Plant Responses

Question 1

Stimulus

Answer 1

Anything that causes a reaction in an organism or in any of its parts

Question 2

Response

Answer 2

The activity of a cell or organism as a result of a stimulus

Question 3

Sensitivity

Answer 3

The ability to detect change and to respond

Question 4

External factors affecting growth

Answer 4

~ Light : Provides energy for photosynthesis
~ Day length : Affects fruit and seed formation as well as dormancy
~ Gravity : Causes roots to grow down and shoots to grow up
~ Temperature : Changes the rate of enzyme production

Question 5

Internal factors

Answer 5

~ Plants produce chemicals called growth regulators
~ Plant growth regulators = Growth promoters and Growth inhibitors

Question 6

Plant growth inhibitors

Answer 6

Chemicals that cause a reduction in growth in plants e.g. ethene

Question 7

Ethene

Answer 7

~ Promotes ripening of fruit
~ Promotes the fall of leaves, flowers and fruits

Question 8

Plant growth promoters

Answer 8

Chemicals that causes growth in plants

Question 9

Tropisms

Answer 9

~ A growth response to a stimulus
~ Positive tropism = the growth is towards the stimulus
~ Negative tropism = the growth is away from the stimulus

Question 10

Phototropism

Answer 10

~ Is a change in growth in response to light
~ Positive = stem grows towards the light which increases the amount of photosynthesis
~ Negative = roots grow away from the light

Question 11

Geotropism

Answer 11

~ Is a change in growth in response to gravity
~ Positive = root grows towards gravity
~ Negative = shoots grow away against gravity

Question 12

Thigmotropism

Answer 12

~ Is growth in response to touch
~ e.g. ivy climbs up walls which help support the plant

Question 13

Hydrotropism

Answer 13

~ Is growth in response to water
~ Positive tropism as roots and pollen tubes grow towards water

Question 14

Chemotropism

Answer 14

~ Is growth in response to chemicals
~ Both positive and negative tropism
~ Roots grow towards favourable minerals but against heavy metal (lead & zinc)

Question 15

What are plant growth regulators?

Answer 15

~ Is a chemical that controls the growth of a plant
~ Regulators that promote growth e.g. auxins (IAA), gibberellins
~ Regulators that inhibit growth e.g. auxins (IAA) and ethene

Question 16

Plant growth regulators in detail

Answer 16

~ Active in very small amounts
~ Produced in the meristems; tips of shoots and roots
~ Transported in the xylem and phloem (vascular bundle)
~ Their effects are dependent on concentration i.e. a small amount of growth regulator can have a great effect on growth

Question 17

How are plant growth regulators similar to animal hormones?

Answer 17

~ Chemical nature
~ Prolonged effects
~ Produced in one place and act in another

Question 18

Use of plant regulators (in horticulture)

Answer 18

~ Weed killers
~ Seedless fruit
~ Tissue culturing : micropropagation
~ Fruit ripening by spraying it with ethene

Question 19

Where are auxins (IAA) produced?

Answer 19

Meristematic tissue in the tips of shoots and roots

Question 20

What are the functions of auxins (IAA)?

Answer 20

~ Stimulating stem elongation and root growth
~ Developing fruit
~ Causes phototropism and geotropism

Question 21

What are the effects of auxins (IAA)?

Answer 21

~ Tropisms
~ Apical dominance : Prevents the growth of lateral buds and side branching
~ Fruit formation and root growth

Question 22

Role of auxins (IAA) in phototropism

Answer 22

~ IAA is produced in the meristems
~ If the stem is exposed to light from one side IAA will diffuse down the shaded side
~ The concentration of IAA present in the shaded cells causes them to elongate more than the cells on the bright side of the stem
~ As a result of the uneven elongation, the stem bends towards the light

Question 23

Adverse external environment

Answer 23

Surroundings that are harmful to organisms

Question 24

Plant adaptations for protection - structural

Answer 24

~ Cuticle and stomata closing reduces water loss
~ Thorns or stinging hair protects them from being eaten by animals

Question 25

Plant adaptations for protection - chemical

Answer 25

~ Heat-shock proteins prevent proteins e.g. enzymes from denaturing when the plant is in high temperatures
~ Stress proteins are produced by a plant when it is infected by a micro-organism, they attack the micro-organism and kill them