5.5 Plant responses

Question 1

What is the name for a plant response

Answer 1

tropism

Question 2

What are the types of tropism?

Answer 2

Geotropism
Hydrotropism
Phototropism
Thigmotropism
Chemotropism

Question 3

Explain geotropism

Answer 3

Positive - growing down with gravity e.g. Roots
Negative - growing up against gravity e.g. Shoots

Question 4

Explain hydrotropism

Answer 4

Positive - growing towards water
Negative - growing away from water

Question 5

Explain phototropism

Answer 5

Positive - growing towards light e.g. Shoots
Negative - growing away from light e.g. Roots

Question 6

Explain thigmotropism

Answer 6

Positive - growing towards a touch / solid e.g. A creeping plant
Negative - away from solid touch
—> e.g. venus fly trap

Question 7

Explain chemotropism

Answer 7

Positive - growing towards a chemical
Negative - growing away from a chemical

Question 8

How does auxin act in plant cells?

Answer 8

• Auxin lowers the pH of the cell by causing H+ ions to accumulate (more acidic conditions)
• This enables enzymes that produce or stretch cellulose to function
• Once there is no auxin, the cells can no longer elongate
  Auxin results in cell elongation

Question 9

What is a nastic movement?

Answer 9

A response which is not related to the direction of the touch stimulus
—> e.g. Mimosa leaves snapping shut

Question 10

How do plants respond to herbivory?

Answer 10

Produce chemicals:
Alkaloids
Pheromones

Question 11

What are alkaloids?

Answer 11

Nitrogenous compounds: bitter tasting + can be toxic
–> e.g. caffeine + nicotine ( a neurotoxin)

Question 12

What are pheromones?

Answer 12

Chemicals released by one plant which affects the behaviour/physiology of another:
—> oxides of ethene switch on genes to produce chemicals that deter insects

Question 13

What are responses to abiotic stress?

Answer 13

1. Plants can respond to drought by shutting their stomata
2. producing an antifreeze chemical in their cells, that decreases the formation of ice crystals in cells

Question 14

What is the function of auxin

Answer 14

Results in cell elongation in response to phototropism

Question 15

What is the function of ethene

Answer 15

Results in ripening of fruits

Question 16

What is the function of gibberellins?

Answer 16

Results in stem elongation

Question 17

Which hormones contribute to leaf loss?

Answer 17

Ethene + auxin

Question 18

What is the abscission layer

Answer 18

The abscission layer develops at the base of the leaf stalk
—-> a layer of parenchyma cells with thin walls, making them weak and easy to break

Question 19

What causes leaf loss?

Answer 19

1. Lack of daylight causes a drop in cytokinin level
2. This causes a drop in auxin level, which starts production of ethene
3. Ethene switches on genes in the abscission zone
4. This causes cellulase enzymes to be produced which digest cell wall in the Abscission zone
5. Vascular bundles sealed off and a waxy scar forms
6. Inner layer of cells take in water - swell and the leaf breaks off

Question 20

How does auxin contribute to leaf loss?

Answer 20

1. Usually, auxins inhibit leaf loss as they are antagonistic to ABA (which encourages aging) and are produced in young leaves, making the leaf stalks insensitive to ethene
2. The concentration of auxins in leaves decreases as they age until leaf loss can once again occur in response to ethene

Question 21

Why do plants close their stomata?

Answer 21

Times of water stress,
—> Certain environmental conditions can cause water stress, such as very high temperatures or reduced water supplies

Question 22

Which hormones contribute to stomatal closure?

Answer 22

Abscisic acid

Question 23

What steps lead to stomatal closure?

Answer 23

1. Lack of water in roots causes ABA to be produced and transported to the leaves
2. ABA binds to receptors on guard cells causing removal of ions from the guard cells
3. Water potential increase in the guard cell, so water moves out by osmosis
4. this leads to a loss of turgor in the guard cell
5. stomata close as guard cell collapses

Question 24

How does ABA cause stomatal closure?

