Practicals

Question 1

Mitotic Index =

Answer 1

Number of cells in mitosis ÷ Total number of cells in field of view

Question 2

1 mark

Why did he put the plant root in HCl?

Answer 2

To hydrolyse the cell wall

Question 3

2 marks

State 2 precautions required when working with hydrochloric acid

Answer 3

1. Wear goggles
2. Wear gloves

Question 4

3 marks

A student did an investigation. He did not have a colorimeter.
Describe a method this student could use to prepare colour standards and use them to give data for the total pigment extracted.

Answer 4

1. Use known concentration of pigment
2. Prepare dilution series
3. Compare results with colour standards to give concentration

Question 5

2 marks

Describe two precautions the student should take when clearing away after the dissection.

Answer 5

1. Carrysharp instruments by pointing away from body
2. Disinfect instruments/surfaces
3. Wash hands with soap and water
4. Put organ/gloves/paper towels in a separate bin bag to dispose

Question 6

5 marks

An environmental scientist investigated a possible relationship between air pollution and the size of seeds produced by one species of tree. He was provided with a very large number of seeds collected from a population of trees in the centre of a city and also a very large number of seeds collected from a
population of trees in the countryside. Describe how he should collect and process data from these seeds to investigate whether there is a difference in seed size between these two populations of trees.

Answer 6

1. Use a random sample of seeds from each population
2. Use large enough sample to be representative of whole population
3. Indication of what size was measured e.g. mass
4. Calculate and mean and SD for each population
5. Use the students t-test
6. Analyse whether there is a significant difference between the means of the 2 populations

Question 7

5 marks

The sundew is a small flowering plant, growing in wet habitats such as bogs and marshes. The soil in bogs and marshes is acidic and has very low concentrations of some nutrients. The sundew can trap and digest insects.
Describe how you could estimate the size of a population of sundews in a small marsh.

Answer 7

1. Use a grid
2. Use random number generator to obtain random coordinates
3. Count number/ frequency in each quadrat
4. Large sample (repeat 30 times) and calculate mean
5. (Mean number of plants per qudrat/ area of marsh) x number of quadrats in the marsh

Question 8

3 marks

Describe how the scientists could use aseptic techniques to transfer 0.3 cm3 of C. difficile in liquid culture from a bottle onto an agar plate

Answer 8

1. Wash hands with soap/ Disinfect surfaces
2. Use sterile pipette to transfer bacteria
3. Remove bottle lid and flame neck of bottle
4. Lift lid of agar plate at an angle
5. Work close to upward air movement
6. Use sterile spreader
7. Place pipette into disinfectant immediately after use

Question 9

A microbiologist investigated the ability of different plant oils to kill the bacterium Listeria monocytogenes. She cultured the bacteria on agar plates. She obtained the bacteria from a broth culture. 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 9

1. Open lid of Petri dish as little as possible.
2. To prevent unwanted bacteria contaminating the dish.
3. Wear gloves/ wash hands
4. To prevent contamination from bacteria on hands / mouth
5. Use sterile pipette
6. To maintain a pure culture of bacteria

Question 10

3 marks

Describe the aseptic techniques that would be used when flooding the agar plate with bacteria.

Answer 10

1. Sterilisation of equipment
2. Use of pipette to transfer culture suspension to plate
3. Use of spreader

Question 11

2 marks

One way of testing the effectiveness of antibiotics is to use discs of paper soaked in antibiotic. These are placed in the centre of an agar plate covered by bacteria. A clear zone forms around the disc if the antibiotic is effective. Give two reasons why it would be important to use sterile techniques during this investigation.

Answer 11

1. To prevent contamination of apparatus with other microorganisms / bacteria
2. To prevent personal contact with bacteria
3. To prevent release of bacteria into air

Question 12

1 mark

The antibiotic reached the bacteria by diffusion. Suggest why an effective antibiotic may produce only a small clear zone.

Answer 12

Diffuses slowly

Question 13

2 marks

Give two factors, other than cost, that should be considered when selecting an antibiotic to treat a bacterial disease

Answer 13

1. Side effects / allergic reactions
2. Interaction with other drugs
3. Should only act on the problem bacteria
4. How much resistance the bacteria have built up

Question 14

2 marks

The minimum inhibitory concentration (MIC) is the lowest concentration of a substance that prevents the growth of a microorganism. When antibiotics are prescribed for treating patients, higher doses than the MIC are recommended. Suggest two reasons why

Answer 14

1. May not all be absorbed
2. May be broken down quickly
3. To kill the microorganisms / bacteria
4. Reference to antibiotic resistance

Question 15

3 marks

A bacterial infection that cannot be treated with one sort of penicillin can be treated with a
different sort. Use your knowledge of enzyme action to explain why the different sort of penicillin is effective in treating the infection.

