5.1.5 - Plant And Animial Responses (set A - Plant Hormones)

Question 1

Define tropisms?

Answer 1

Directional growth in response to environmental cues - e.g. light - phototropism and gravity - geotropism

Can be positive (growth response towards the stimulus) or negative (away from stimulus)

Question 2

Give 4 important plant hormones?

Answer 2

• auxins
• gibberellin
• ethene
• abscisic acid (ABA)

Question 3

Give the role of auxin in plants?

Answer 3

• controls cell elongation and involved in tropisms
• prevents leaf fall (abscission)
• maintains apical dominance
• stimulates release of ethene (for fruit ripening)

Question 4

Give the role of Gibberellin in plants?

Answer 4

• cause stem elongation
• trigger mobilisation of food stores in a seed at germination
• stimulate pollen tube growth in fertilisation

Question 5

Give the role of ethene in plants?

Answer 5

• causes fruit ripening
• promotes abscission in deciduous trees

Question 6

Give the role of ABA in plants?

Answer 6

• maintains dormancy of seeds and buds
• stimulates cold protective responses (e.g. antifreeze production and stomatal closing)

Question 7

Explain why scientists are still unsure about the details of many plant responses - give 2 reasons?

Answer 7

• plant hormones work at very low concentration - hard to isolated them and measure changes
• multiple interactions between different chemical control systems - hard to isolate role of single chemical in a specific response

Question 8

Outline and explain hormones are involved in seed germination - specifically gibberellins?

Answer 8

• seed absorbs water and grows, embryo activated and produces gibberellins which stimulate production of digestive enzymes (amylase and protease) by switching on genes that code for them
• enzymes break down food stores to produce ATP for building material so it can grow and break through seed coat

evidence suggest ABA also involved and levels of broth hormones determine when seed germinates

Question 9

Provide and outline experimental evidence for the role of plant hormones in seed germination - focus on giberellins?

Answer 9

• mutant variants of seeds bred which lack genes to make gibberellins - these seeds do not germinate unless gibberellins were applied to seed externally
• if gibberellin inhibitor applied seeds do not germinate, if inhibition is removed the seeds germinate

Question 10

Explain how auxins are involved in plant growth - specifically their function regarding cell growth by elongation - where are they produced?

Answer 10

Auxins specifically IAA are made in cells at the tip of roots and shoots and in meristems move via transport tissue and from cell to cell

• stimulate growth of the main apical shot by affecting the plasticity of the cell wall - presence of auxins cause cell wall to stretch easily

Question 11

Explain how auxins affect the plasticity of cell walls and control growth?

Answer 11

• presence of auxin makes cell wall stretch more easily - bind to specific receptor sites in the plants cell membrane causing a fall in pH (about 5 - optimum for enzymes needed to keep walls flexible)

As cell matures auxin is destroyed - pH rises so enzymes become inactive - wall becomes rigid and more fixed (cells no longer expand and grow)

Question 12

Explain the need for plants to respond to the environment?

Answer 12

Plants responses have evolved because they provide the plant with some type of selective advantage - making them better adapted to survive and reproduce in their environment, for example by producing toxic substances to avoid predation or to cope with changing environmental conditions and avoid abiotic stress

• environmental stimuli to which they respond can be abiotic (non-living) or biotic (living)

Question 13

Give examples of plant responses - provide 4?

Answer 13

• tropisms
• responses to touch
• responses to herbivore
• responses to abiotic stresses

Question 14

Explain how apical dominance occurs in plants - reference concentration of auxin and give an example?

Answer 14

High concentrations of auxins suppress the growth of lateral shoots - results in apical dominance

• growth in main shoot stimulated by auxins produced in tip - lateral shoots inhibited by hormone that moves back down the stem so they do not grow well (lateral shoots lower down grow more strongly due to lower auxin conc)

Evidence comes from the fact that if the apical shoot is removed (so auxin-producing cells also removed) lateral shoots are freed from apical dominance and grow faster

Question 15

Explain how auxin concentration impacts root growth?

Answer 15

Low conc of auxin promotes root growth (auxin produced by root tips and also reaches the roots in low conc from growing shoots)

If apical shoot is removed the amount of auxin reaching the roots is reduced - root growth slows and stops (high auxin conc Inhibit root growth)

Question 16

Explain the role of gibberellins regarding elongation of plant stems + give evidence?

