Practical Skills: Written Assessment (OCR A Level Biology)

A student investigated the effect of different sugars on the respiration rate in yeast using the following procedure.

The student calculated mean values for each sugar and plotted the results on a graph.

The graph is shown below.

The student wanted to study the effect of the different sugars on the rate of growth of a yeast population.

They used a colorimeter to measure the absorbance of a culture of yeast cells.

The absorbance of the yeast culture is proportional to the concentration of yeast cells.

As the yeast multiplied, it was necessary to dilute the sample to obtain a reading on the colorimeter.

A different starter culture was prepared containing 1 × 10⁷ cells.

Use the axes below to sketch the growth curve you would expect for the yeast culture over the first 15h.

A student carried out an investigation into the effect of ethanol on the permeability of cell membranes in beetroot.

The student’s method comprised the following five steps:

Assumption A relates to step .............

Assumption B relates to step .............

Assumption C relates to step .............

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Fig. 20.1 shows the graph plotted by the student.

Fig. 20.1

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A student carried out an investigation into the production of CO₂ in five different species of yeast.

The yeast cells were placed in different environments and the CO₂ production was measured.

Table 20 shows the results of the experiment. The mean values for these data are also represented as a graph in Fig. 20.

Table 20

Using the information in Table 20, calculate the standard deviation for the number of CO₂ bubbles produced by A. pullulans in anaerobic conditions.

Give your answer to one significant figure. Show your working.

Fig. 20 is a graph showing the mean values of the data from Table 20.

Plot the standard deviations for all data on Fig. 20.

Calculate the mean percentage change in CO₂ production for S. cerevisiae when moving from anaerobic to aerobic respiration. Give your answer to four significant figures.

Show your working.

For each conclusion, state and explain whether the student is correct.

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Which type of error is suggested by the student’s data? Justify your answer.

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Anaerobic respiration in yeast cells requires enzymes.

Which organelle is responsible for synthesising these enzymes?

A student investigated the heart rates of smokers and non-smokers.

• Each test subject had their resting heart rate measured using an electronic heart rate monitor.
• They ran 1km on a running track and their heart rate after running 500m was recorded.
• Their heart rate was recorded for a third time 3 minutes after the completion of the exercise.

All test subjects were 18 years old. Subjects were tested between 9am and 4pm on one day, one at a time. Each test lasted approximately 20 minutes in total. The tests were repeated one week later using the same method. Mean heart rates were calculated for each subject.

The student’s plan was to compare the heart rates of smokers and non-smokers using Student’s t-test.

The student’s results are shown in Table 6.

Table 6

Suggest and explain improvements that the student could make to his experimental method and his presentation of data.

In your answer you should explain the benefits of your suggested improvements.

Water moves by osmosis in living organisms.

The apparatus shown in Fig. 16 can be used to demonstrate osmosis.

Fig. 16

When the capillary tube with visking tubing bag was placed in solution Y, the level of solution X inside the capillary tube rose from 10.5mm to 26.5mm.

uncertainty = ............................. % [2]

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A group of students used the following method to investigate osmosis in plant cells.

• Cut pieces of plant material of equal surface area ensuring no skin is present.
• Rinse to remove cell debris.
• Gently pat the plant pieces dry with a paper towel.
• Weigh each piece and record mass.
• Put the plant piece in a 200cm³ beaker.
• Cover plant piece with 50cm³ of sucrose solution.
• Use sucrose solutions of 0, 0.1, 0.3, 0.5, 0.7moldm^–3.
• Leave for 24h.
• Remove the piece of plant material.
• Dry carefully using a paper towel.
• Weigh the plant piece and record the mass.
• Calculate the percentage change in mass for each piece.
• Repeat twice for each sucrose concentration.

The students investigated material from three different plants: carrot, courgette and potato.
Their results are shown in Table 16.

Table 16

Plant cloning is often used by farmers to produce new plants.

A plant that is often cloned by taking cuttings is lavender, Lavandula angustifolia.

