B6-STIMULI

Question 1

DEFINE stimulus

Answer 1

a change in the internal or external environment

Question 2

DEFINE tropism

Answer 2

a directional response, growth of a plant toward or away from a stimulus

Question 3

DEFINE positive response

Answer 3

movement towards stimulus

Question 4

DEFINE negative response

Answer 4

movement away from stimulus

Question 5

DEFINE phototropism

Answer 5

a growth response to light

Question 6

DEFINE gravitropism

Answer 6

a growth response to gravity

Question 7

DEFINE hydrotropic

Answer 7

a growth response to water

Question 8

Give an example of taxis.

Answer 8

• Single celled algae moving towards light (positive phototaxis).
• Earthworms moving away from light (negative phototaxis).
• Both increases their chance of survival.

Answer 9

Kinesis is a form of response where an organism changes the speed at which it moves and the rate at which it changes direction.
- It is important when a stimulus is less directional such as humidity and temperature.

Question 10

Why is kinesis carried out? Give an example of kinesis.

Answer 10

• It is carried out in order to increase the chance that the organism will enter more favourable conditions more rapidly.
• A woodlouse will speed up and change direction more rapidly in dry conditions to increase its chance of entering a damp region.

Question 11

What is tropism?

Answer 11

Tropism is the growth of part of a plant in response to a directional stimulus.

Question 12

What tropisms do plant shoots show?

Answer 12

• Plant shoots grow towards the light (positive phototropism).
• They grow away from gravity (negative gravitropism).
• This allows the leaves to be in an optimum position for capturing light.

Question 13

What tropisms do roots show?

Answer 13

• Plant roots grow away from light (negative phototropism).
• They grow towards gravity (positive gravitropism).
• This increases the probability roots will grow into the soil.
• Plant roots also grow towards water (positive hydrotropism).

Question 14

Difference between plant SHOOTS and ROOTS

Answer 14

SHOOTS=towards light
ROOTS= away light

Question 15

What are plant growth factors?

Answer 15

• They are hormones that exert their influence by affecting growth and are made by cells
• Example: Indoleacetic acid (IAA) - Auxin.

Question 16

how IAA works

Answer 16

causes active transport of H+ into cell walls of plants, this lowers the pH and disrupts the hydrogen bonding between microfibrils, makes cell wall more elastic and so can elongate

Question 17

In flowering plant shoots, where is IAA produced and where is it transported?

Answer 17

• Cells in the tip produce IAA, which is transported down the shoot.
• Initially transported throughout the shoot evenly.

Question 18

In terms of IAA, what happens when asymmetric illumination is detected by the plant?

Answer 18

• Asymmetric illumination is detected causing IAA to move to the darker side of the plant.
• IAA promotes cell elongation on the darker side.
• The darker side elongates faster than the lighter causing the shoot tip to bend towards the light.

Question 19

In plant roots, where is IAA produced and where is it transported?

Answer 19

• Cells in the tip of the root produce IAA, which is then transported along the root.
• It is initially transported evenly.

Question 20

How does gravity affect IAA in the roots and how does this influence the growth of the roots?

Answer 20

• Gravity influences the movement of IAA from the upper side to the lower side of the root.
• IAA inhibits the elongation of root cells so the lower side elongates less than the upper side.
• This causes the root to bend downwards as there is a greater elongation of cells on the upper side.