Answer 16

Gibberellins affect the length of the internodes (region between leaves on a stem)

• plants that have short stems produce few/no gibberellins
• scientists have bred dwarf varieties of plants where gibberellin synthesis pathway is interrupted - plant stems are much shorter (reduces water and vulnerability of plant to nature)

Question 17

Explain what synergism is?

Answer 17

Interaction of different hormones working together and complementing each other to give a greater response then they would on their own

Opposite affect is antagonism (eg one substance promoting growth and other inhibiting it)

Question 18

outline the reason for leaf loss in deciduous plants as a response to abiotic stress?

Answer 18

seasonal changes have a big impact on photosynthesis (due to effect of light + temp)

• point comes when amount of glucose needed for respiration to maintain the leaves and produce chemicals for chlorophyl is greater then the amount of glucose produced
• loose all their leaves until days lengthen and temps rise

Question 19

outline the process of abscission (leaf loss)?

Answer 19

less light (winter) causes fall in conc of auxin - this causes leaves to produce hormone ethene

• ethene involved in production of enzymes which digest/weaken cell walls in outer layer of abscission zone
• vascular bundles sealed off + fatty material deposited in the cells to form a protective scar (preventing pathogens entering) cells in separation zone retain water and swell placing strain on weakened outer layer - environment finish’s the process (e.g. wind)

Question 20

explain how abscission occurs - with specific focus on the abscission zone?

Answer 20

• ethene initiates gene switching in cells located at the base of the leaf stalk - results in production of enzymes
• enzymes digest/weaken cell wall in outer layer of abscission zone (called separation layer)
• vascular bundles are sealed off + fatty material deposited in the cells on the stem side of separation layer (forms protective layer, prevents pathogens)
• cells within separation zone respond to hormones - start to retain water and swell - places strain on weakened layer

Question 21

outline how plants are able to respond to abiotic factors like cold temperatures?

Answer 21

cytoplasm of cells and sap in vacuoles contains solutes which lower freezing point

• some plants produce substances (e.g. proteins/sugars) act as antifreeze to prevent the cytoplasm from freezing
• plants genes suppressed and activated depending on weather to produce necessary chemicals

Question 22

explain the plant repone stomatal control for responding to abiotic stresses?

Answer 22

ABA largely controls opening and closing of stomata - released by leaf cells when under abiotic stress + by roots which release ABA when soil water levels fall

• ABA transported to leaves, binds with receptors on plasma membrane of the stomatal guard cells - changes ionic conc of guard cells which reduces water potential and thus turgor of cells
• reduced turgor causes the guard cell to close the stomata

Question 23

give 4 physical defences of plants?

Answer 23

• barbs/spikes
• spiny leaves
• thorns
• inedible leaves

Question 24

Outline 4 chemical defences of plants and briefly explain them?

Answer 24

Tannins - part of group of compounds called phenols, toxic to insects and put of animals eating leaves

Alkaloids - group of bitter tasting nitrogenous compounds which impact metabolism of animals

Terpenoids - group of compounds produced by plants, form oils acting as toxins to insects/fungi

Pheromones - chemical made by organisms which affects social behaviour of other members of same species

Question 25

Explain the role and function of tannins and terpenoids as chemical defences?

Answer 25

Tannins - phenols produced by leaves (make up most of dry weight of leaves) bitter taste puts animals off eating leaves (toxic to insects as they bind to enzymes in saliva and inactive them) eg red wind and tea

Terpenoids - form essential oils which act as toxins to insects/fungi - some types can act as insect neurotoxins

Question 26

Explain the role and function of alkaloids as chemicals defenses?

Answer 26

• group of very bitter tasting nitrogenous compounds
• impact metabolism of animal - can poison them
• include caffeine, nicotine and cocaine
• caffeine is toxic to fungi and insects and prevents germination of other plants by spreading through soil (Protects against rivals)

Question 27

Explain the role and function of pheromones as chemicals defenses?

Answer 27

• chemical made by organism which affects the social behaviour of other members of same species
• maple tree releases pheromones when attacked - signals to surrounding trees to produce chemicals like callose to protect them

Question 28

Explain the function of folding in response to touch as a response to herbivory?

Answer 28

• some plants leaves fold down and collapse when touched - frightens off larger herbivores and dislodges smaller insects
• involves potassium ion movement and osmotic water movement

Question 29

Outline phototropism in plants?