Answer 15

1. Different penicillin has different tertiary structure / active site has specific tertiary structure
2. No longer binds to active site / not complementary to active site / does not form E-S complex
3. Different) penicillin not broken down

Question 16

2 marks

Farmers often keep large numbers of cattle together. Farmers used to give cattle food which had antibiotics added to it. Suggest how adding antibiotics to the food of the cattle increased profit for the farmers.

Answer 16

1. Kills pathogenic / harmful bacteria
2. Disease less likely / improves health / animals healthier
3. Faster growth / more productive animals / more food converted to meat/ increased yield

Question 17

2 marks

Adding antibiotics to the food of cattle is now banned in many countries. Use your knowledge of selection to explain why adding antibiotics was banned.

Answer 17

1. Adding antibiotics selects in favour of antibiotic resistance / resistant bacteria more likely to survive
2. Increase in numbers / higher proportion of resistant bacteria

Question 18

3 marks

The scientists used the following null hypothesis.
‘The proportion of plants that produce 2n gametes will not change from one breeding
cycle to the next.’
The scientists tested their null hypothesis using the chi-squared statistical test. After 1 cycle their calculated chi-squared value was 350. The critical value at P=0.05 is 3.841. What does this result suggest about the difference between the observed and expected results and what can the scientists therefore conclude?

Answer 18

1. There is a less than 0.05/5% probability that the differences occurred by chance
2. Calculated value is greater than critical value so the null hypothesis can be rejected
3. The proportion of plants that produce 2n gametes does change from one breeding cycle to the next

Question 19

4 marks

A student used chromatography to separate the different photosynthetic pigments in a chlorophyll solution. She had the following materials.
Chromatography paper
A ruler and pencil
Suitable glassware
A solvent (solvent A)
2 cm3 of the chlorophyll solution
Describe how she could use these materials to separate the photosynthetic pigments by chromatography.

Answer 19

1. Draw line on chromatography paper using ruler and pencil
2. Use pipette to add chlorophyll/solution to origin line
3. Add solvent below line
4. Remove from glassware before solvent reaches end of chromatography paper

Question 20

Using solvent A, the student separated five pigments. She then repeated her method using a different solvent, B. Using solvent B, she separated six pigments.
Explain the difference between these results.

Answer 20

one pigment is only soluble in solvent B

Question 21

1 mark

Explain why a log scale is used to record the number of cells.

Answer 21

Large range

Question 22

1 mark

Explain why the student marked the origin using a pencil rather than using ink.

Answer 22

Ink and pigment would mix

Question 23

2 marks

Describe the method the student used to separate the pigments after the solution of pigments had been applied to the origin.

Answer 23

1. Level of solvent below origin line
2. Remove before solvent reached top

Question 24

1 mark

The pigments in leaves are different colours. Suggest and explain the advantage of having different coloured pigments in leaves.

Answer 24

Absorb different wavelengths for photosynthesis

Question 25

# 2 marks Apart from temperature and pH, give two variables the scientist would have controlled when preparing the liquid medium cultures.

Answer 25

1. Volume of stock bacteria culture 2. Concentration of stock bacteria culture 3. Concentration of glucose 4. Volume of ammonium chloride 5. Time bacteria/culture left to divide 6. Concentration/volume of oxygen 7. Concentration/volume of nitrogen

Question 26

# 5 marks Explain what limits the size of populations in a climax community.

Answer 26

1. nutrient availability such as nitrogen, phosphorus 2. numbers of producers (plants, algae) providing energy for a food chain / light intensity affecting the rate of photosynthesis 3. disease killing weaker members of species 4. space for nest building / niches 5. competition for a named limited resource- food, space, water, or mates

Question 27

# 4 marks The shore crab has recently spread to, and has established large populations in, the coastal waters of the USA, where it is not a native species. Explain how the shore crab has been able to establish large populations and why this is causing concern to ecologists in the USA

Answer 27

1. Shore crab rapidly colonises (rapid growth) 2. Ability to live different environments 3. Shore crab better comptitor/more aggressive 4. Decreased population of prey species 5. Ecosystem less stable

Question 28

# 2 marks With some samples, the scientists decided they needed to carry out a series of dilutions of the sample before counting the bacteria. Explain why dilutions were necessary for some samples but not for others. | log scale graph

Answer 28

1. Log scale on graph shows big range in number of bacteria 2. Some samples too many to count (so dilute) but some countable (so don’t dilute)

Question 29

# 3 marks Outline a method the ecologists could have used to determine the plant species richness at one site.

Answer 29

1. Use a grid and random number generator to generate coordinates 2. Use of quadrat 3. Identify plant species in each quadrat/ Count number of different plant species in each quadrat

Question 30

# 4 marks Plantations of trees are often created to remove carbon dioxide from the atmosphere, to help to balance the carbon dioxide released by burning fossil fuels. For different species of tree, information is available for: * the relationship between diameter of trunk and freshly cut biomass * the percentage of water in fresh-cut wood * the mean dried density of wood. Suggest how the mass of carbon in the wood of a plantation of trees of a particular species could be estimated. Start with measuring the diameter of a large number of trees. Assume that the dry biomass of a tree consists of biological molecules that contain carbon.

