Topic 6 – Plant Structures and their Functions-Controlling Plant Growth

Question 1

What are plant tropisms?

Answer 1

The growth of a plant to in response to a stimulus

Question 2

What is positive tropism?

Answer 2

The growth of a plant towards a stimulus

Question 3

What is negative tropism?

Answer 3

The growth of a plant away from a stimulus

Question 4

What are the two main types of plant tropism?

Answer 4

Phototropism

Gravitropism

Question 5

Define phototropism

Answer 5

A plant’s growth response to light

Question 6

Define gravitropism

Answer 6

A plant’s growth response to gravity

Question 7

What are auxins?

Answer 7

● A group of plant hormones involved in plant tropisms

● Control growth in plant roots and shoot tips

Question 8

Compare the effect of auxins in plant shoots and roots

Answer 8

Auxins stimulate growth in plant shoots

Auxins inhibit growth in plant roots

Question 9

Where are auxins produced?

Answer 9

Root and shoot tips

Question 10

How do auxins move through a plant?

Answer 10

They diffuse through the plant in solution

Question 11

Are plant shoots positively or negatively phototropic?

How does this affect shoot growth?

Answer 11

● Positively phototropic

● Plant shoots grow towards the light

Question 12

Explain why plant shoots are positively phototropic

Answer 12

● Shoot tip exposed to light
● On the shaded side of the shoot, auxin accumulates
● Elongation of cells on the shaded side
● Shoot tip bends towards the light

Question 13

Are plant shoots positively or negatively gravitropic?

How does this affect shoot growth?

Answer 13

● Negatively gravitropic

● Plant shoots grow upwards, away from gravity

Question 14

Explain why plant shoots are negatively gravitropic

Answer 14

● Shoot placed horizontally
● Due to gravity, auxin accumulates on the lower side of the shoot
● Elongation of cells on the lower side
● Shoot bends upwards growing away from gravity

Question 15

Are plant roots positively or negatively phototropic? How does this affect root growth?

Answer 15

● Negatively phototropic

● Plant roots grow away from the light

Question 16

Explain why plant roots are negatively phototropic

Answer 16

● Root exposed to light
● On the shaded side of the root, auxin accumulates
● Inhibition of cell growth on the shaded side
● Root grows away from the light

Question 17

Are plant roots positively or negatively gravitropic? How does this affect root growth?

Answer 17

● Positively gravitropic

● Plant roots grow downwards, towards gravity

Question 18

Explain why plant roots are positively gravitropic

Answer 18

● Root placed horizontally
● Due to gravity, auxin accumulates on the lower side of the root
● Inhibition of cell growth on the lower side
● Root bends downwards growing towards gravity

Question 19

Explain how plant tropisms increase the chance of survival

Answer 19

● They enable plants to respond to their environment
● Shoot growth towards the light maximises light absorption ● Root growth downwards increases the uptake of water
and minerals from the soil and enables anchorage of the plant body to the ground

Question 20

Describe the role of auxins in commercial plant cultivation (higher)

Answer 20

● Rooting powders contain auxins that stimulate the growth of roots in cuttings. Enables rapid plant cloning.
● Used in selective weedkillers that target and alter growth patterns in broad-leaved plants (most weeds), killing them.

Question 21

What are gibberellins? (higher)

Answer 21

Plant hormones that control germination and flowering

Question 22

Describe the role of gibberellins in commercial plant cultivation (higher)

Answer 22

● Initiate germination in seeds at times of the year when they naturally wouldn’t. Ensure all seeds in a batch germinate.
● Trigger flowering in plants under irregular conditions. Also increase fruit size by reducing the number of flowers produced by plants.
● Produce seedless fruits by stimulating the growth of fruit from unpollinated flowers.

Question 23

How do gibberellins trigger germination? (higher)

Answer 23

In the presence of water, gibberellins break seed dormancy, initiating germination.

Question 24

Why is ethene important in commercial plant cultivation? (higher)

Answer 24

● Ethene stimulates enzymes that control fruit ripening
● Enables fruits to be picked while they are unripe and less easily damaged. They can be stimulated to ripen during transportation to shops.