Practical Skills: Endorsement Assessment (OCR A Level Biology)

 A group of students wanted to test an unknown solid sample for proteins. 

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Explain why the unknown sample might still contain amino acids even if there is a negative result.

Lipids include triglycerides and phospholipids, which are found in cell membranes.  

An emulsion test can be used to test for lipids in a sample. The sample is shaken with ethanol before water is added. If lipids are present, a cloudy emulsion appears. 

Organic solvents affect the permeability of cell membranes. A student wanted to investigate how solvent concentration affected the permeability of beetroot cell membranes. Beetroot cells contain a red-violet pigment called betacyanin. They cut cubes of beetroot and put them in a range of ethanol solutions of different concentrations, then measured the amount of pigment that left the cells using a colorimeter. 

The results of the experiment are presented in Figure 3.1.

Figure 3.1

Describe and explain the shape of the graph.

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Computer simulations can be used to model the effects of natural selection on populations. A scientist wanted to test the effects of selection on a population of white butterflies. 

Suggest the effect on the butterfly population if the scientist changed the abiotic environment in the simulation. 

The scientist added a dominant mutation into the butterfly population making it bright red and toxic to a predator in the simulation environment.
Explain what would likely happen to the mutation in the simulation.

The scientist changed the mutation to be recessive.

Explain how the simulation with this allele would differ from your answer in (b).

Discuss the advantages of using computer modelling for studying natural selection.

A scientist wanted to determine the concentration of a stock bacterial suspension. They created a serial dilution, taking 1 ml of stock solution, adding 9 ml of nutrient broth. This was repeated 6 times, as shown in Figure 5.1. 

Figure 5.1

1 ml of each dilution was plated on an agar plate and the number of colonies was counted. 

The number of colonies from Dilution 5 was 3.

Calculate the estimated number of bacterial colonies in the entire Dilution 4 tube.

Dilution 3 had 311 colonies.

Calculate the concentration of bacterial colonies of the stock solution. Write your answer in standard form.

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Two students investigated the growth of bacteria at different temperatures.

Three flasks containing identical solutions of nutrient broth were used.

• Flask 1: inoculated with 1cm³ of broth containing the bacterium Bacillus subtilis and incubated at 20°C.
• Flask 2: inoculated with 1cm³ of broth containing B. subtilis and incubated at 30°C.
• Flask 3: inoculated with 1cm³ of broth containing no bacteria and incubated at 30°C.

Aseptic techniques were used throughout.

At set times over the next 3 days the students removed samples from each flask and measured the number of viable bacteria present.

State one further variable the students should have controlled in their investigation in order to produce valid results.

The students used the following procedure to determine the number of viable bacteria in each flask at a given time.

From each flask, 0.1cm³ was removed and mixed with 9.9cm³ of sterile water in a test tube.
This was labelled Tube A. A serial dilution then proceeded, as shown in Table 19.1.

Table 19.1

From each tube, A–F, 0.1cm³ of mixture was cultured on nutrient agar for 24 hours at 30°C.

The results from Flask 2 after 7 hours of incubation are shown in Fig. 19.

Fig. 19

The students used Tube F to calculate the number of viable bacteria present in the original sample.

The processed results from the students’ investigation are shown in Table 19.2.

Table 19.2

Fig. 1.1 shows the structure of the amino acid leucine.

Thin layer chromatography can also be used to separate photosynthetic pigments.

When sequencing DNA, fragments of DNA are separated by electrophoresis.

Describe three differences between the process of thin layer chromatography and the form of electrophoresis used to sequence DNA.

Laboratory techniques are used by workers in various professions, and by scientists.

A patient has been coughing blood, and it is suspected that bacteria will be found in the blood.

A medical technician cultures the blood on an agar plate. What measures should the technician take to keep the agar plate culture sterile?

Tissue traces from a crime scene often need to be identified. DNA from the tissue is ‘amplified’ by the polymerase chain reaction (PCR) to get samples large enough for further analysis.

Modern PCR technique uses DNA polymerase from the bacterium Thermus aquaticus. Why is this enzyme chosen?

Valine, citrulline, hydroxyproline and glutamic acid are amino acids that are normally found in considerable amounts in urine. Following certain diets can result in a change in the amino acids present in the urine of some people.

Plan a method to compare the amino acids present in the urine of a person who has been following one of these diets with that of a person who has not.

Bacteria and fungi can be used to make food for human consumption. The use of microorganisms in food production creates fewer ethical issues than the use of animals.

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Plants lose water by transpiration.

The rate of transpiration varies between different species of plant.

The rate of transpiration can be measured using a potometer.

Plan an investigation into the rate of transpiration in two species of plant that would allow valid data to be collected.

Details of how to set up a potometer are not required.

A student planned to carry out a dissection of insect and fish gaseous exchange systems.

The student planned to complete diagrams of the different tissues. They were advised to observe the following guidelines:

• use a sharp pencil
• use ruled label lines
• include a scale bar.

Suggest two further guidelines for the student to follow to ensure they present their diagrams clearly and accurately.

1 ..........................................................

2 ..........................................................

Lactose is a reducing sugar.

Benedict’s reagent can be used to detect the presence of lactose in a solution.

A colorimeter can be used to measure the concentration of lactose.

The colorimeter first needs to be calibrated.

Describe how a method that uses Benedict’s reagent and a colorimeter could be calibrated to measure the concentration of lactose in an unknown sample.

A group of students set up a simple respirometer, as shown in Fig. 1.1, and used it to determine the rate of respiration in germinating mung beans.

• They placed a small muslin bag of soda lime into the syringe and then added five germinating mung beans, which were held in place with the syringe plunger.
• The students measured the movement of the red fluid in the capillary tube.
• After each set of readings the plunger was reset to return the fluid to its original position.

Fig. 1.1

The results are shown in Table 1.1.

Table 1.1

Give one limitation of using this method to investigate respiration rate.

Read the procedure carefully. Identify one variable that had not been controlled in this experiment and suggest an improvement to control that variable.

Describe how you would add the red fluid to the capillary tube at the start of the experiment.

The data shows an anomalous result at 60 seconds.

Explain why the result is considered to be anomalous and describe one correct way of dealing with this type of result.

Using the data the student obtained, calculate the mean rate of respiration for germinating mung beans between 90 and 150 seconds. 

What additional information would be needed to calculate:

The group of students wanted to find out if the rate of respiration of a small invertebrate animal was comparable to that of the mung beans.

Adapt the procedure used to investigate the respiration rate of a small invertebrate, such as a woodlouse or caterpillar, with that of mung beans.

Comment on the results you might expect from this experiment and the conclusions you might draw.