tropisms

Question 1

define ‘tropism’

Answer 1

when plants respond to stimuli via growth
- triggered by chemical messages produced in response to the stimulus

Question 2

what 3 factors do plants respond to?

Answer 2

• gravity
• water
• light

Question 3

what are tropisms controlled by?

Answer 3

• growth factors
• eg indoleacetic acid (IAA)

Question 4

describe IAA

Answer 4

• type of auxin
• controls cell elongation in shoots
• inhibits cell growth in roots
• made in tip/root/shoot
• can diffuse to other cells

Question 5

what is a positive tropism?

Answer 5

growth towards a stimulus

Question 6

what is a negative tropism?

Answer 6

growth away from a stimulus

Question 7

describe phototropism in the shoots

Answer 7

• positive
• light is required for light dependent reactions in photosynthesis so plant bends towards light

Question 8

summarise the process of phototropism in the shoots

Answer 8

• shoot tip cells produce IAA
• causes cell elongation
• IAA diffuses to other cells
• if light is unilateral, IAA diffuses to shade

Question 9

describe phototropism in the roots

Answer 9

• negative
• roots don’t photosynthesise so don’t need light
• they anchor the plant in soil
• high conc of IAA inhibits cell elongation so cells elongate more on lighter side and bend away from light

Question 10

describe gravitropism in shoots

Answer 10

• IAA diffuses from upper to lower side
• if plant is vertical, cells elongate and plant grows upwards
• if plant is horizontal, IAA diffuses to lower side, shoot bends upwards (negative tropism)

Question 11

describe gravitropism in the roots

Answer 11

• IAA moves to lower side of root
• upper side elongates
• lower side bends downwards towards gravity
• positive

Question 12

define ‘phototropism’

Answer 12

growth of plants in response to light
- towards or away

Question 13

define ‘geotropism’

Answer 13

plant response to gravity

Question 14

which hormone is most commonly associated with phototropism?

Answer 14

auxins
IAA

Question 15

how will plants grow if they are grown in bright, all round light in normal gravity conditions?

Answer 15

straight upwards
- faster
- taller

Question 16

how will plants grow if they are exposed to light which is brighter on one side than the other?

Answer 16

• shoots grow towards light
• roots grow away

Question 17

shoots are …. phototrophic
roots are ….. phototrophic

Answer 17

positively
negatively

Question 18

describe how IAA control tropisms

Answer 18

• IAA is produced in cells at shoot tip
• light causes IAA to gather in shaded side of shoot
• IAA in shoot causes elongation
• one side of shoot elongates faster, uneven growth causes bending
• at root, high conc of IAA inhibits growth and low conc promotes growth

Question 19

state what happens in each of Darwin’s experiments in discovering the role of IAA

Answer 19

1. intact
2. apex excised
3. apex shielded

Question 20

what happened in Darwin’s expt 1: intact ?

Answer 20

• auxins gather
• grows towards light

Question 21

what happened in Darwin’s expt 2: apex excised?

Answer 21

• no auxins if no tip
• grows upwards

Question 22

what happened in Darwin’s expt 3: apex shielded?

Answer 22

• IAA not produced by auxins as no light
• grows upwards

Question 23

what happened in Darwin’s expt 4: light passes through tip?

Answer 23

• auxins pass down to shaded side
• grows towards light

Question 24

what happened in Darwin’s expt 5: barrier on shaded side?

Answer 24

• movement of auxins prevented
• no response

Question 25

what happened in Boysen-Jensen’s expt: gelatine inserted & tip replaced?

Answer 25

• auxins pass down to shaded side
• gelatine allows auxins to pass through but not electrical messages
• grows towards light