M5, C16 Plant Responses

Question 1

what are plant hormones

Answer 1

chemicals produced by one region of the plant, transported to another region and have an effect on that region

Question 2

give 4 examples of plant hormones

Answer 2

auxins
gibberellins
abscisic acid
ethene

Question 3

what is the purpose of the endosperm in a seed

what is the purpose of the seed coat

Answer 3

food store

protection

Question 4

what are the roles of gibberellins

Answer 4

seed germination

elongation of plant stems (the longer the stem, the more gibberellins)

Question 5

give a step-by-step process of seed germination

Answer 5

1) seed absorbs water
2) embryo activated and begins to produce gibberellins
3) the production of enzymes is stimulated which breaks down food sources found in the seed
4) embryo plant uses these food sources to produce ATP for building materials so it can grow and break through the seed coat
5) gibberellins switch on genes which code for amylases and proteases (digestive enzymes of germination)
6) ABA is another hormone which interferes with gibberellins to determine when the seed will germinate

Question 6

what evidence do they have for the role of gibberellins in seed germination

Answer 6

• a mutant variety of seed lacking the gene to make gibberellin - it doesn’t germinate
• apply gibberellins externally to mutant variety and they germinate
• add gibberellin inhibitor to seed and it doesn’t germinate
• when you remove the inhibitor it germinates

Question 7

why do trees of temperate climates lose their leaves in the winter

Answer 7

In winter light intensity is less and for shorter periods and temperature is lower which affect photosynthesis.
The amount of glucose needed is larger than the amount of glucose made because there is less photosynthesis happening.
Glucose is needed in the leaves to maintain them (respiration) and to produce chemicals from the chlorophyll. So to conserve the glucose and energy, the leaves fall off.
Also leaves are more likely to be damaged in winter gales anyway.

Question 8

give the process of leaf abscission (leaf fall)

Answer 8

1) In winter, light levels decrease which mean levels of auxin decrease.
2) Leaves produce the produce ethene.
3) Ethene initiates gene switching in the abscission zone (base of leaf stalk).
4) New enzymes are produced which digest and weaken the cell walls in the outer layer of the abscission zone (separation layer).
5) Vascular bundles are sealed off.
6) Fatty material is deposited in cells of the separation layer. This prevents entry of pathogens when leaves fall.
7) Deeper cells retain water and swell putting strain on the outer layer.
8) The strain becomes too much and the leaf separates from the plant.
9) Waterproof scar is left behind.

Question 9

describe the relationship between ABA concentration and stomatal resistance during a drought

Answer 9

When ABA concentration increases because of a lack of water, stomatal resistance increases so more stomata close.

Question 10

explain the importance of more stomata closing as water potential decreases

Answer 10

As water potential decreases, there’s less water for the plant to absorb for processes like photosynthesis or chemical reactions like hydrolysis.
The stomata closing mean water isn’t lost in transpiration and it can be conserved for these reactions.

Question 11

name some physical defences plants have to prevent herbivores eating them

Answer 11

spikes
thorns
hairs
stings

Question 12

name three chemical defences plants could have to stop herbivores eating them
what are the main features of them

Answer 12

alkaloids
tanins
terpenoids

toxic or taste unpleasant

Question 13

how do tanins work

Answer 13

• produced by many plants
• very bitter taste which puts animals off
• toxic to insects as they inactivate digestive enzymes in their saliva

(eg. tea and red wine)

Question 14

how do plants use alkaloids to deter herbivores

Answer 14

• bitter tasting, nitrogenous compounds
• act as drugs affecting the animal’s metabolism, sometimes poisoning them
• (eg. caffeine, nicotine, morphine and cocaine)

Question 15

what effects do terpenoids have on herbivores

Answer 15

act as toxins to insects and fungi
can interfere with an insect’s nervous system

some are insect repellents eg. citronella

Question 16

define pheromones

Answer 16

chemicals made by an organism which affect the social behaviour of other members of the same species

Question 17

define VOCs (volatile organic compounds)

Answer 17

chemicals produced by plants which communicate with other organisms

Question 18

why is it important for pheromones and VOCs to be volatile

Answer 18

means they are easily evaporated at normal temperatures so can quickly travel through the air to the other organisms

