Tropisms

Figure 3 shows Gammarus pulex, commonly known as a freshwater shrimp.

A student used a choice chamber to investigate the effect of light intensity on the movement of G. pulex.

The student:

• collected 30 shrimps from a freshwater stream

• added freshwater and gravel substrate from the same stream to each segment

• used layers of black paper on the lid of the choice chamber to reduce the percentage light intensity in 3 segments of the choice chamber as shown in

Figure 4

• placed 30 shrimps at the centre of the choice chamber

• waited 10 minutes before counting the number of shrimps in each segment.

Figure 4

The student stated that ‘the procedure used in this investigation helps to ensure the validity of any conclusions made’.

Suggest and explain three features of this investigation that justify this statement.

Table 3 shows the student’s results.

The student observed directional movement by G. pulex.

Name the type of behaviour shown by G. pulex and suggest an advantage to G. pulex of this behaviour.

Use the information in Table 3 in your answer.

Type of behaviour

The student analysed the results in Table 3 using the chi-squared test (χ²) and

Table 4 shows the critical values of χ² at different probability (P) values for different degrees of freedom.

State a null hypothesis for this investigation and explain why the χ² test was the appropriate statistical test to use.

Explanation for using a χ₂ test

Using the calculated χ² value of 11.87 and the information in Table 4, what can you conclude from this investigation?

You should include the terms probability and chance in your answer.

Write an essay on one of the topics below.

Either

The importance of condensation and hydrolysis reactions in biological systems.

Or

The importance of responses to changes in the internal and external environment of organisms.

Plant cells are surrounded by a rigid cell wall. Cell wall plasticity refers to the ability of a cell wall to stretch permanently. IAA is a growth factor which can activate enzymes that loosen the cell wall.

Scientists investigated the effect of IAA concentration on cell wall plasticity and the growth of stem segments.

Figure 6 shows some of the scientists’ results.

Use the information provided to explain the relationship between IAA concentration, cell wall plasticity and mean growth of the stem segments.

Use Figure 6 to calculate the percentage increase in mean growth of the stem segments when the IAA concentration was increased from 10^–8 mol dm^–3

Give your answer to 2 significant figures.

Show your working.

Gibberellic acid (GA) is another plant growth factor. A student was asked to design

and carry out an investigation into the effect of different concentrations of GA on the growth of stem segments.

The student was provided with:

• 10^–1, 10^–2, 10^–3, 10^–4 and 10^–5 mol dm^–3 concentrations of GA solution

• distilled water

• 6 Petri dishes and access to glassware

• 60 stem segments of different lengths.

A technician produced the different concentrations of GA solution from a stock

Describe how the technician produced the 10^–3 mol dm^–3 solution.

Describe a suitable procedure the student could follow to investigate the effect of different concentrations of GA on the growth of stem segments.

In your answer, you should include the variables that should be controlled in this investigation.

Figure 1 shows an investigation into growth factors in plants.

Use your knowledge of indoleacetic acid (IAA) to explain the growth curvature shown in Figure 1.

A bioassay is a method to determine the concentration of a substance by its effect on living tissues.

Figure 2 shows the practical procedure used in a growth curvature bioassay to determine the concentration of IAA in shoot tips.

Figure 3 shows the calibration curve for this growth curvature bioassay.

Using the procedure in Figure 2 and the calibration curve in Figure 3, describe how you could compare the IAA concentration in shoot tips from two different plant species.

In your answer you should refer to all the variables that should be controlled to produce a valid comparison.

A scientist investigated the effect of a directional light stimulus on the distribution of IAA in shoot tips. The scientist set up three experiments as shown in Figure 4. All variables were controlled apart from exposure to light.

She then used the growth curvature bioassay to compare the IAA concentrations in the agar blocks from:

• experiment 1

• experiment 2

• experiment 3 section A

• experiment 3 section B.

Table 1 shows the scientist’s results.

State two conclusions about IAA that you can make from the results shown in Table 1.

Scientists investigated movement in adult pine beetles. Adult beetles emerge from cracks in tree bark.

The scientists released a newly emerged adult beetle, G, from the centre of a sample area that had a single light source coming from one direction. They made a drawing of the beetle’s path of walking. They repeated this with three more beetles, J, P and R.

Figure 2 shows the scientists’ results.

Name the type of behaviour shown by beetles G, J, P and R, and suggest one advantage to adult beetles of the type of behaviour shown.

At higher temperatures and higher light intensities, adult pine beetles normally

• move more

• fly rather than walk.

When preparing to fly, these adult beetles walk slowly. The scientists investigated the movement of adult beetles at different temperatures, and in the light and the dark. They created a box that was half in the light and half in the dark. They released an adult beetle at the midpoint of the central dividing line between light and dark areas. They recorded the path of the beetle’s movement and its location after 5 minutes. From this, they calculated the mean speed of movement. They repeated the experiment with many beetles and at several temperatures.

Figure 3 shows the scientists’ results.

After studying these experiments, a student concluded:

• there is a significant change in movement between 35 °C and 37.5 °C

• between 35 °C and 37.5 °C, more beetles move away from the light

• between 35 °C and 37.5 °C, more beetles have a slower walking speed.

Suggest reasons why these conclusions might not be valid.

Explain why the student removed the shoot tip from each seedling.

Explain why the student added glucose solution to each Petri dish.

Explain why the lids were kept on the Petri dishes.

Describe and explain the results shown in Table 1 and suggest how the results might have differed if lengths of root had been used.

The student produced the different concentrations of IAA using a stock 1 g dm^–3 solution of IAA (1 g dm^–3 = 1 part per thousand) and distilled water.

Complete Table 2 with the volumes of stock IAA solution and distilled water required to produce 40cm³ of 10 ppm (parts per million) IAA solution.

The crown-of-thorns starfish (COTS) is one of the main causes of the decline of the world’s coral reefs.

Marine biologists used a choice chamber to investigate the effects of flashing and constant light on the behaviour of COTS.

Table 1 shows their results as they presented them. The P values show results from a statistical test.

State a null hypothesis the marine biologists tested in this investigation.

The natural habitat of COTS is coral reefs of tropical oceans.

Suggest two factors that should be kept constant in the choice chambers so that COTS display normal behaviour.

A journalist studying Table 1 suggested that either type of light could be used to cause COTS to move away from coral reefs.

Evaluate the journalist’s suggestion.

One of the reasons COTS can destroy coral reefs in a short time is because COTS move quickly, allowing them to move from one reef to another.

Table 2 shows the maximum speeds recorded of COTS in constant light.

Calculate the shortest time one COTS would take to move up a coral reef from 66 m under water to 18 m under water in hours of daylight.

Give your answer to the nearest hour.