Plant & Animal Responses (OCR A Level Biology)

Gibberellin causes stem elongation in plants.

Fig. 17.1 shows the effect of gibberellin on cabbage plants.

Fig. 17.1

Gibberellin causes an increase in the distance between the leaves on the stem, which is known as the internodal length.

Explain why gibberellin is classed as a plant hormone.

A scientist carried out an investigation into the effect of gibberellin on cabbage plants.

The scientist applied a range of volumes of gibberellin and measured the rate of increase of internodal length over 30 days.

Table 17 shows the scientist’s results.

Table 17

Plot the results from Table 17 as a suitable graph.

[4]

[1]

A student carried out chemical tests on cabbage leaves to investigate which molecules were present.

The student’s method was as follows:

[2]

Gestational diabetes is a medical condition that affects pregnant women. It results in high levels of glucose in the blood, even though the woman produces normal levels of insulin.

Skeletal muscle is one of the main tissues where glucose is removed from the blood in response to insulin.

During late pregnancy, women find ventilation more difficult, as the developing foetus reduces the volume of the thorax. This can lead to tiredness and difficulty breathing.

A student used a spirometer to measure ventilation in a woman who was 36 weeks pregnant.

Fig. 16.1 shows the trace produced.

Fig. 16.1 

Mean oxygen uptake rate at rest in women is around 0.020

Using these data, the student made the following conclusion:

    My data show that being pregnant reduces rate of oxygen uptake by up to 20%

Evaluate this claim, using the data in Fig. 16.1.

Plant hormones affect the growth of plant tissues in different ways.

One such effect is to promote the formation of seedless fruit.

Cytokinins are a group of plant hormones.

A commercial plant hormone firm carried out research into three different cytokinins: kinetin, zeatin and diatin.

The firm investigated the effect of adding different volumes of each cytokinin on the production of seedless fruit.

The cytokinins were sprayed on the flowers of different plants. Over time, the mass of seedless fruits produced by the plants was measured.

Fig. 21 is a summary of their results.

Fig. 21

On the basis of these results, the firm decided to use diatin in their new plant spray.

The firm made the following claim on their packaging:

Diatin is scientifically proven to cause production of seedless fruit when applied to flowers.

Evaluate the firm’s claim, using the evidence in Fig. 21.

Another response affected by plant hormones is phototropism.
A student completed an investigation into phototropism in cress seeds.
This was the method used:

• Place 50 cress seeds (Lepidium sativum) on a sterile paper towel in a petri dish.
• Water with 10cm³ of distilled water.
• Repeat for 3 different sets of seeds:
  • Set 1 is placed in a box to prevent light shining on the seeds.
  • Set 2 is placed in a box with light from above only.
  • Set 3 is placed in a box with light from the right hand side only.
• Keep all 3 sets at 25°C.
• After 72 hours, remove 20 of the seedlings from each set and count how many have bent.

Identify two limitations of the student’s method.

For each limitation, explain how it limits the validity of conclusions that can be drawn and suggest an improvement that would improve the validity of conclusions made.

limitation 1: .........................................................

explanation: .........................................................

improvement: .........................................................

limitation 2: .........................................................

explanation: .........................................................

improvement: .........................................................

Multiple sclerosis (MS) is an autoimmune disease that damages the nervous system.

Guillain–Barré syndrome is another autoimmune condition in which neurones are damaged and the rate of nervous impulses is reduced.

MS affects the central nervous system.

Guillain–Barré syndrome affects the peripheral nervous system.

Herbicides work in a number of different ways.

(i)

Some herbicides, known as phenoxy herbicides, mimic the action of the auxin, indoleacetic acid (IAA).

What is the normal action of IAA in plant cells?

[1]

(ii)

The herbicide atrazine works by disabling plastoquinone, one of the proton pumps in photosystem II.

Explain how atrazine would kill a susceptible plant.

[5]