5.1.5 Plant

Question 1

What are tropisms?

Answer 1

Directional responses of plants to environmental cues

Question 2

What are the roles of auxins in plants?

Answer 2

Control cell elongation, prevent abscission, maintain apical dominance, involved in tropisms, stimulate insulin release, involved in fruit ripening

Question 3

What the roles of gibberellins in plants?

Answer 3

Cause stem elongation, trigger mobilisation of food stores at germination, stimulate pollen tube growth in fertilisation

Question 4

What are the roles of ethene in plants?

Answer 4

Causes fruit ripening, promotes abscission in deciduous trees

Question 5

What are the roles of ABA in plants?

Answer 5

Maintains dormancy of seeds and buds, stimulates cold protective responses, stimulates stomatal closing

Question 6

Describe the process of germination

Answer 6

The seed absorbs water, activating the embryo, causing production of gibberellins. They stimulate the production of enzymes that break down food stores found in the seed. The embryo plant uses these food stores to produce ATP so it can break out of the seed coat. ABA acts as an antagonist to gibberellins

Question 7

Describe the process of stomatal closing

Answer 7

ABA binds to receptors at surface of plasma membrane of guard cells. Receptors cause transfer of Ca+2 from the vacuole of cytosol, and a rise in the pH of cytosol. This causes the loss of K+, NO3-, and Cl-. The loss of solutes reduces osmotic pressure and so turgor, resulting in stomatal closure

Question 8

Describe the process of abscission

Answer 8

Falling light levels result in auxin concentration decreasing, so ethene is produced. Ethene initiated gene switching in the abscission zone causing in enzyme production. These weaken the cell wall in the outer layer of the abscission zone, known as the separation layer. The vascular bundles into the leaf are sealed off. The cells in the separation zone retain water and swell, putting more strain, eventually causing the leaf to fall. A protective scar forms when the leaf falls, preventing entry of pathogens.

Question 9

How was the impact of auxin on phototropism explored?

Answer 9

First, nothing was done to the shoot, and it grew toward the light. Then, the tip was removed, but there was no response. Then, a lightproof cover was placed, but no response. Then, mica was put on the side of the light, but it grew, but when put on the other side it did not grow. Putting a gelatin block (blocks electrical messages) still allowed it to grow as normal. Therefore, it was conclude that a hormone on the shaded side in the tip of the plant caused the phototropism (auxin).

Question 10

Describe chemical defences of plants to herbivory

Answer 10

Tannins, alkaloids, terpenoids, pheromones

Question 11

Describe the experimental evidence that supports the role of gibberellins in the germination of seeds

Answer 11

• Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins. They do not germinate until gibberellins is applied externally.
• If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate, but if it is removed, they germinate

Question 12

Describe the role of gibberellins in stem elongation

Answer 12

Gibberellins affect the length of the internodes, which are the regions between the leaves on a stem.

Question 13

Describe the experimental evidence that supports the role of gibberellins in stem elongation

Answer 13

Scientists have bred many dwarf varieties of plants where the gibberellin synthesis pathway is interrupted. Without gibberellins the plant stems are much shorter

Question 14

Give some commercial uses of auxin

Answer 14

Seedless fruit production, root growth stimulation (for micropropagation), weedkillers,

Question 15

Give commercial uses of ethene

Answer 15

Ripening of climacteric fruit (fruit that ripen after being picked) and promotion of fruit dropping in plants such as cotton, walnuts, and cherries.

Question 16

Describe a commercial use of cytokinins

Answer 16

Prevent ageing of ripened fruit and products such as lettuces, and in micropropagation to control tissue development

Question 17

Describe commercial uses of gibberellins

Answer 17

Delay ripening and ageing in fruit, improve the size and shape of fruits, and in beer brewing to speed up the malting process

Question 18

Describe the process by which plants prevent freezing

Answer 18

The cytoplasm of the plant cells and the sap in the vacuoles contain solutes which lower the freezing point. Some plants produce sugars, polysaccharides, amino acids and proteins which act as antifreeze to prevent the cytoplasm from freezing.