Plant Growth Factors

Question 1

What is tropism?

Answer 1

the response of a plant to a stimuli, involving growth

Question 2

What are tropisms controlled by?

Answer 2

growth factors

Question 3

What does IAA stand for and what is it an example of?

Answer 3

indoleacetic acid, a growth factor which is a type of auxin

Question 4

What does IAA do?

Answer 4

control cell elongation. It stimulates it in the shoots and inhibits it in the roots

Question 5

Where is IAA made?

Answer 5

in the roots and shoots, but it can diffuse to other cells

Question 6

Why is it important that plants grow towards light?

Answer 6

as light is needed for the LDR for photosynthesis

Question 7

What is it called when plants grow TOWARDS light?

Answer 7

positive phototropism

Question 8

What direction does IAA diffuse in?

Answer 8

diffuses towards the shaded side

Question 9

Describe how positive phototropism occur

Answer 9

• unilateral light causes IAA to diffuse towards the shaded side of the plant
• unequal concentrations of IAA
• causes cells on shaded side to elongate more than those in the light
• this causes the shoot to bend towards the light

Question 10

Why does negative phototropism happen in the roots?

Answer 10

no need for light as roots do not photosynthesise, growing away from the light helps the roots anchor the plants and increases surface area for absorption of minerals and water

Question 11

Describe how negative phototropism occurs

Answer 11

• IAA diffuse to the side of the roots that is furthest away from the light source
• uneven distribution and unequal; concentration of IAA
• this inhibits cell elongation in cells on shaded side
• so the roots bend downwards, away from the light

Question 12

Describe how gravitropism will occur in the shoots

Answer 12

• IAA will diffuse downwards
• if vertical, this causes the cells to elongate so the plant grows upwards
• if the shoot is on its side, it will cause the shoot to bend upwards (due to uneven distribution of IAA)

Question 13

What is negative gravitropism?

Answer 13

when plants grow against the force of gravity

Question 14

what is positive gravitropism?

Answer 14

when plants grow towards the direction of gravity

Question 15

Describe positive gravitropism

Answer 15

• in the roots
• IAA diffuses towards direction of gravity (downwards)
• unequal concentration of IAA, higher on the lower side
• this inhibits cell elongation on the lower side, whilst cells on the upper side still elongate faster
• causes the plant to bend downwards

Question 16

What do plants respond to?

Answer 16

• light
• water
• gravity

Question 17

Why is positive gravitropism beneficial to plants?

Answer 17

• roots grow into soil
• anchor the plant
• access water needed for photosynthesis

Question 18

Explain how the movement of IAA in the shoots helps a plant to survive

Answer 18

• moves away from light/to shaded area
• cells on shaded side elongate faster
• shoot bends towards the light
• maximises light intensity for photosynthesis -> more organic molecules made so more likely to survive

Question 19

How is the easy synthesis and absorption of IAA as well as its lethal properties for some plants important in agriculture?

Answer 19

• useful as a herbicide/selective weed killer
• affects broader-leaved plants (not narrow-leaved crops grown in agriculture)

Question 20

What would happen if the tip was cut off of a shoot and there was unilateral light?

Answer 20

• no IAA produced
• no uneven cell elongation
• so no response to light

Question 21

When investigating plant growth, what variables need to be controlled?

Answer 21

• size of shoot tip
• type of agar
• time period
• temperature
• age of shoots/stage in development

Question 22

Why would a lid be kept on the petri dish when investigating the affect of IAA?

Answer 22

• prevents evaporation
• which affects IAA concentration