B9.5 : Tropic Responses

Question 1

Tropisms

Answer 1

1. Tropisms are controlled by chemicals including the plant hormone auxin
2. Tropisms can be split into phototropism and gravitropism
3. Tropisms are a growth/response by part of a plant to a directional stimulus light light or gravity

Question 2

Auxin

Answer 2

1. Auxin cases growth but the chemical 2,4D is similar to auxin and can be used as a weedkiller
2. It selectively builds up in weeds and kills them, leaving grass unharmed, so its used on lawns
3. Tropisms are controlled by auxins
4. produced at the apex of the stem

Question 3

Phototropism

Answer 3

1. Can be negative or positive
2. Positive phototropism are responses that involve growth towards a light source
3. Negative phototropism are responses that involve growth away from a light source
4. These involve growth either towards (positively phototropic) or away from (negatively phototropic) a light source.

Question 4

Gravitropism

Answer 4

1. Can be negative or positive
2. Positive gravitropism are responses that involve growth towards the direction of gravity
3. Negative gravitropism are responses that involve growth away from the direction of gravity
4. Growth either towards (positively gravitropic) or away from (negatively geotropic) the direction of gravity.

Question 5

The Distribution of Auxin and Phototropism

Answer 5

1. One side of the shoot is exposed to more light.
2. Auxin is redistributed within the shoot so that the concentration is greater on the shaded side through diffusion
3. This unequal auxin distribution will result in increased cell elongation (grows longer) on the shaded side.
4. In shoots, auxins promote the elongation of cells.
5. However, in roots, auxins prevent the elongation of cells.

Question 6

Auxins and Gravitropism

Answer 6

In shoots, auxins promote the elongation of cells, whereas in roots, auxins prevent the elongation of cells.

Question 7

Light Investigation

Answer 7

1. The effect of light on seedlings can be demonstrated with mustard or cress seeds
2. Place the seeds in petri dishes with damp cotton wool, and leave them in a warm place to germinate (develop).
3. Randomly divide the petri dishes into three groups and assign each group to one of the following light conditions: Full sunlight in a windowsill, Darkness, Partial light
4. Measure the height of the plants daily for a week.
5. Draw and label biological drawings to show the effects.
6. The independent variable is the lighting conditions while the dependent variable is the growth of the plant.
7. Phototropism can also be demonstrated by placing a seedling in a container that is open only on one side.

Question 8

Advantage of phototropism

Answer 8

1. The advantage of phototropism is that it helps position the leaves in such a way that leaves are exposed to the maximum surface area to light
2. This means there is more photosynthesis taking place