Animal And Plant Responses - Plant Hormones

Question 1

What stimuli do plants respond to?

Answer 1

Abiotic stress - non living

Biotic stress - Herbivory

Tropisms

Question 2

Examples of abiotic stress for plants and e.g how they respond to these?

Answer 2

Freezing
Drought
Increased soil water salinity
Presence of heavy metals

RESPONSE TO DROUGHT: shut stomata/lose leaves - reduce water loss by transpiration
RESPONSES TO LOW TEMPS: produce antifreeze chemical in cells - decrease formation of ice crystals

Question 3

Chemical defences to herbivory (being eaten by herbivores)?

Answer 3

Produce chemicals:

ALKALOIDS (caffeine/nicotine) - bitter/toxic - deters or kills herbivores
PHEROMONES - signal to nearby plants of same species that they are under attack from herbivores - triggers other defences
- signal can attract herbivorous insect’s predator

Question 4

Additional response to herbivory?

Answer 4

Responds to touch:
Touch sensitive leaves FOLD rapidly when touched
- can protect against herbivorous insects/reduce transpiration
Most likely caused by bioelectrical signals

Question 5

Additional response to herbivory?

Answer 5

Responds to touch:
Touch sensitive leaves FOLD rapidly when touched
- can protect against herbivorous insects/reduce transpiration
Most likely caused by bioelectrical signals

Question 6

What are tropisms?

Answer 6

Growth response of a part of a plant in response to a directional stimulus
- can be towards stimulus (+) or away from stimulus (-)

Question 7

Types of tropisms and their stimulus type?

Answer 7

PHOTOTROPISM: response to light - ABIOTIC - maximise light access for max photosynthesis

GEOTROPISM : response to gravity - ABIOTIC
- ensure shoots/roots grown in right directions

HYDROTROPISM: response to moisture - ABIOTIC
- root tips grow to damper areas - increase water access

THIGMOTROPISM : response to touch - ABIOTIC/BIOTIC - good for climbing plants - help get access to light/detect a support and curl around it

CHEMOTROPISM: response to chemicals - ABIOTIC/BIOTIC

Question 8

Darwin’s experiment to investigate phototropism?

Answer 8

Removed tip of coleoptile - stopped phototropic response to unidirectional light source (light from one side) from occurring
- to ensure it was not due to wound in of plant - covered tip of coleoptile with opaque cover/cap to block out light - also stopped phototropic response from occurring (shows tip is responsible for detecting light)

Question 9

Boysen-Jensen’s Experiment to investigate phototropism?

Answer 9

Found that phototropic response restored when he replaced the cut tip back on to coleoptile and a gelatin block in between
- then inserted mica barrier (impermeable to chemicals) halfway through coleoptile on lit side and shaded side :
On lit side : phototropic response occurred
Shaded side : response didn’t occur

Confirmed the stimulus for growth was a hormone (chemcial) produced at tip and travelled down opposite side of stimulus - shaded side - causes growth on shaded side

Question 10

Boysen-Jensen’s Experiment to investigate phototropism?

Answer 10

Found that phototropic response restored when he replaced the cut tip back on to coleoptile and a gelatin block in between
- then inserted mica barrier (impermeable to chemicals) halfway through coleoptile on lit side and shaded side :
On lit side : phototropic response occurred
Shaded side : response didn’t occur

Confirmed the stimulus for growth was a hormone (chemcial) produced at tip and travelled down opposite side of stimulus - shaded side - causes growth on shaded side

Question 11

Paál’s experiment to investigate phototropism?

Answer 11

Paál cut off tip of coleoptile +replaced it off centre in the dark
- the side that the tip was placed on grew more than other side - cause coleoptile to curve

Showed that, in light, phototropic response was caused by hormone diffusing through plant tissue /stimulates growth of tissue

Question 12

Went’s experiment to investigate phototropism ?

Answer 12

Went placed cut tip of coleoptile on gelatin block, allowing the hormones from the tip to diffuse into block
- block was placed on coleoptile, off-centre and in the dark
- the side of the coleoptile that the block was placed on grew more than the other side, causing the coleoptile to curve
The greater the concentration of hormone present in the block, the more the coleoptile curved

Question 13

How does auxin cause cell elongation?

Answer 13

Indole-3-acetic acid (IAA) - an auxin , made in growing tips of roots/shoots (meristems)
- IAA molecules activate proteins in cell wall - EXPANSINS
- they loosen bonds between cellulose microfibrils - cell wall more flexible
- cells can elongate

Question 14

How does phototropism occur?

Answer 14

• affects shoots/top of stems
  In shoots - IAA moves from illuminated side of shoot to shaded side
  higher conc of IAA on shaded side causes greater rate of cell elongation
  Causes shoot to bend towards light

Question 15

What happens when conc of IAA is not uniform on either side of root/shoot?

Answer 15

Uneven growth occurs

Question 16

How does geotropism occur in shoots and roots?

