plant responses

give some limitations of plant responses

1. plants are rooted to the spot so they are not mobile

2. plants do not have rapidly responding nervous systems

how do plants respond to their environment

by producing hormones

how do hormones help plants carrying out a response

they are transported throughout the plant and affect different parts of the plant

what are gibberellins

plant growth regulators

what do gibberellins do

stimulate seed germination and regulate plant height

via what do gibberellins regulate plant height

stem growth

give an example of how gibberellins cause elongation of plant stems

they increase the length of the internodes, which are the regions between the leaves on a stem

what is a feature of plants that are unable to synthesise many gibberellins

shorter stems

why may plants having shorter stems be beneficial

it makes them less vulnerable to damage by weather and harvesting

what do seeds in their mature state exhibit

a condition known as dormancy

what is dormancy

a period of inactivity that allows the seed to withstand various adverse environmental conditions

what is the role of gibberellins in dormancy

they end the dormant state by initiating the germination process

what is the embryo of a seed

the developing plant

what is the endosperm of the seed

an energy store that contains starch and surrounds the embryo

describe the steps of germination

1. water enters the seed

2. this absorption of water causes the embryo to synthesise gibberellins

3. in response, amylase and maltase enzymes are synthesised

4. the amylase enzymes hydrolyse the starch stored in the endosperm into maltose

5. this can then be broken down into glucose, which provides the embryo with sugars that are essential for energy and growth

what does the absorption of water do during germination

it causes the embryo to synthesise gibberellins

what do the amylase enzymes hydrolyse the starch stored in the endosperm into

maltose

what can maltose be broken down into

glucose

what does glucose provide the embryo with

sugars that are essential for energy and growth

through the mechanism of germination, what do gibberellins do

they activate the mobilisation of stored starch reserves, starting germination

for seeds that cannot produce gibberellins, how else can their seeds be germinated

gibberellins can be applied to the seeds externally

(this also provides evidence for the role of gibberellins in germination)

what are plant growth factors/plant hormones/plant growth regulators

they are chemicals that influence plant development mainly by modifying cell elongation and division

what does it mean for plant hormones to be synergistic

when different hormones work together, giving a greater response than they would on their own

what are antagonistic plant hormones

when different hormones have opposing effects, like one promoting growth and one inhibiting it.

the balance between them determines the response of the plant

what plant hormone is a crucial plant growth factor

auxins, particularly indoleacetic acid (IAA)

where are auxins like IAA synthesised

in the growing tips (meristems) of shoots and roots, where the cells are dividing

what aspects of plant growth do auxins regulate

• cell expansion and differentiation

• suppression of lateral bud growth (apical dominance)

• directional growth responses (tropisms)

what is the effect of high auxin concentration on plant tissue

inhibits root growth

promotes shoot growth

what is the effect of low auxin concentration on plant tissue

promotes root growth

inhibits shoot growth

list the steps of IAA in cell elongation

1. auxins are synthesised by the meristem cells in shoot tips

2. auxins diffuse down the shoot away from the tip

3. auxins bind to receptor sites on cell-surface membranes

4. a low pH develops in cell walls

5. the cells absorb water by osmosis, forming vacuoles and increasing the internal pressure, causing the cell walls to expand

6. cells elongate and the plant grows

what causes a lower pH to develop around the plant cell walls

auxins binding to receptor sites on cell-surface membranes

how are vacuoles formed

by cells absorbing water by osmosis

what is apical dominance

when IAA stimulates the growth of the main, apical shoot and suppresses the growth of the lateral shoots

what does apical dominance suppress the growth of

lateral shoots

where is the source of IAA

the apical bud

what would happen if the apical bud is removed

• the source of IAA goes

• stops the inhibition of lateral bud growth

• plant gets bushier

what are abiotic stresses

non-living factors in the environment that can negatively affect an organisms survival

give some examples of abiotic stresses

• changes in day length

• excessive cold and heat

• lack of water or excess water

• high winds

• changes in salinity

what do plants usually have to overcome abiotic stresses

adaptations

what is photoperiodism

the sensitivity of plants to the levels of light in their environment

what are deciduous tress

plants that lose their leaves in autumn

how does photoperiodism align the plant with the changing seasons

1. in summer- daylight hours are at a maximum and deciduous trees are in full leaf, maximising photosynthesis and growth

2. in autumn- lengthening of the dark period triggers leaf fall in deciduous trees

3. in winter- daylight hours are at a minimum and deciduous trees maintain a period of dormancy

4. in spring- days lengthen and temperatures rise, curing deciduous trees to come out of dormancy

what is phytochrome

a pigment used to sense the duration of light and darkness

what is the name of the pigment that is light-sensitive

phytochrome

what are the two forms of phytochrome

• Pr and Pfr

• they absorb different types of light

what does the ratio of Pr to Pfr change according to

light level

how does phytochrome affect many plant responses (including photoperiodism)

• it breaks dormancy in buds

• it initiates tuber formation

• it times the flowering phase

define abscission

when plants lose their leaves

list the things that abscission is triggered by

• lengthening of the dark period during autumn, which leads to reduced photosynthesis rates

• the energy demands for respiration outweighing glucose availability

• the need for dormancy to avoid winter-related damage

list the stages of abscission

1. a decrease in light prompts a decease in auxin levels

2. the hormone ethene is released by the leaves

3. ethene helps switch on genes in cells in the abscission zone at the base of the leaf stalk

4. enzymes digest and weaken the cell walls in the separation layer

5. vascular tissues are sealed off, blocking transport to the leaf

6. fatty material is deposited in the cells in the inner, protective layer of the abscission zone

7. cells within the separation layer swell with water, straining the weakened walls