Question 19

some plants fold immediately after being touched

why is this beneficial

Answer 19

frightens off large herbivores

dislodges small insects which have landed on the leaves

Question 20

what are auxins

where are they made

Answer 20

growth stimulants produced in plants

made in cells at tip of roots and shoots and meristems

Question 21

how does auxin cause growth

Answer 21

stimulates growth of the main, apical shoot

affect the plasticity of cell wall - makes it stretch more easily

it binds to receptors on the cell-surface membrane causing the pH to fall to an optimum for the enzymes

Question 22

what happens to auxins and plant growth as a plant matures

Answer 22

as cells mature, auxin is destroyed
as hormone levels fall, pH rises so enzymes become inactive
cell walls become rigid and more fixed in shape so can no longer expand and grow

Question 23

what happens when there is high concentrations of auxin in the plant

Answer 23

Apical dominance - suppress growth of lateral shoots.

• Growth in the main shoot is stimulated by the auxin so it grows quickly
• Lateral shoots are inhibited by the hormone that moves back down the stem so they don’t grow very well
• Further down the stem, auxin concentration is lower so lateral shoots grow more strongly

Question 24

what is the evidence for apical dominance (high concentrations of auxin suppress growth of lateral shoots)

Answer 24

If apical shoot is removed, auxin-producing cells are removed so no auxin is made
Lateral shoots grow faster.
If the auxin is applied artificially, apical dominance is reasserted, lateral shoot growth is suppressed

Question 25

what happens when there is low concentrations of auxin in the plant

what is the evidence

Answer 25

Promote root growth
Up to a given concentration, more auxin reaches the roots, more they grow.

If the apical shoot is removed, the amount of auxin reaching the roots is greatly reduced and root growth slows and stops.
Replacing the auxin artificially at the cut apical shoot restores growth of roots. High auxin concentrations inhibit root growth

Question 26

what is meant by a tropism in a plant

Answer 26

the direction of the response is related to the direction from which the stimulus comes

Question 27

what is phototropism

Answer 27

result of the movement of auxins across the shoot or root when it’s exposed to light that is stronger on one side than the other

Question 28

Shoots are said to be ________ phototropic.

Roots are said to be ________ phototropic.

Answer 28

Shoots are said to be positively phototropic.

Roots are said to be negatively phototropic.

Question 29

how does the auxin actually bend the shoot towards the light

Answer 29

Light causes auxin to move laterally across the shoot, so there is a greater concentration on the unilluminated side.
Stimulates cell elongation and growth on the dark side - growth towards light.
Stimulus is removed when the shoot is growing directly towards the light.

Question 30

what is geotropism (gravitropism)

Answer 30

plants are sensitive to gravity
gravitational stimulus - acts downwards

shoots are negatively geotropic
roots are positively geotropic

Question 31

what are some commercial uses of the plant hormone ethene

Answer 31

Ripening of fruit eg. bananas, mangoes, tomatoes

Promotes plants to drop fruits

Question 32

what are some commercial uses of the plant hormone auxin

Answer 32

Grows new plants from cuttings

Weedkillers - they absorb the hormone and the metabolism is affected so they unsustainably grow and die

Produce seedless fruit

Question 33

what are some commercial uses of the plant hormone cytokinins

Answer 33

Prevents ageing of ripened fruits

Controls tissue development in micropropagation

Question 34

what are some commercial uses of the plant hormone gibberellins

Answer 34

delay ripening and ageing in fruit

improve size and shape of fruits

beer brewing - speeds up the malting process

Question 35

what hormones are responsible for these processes:

1) seed germination
2) leaf abscission
3) stomatal closure
4) apical dominance

Answer 35

1) seed germination - gibberellins
2) leaf abscission - ethene
3) stomatal closure - abscissic acid (ABA)
4) apical dominance - auxin

Question 36

give the process of stomatal closure in leaves when there’s a drought

Answer 36

1) water availability for the plant decreases
2) plant increases the production of abscissic acid (ABA)
3) ABA binds to receptors on the plasma membrane of the guard cell
4) causes calcium ion channels to open and calcium ions diffuse from the vacuole to the cytoplasm
5) potassium ion channels open and potassium ions diffuse from the cytoplasm out of the guard cell
6) this increases the water potential in the guard cell
7) water diffuses out of the cell and the cell becomes flaccid
8) this closes the stomata
9) the rate of transpiration is reduced, water in the plant is conserved