plant hormones

Question 1

what is the advantage of apical dominance to plants

Answer 1

• prevents side shoots from growing which saves energy, and prevents side shoots from the same plant competing with the shoot tip and lower leaves for light.

This allows plants to grow tall very fast

Question 1

what is the role of the plant hormone auxin

Answer 1

stimulates growth of the apical bud, inhibits growth of side shoots from lateral buds, therefore promotes apical dominance

Question 2

how does the concentration of auxin differ down the plant

Answer 2

auxin becomes less concentrated further away from the apical bud, therefore side shoots grow at the bottom

Question 3

what is the role of giberellins (2)

Answer 3

stimulates the stems of plants to grow via stem elongation in the internodal regions.

stimulates seed germination and flowering.

Question 4

outline how giberellin stimulates seed germination

Answer 4

seed germination is activated by water, radical and pineal cells produce giberellin which activates genes to produce amylose which breaks down starch in the endosperm to glucose, which is used for respiration

Question 5

explain the 2 terms that can describe how auxins and giberellins work together

Answer 5

they are synergistic - work together to have a large effect, larger than they would have working independantly eg. help plants grow very tall

they are antagonistic - oppose each others actions eg. giberellins stimulates growth of side shoots, auxin inhibits the growth of side shoots.

Question 6

why is leaf loss in deciduous plants an advantage

Answer 6

losing leaves in winter helps conserve water which may be difficult to absorb from the soil.

reduces energy needed to maintain leaves in the winter, fallen leaves insulate roots, rotten leaves provide minerals in the soil

Question 7

outline and explain the 2 hormones involved in leaf loss

Answer 7

leaf loss is triggered by shortening day lengths which is detected by receptors called phytochromes.

auxins inhibits leaf loss
ethene stimulates leaf loss, a layer of cells called the abscission layer develops at bottom of leaf stalk. Ethene stimulates the cells in the abscission layer to exapnd and activates genes to produce digestive enzymes which break down the cell wall, causing the leaf to fall off

Question 8

what plant hormone is involved in stomatal closure

Answer 8

abscisic acid (ABA)

Question 9

outline how the hormone ABA triggers stomatal closure

Answer 9

• ABA binds to receptors on guard cell membrane
• ion channels open, Ca 2+ ions enter the cytosol from the vacuhole
• increased conc. of Ca2+ ions causes more ion channels to open, which allows ions to leave the guard cell.
• water potential increases
• water leaves the guard cell via osmosis.
• cell becomes flaccid and stomata closes

Question 10

give an example of how a plant may respond to abiotic stress

Answer 10

carrots produce antifreeze proteins, these bind to ice crystals and lower the temp that water freezes at, preventing more icecrystals from forming.

Question 11

what are alkaloids, give an example

Answer 11

chemicals with a bitter taste, noxious smell or poisonous characteristics that deter or kill herbivors.

Tobacco plants produce the alkaloid nicotine, which is highly poisonous to insects.

Question 12

what are tannins

Answer 12

plants produce tannins in response to herbivory, they have a bitter taste, and in some herbivors can bind to proteins in the gut making them hard to digest.

Question 13

what are pheremones

Answer 13

signalling chemicals - plants can release alarm pheremones in the air in response to herbivore grazing, this causes nearby plants to start maming chemical defences.

Question 14

give an example of how pheremones deter herbivores

Answer 14

corn plants are eaten by catepillars, they release pheremones which causes parasitic wasps to kill the catepillars

Question 15

what is something some plants can physically do to deter herbivores

Answer 15

some plants can fold up in response to touch for example, the mimosa pudica, this helps to knock of small insects.

Question 16

how do plants respond to external stimuli

Answer 16

by regulating growth

Question 17

what is. the definition of a tropism

Answer 17

response of a plant to a directional stimulus.

Question 18

what is phototropism

Answer 18

growth of a plant in response to light

• shoots are positvely phototropic - grow towards
• roots are negatively phototropic - grow away

Question 19

what is geotropism

Answer 19

growth in response to gravity

• shoots are negatively geotrophic - grow upwards
• roots are positively geotrophic - grow downwards

Question 20

apart from geotropism and phototropism, what are other types of tropisms

Answer 20

hydrotropism- growth in response to water
thermotropism - temperature
thigmotropism - growth in response to touch

Question 21

outline and explain how auxin causes growth of shoots and what its effect is on roots.

Answer 21

auxin is produced in the tips of shoots (meristems) in response to light, and diffuses backwards into the zone of elongation.

Auxin binds to receptors on plant cell, pH inside the cell lowers as H+ ions are actively transported into the cell, creating optimum conditions for expansion enzymes.

Expansins break bonds in the cell wall, increasing the flexibility and allowing the vacuhole to absorb more water.

central vacuhole forms, auxins are destroyed and cell wall becomes rigid. no more elongation is possible.

high concentrations of auxin inhibits growth of roots.

Question 22

what is Indoleacetic acid, how is it transported around the plant

Answer 22

IAA is a type of auxin.

moves via the phloem for long distances and diffusion + active transport for short.

this results in uneven distribution of IAA and therefore uneven growth of plant.

Question 23

outline how IAA responds to directional light

Answer 23

IAA diffuses to the shaded side of the shoot, causing uneven growth and the shoot to bend towards the light.

IAA diffuses to the underside/more shaded side of the root, growth here is inhibited to root bends away from the light.

Question 24

outline how IAA controls geotropism

Answer 24

IAA also regulates geotropism by accumulating on the underside of shoots and roots. In shoots, IAA causes cell elongation on the underside of the shoot, causing it to bend upwards, away from the force of gravity. In roots, IAA inhibits cell growth, causing roots to grow downwards, towards the force of gravity

Question 25

what are cytokinins

Answer 25

stimulate cell division and cell differentiation.

Question 26

outline how IAA actually causes cell elongation and growth

Answer 26

It works by entering the nucleus of plant cells and binds to the promoter regions of DNA. It then acts as a transcription factor, activating or inhibiting the transcription of genes which code for proteins involved in cell elongation and growth.

Question 27

describe and explain the commercial use of ethene

Answer 27

stimulates enzymes that breaks down cell walls and converts starch into sugar. This makes fruit soft, ripe and ready to eat.

Question 28

describe and explain the different commercial uses of the plant hormone auxins.

Answer 28

used in selective weedkillers - make them produce long stems, this means the weed grows too qucikly so can’t get enough water or nutrients causing them to die.

• used as a rooting hormone to make a cutting grow roots. Means clones can be grown quickly.

Question 29

outline the commercial use of giberellin

Answer 29

produces seedless fruit, can be used to make unpollinated flowers develop fruit.

can delay ripening and aging of fruit.

Question 30

what is the experimental evidence for the role of giberellin in seed germination

Answer 30

Seeds of mutant varieties of the Arabidopsis plant that do not produce gibberellins can be induced to germinate if gibberellins are applied

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