Required Practicals 1-12

Question 1

RP2
Describe how temporary mounts of plant tissue are made (1)

Answer 1

Thin slice on slide;
Add stain and add coverslip

Question 2

RP2
Describe how a scientist could have used the temporary mounts of leaves to determine the mean number of chloroplasts in mesophyll cells of a leaf (1)

Answer 2

Large field of view - large scope with small magnification
OR
Small POV (few cells) with a high magnification

Question 3

RP2
A student cut thin sections of tissue to view with an optical microscope. Explain why it was important that the sections were thin (2)

Answer 3

1.Light microscopes have a low resolution
SO
2. Thin specimens let more light through
/Thick specimens are non-permeable
to light

Question 4

RP2
Why should the coverslip not move sideways when preparing microscope slides? (1)

Answer 4

No damage to cell, moving coverslip could break the thin tissue specimen.

Question 5

RP2
Explain why 200 cells were examined (1)

Answer 5

(A large field of view so) a representative sample

OR to see more cells in mitosis and not interphase.

Question 6

RP2
Name the purpose of heating the acid and name an alternative to heating (2)

Answer 6

• Heating increases rate of diffusion so breaks down the cell wall faster.
• Alternative = Higher conc of acid
OR leave in acid longer.

Question 7

RP2
A student prepared a stained squash of cells from the tip of an onion root and observed it using an optical microscope. Explain why the student:

a) used only the first 5mm of the tip of the root (1)
b) pressed down firmly on the coverslip (1)

Answer 7

a) Tip of the root = meristem (site of mitosis)

b) To avoid breaking chromosomes in tissue in the wrong direction.
(To remove bubbles)

Question 8

RP2
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)

Answer 8

1. Examine a large field of view with many cells - ensures representative sample
2. Repeat the count - to standardise counting.

Question 9

RP2
A scientist treated growing onion roots with a chemical that stops roots growing. After 24 hours, he prepared a stained squash of these root tips.

Explain how the chemical stops the growth of roots.

Answer 9

1. Stops mitosis by stopping spindle fibres forming;
2. So sister chromatids don’t separate to opposite poles;
3. So no cell division occurs, no new daughter cells produced.

Question 10

RP2
Why is a stain used? (1)

Answer 10

• To make chromosomes visible to an optical microscope.

Question 11

RP2
A different set of results was obtained when the count was repeated with a different garlic root tip. Give two reasons for the difference in results.

Answer 11

1. Different size of root tip
2. Age of root tip

Question 12

RP2
Describe the METHOD of making a temporary mount of root tissue to observe cells in the cell cycle (6)

Answer 12

1. Place the root tip in HCL (for 5 mins)
2. To soften the tissue and break the cell wall
3. Rinse root tips in distilled water in a watch glass
4. Add Toludine blue stain & cut down to 5mm
5. Macerate suspended needle in vertical direction
6. Add coverslip to squash & view under microscope

Question 13

RP7
What is the purpose of chromatography?

Answer 13

To separate different components in a sample

Question 14

RP7
State the factors affecting the rate of migration of different pigments (3)

Answer 14

Solubility
Mass
Affinity to paper

Question 15

RP7
What is the formula for Rf value?

Answer 15

Distance moved by pigment (origin-middle)
—————————————
Distance moved by solvent (origin-solv front)

Question 16

RP7
What is the purpose of finding the Rf value of a pigment?

Answer 16

Rf value can be compared to a standard Rf value in a database to identify the pigment

Question 17

RP7
Outline the procedure of using chromatography to separate photosynthetic pigments (6)

Answer 17

1. Draw the origin pencil line 1cm from the bottom
2. Add 2cm^3 of acetone and use the mortar and pestle to grind up leaf sample and release the pigments
3. Use a capillary tube to transfer the pigment onto the origin
4. Place filter paper in solvent (below pencil line) and leave until solvent is 1cm from top
5. Remove paper from solvent and draw solvent front
6. Calculate Rf value for each spot

Question 18

RP7
State the hazards and precautions of this practical

Answer 18

Solvents are irritants and flammable SO
Keep away from naked flames
Wear eye protection
Avoid inhaling solvent vapour

Question 19

RP7
You could use Rf values to find out if a pigment on your chromatogram was the same as a pigment on another chromatogram. Explain why it is better to use the Rf value to do this and not the distances moved by the pigment. (3)

Answer 19

1. The solvent front moves a different distance;
2. Rf value is distance moved (by spot) divided by distance moved by solvent front;
3. Rf values are constant so COMPARABLE.