A farmer had two fertiliser solutions, solution A and solution B, and wanted to investigate which one to use on lavender plants. In order to ensure the investigation would be valid, two cuttings were needed from the same parent plant.

Describe how to clone a plant by taking a cutting.

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• growing the plants in the same type of soil
• exposing the plants to the same light intensity.

After a set period of time the farmer measured the increase in height of the lavender plants. The farmer’s results are shown in the table below.

The farmer concluded that solution A increased the height of lavender more. A student said that, even though the investigation was valid, the results did not give strong support to the farmer’s conclusion.

Describe and explain how the investigation could be improved in order to have more confidence in any conclusions drawn from the results.

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Fig. 2.3 shows the relationship between body mass and lifespan in a range of mammal species.

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Fig. 16 shows pressure changes during the cardiac cycle.

Fig. 16

The heart supplies oxygenated blood to the tissues.

VO₂^max is a measurement of the maximum volume of oxygen that an individual can use during intense exercise in a given time.

Smart watches can estimate the VO₂^max of an individual by measuring heart rate while exercising.
Having a higher VO₂^max is associated with improved aerobic fitness.

Two male students exercised for 30 min and used smart watches to record their VO₂^max.
Table 16 shows their masses and the VO₂^max values they recorded.

                                       Table 16

Student 1 drew the following conclusion from this result:

Student 2 said that this conclusion is invalid because several variables have not been controlled.

State three variables necessary for a valid comparison that have not been controlled in the above experiment.

A scientist used a respirometer to investigate the rate of respiration and photosynthesis of maize in different light intensities.

• The scientist placed ten maize seedlings in a respirometer and kept it in the dark for three hours. 
• The respirometer contained soda-lime to remove any CO₂ produced by the seedlings.
• The scientist placed ten maize seedlings in a separate respirometer without soda-lime and placed it in different light intensities for three hours at a time.

Table 22.1

Answer = ............................................mm³h^-1 [3]

Answer = ...................................................... % [2]

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The scientist made the following claim:

These results suggest that, in maize seedlings, the rate of photosynthesis only exceeds the rate of respiration when the light intensity is above 1510 lux.

Use the data in Table 22.1 to explain why the scientist made this claim.

Temperature and light intensity are two factors that affect the rate of photosynthesis.

A student investigated how temperature and light intensity affected the rate of photosynthesis in the aquatic plant Elodea canadensis. The rate of photosynthesis was measured by counting the number of bubbles produced by the plant per minute.

The student’s results are shown in Table 3.

Table 3

The Madidi National Park, in the South American rainforest, is home to a wide variety of species.
The largest predator in the area is the jaguar. These large cats are well camouflaged and hunt mostly at night. A single individual can cover a very large area.

In 2007 the Wildlife Conservation Society (WCS) attempted to estimate the population of jaguars in the Madidi National Park.

• Digital camera traps were placed in areas that jaguars were likely to visit.
• If an infrared beam was broken by an animal, the camera was activated.
• The camera then took a photograph of the animal.

• 100 camera traps were set up covering an area of 271km².
• 28 images of 9 different jaguars were recorded.

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The potato plant, Solanum tuberosum, is a staple food plant in many parts of the world.

Potatoes are susceptible to infection by a pathogen called Phytophthora infestans, which causes a disease known as potato late blight. The most visible sign of the disease is a brown discolouration of the leaves.

Some varieties of potato are resistant to infection by P. infestans.

State two ways in which an individual S. tuberosum plant could respond to infection by P. infestans.

The resistance of different varieties of S. tuberosum to infection by P. infestans was investigated.

• Three different clones, A, B and C, of S. tuberosum were used.
• The clones were grown in adjacent fields over the same time period.
• The percentage of leaf area affected by the disease was estimated at regular intervals.

The results are shown in Fig. 18.

Fig. 18

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The effect of wave action on the height of the shells of the dog whelk (Nucella lapillus) was investigated by comparing an exposed shore and a sheltered shore.

