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 to 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 toward 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 - 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)

Question 11

describe gravitropism in the roots

Answer 11

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

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 bright 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

Question 19

what happened in Darwin’s exp 1: intact?

Answer 19

• shoot bends towards unilateral light
• so shoot is positively phototropic

Question 20

what happened in Darwins exp 2: apex excised

Answer 20

• tip removal prevents response

Question 21

what happened in Darwin’s exp 3: apex shielded

Answer 21

• lightproof cover is placed over tip of shoot
• no response

Question 22

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

Answer 22

• thin, impermeable layer of mica as barrier
• chemical moves down shaded side
• bends towards light

Question 23

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

Answer 23

• movement of chemical down shaded side is prevented by mica
• no response

Question 24

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

Answer 24

• tip removed, gelatin block placed and tip replaced
• gelatin allows chemical messages to pass through
• plant bends towards light