Question 20

RP7
Suggest why you shouldn’t move the bottle once the TLC paper is in it (1)

Answer 20

• Solvent may not run straight / pigments could wash off (origin)

Question 21

RP7
Explain why you measure the centre of the pigment spot to find the distance moved by the pigment (1)

Answer 21

1. To standardise the method to allow comparison;
   OR
2. Pigment could be elongated/spread out.

Question 22

RP7
Explain why extracting pigments from xerophytic leaves may not produce good results (2)

Answer 22

1. Xerophytic leaves have spines;
2. Little-no pigment

OR 1. Have a (thick) waxy cuticle;
2. So harder to transfer pigment to paper;

OR 1. Leaves are thicker / stronger;
2. So harder to crush to release pigment.

Question 23

RP7
Describe how you could use chromatography to show that chlorophyll is lost from Autumn leaves (2)

Answer 23

1. Carry out chromatography at different times (e.g seasons);
2. Find Rf value for chlorophyll (in normal leaf);
3. Look for substance at predicted position / green pigment.

Question 24

RP7
Holly has dark green leaves but some cultivated varieties have yellow leaves. Explain why the Holly with yellow leaves grow more slowly than Holly with dark green leaves (3)

Answer 24

1. Yellow leaves have less chlorophyll;
2. Less photosynthesis;
3. Named Products of photosynthesis e.g GLUCOSE needed for growth / synthesis /aerobic respiration

Question 25

RP7 Describe the method used after the solution of pigments had been applied to the origin (2)

Answer 25

1. Add the paper to the solvent below the origin line; 2. Remove before solvent front reaches top.

Question 26

RP7 Give one dependent variable you could measure in order to determine the rate of photosynthesis in an aquatic plant (1)

Answer 26

• Count the number of bubbles OR Measure the volume of gas/CO2/change in pH

Question 27

RP7 - A suspension of chloroplasts was isolated from an aquatic plant and a blue reagent was added which is blue when oxidised and colourless when reduced. i) The suspension in blue reagent was exposed to sunlight and the blue colour disappeared. Use your knowledge of the LDR to explain why (2) ii) Small quantities of ADP and Pi were added to another suspension and exposed to light. The blue colour disappeared more quickly. Explain why (2)

Answer 27

i) • Chlorophyll becomes oxidised / reduced NADP formed • Electrons from chlorophyll / NADPH change dye colour. ii) • ADP and Pi are required to produce ATP in the LDR • (so) ADP levels are a limiting factor.

Question 28

RP8 What is the function of dehydrogenase in plants?

Answer 28

It catalyses the acceptance of electrons by NADP in the LDR

Question 29

RP8 What is the purpose of DCPIP?

Answer 29

It is a redox indicator dye that acts as an alternative electron acceptor instead of NADP It turns from blue -> colourless when reduced (gain H)

Question 30

RP8 Why is the plant extract chilled in an ice water bath?

Answer 30

To lower enzyme activity to prevent them from breaking down the chloroplasts

Question 31

RP8 How is the control set up?

Answer 31

Fill a cuvette with chloroplast extract and distilled water

Question 32

RP8 How is light intensity controlled?

Answer 32

Adjust the distance of the lamp from the set up; Perform practical in dark room so that the only light source is the lamp

Question 33

RP8 What is the function of the muslin cloth?

Answer 33

To filter out debris from the ground leaf mixture but allowing any debris to pass through

Question 34

RP8 Why are the stalks of leaves removed before grinding?

Answer 34

The stalks do not contain chloroplasts

Question 35

RP8 Outline the procedure of investigating the effect of light intensity, after chloroplast extract has been obtained

Answer 35

1. Set the colorimeter to the red filter. Zero using a cuvette containing chloroplast extract and distilled water; 2. Place test tube in the rack 30cm from light source and add DCPIP, Immediately take a sample and add to cuvette. Measure the absorbance of the sample; 3. Take a sample and measure its absorbance every 2 minutes for 10 minutes; 4. Repeat for different distances from lamp up to 100 cm.

Question 36

RP9 What is the function of Methylene blue?

Answer 36

Redox dye so acts as an alternate electron acceptor (during ATP synthesis); Turns from blue - colourless (+ result colour change)

Question 37

RP9 Outline the procedure to investigate the effect of temperature on the rate of respiration of yeast

Answer 37

- Set up a water bath at 35c; - Add equal volumes of yeast and glucose solution to (3-5) test tubes; - Place test tubes in water bath and leave to equilibrate for 10 mins; - Add 2cm^3 methylene blue and start timer - SHAKE (for 10 seconds then place back into water bath) - Record how long it takes for methylene blue to turn colourless - Repeat experiment at 45, 55, 65c

Question 38

RP9 How are the results used to calculate the rate of respiration at each temperature?

Answer 38

Rate = 1/time taken for methylene blue to decolorise

Question 39

RP9 Why does the yeast solution need to be buffered?

Answer 39

To maintain a constant pH - enzymes functioning at optimum pH

Question 40

RP9 What is the effect of temperature on the rate of respiration?

Answer 40

As the temp increases, rate of enzyme activity increases so rate of respiration increases Above optimum temperature, enzymes denature so rate of enzyme activity plateaus/decreases.