Answer 24

1. Guard cells have ABA receptors on their cell surface membranes
2. ABA binds with receptors, inhibiting proton pumps and stops active transport of H+ ions out of the guard cells
3. ABA causes Ca2+ ions to move into cytoplasm of guard cells through cell surface membranes
4. Ca2+ ions act as second messengers:
   —> cause channel proteins to open, allows negatively charged ions to leave the guard cells
   —> stimulates opening of channel proteins that allow K+ ions to leave guard cells
   —> Ca2+ ions stimulate closing of channel proteins that allow K+ ions to enter the guard cells
   —> loss of ions increases WP of guard cells
   —> Water leaves the guard cells by osmosis
   The guard cells become flaccid, causing the stomata to close

Question 25

Which hormones are involved in seed germination and ending dormancy ?

Answer 25

gibberellins

Question 26

What does a seed contain? and how is it specialised

Answer 26

When dropped from parent plant it is in a state of dormancy - to survive harsh weather/winter Contains: embryo, endosperm, aleurone layer

Question 27

What are the steps to seed germination?

Answer 27

1. seed starts to absorb water 2. stimulates embryo to produce gibberellins 3. Gibberellin molecules diffuse into aleurone layer, stimulate cells there to synthesise amylase 4. gibberellin stimulates production of amylase 5. amylase hydrolyses starch molecules in endosperm, producing soluble maltose molecules 6. maltose is converted to glucose and transported to the embryo 7. glucose can be respired by embryo, breaking dormancy and providing the embryo with the energy needed for it to grow

Question 28

What is the start of germination determined by?

Answer 28

Levels of abscisic acid acid and gibberellins as abscisic acid has the opposite effect to gibberellins, maintaining dormancy by inhibiting amylase production

Question 29

What is apical dominance?

Answer 29

When the stems of a plant grows upwards and lateral buds are not produced

Question 30

What is the apex?

Answer 30

The tip of the plant - where auxins are produced

Question 31

What happens if the apex is removed?

Answer 31

lateral buds grow from the top of the plant, as the source of auxins has been removed and there is no longer apical dominance

Question 32

What can be inferred if when the apex is cut off, lateral buds start forming?

Answer 32

Auxins from the apical bud prevents lateral buds from growing

Question 33

How can apical dominance be shown experimentally?

Answer 33

1. Apical bud of first test plant is removed ---> This allows the lateral buds to grow 2. A second - genetically identical test plant is decapitated but this time the cut tip is immediately replaced with an agar block containing auxin ---> This restores the inhibition of lateral bud growth and no lateral buds grow

Question 34

How can auxin paste and auxin transport inhibitor show the role of auxins?

Answer 34

Plant 1: Apex cut off and auxin paste IAA applied to cut shoot ---> no lateral bud growth Plant 2: Auxin transport inhibitor applied below apex of shoot ---> Lateral bud growth Conclusion: low auxin levels promotes growth of lateral buds, but normal to high levels of auxins inhibits growth

Question 35

How does auxin level affect abscisic acid?

Answer 35

High levels of auxins keep high levels of ABA When auxin levels drop, ABA levels drop and buds begin to grow

Question 36

What is some experimental evidence that gibberellins stimulates germination?

Answer 36

1. Seeds of mutant varieties of the Arabidopsis plant that do not produce gibberellins can be induced to germinate if gibberellins are applied 2. Seeds certain lettuce varieties that require light to germinate can be made to germinate in the dark if gibberellins are applied

Question 37

What is some experimental evidence that gibberellins stimulates stem elongation

Answer 37

1. Dwarf plant varieties have very low levels of gibberellins ---->This is often due to a mutation in a gene involved in the synthesis of gibberellin ---> Gibberellins can be applied and they grow to the same height as usual plants of the same species

Question 38

How do gibberellins affect fruit production?

Answer 38

Promotes cell elongation - stalks can elongate so can hold larger fruit

Question 39

How can auxins be used as weed killer?

Answer 39

Promotes rapid shoot growth - the stem cannot support this, buckles and dies