8. environmental factors such as winds separate the leaf from the plant

9. the protective layer that remains forms a scar to prevent the entry of pathogens

flip for a simple picture of abscission

what change to light prompts a decrease in auxin levels

a decrease in light

what does ethene help to do during leaf abscission

it switches on genes in the cells in the abscission zone

what do enzymes do in the separation layer

they digest and weaken the cell walls

how is transport to the leaf blocked off

vascular tissues are sealed off

what happens to cells within the separation layer

they swell with water

what is the purpose of the protective layer that remains

it prevents the entry of pathogens

give some examples of plant responses to freezing temperatures

• synthesising compounds like sugars, amino acids, and proteins that act as antifreeze and prevent the cytoplasm from freezing

• modifying gene expression to strengthen cold resistance

• adjusting sap solute concentrations to lower the freezing point

what hormone to plants use as a signal for conserving water during periods of heat and drought

abscisic acid (ABA)

give the mechanism of ABA in stomatal regulation

1. low water availability triggers the production of ABA

2. ABA moves from the roots to the leaves

3. ABA binds to receptors on guard cells

4. ABA activates changes in the ionic concentration of guard cells, reducing the water potential and turgor of the cells

5. this results in stomatal closure, minimising water loss

in what direction does ABA move

from the roots to the leaves

what does ABA activate changes in

it activates changes in the ionic concentration of guard cells

what feature allowed roots to induce an early protective response

they can pre-emptively synthesise and send ABA to the leaves

how does opening stomata allow plants to cool down

water evaporating from the leaves via transpiration removes some heat

why do plants choose to close their stomata when temepratures are too high, even when water loss removes heat

higher temps result in more evaporation and water loss

define herbivory

the process by which herbivores eat plants

give some examples of physical defences against herbivores

• thorns, barbs, or spikes

• tough, fibrous tissue

• stinging hairs

• densely hairy leaves

how do thorns, barbs, or spikes work

these sharp structures can cause physical harm to herbivores, deterring them from eating the plant

how to tough, fibrous tissues work

they make the plant hard to chew and digest

how to stinging hairs work

they inject irritating substances into herbivores

how do densely hairy leaves work

they create a physical barrier that is difficult for many herbivores to overcome

why does the effectiveness of physical defences vary

because it depends on the herbivores size and feeding habits

list the three main types of chemical defences

• tannins

• alkaloids

• terpenoids

where are tannins, alkaloids and terpenoids usually found in the plant

places that are the most vulnerable to herbivory, like leaves and seeds

what are alkaloids

bitter nitrogen-containing compounds

list the effects of alkaloids

1. alkaloids like morphine and cocaine act as drugs and affect the metabolism of animals, discouraging some herbivores from consuming the plant that produces them

2. alkaloids like caffeine and nicotine are toxic or poisonous to animals, discouraging some herbivores from consuming the plant or even killing them.

what do VOCs stand for

volatile organic compounds

when and where do plants release VOCs

they release them into the air when they detect herbivore attacks

in what ways are VOCs similar and different to pheromones

• similar- chemicals made by an organism that affect the social behaviour of other members of the same species

• different- VOCs can affect members of different species too

what are the three purposes of VOCs

1. attracting predators

2. repelling herbivores

3. signalling neighbouring plants

give an example of VOCs attracting predators

VOCs from pest-damaged cabbages can attract wasps that prey on the pests

how can VOCs repel herbivores

VOCs can discourage other herbivores from feeding on the plant

how can VOCs be useful when signalling to neighbouring plants

VOCs can alert nearby plants to prepare their defences, even before they are attacked themselves

what is another defence mechanism that some plants respond to touch with

they rapidly move e.g. defence mechanism against small insect herbivores

give a plant example that exerts movement as defence

Mimosa pudica

when touched, what happens to the leaves of Mimosa pudica and why do they do this

they quickly fold up and the plant droops

this can dislodge or deter small insects, or make the plant appear smaller and less appetising

what are tropisms

directional growth responses of organisms to a specific stimuli

list some examples of tropisms in plants

• phototropism

• geotropism

• chemotropism

• thigmotropism

what is a phototropism

growth towards or away from light

what is geotropism

growth towards or away from gravity

what is chemotropism

growth towards or away from certain chemicals

what is thigmotropism

growth towards or away from touch or physical contact

why do plants have phototropism

to maximimise the ability of the plants shoots and leaves to capture light for photosynthesis

descrie the two types of phototropism

• positive phototropism- when the shoots grow towards the light

• negative phototropism- when the roots grow away from the light

what is another name for geotropism

gravitropism

why do plants have geotropism

to help the plant establish its roots in the optimal orientation- helps roots anchor into the soil to support the plant and obtain minerals and nutrients

describe the two types of geotropism

• positive geotropism- when the roots grow downwards, towards the gravitational pull

• negative geotropism- when the shoots grow upwards, away from the gravitational pull

name the type of auxin that plays a central role in directing phototropisms and geotropism

IAA

describe the steps of positive phototropism in shoots

1. IAA is produced in cells in the tip of the plant shoot

2. IAA is transported down the plant shoot

3. light stimulates IAA to move from the light side of the shoot to the shaded side

4. IAA becomes concentrated and stimulates more cell elongation on the shaded side of the shoot

5. the shoot bends towards the light

describe the steps of a negative phototropism and positive geotropism in plant roots

1. IAA is produced in cells in the tip of the plant root

2. IAA is transported along the plant root

3. any light available stimulates IAA to move from the light side of the root to the shaded side

4. gravity also pulls IAA from the upper side of the root to the lower side of the root

5. IAA becomes concentrated and inhibits cell elongation in the lower, shaded side of the root

6. the root bends away from any available light and downwards towards the pull of gravity