Answer 30

1. Calculate a mean diameter 2. Use this to determine the mean fresh biomass of trees 3. Use the percentage water content to find the dried biomass 4. Use the dried density to calculate the mass of trees 5. Count the number of trees in plantation and multiply by mean carbon content to find total carbon

Question 31

An environmental scientist investigated a possible relationship between air pollution and the size of seeds produced by one species of tree. He was provided with a very large number of seeds collected from a population of trees in the centre of a city and also a very large number of seeds collected from a population of trees in the countryside. **Describe** how he should collect and process **data from these seeds** to investigate whether there is a **difference** in seed size between these two populations of trees.

Answer 31

1. Use random sample of seeds from each population 2. Use large sample to be representative of whole population 3. Indication of what size was measured e.g. mass 4. Calculate a mean and standard deviation (for each population) 5. Use the Student’s t-test 6. Analyse whether there is a significant difference between the means of the two populations

Question 32

# 2 marks The values for the biomass of the barley plants are different in each quadrat. Suggest an explanation for this difference.

Answer 32

1 Quadrats from different parts of field 2 Biotic & abiotic factors different

Question 33

# 1 mar The researchers measured the dry biomass of the barley plants and the barley grain. What is the advantage of using dry biomass for these measurements?

Answer 33

Water content varies

Question 34

# 2 marks A farmer uses a herbicide which reduced biomass of weeds in field. Explain why would this herbicide give a higher crop yield.

Answer 34

1 Fewer weeds left 2 Fewer weeds to produce seeds 3 Less contamination of crop by weeds 4 Fewer weeds to separate from crop 5 Less competition between crop and weeds

Question 35

# 2 marks 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. The student put the same volume of water in each tube. **Explain** why it was important that he controlled this experimental variable.

Answer 35

1. If too much water, conc of pigment will be lower (more light will pass through). If too little water, conc of pigment will be greater (less light passes through) 2. To allow for comparison

Question 36

# 1 mark Describe a method the student could have used to monitor the temperature of the water in each tube

Answer 36

Take reading**s** using a thermometer

Question 37

# 2 marks You were told to count the number of stomata in different fields of view of the same leaf Explain why this was important.

Answer 37

1. Density/number of stomata may not be uniform 2. To get results representative of the whole leaf/epidermis

Question 38

# 3 marks A student determined their pulse rate when sitting down. Describe how they could have investigated whether their results were typical of all students of their age.

Answer 38

1. Obtain pulse rates from a large number of students 2. From a range of different ethnic groups, sexes 3. Calculate mean and SD 4. See if their mean lies within SD

Question 39

# 3 marks How to investigate the optimum temperature for urease activity

Answer 39

1. Set up a range of water baths at different temperatures (e.g. 20°C, 30°C, 40°C, 50°C) to maintain specific temperatures for each test tube. 1. In each test tube, mix equal volumes of urea solution and urease enzyme. 1. Add a pH indicator to each tube. 1. Measure the time taken for the solution to turn blue, indicating an increase in pH due to ammonia production — this is used as a measure of rate of enzyme activity. 1. Keep all other variables constant: volume of solutions, starting pH, enzyme concentration, and incubation time.

Question 40

# 6 marks Test for non reducing sugars

Answer 40

1. Add Benedict’s solution to your sugar solution and heat in a water bath (≥60°C). 1. If the solution stays blue, no reducing sugar is present → it might be a non-reducing sugar. 1. Boil a fresh sample with acid to hydrolyse glycosidic bonds and break the sugar into reducing sugars. 1. Neutralise the acid using an alkali 1. Then add Benedict’s solution again and heat in a water bath. 1. If a red/orange precipitate forms now, it means a non-reducing sugar was present originally.

Question 41

What is a non reducing sugar

Answer 41

* A reducing sugar (like glucose or maltose) can react directly with Benedict’s solution and give a red/orange precipitate. - monosaccharide * A non-reducing sugar (like sucrose) doesn’t do this — unless you break it apart first.