5.3 Practical Assessment (AS Only)

A biology student wanted to investigate the stages of mitosis in onion root tips. To do this they prepared a stained squashed sample of onion root tip cells. They used the following method:

• Cut first 4-6 mm from onion root tip and place on glass slide (shown in Figure 1)
• Add stain solution to the tip and lay cover slip over tip
• Press down firmly on cover slip

 Figure 1

Explain why the student:

The biology student looked at their stained squash of onion root tip cells under the microscope and obtained the data shown in Table 1.

 Table 1

Calculate the percentage of cells in which the chromosomes are visible and are lined up along the middle of the cell. Show your working.

The biology student wanted to repeat the procedure in parts (a) and (b) with more onion root tips (from genetically identical onion plants) to check the accuracy of their results in part (b).

Suggest two variables that the student should control to ensure similar root growth in the other onion plants. For each variable, explain why it should be controlled.

The student in parts (a), (b) and (c) conducted further studies on different sections of onion roots. They found that the proportion of dividing cells decreased with increasing distance from the root tip.

Explain why they found this result.

A biologist was investigating the effect of three types of antibiotic on the growth of Streptococcus bacteria. The student biologist carried out the following procedure:

• Boil the agar before pouring the agar plates
• Flood agar plates with a liquid culture of Streptococcus
• Place four sterile paper discs, A, B, C and D onto the agar plates (three soaked in different antibiotics, one in sterile water)
• Incubate agar plates at 37°C for three days

Explain why the biologist:

The biologist in part (a) should use aseptic techniques to transfer the liquid culture of Streptococcus onto the agar plate.

Describe two aseptic techniques that the biologist should use and for each, explain why these techniques are important.

Certain factors can affect the rate at which the antibiotics from each paper disc diffuse through the agar.

State two of these factors and for each one, describe the effect it would have on the rate of diffusion through the agar.

By what process do bacterial cells divide?

A biologist carried out an investigation to find the water potential of carrot tissue. The biologist started with a carrot and 100 cm³ of a 1.0 mol dm⁻³ sucrose solution. They made a series of different concentrations of sucrose solution using the 1.0 mol dm⁻³ solution. They cut and weighed cylinders of carrot tissue and left them in the sucrose solutions for a set time. They then removed the carrot cylinders and blotted them dry
before reweighing them.

Explain why the biologist blotted the carrot cylinders dry before reweighing them.

Describe how the biologist in part (a) could use a 1.0 mol dm⁻³ solution of sucrose to produce 60 cm³ of a 0.30 mol dm⁻³ solution of sucrose. Show your working.

The biologist presented the results of their investigation from part (a) in Table 1 below:

 Table 1 

Describe how the biologist could use their results in Table 1 to find the water potential of the carrot tissue.

The biologist in part (a) calculated the percentage change in mass rather than just the change in mass. Explain why it might have been advantageous to do this.

Betalains are pigments found in the cell vacuole of beetroot cells and are responsible for the dark purple-red colour of beetroot tissue. A student wanted to investigate how alcohol concentration affected the loss of these pigments from beetroot tissue. They obtained the results shown in Table 1 below:

 Table 1

Suggest a suitable method that the student could use to obtain these results.

What conclusions can you draw from the results shown in Table 1?

Another student wanted to investigate the effect of temperature on the loss of pigment from beetroot. They put beetroot discs into test tubes containing water and placed the tubes in water baths at a different temperature. After 40 minutes, they measured the percentage of light passing through the water in each tube.

Explain why it was important that the student made sure each test tube contained the same volume of water to begin with.

The student in part (c) found that the higher the temperature, the more pigment was released from the beetroot and the lower the percentage of light passing through the water in each tube.

Suggest how an increase in the temperature of the water caused pigment to be released from the beetroot.

Insects have a system of very small air-filled pipes called tracheae which they use for gas exchange. A student wants to dissect a grasshopper to view the tracheae.

Give two safety precautions the student should take when carrying out this dissection.

Describe one ethical issue involved in the dissection of animals for scientific purposes.

The tracheal system of the grasshopper that the student dissected in part (a) is adapted for efficient gas exchange. Explain how.

Insect spiracles have valves that enable them to be closed by the insect. In many insect species, the spiracles are also surrounded by tiny hairs. Suggest the advantage of each of these features for an insect in hot weather.