EduNinja
[Maximum number: 3]

The scientist Gregor Mendel investigated differences in the length of the stem in the pea plant, Pisum sativum. In 1866, he published the results of his investigation into this trait (characteristic).

Fig. 2.1 shows a diagram of a pea plant.

Fig. 2.1

Fig. 2.1

Mendel observed that the pea plants he grew either had tall stems or dwarf (short) stems. In his investigation, Mendel carried out crosses using pea plants with these two phenotypes.

(a)

The scientists P W Brian and H G Hemming identified that the difference in the length of the stem in pea plants was associated with the presence or absence of gibberellin. They published their findings in 1955.

[ 3 ]
(i)

Gibberellin leads to a response in plant cells by binding to specific receptor molecules.

State the term used to describe a molecule, such as gibberellin, that binds to specific receptor molecules and leads to a response in cells.

[ 1 ]
(ii)

Suggest the response of the cells in the internode region of the stem, as labelled in Fig. 2.1, to the presence of gibberellin and describe how this response affects the trait investigated by Mendel.

[ 2 ]
[Maximum number: 11]

One example of chemical control and co-ordination in plants is in the breakdown of food reserves during the germination of cereal grains, such as wheat.

(a)

Fig. 3.1 is a diagram of a grain of wheat.

The numbered arrows indicate the sequence of events that occurs after the uptake of water by the grain of wheat.

Fig. 3.1

Fig. 3.1

[ 6 ]
(i)

With reference to the three numbered arrows in Fig. 3.1, describe the sequence of events that follows the uptake of water by the grain of wheat.

[ 6 ]
(b)

Growth in plants occurs in areas such as shoot and root tips. Growth occurs by cell division and cell elongation.

Fig. 3.2 shows the effect of the plant growth regulator auxin on mean cell length in the shoot tips of wheat seedlings.

Fig. 3.2

Fig. 3.2

[ 5 ]
(i)

The pH of the cell walls decreased after the application of auxin.

Outline the events that occurred to cause this decrease in pH .

[ 2 ]
(ii)

Explain the increase in the length of the cells following the decrease in pH of the cell walls.

[ 3 ]
[Maximum number: 3]

Oryza is a genus of grass plants that includes the rice, Oryza sativa, a food crop.

(a)

Farmers flood fields of rice because this encourages faster growth and higher yields.

[ 3 ]
(i)

Auxin is a plant growth hormone that affects the growth of rice stems.

Explain how auxin affects the growth of rice stems.

[ 3 ]
[Maximum number: 2]

There are more than 600 plant species in the genus Ipomoea. Many species are grown for their attractive flowers, and some species are used as crop plants.

(a)

Scientists are researching whether abscisic acid can be used in crop treatment to increase yield. Evidence suggests that abscisic acid modifies the effect of auxin on elongation growth in plants.

[ 2 ]
(i)

The passage outlines the role of auxin in elongation growth in plants.

Complete the passage by using the most appropriate scientific terms.

The binding of auxin to receptors causes to be pumped into cell walls.

This activates proteins called expansins, which disrupt the links between microfibrils. The cell walls are then able to expand.

[ 2 ]
[Maximum number: 6]

An investigation was carried out into the effects of a plant growth regulator, auxin (IAA), on apical dominance.
- The apical buds of 20 pea plants were cut off and discarded.
- The cut surfaces of 10 pea plants were coated with an inert paste containing auxin.
- The cut surfaces of the other group of 10 pea plants were coated with the inert paste alone.
- A further group of 10 pea plants did not have their apical buds removed and were not coated with paste. This was a control group.

The lengths of the side shoots of plants in each of the three groups were measured at regular time intervals and mean values calculated.

The results are shown in Fig. 7.1.

Fig. 7.1

Fig. 7.1

(a)

Explain why the side shoots increase in length when the terminal buds are removed.

[ 3 ]
(b)

Using data from Fig. 7.1, describe and explain the effect of auxin on the growth of side shoots.

[ 3 ]
[Maximum number: 7]

The passage below summarises the effects of auxin on the growth of a shoot. Complete the passage by using the most appropriate scientific term(s).

Auxin is synthesised in the growing tips of shoots (apical buds). It is transported from here down the shoot by from cell to cell and also to a lesser extent by flow in the

Auxin seems to be involved in determining whether a plant grows upwards or whether it branches sideways. When the apical bud is actively growing, it tends to stop lateral buds from growing. This is called apical The plant grows upwards rather than branching out sideways.

However, if the apical bud is cut off, the lateral buds start to grow. It is thought that removal of the apical bud causes the concentration of auxin in lateral buds to so the buds can now grow by cell and cell .

Answer one question.

(a)

Explain how auxin causes plant cells to elongate.

[ 7 ]
(a)

Explain the role of auxin in cell elongation in plants.

[ 7 ]
(a)

Explain the role of auxin in cell elongation.

[ 8 ]
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