Required Practical 1-6 & How Science Works

Question 1

RP2 (Mitosis)

Mitotic index equation

Answer 1

Question 2

RP2 (Mitosis)

Describe how you would determine a reliable mitotic index from tissue observed with an optical microscope (3 marks).

Answer 2

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;

Question 3

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)

Answer 3

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;

Question 4

RP2 (Mitosis)

State two precautions required when working with hydrochloric acid.

Answer 4

1. Eye protection;

2. Gloves;

3. Add water to spills (immediately);

4. Do not pour away down sink;

Question 5

RP2 (Mitosis)

Pressing the coverslip downwards enabled the student to observe the stages of mitosis clearly.

Explain why (2 marks).

Answer 5

1. To create a single/thin layer of cells;

2. So that light could pass through;

Question 6

RP2 (Mitosis)

Answer 6

Where dividing cells are found / mitosis occurs;

OR

No dividing cells / mitosis in tissue further away / more than 5 mm from tip;

Question 7

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).

Answer 7

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;

Question 8

RP2 (Mitosis)

Answer 8

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);

Question 9

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.

Answer 9

28.8 minutes

Working out:
6 / 200 = proportion in anaphase

Multiply this answer by 16 x 60
(convert hours into minutes).

Question 10

RP2 (Mitosis)

When comparing the mitotic index in the roots of two different species.

Give two considerations to ensure this comparison is valid.

Answer 10

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;

Question 11

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.

Answer 11

Differences in base sequences

OR

Differences in histones / interactions with histones

OR

Differences in condensation / super coiling;

Question 12

RP3 (Osmosis)

Equation for making up a dilution

Answer 12

C1 x V1 = C2 x V2

Question 13

RP3 (Osmosis)

In equation C1 x V1 = C2 x V2 what does C1 represent?

Answer 13

C1 = stock concentration

This was always be the highest concentration available

Question 14

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 14

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

Question 15

How Science Works (Maths)

Hint: C1 x V1 = C2 x V2

Answer 15

Hint: C1 x V1 = C2 x V2

Question 16

RP3 (Osmosis)

Answer 16

Calculations made (from raw data)

OR raw data would have recorded initial and final masses.

Question 17

RP3 (Osmosis)

Answer 17

1. Water potential of solution is less than that of potato tissue;

2. Tissue loses water by osmosis so masses decreases;

Question 18

RP3 (Osmosis)

Answer 18

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);

Question 19

RP3 (Osmosis)

Answer 19

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;

Question 20

RP3 (Osmosis)

Answer 20

1. Name of solution / independent variable in first column;

2. Same number of decimal places in final / column on right;

Question 21

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).

Answer 21

1. Independent variable = concentration of sugar solution;
2. 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

Question 22

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.

Answer 22

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

Question 23

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.

Answer 23

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;

Question 24

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.

Answer 24

White blood cells have a nucleus
(that stains but red cells do not);

Question 25

How science works (AO3)

Suggest why the scientist made repeat measurements.

Answer 25

Increases reliability (of results)

OR

Anomalous results can be identified;

Question 26

How Science Works (AO3)

Why should sampling be random?

Answer 26

To reduce bias

This also make the results / experiment more reliable

Question 27

How Science Works (AO3)

Why should a sample be large?

Answer 27

To ensure it is representative;

To calculate a mean so results are more reliable;

Identify anomolus results;

Question 28

RP4 (Membrane damage)

What is uncertainty?

Answer 28

The amount of error your measurements might contain

Question 29

RP4 (Membrane damage)

How do you calculate uncertainty?

Answer 29

Is always HALF of the smallest interval you can read with the measuring equipment you are using.

Question 30

RP4 (Membrane damage)

What is the uncertainty of a measuring cylinder where the smallest interval is 20mls?

Answer 30

10mls

Uncertainty is always HALF of the smallest interval

Question 31

RP4 (Membrane damage)

What is the uncertainty of a ruler and why?

Answer 31

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

Question 32

RP4 (Membrane damage)

What is the equation for percentage error?

Answer 32

Question 33

RP4 (Membrane damage)

Calculate the percentage error when measuring 50mls using a cylinder with the smallest interval of 5mls.

Answer 33

(2.5 / 50) x 100 = 5%

Question 34

RP4 (Membrane damage)

What is the uncertainty of this measuring cylinder in mls?

Answer 34

1 ml

Question 35

RP4 (Membrane damage)

What is the percentage error if measuring 15mls using this measuring cylinder? (to 1dp).

Answer 35

(1 / 15) x 100 = 6.7%

Question 36

RP4 (Membrane damage)

Suggest how you could reduce the uncertainty calculated using this measuring cylinder

Answer 36

Use instrument with smaller intervals

Question 37

RP4 (Membrane damage)

Give one way in which a student could ensure their beetroot cylinders were kept at 25 °C throughout the experiment.

Answer 37

Measure temperature (in tube) at intervals and use appropriate corrective measure (if temperature has fluctuated);

Question 38

RP4 (Membrane damage)

In an experiment investigating the impact of increasing temperature on the permeability of beetoot.

List the key control variables.

Answer 38

1. (Of beetroot cylinders):

Length and diameter

OR Surface area

OR Volume

OR Mass/weight (of cylinders);

2. Time in water/solution;
3. The volume of water/solution used
   (so they completely submerge the cylinders)

Question 39

RP4 (Membrane damage)

Answer 39

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;

Question 40

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.

Answer 40

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;

Question 41

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).

Answer 41

1. Damage to (cell surface) membrane;
2. (membrane) proteins denature;
3. Increased fluidity / damage to the phospholipid bilayer;

Question 42

RP1 (Enzymes)

Answer 42

Concentration of substrate solution

Concentration of enzyme solution

pH

Question 43

RP1 (Enzymes)

Calculate the rate of reaction at 25°C.

Answer 43

DY (change in Y) / DX (change in X = time)

Question 44

RP1 (Enzymes)

Describe the differences between the two curves (2 marks)

Answer 44

1. Initial rate of reaction faster at 37 °C;
2. Graph reaches plateau at 37 °C;

Question 45

RP1 (Enzymes)

Explain the differences between the two curves (3 marks).

Answer 45

(Initial rate of reaction faster at 37 °C);

1. Because more kinetic energy;
2. So more enzyme substrate complexes formed;

(Graph reaches plateau at 37 °C);

3. Because all substrate used up.

Question 46

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?
.

Answer 46

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.

Question 47

RP1 (Enzymes)

What is a common control condition that proves a specific enzyme is needed for a reaction?

Answer 47

1. Boiled enzyme
2. At same concentration & volume
3. 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.

Question 48

RP1 (Enzymes)

Answer 48

Mark in pairs, 1 and 2 OR 3 and 4 OR 5 and 6

1. Boil OR Add (strong) acid/alkali;
2. Denatures the enzyme/ATP synthase;
3. Put in ice/fridge/freezer;
4. Lower kinetic energy so no enzyme-substrate complexes form;
5. Add high concentration of inhibitor;
6. Enzyme-substrate complexes do not form;

Question 49

RP1 (Enzymes)

Answer 49

1. Same volume of (each) buffer / pH solution;
2. Same concentration / mass of substrate (at start);
3. Same concentration / mass of denatured enzyme;

Question 50

How Science Works (Maths)

Describe how to determine the median

Answer 50

Put the values for the dependent variable into rank/order (i.e. smallest to largest) and then identify the middle value