what are plant hormones
chemicals produced by one region of the plant, transported to another region and have an effect on that region
give 4 examples of plant hormones
what is the purpose of the endosperm in a seed
what is the purpose of the seed coat
what are the roles of gibberellins
elongation of plant stems (the longer the stem, the more gibberellins)
give a step-by-step process of seed germination
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
what evidence do they have for the role of gibberellins in seed germination
- 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
why do trees of temperate climates lose their leaves in the winter
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.
give the process of leaf abscission (leaf fall)
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.
describe the relationship between ABA concentration and stomatal resistance during a drought
When ABA concentration increases because of a lack of water, stomatal resistance increases so more stomata close.
explain the importance of more stomata closing as water potential decreases
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.
name some physical defences plants have to prevent herbivores eating them
name three chemical defences plants could have to stop herbivores eating them
what are the main features of them
toxic or taste unpleasant
how do tanins work
- 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)
how do plants use alkaloids to deter herbivores
- bitter tasting, nitrogenous compounds
- act as drugs affecting the animal’s metabolism, sometimes poisoning them
- (eg. caffeine, nicotine, morphine and cocaine)
what effects do terpenoids have on herbivores
act as toxins to insects and fungi
can interfere with an insect’s nervous system
some are insect repellents eg. citronella
chemicals made by an organism which affect the social behaviour of other members of the same species
define VOCs (volatile organic compounds)
chemicals produced by plants which communicate with other organisms
why is it important for pheromones and VOCs to be volatile
means they are easily evaporated at normal temperatures so can quickly travel through the air to the other organisms
some plants fold immediately after being touched
why is this beneficial
frightens off large herbivores
dislodges small insects which have landed on the leaves
what are auxins
where are they made
growth stimulants produced in plants
made in cells at tip of roots and shoots and meristems
how does auxin cause growth
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
what happens to auxins and plant growth as a plant matures
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
what happens when there is high concentrations of auxin in the plant
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
what is the evidence for apical dominance (high concentrations of auxin suppress growth of lateral shoots)
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
what happens when there is low concentrations of auxin in the plant
what is the evidence
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
what is meant by a tropism in a plant
the direction of the response is related to the direction from which the stimulus comes
what is phototropism
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
Shoots are said to be ________ phototropic.
Roots are said to be ________ phototropic.
Shoots are said to be positively phototropic.
Roots are said to be negatively phototropic.
how does the auxin actually bend the shoot towards the light
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.
what is geotropism (gravitropism)
plants are sensitive to gravity
gravitational stimulus - acts downwards
shoots are negatively geotropic
roots are positively geotropic
what are some commercial uses of the plant hormone ethene
Ripening of fruit eg. bananas, mangoes, tomatoes
Promotes plants to drop fruits
what are some commercial uses of the plant hormone auxin
Grows new plants from cuttings
Weedkillers - they absorb the hormone and the metabolism is affected so they unsustainably grow and die
Produce seedless fruit
what are some commercial uses of the plant hormone cytokinins
Prevents ageing of ripened fruits
Controls tissue development in micropropagation
what are some commercial uses of the plant hormone gibberellins
delay ripening and ageing in fruit
improve size and shape of fruits
beer brewing - speeds up the malting process
what hormones are responsible for these processes:
1) seed germination
2) leaf abscission
3) stomatal closure
4) apical dominance
1) seed germination - gibberellins
2) leaf abscission - ethene
3) stomatal closure - abscissic acid (ABA)
4) apical dominance - auxin
give the process of stomatal closure in leaves when there’s a drought
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