• A random sampling technique was used to collect 50 shells from an exposed shore.
• The shell height was measured from the base to the conical tip. The whelk was returned to its location.
• The process was repeated for the sheltered shore.
• All the results were recorded in Table 3.1.

Table 3.1

The t test can be used to determine the significance of the differences between shell height on the exposed shore and the sheltered shore.

Calculate the t value for the data using the formula:

where,

is the difference in mean values of sample 1 and sample 2

and are the squares of the standard deviations of the samples

n₁ and n₂ are the sample sizes.

Give your answer to two decimal places.

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The null hypothesis is that there is no difference between the means of the two shell populations.

The critical values at 98 degrees of freedom are shown in Table 3.2.

Table 3.2

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The students organised the data from Table 3.1 into classes.

The organised data is shown in Table 3.3.

Sheltered shore			Exposed shore
Height (mm)	Tally	Total	Height (mm)	Tally	Total
23–26		5	11–14		4
27–30		22	15–18		7
31–34		11	19–22		12
35–38		9	23–26		12
39–42		3	27–30		4

Plot the most suitable graph of the data given in Table 3.3.

Use the data and graph to discuss any correlation between the height of the whelk shell and the type of shore.

Suggest explanations for your findings.

Suggest a limitation of the procedure used to gather the data in this experiment and recommend how you could improve this.

How could the students improve the accuracy of their data?

Discuss the validity of the conclusions you have made during this experiment.

A student wanted to investigate how length of day period affects the rate of plant growth. She decided to use common bean plants and set up her experiment in a room in the school laboratory. 

Explain how the student could carry out her experiment in the laboratory. Give details of how she could collect data.

List three control variables in this experiment AND explain how each could be controlled. 

Suggest the most appropriate way to present the data.

Two students wanted to test the effects of light intensity on the rate of photosynthesis in pond weed. 

Student 1 used the setup in Figure 2.1, while Student 2 used the setup in Figure 2.2. 

Figure 2.1

Figure 2.2

Compare how the two experimental setups would affect the validity and accuracy of the results. Give reasons for your answers.

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Suggest how the students could adapt the experiment to test the effects of temperature on photosynthesis.

A student wanted to use a 0.5 m x 0.5 m quadrat to carry out a line transect up a 300 m stretch of shore perpendicularly to the water line. They carried out quadrats every 5 metres along the transect.

Calculate the total area covered by the transect quadrats in cm².

A student investigated the effects of surface area on osmosis in different sized potato cubes. They cut sets of cubes of the following sizes:

• 1 cm³
• Sides of 5 mm each
• Sides of 0.25 cm each

They then weighed equal masses of each cube size and put them in different sugar solutions for 10 minutes. They then removed the cubes from the solution, dabbed off the excess moisture with tissue paper and weighed each set of cubes. 

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In a clinical trial for a new drug affecting insulin sensitivity, 20 subjects were given 100 mg tablets of either the drug or a placebo. Subjects’ blood glucose response was tested before and after the medication. 

Calculate how many tablets would be needed to provide a drug dose of 5 mg per kilo for an 80 kg man. 

The scientists found that this dose was too low to produce a significant effect. In a previous experiment 1000 mg was found to be effective but created too many side effects. 

Design an experiment to test what the lowest effective dose would be. Include the variables that should be controlled.

The glucose monitors used to measure subjects’ insulin responses had a percentage error of 6%. 

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An ecologist wanted to study the change in arthropod species in a 100 m x 100 m woodland over the course of 5 years. Each year, they conducted sweep-net samples and pitfall trap samples and recorded the mean abundance of 4 key species across 10 randomly-selected 1 m² sample sites. Their results are presented in Table 1 below.

Table 1

Explain the measures the ecologist could take to ensure that their data was valid.

Describe the trends that can be observed in the data over time.

The ecologist wanted to use their data to understand changes in species abundance over the whole study site.

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