Answer 29

Growth of plants in response to light - plants grow towards light (shoots are positively phototropic and roots are negative)

• involves movement of auxin across the shoot or root when exposed to light
• unilluminated side has more auxin and grows by cell elongation

Question 30

Explain the effect of unilateral light on plant growth - phototropism?

Answer 30

• side of shoot exposed to light contains less auxin than side which is not illuminated - auxin moves laterally across shoot
• greater concentration on unilluminated side - stimulates cell elongation and growth on dark side
• results in growth towards light - when unilateral stimulus is removed, transport of auxin stops and shoot grows straight towards light

Question 31

Explain how plants grow in the dark?

Answer 31

• plants grow more rapidly in dark - will grow upwards rapidly in bid to reach light
• evidence suggests gibberellins responsible for extreme elongation of internodes during growth in dark
• upwards growth slows in light to save resources

Question 32

Outline geotropism in plants?

Answer 32

• plants sensitive to gravity - shoots are negatively geotropic (grow away from gravity) and roots are positively geotropic
• ensures roots grow down in soil

Question 33

Explain how ripening can be controlled in plants?

Answer 33

• ethene involved
• climacteric fruits continue to ripen after harvesting, linked to peak of ethene production which trigger increased respiration rates
• fruits harvested when fully formed, cooled, stored and transported (much harder, less likely to damage) and then exposed to ethene gas under controlled conditions - when its time to be sold

Question 34

Explain the benefits of controlling fruit ripening?

Answer 34

• increases time available for them to be sold
• prevents a lot of wastage of fruits during transport - unripe fruits are less likely to damage as they are harder to

Question 35

Explain how root powders work?

Answer 35

• use auxins to affect growth of shoots and roots (more likely for roots to grow)
• cutting of stem dipped in rooting powder increases chance of root forming and successful propagation

Question 36

Explain how hormonal weedkiller’s work?

Answer 36

• use synthetic auxins which are absorbed by the weeds broad-leaves and impact their metabolism
• growth rates increase and become unsustainable - causing death
• narrow-leaved crop plants are not affected, grow normally

Question 37

Outline and explain 2 other uses of plant hormones?

Answer 37

• gibberellins - used to delay ripening and ageing in fruit - improves size and shape of fruits
• cytokinins - used to prevent ageing of ripened fruit

Question 38

Explain how geotropism works in plants - focus on roots?

Answer 38

• Cells in tip of root produce IAA (auxin) - transported along the root.
• IAA initially transported to all sides of the root.
• Gravity influences the movement of IAA from upper side to lower side of
  the root
• greater concentration of IAA builds up on the lower side of the root than on the
  upper side (More auxin in a root means LESS elongation)
• IAA inhibits the elongation of root cells, and there is a greater concentration of
  IAA on the lower side, the cells on this side elongate less than those on the upper
  side
• causes the root to bend downwards towards the force of gravity.

Question 39

Explain how geotropism works in plants - focus on shoots?

Answer 39

• Auxin moves to the lower side.
• The cells of the shoot grow more on the side with most auxin, so it stimulates cells to grow more
  here.
• makes the shoot bend and grow away from the ground.
• beneficial as light levels are likely to be higher further away from the ground

Question 40

Outline went’s experiment on phototropism - using a gelatine block?

Answer 40

• placed the cut tip of a shoot on a gelatine block - allows hormone from tip to diffuse down
• block than placed off centre and in the dark
• side of the shoot that the block was placed on grew more rapidly than other side causing a curve (due to greater concentration of hormones present in the block)

Question 41

Outline Paal’s experiment on phototropism - removing and replacing the tip?

Answer 41

• cut off tip of a shoot and replace it off centre and in dark
• side of shoot where tip was placed, grew more than other side causing a curve
• showed that in light the photographic response was caused by diffusion of hormone through tissue stimulating growth

Question 42

Outline boysen-Jensen experiment on phototropism - using a mica barrier?

Answer 42

• inserted a mica barrier (impermeable to chemicals) halfway through shoot, below tip - first on lit and than shaded side
• when inserted into lit side, photographic response occurred, when inserted into shaded side there was no response
• confirmed that stimulus for growth was a chemical hormone, produced at the tip that traveled on opposite side of stimulus and that stimulus acted by causing growth on shaded side