damage)
Calculate the percentage error when measuring 50mls using a cylinder with the smallest interval of 5mls.
(2.5 / 50) x 100 = 5%
RP4 (Membrane damage)
What is the uncertainty of this measuring cylinder in mls?
1 ml
RP4 (Membrane damage)
What is the percentage error if measuring 15mls using this measuring cylinder? (to 1dp).
(1 / 15) x 100 = 6.7%
RP4 (Membrane damage)
Suggest how you could reduce the uncertainty calculated using this measuring cylinder
Use instrument with smaller intervals
RP4 (Membrane damage)
Give one way in which a student could ensure their beetroot cylinders were kept at 25 °C throughout the experiment.
Measure temperature (in tube) at intervals and use appropriate corrective measure (if temperature has fluctuated);
RP4 (Membrane damage)
In an experiment investigating the impact of increasing temperature on the permeability of beetoot.
List the key control variables.
- (Of beetroot cylinders):
Length and diameter
OR Surface area
OR Volume
OR Mass/weight (of cylinders);
- Time in water/solution;
- The volume of water/solution used
(so they completely submerge the cylinders)
RP4 (Membrane damage)
1. Water/25oC caused no damage/no pigment release (in E);
2. Damage to cell-surface membrane / phospholipid bilayer;
3. Ethanol/acid caused some/similar/identical damage
OR 70oC caused most damage;
Accept description of ‘pigment release’ for ‘damage’
4. (By) ethanol dissolving phospholipid bilayer
5. (By) 70oC denaturing/altering membrane protein
OR (By) 70oC increasing fluidity/permeability of membrane;
RP4 (Membrane damage)
The cells of beetroot contain a red pigment. A student investigated the effect of temperature on the loss of red pigment from beetroot. He put discs cut from beetroot into tubes containing water. He maintained each tube at a different temperature.
After 25 minutes, he measured the percentage of light passing through the water in each tube.
(a) The student put the same volume of water in each tube.
Explain why it was important that he controlled this experimental variable.
1. (If) too much water the concentration of pigment (in solution) will be lower / solution will appear lighter / more light passes through (than expected);
OR
(If) too little water the concentration of pigment (in solution) will be greater / solution will appear darker / less light passes through (than expected);
2. So results (from different temperatures) are comparable;
RP4 (Membrane damage)
The decrease in the percentage of light passing through the water between 25 °C and 60 °C is caused by the release of the red pigment from cells of the beetroot.
Suggest how the increase in temperature of the water caused the release of the red pigment (2 marks).
- Damage to (cell surface) membrane;
- (membrane) proteins denature;
- Increased fluidity / damage to the phospholipid bilayer;
RP1 (Enzymes)
Concentration of substrate solution
Concentration of enzyme solution
pH
RP1 (Enzymes)
Calculate the rate of reaction at 25°C.
DY (change in Y) / DX (change in X = time)
RP1 (Enzymes)
Describe the differences between the two curves (2 marks)
- Initial rate of reaction faster at 37 °C;
- Graph reaches plateau at 37 °C;
RP1 (Enzymes)
Explain the differences between the two curves (3 marks).
(Initial rate of reaction faster at 37 °C);
- Because more kinetic energy;
- So more enzyme substrate complexes formed;
(Graph reaches plateau at 37 °C);
- Because all substrate used up.
RP1 (Enzymes)
When investigating factors that affect enzyme-controlled reactions, enzymes are often ‘isolated’ from a cell (e.g. a bacterial cell).
Explain why this is a limitation?
.
The process of isolation may change the enzyme’s activity
Outside the optimum conditions of the host cell, the enzyme’s activity may also change.
RP1 (Enzymes)
What is a common control condition that proves a specific enzyme is needed for a reaction?
- Boiled enzyme
- At same concentration & volume
- Same concentration & volume of buffer if this is required for the reaction
This denatures the enzyme and the reaction will not take place in this condition.
RP1 (Enzymes)
Mark in pairs, 1 and 2 OR 3 and 4 OR 5 and 6
- Boil OR Add (strong) acid/alkali;
- Denatures the enzyme/ATP synthase;
- Put in ice/fridge/freezer;
- Lower kinetic energy so no enzyme-substrate complexes form;
- Add high concentration of inhibitor;
- Enzyme-substrate complexes do not form;
RP1 (Enzymes)
- Same volume of (each) buffer / pH solution;
- Same concentration / mass of substrate (at start);
- Same concentration / mass of denatured enzyme;
How Science Works (Maths)
Describe how to determine the median
Put the values for the dependent variable into rank/order (i.e. smallest to largest) and then identify the middle value
