16.4 Tropisms in plants

Question 1

what are plant tropisms

Answer 1

growth responses to a unidirectional stimulus

Question 2

what is the growth of plants in response to unidirectional light called

Answer 2

phototropism

Question 3

whats the growth of plants in response to gravity called

Answer 3

geotropism

Question 4

what type of phototropism do plant shoots show

Answer 4

positive phototropism

Question 5

what type of phototropism do plant roots show

Answer 5

negative phototropism

Question 6

explain how a plant responds to unidirectional light

Answer 6

• unidirectional light causes auxin to move to the darker (more shaded) side of the shoot
• auxin stimulates elongation of cells in darker side of shoot
• therefore shoot grows towards the light showing positive phototropism

Question 7

explain the results of a phototropism experiment in which the shoot tip is divided in half

Answer 7

shoot without division
- auxin can diffuse away from light towards the darker side so the higher concentration of auxins in on the LHS

shoot with division
- auxins cannot diffuses to the darker side of shoot despite unidirectional light due to division so the auxin concentration remains similar on both sides

Question 8

outline a practical investigation into plant phototropisms

Answer 8

• germinate and grow seedlings in dark, all round and unidirectional light record directions of growth of shoots and roots
• germinate and grow seedlings in unilateral light with different colour filters to see which wavelengths trigger phototropic response

Question 9

what would you expect to see
1. unilateral light
2. unilateral light + shoot tip removed
3. unilateral light and cover on shoot tip
4.unilateral light and thin impermiable barrier of mica on light side of shoot tip
5. unilateral light and mica on shaded side
6. unilateral light and tip removed gelatin inserted the tip replaced

Answer 9

1. auxins are able to diffuse away from light source, cause elongation of cells on dark side & positive phototropsim
   2.no response. no elongation since auxins are produced by tip of shoot which has been removed.
2. no response, since light must be detected by the tip of shoot.
3. positive phototropism. Auxin acts on the dark side of shoot.
   5.no response. Diffusion of auxin down shaded side is prevented.
   6.Gelatin allows chemicals through but not electrical impulses. Shoot grows towards light since auxin is chemical hormone.

Question 10

why may plants grow faster in the dark

Answer 10

they can avoid competition with other seedlings by being the first to grow out of the soil and sprout leaves

Question 11

what type of geotropism to plant shoots show

Answer 11

negative geotropism (they grow away from gravitational pull)

Question 12

what type of geotropism do plant roots show

Answer 12

positive geotropism (they grow along side gravitational pull)

Question 13

outline a practical investigation into plant geotropisms

Answer 13

• plants grown on clinostat, so gravitational pull is applied evenly
  -seeds grown in petri dished and rotated 90* periodically