RP 1-6

Question 1

RP1: You used a buffer in your investigation. Explain why buffer solutions are used

Answer 1

Maintain constant pH

Question 2

RP1: You left the test tubes in the water bath for 10 minutes before you added the enzyme to the milk powder solutions. Explain why you did this.

Answer 2

To equilibrate/reach temperature at which reaction will take place

Question 3

RP1: Describe and explain what you did to make sure the temperatures of the water baths were as reliable as possible.

Answer 3

1. Measure temperature of water bath at beginning and end of reaction period (as a minimum number of times);
2. To assess the effect of any temperature changes during the reaction/to show that there was no/little variation in temperature;
   OR
3. Measure temperature (several times) and add hot or cold water as appropriate;
4. To try to keep the temperature close to that required;

Question 4

RP1: Suggest a more appropriate control for this enzyme-controlled investigation.

Answer 4

Boiled trypsin; / Denatured enzymes.

Question 5

RP1: It is difficult to decide when the solution of milk powder goes clear. Suggest one better way of determining when the solution of milk powder goes clear.

Answer 5

• Use a colorimeter;
• Record time taken to reach constant/set value (of absorbance / transmission)

Question 6

RP1: How should the students make sure that the pH of the protease solution did not change?

Answer 6

Use buffer / test pH (at end / at intervals);

Question 7

RP2: Describe how temporary mounts of plant tissue are made. [3]

Answer 7

1. Thin slice/section;
2. Put on slide in water / solution / stain;
3. Add cover slip;

Question 8

RP2: 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. [3]

Answer 8

1. Select large number of cells
2. 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
3. Count number of chloroplasts;
4. Divide number of chloroplasts by number of cells;

Question 9

RP2: Why did the student cut thin sections of tissue to view with an optical microscope.

Answer 9

1. 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

Question 10

RP2: When the student was making their microscope slides, they were told to ensure they did not move the coverslip sideways. Explain why.

Answer 10

1. To prevent cells from rolling on top of each other / to not damage the chromosomes

Question 11

RP2: Mitosis is important in the life of an organism. Give two reasons why.

Answer 11

1. Growth / increase in cell number;
   Ignore growth of cell
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;

Question 12

RP2: Explain why
(i) a root tip was used;

(ii) a stain was used;

(iii) the root tip was firmly squashed.

Answer 12

(i) where mitosis / division / growing / occurs
(reject growing cells)
(ii) to distinguish chromosomes / chromosomes not visible without stain;
(iii) to let light through / thin layer;

Question 13

RP2: Define Mitotic index

Answer 13

Number of cells with visible or condensed chromosomes DIVIDED BY
Total number of cells (in the field of view)

Question 14

RP2: Why is the mitotic index expressed as a decimal?

Answer 14

It is a proportion of cells within a sample.

Question 15

RP2: Suggest why the student soaked the root tips in hydrochloric acid [2]

Answer 15

1. To break down links between cells/cell walls OR
   To separate cells/cell walls OR To break down/hydrolyse cellulose/cell wall;

Ignore references to any bonds

2. Allowing the stain to pass/diffuse into the cells OR Allowing the cells to be (more easily) squashed;
3. To stop mitosis; Or To stop cell division/cell cycle

Question 16

RP2: Other students in the class followed the same method, but calculated different mitotic indices. Apart from student errors, suggest two explanations why.

Answer 16

1. (Garlic roots) are a different age OR (Garlic) grown in different conditions;

Accept suitable descriptions of conditions, eg in different temperatures

2. (Root tips) from different (garlic) plants/bulbs/species;
3. Single field of view is not representative of a root tip OR (Other) students may have looked at more fields of view OR (Other) students may have calculated a mean;

Accept ‘samples’ for ‘fields of view’

4. (Different fields of view are from) different parts of the root tip;

Reject different sized fields of view

Reject different number of cells (per field of view)

5. Cells/roots undergo mitosis/cell division at different times/rates;

Question 17

RP6: Describe two aseptic techniques you would use when transferring a sample of broth culture (bacteria) on to an agar plate. ​

Explain why each was important. [4 marks]

Answer 17

1. Keep lid on Petri dish OR Open lid of Petri dish as little as possible (at a 45 degree angle only);
2. To prevent unwanted bacteria contaminating the dish OR prevent the bacteria getting out;​
3. Wear gloves / Wear mask / Wash hands with soap;
4. To prevent contamination of agar with bacteria on hands/mouth OR Prevent spread of bacteria from agar to outside the lab;
5. Use sterile pipette / Flame the loop / Flame the neck of the container of the culture;
6. To maintain a pure culture of bacteria;

Question 18

RP6: After 2 days, she counted the number of colonies of bacteria on each agar plate.
(a) Explain the purpose of:
boiling the agar

Answer 18

Bacteria killed / destroyed. / So no contamination other bacteria;
Accept: no living bacteria.

Question 19

RP6: Explain the purpose of transferring the same volume of liquid culture onto each agar plate.

Answer 19

So same number of bacteria transferred to allow comparison;

Question 20

RP6: Name the process by which bacterial cells divide.

Answer 20

Binary Fission

Question 21

RP6: Describe two aseptic techniques she would have used when transferring a sample of broth culture on to an agar plate.

Explain why each was important.

Answer 21

1. Keep lid on Petri dish OR Open lid of Petri dish as little as possible.
2. To prevent unwanted bacteria contaminating the dish.
   OR
   Bacteria may be dangerous / may get out.
   OR
3. Wear gloves OR Wear mask OR Wash hands;
4. To prevent contamination from bacteria on hands / mouth
   OR
   Prevent spread of bacteria outside the lab;
   OR
5. Use sterile pipette OR Flame the loop OR
   Flame the neck of the container of the culture;
6. To maintain a pure culture of bacteria

Question 22

RP6: A student used a dilution series to investigate the number of cells present in a liquid culture of bacteria.

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. [3]

Answer 22

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);
3. Repeat using 9 parts fresh (sterile) liquid and 1 part of 10–1 and 10–2 dilutions to make 10–3 dilution; OR

Add 1 part 10–1 (suspension) to 99 parts (sterile) liquid (to make 10–3 dilution);

Question 23

RP6: Explain why there is a clear zone around each paper disc.

Answer 23

Bacteria killed;

Question 24

RP6: Starting with a single bacterium, calculate how many generations it would take to produce
4 194 304 bacteria

You can assume no bacteria die.

You could use log button on your calculator to calculate your answer.

Answer 24

Log (2) [4194304]

= 22 generations

Question 25

RP6: Express 4194304 as a power of 10^

Answer 25

Log(10) [4194304] = 10^6.62

Question 26

RP2: State two precautions required when working with hydrochloric acid

Answer 26

1. Eye protection; wear goggles
2. Gloves;
3. Add water to spills (immediately);
4. Do not pour away down sink;