Answer 16

SHOOTS - negative geotropism
- gravity modifies distribution of IAA so accumulates on lower side on shoot
- IAA causes increase in rate of growth —> shoot grows upwards

ROOTS - positive geotropism
- in roots, higher conc of IAA = lower rate of cell elongation
IAA that accumilates on lower side of root inhibits cell elongation - lower side grows at slower rate than upper side of root
Causes root to bend downwards

Question 17

Role of hormones in leaf loss?

Answer 17

Layer of cells - ABSCISSION LAYER - develops at base of leaf stalk
- a layer of parenchyma cells with thin walls - makes them weak/easy to break
ETHENE stimulate breakdown of cell walls in abscission layer - leaf drops off
AUXIN can inhibit leaf loss (produced in young leaves) - less sensitive to ethene , but conc of AUXIN decreases with age - leaf loss occurs

Question 18

Role of hormones in stomatal closure?

Answer 18

Hormone ABSCISIC ACID (ABA) produced during times of water stress - stimulate closing of stomata
Guard cells have ABA receptors on their cell surface membranes
ABA binds to these receptors - INHIBITING PROTON PUMPS , therefore stopping active transport of H+ ions OUT guard cells
ABA also causes Ca2+ ions to move into cytoplasm of guard cells
- calcium ions act as second messengers:
They cause channel proteins to open- allow negatively ions to leave guard cells
This stimulates the opening of further channel proteins that allow (K+) ions to leave guard cells
Ca2+ ions also stimulate the closing of channel proteins that allow (K+) ions to enter the guard cells
- loss of ions increases the water potential of the guard cells
Water leaves the guard cells by osmosis
The guard cells become flaccid, causing the stomata to close

Question 19

Role of hormones in seed germination?

Answer 19

Gibberellins involved in controlling seed germination/ stem elongation
- in good conditions - seed starts to absorb water to begin germination
- stimulates embryo in seed to PRODUCE GIBBERELLINS
- gibberellin molecules diffuse into aleurone layer + stimulate cells there to synthesis AMYLASE
- amylase hydrolyses STARCH molecules in endosperm - produces soluble maltose molecules
- maltose converted to glucose +trasnprted to embryo
- glucose respired by embryo, breaking dormancy and proving energy for growth

Question 20

Why do deciduous plants lose leaves?

Answer 20

In hot/dry conditions - reduce water loss
Winter - absorption of water is difficult due to frozen soils /photosynthesis is limited by low temps and sunlight

Question 21

How does ABA have opposite effect to gibberellins?

Answer 21

Maintains dormancy by inhibiting amylase production

Question 22

How does ABA have opposite effect to gibberellins?

Answer 22

Maintains dormancy by inhibiting amylase production

Question 23

What is apical dominance?

Answer 23

Auxins produced at growing tip at the APEX (TOP) of plant stem causes STEM TO GROW UPWARDS and STOPS lateral buds from growing

Question 24

What happens if growing tip at apex of plant is removed?

Answer 24

Lateral buds grow from top of plant (auxin source removed/no apical dominance)
- lateral shoots that grow from lateral buds curl up towards light - plant continues to grow upwards

Question 25

Experimental evidence for role of gibberellins in stem elongation?

Answer 25

Dwarf plant varieties shown to have very low levels of gibberellins - due to mutation
Under experimental conditions , treating the dwarf varieties with gibberellins - they grow to same height as normal varieties

Question 26

Experimental evidence for role of gibberellin in seed germination?

Answer 26

Seeds of mutant varieties of the Arabidopsis plant that don’t produce gibbeellins can be induced to germinate if gibberellins applied

Seeds of certain lettuce varieties that need light to germinate can be made to germinate in dark if gibberellins are applied

Question 27

How can response to the removal of source of auxins be shown experimentally?

Answer 27

• apical bud of the 1st test plant is removed (decapitated)
• allows the lateral buds to grow
  A second (genetically identical) test plant is decapitated but 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 28

How are plant hormones used commercially?

Answer 28

• selective weed killers
  -rooting powders
  -control ripening

Question 29

How are plant hormones used in selective weed killers?

Answer 29

High conc of auxins cause very rapid growth that plant tissues become distorted +damaged —> pathogens can enter
- use synthetic auxins (100x greater in conc than natural hormones in plants) - effective against weeds (grasses are less sensitive to these killers - don’t die)

Question 30

How are plant hormones used in rooting powders?

Answer 30

Low doses of auxin use to stimulate cuttings to grow new roots
- these auxins sold as rooting powders
Lower end of cutting dipped in powder, then planted —> good environment allows roots to grow

Question 31

How are plant hormones used to control ripening?

Answer 31

Ethene stimulates fruit to ripen
- delicate/soft fruits harvested when unripe , transported and ripened artificially using ethene during transport

Question 32

Extra use of auxins and gibberellins?

Answer 32

• used to make unpollinated flowers develop fruit (normally only occur when flower is pollinated/fertilised)
• used to produce seedless fruit
  Known as PARTHENOCARPIC FRUITS - fruits formed without fertilisation

Auxin used to stop trees dropping fruit before its harvested (cant be used once dropped)