5.1.5 - Plant and animal responses Flashcards
What kind of system are plants ?
- Plants are dynamic systems
- Not only photosynthesising and producing food but also responding to their environment in many different ways.
How have plants evolved ?
- To cope with abiotic stress such as a lack of water
- They have a range of adaptations to protect them against the attention of herbivores.
What is a tropism ?
Showing directional growth response to environmental cues such as light and gravity
What is the key limitation to plants ?
They are rooted, not mobile, and do not have a rapid responding nervous system
What kind of system have the plants developed ?
- A system of hormones
- They are produced and released in one part of the plant and transported both through the transport tissues and from cell to cell and have an effect in another part of the plant
What are some of the hormones in plants ?
- Auxins
- Gibberellins
- Ethene
- Abscisic acid (ABA)
What are the roles of auxins ?
- Controls cell elongation
- Prevent leaf fall, abscission
- Maintain apical dominance
- Involved in tropisms
- Stimulate the release of ethene
- Involved in fruit ripening
What are the roles of gibberellins ?
- Cause stem elongation
- Trigger the mobilisation of food stores in a seed at germination
- Stimulate pollen tube growth in fertilisation
What are the roles of ethene ?
- Cause fruit ripening
- Promotes abscission in deciduous trees
What are the roles of abscisic acid (ABA) ?
- Maintains dormancy of seeds and buds
- Stimulates cold protective responses (antifreezing)
- Stimulates stomatal closing
Why are scientists still unsure about some plant hormones ?
- Plant hormones work at very low concentrations, so isolating them and measuring changes in concentration is not easy
- The multiple interactions between the different chemical control systems also make it very difficult for researchers to isolate the role of a single chemical in a specific response
What must happen to the seeds for the plant to start growing ?
They must germinate
What happens when the seed absorbs water ?
The embryo is activated and begins to produce gibberellins.
What do the gibberellins stimulate the production of ?
Enzymes that break down the food stores found in the seed.
Where are the food stores found ?
The food store is in the cotyledons in dicot seeds and the endosperm in monocot seeds
How does the embryo plant use the food stores ?
The embryo plant uses these food stores to produce ATP for building materials so that it can grow and break through the seed coat
What does evidence suggest that gibberellins do regarding genes ?
Gibberellins switch on genes which code for amylases and proteases, the digestive enzymes required for germination
What does ABA act as in relation to gibberellins ?
- It acts as an antagonist to gibberellins
- It is the relative level of both hormones which determines when a seed will germinate
Give an example of an auxin
Indoleacetic acid
What do auxins do ?
- They are growth stimulants produced in plants
- Small quantities can have large effects
Where are auxins made ?
They are made in cells at the tip of roots and shoots, and in the meristems
Where can the auxins move to ?
They can move down the stem and up the root both in transport tissue and from cell to cell
What can auxins stimulate ?
They can stimulate the growth of the main apical shoot
What do auxins affect regarding the cell wall ?
They affect the plasticity of the cell wall, the presence of auxins means that the cell wall stretches more easily
How do auxins stimulate the growth of the main apical shoot ?
- They bind to specific receptor sites in the plant cell membrane, causing a fall in the pH to about 5
- As the cells mature, auxin is destroyed
- As the hormone levels fall, the pH rises so the enzymes maintaining plasticity become inactive
- As a result, the wall becomes rigid and more fixed in shape and size and the cells can no longer expand
What can high concentrations of auxin do ?
They suppress the growth of the lateral roots
What does suppressing the growth of the lateral shoots lead to ?
This results in apical dominance
How does apical dominance arise ?
- Growth in the main shoot is stimulated by the auxin produced at the tip so it grows quickly
- The lateral shoots are inhibited by the hormone that moves back down the stem, so they do not grow as well
- Further down the stem, the auxin concentration is lower and so the lateral shoots grow more quickly
What happens if the apical shoot is removed ?
- The auxin producing cells are removed and so there is no auxin.
- As a result, the lateral shoots, freed from the dominance of the apical shoot, grow faster
How can we reassert apical dominance if the apical shoot is cut ?
- Apply auxin artificially to the cut apical shoot
- Apical dominance is reasserted and lateral shoot growth is suppressed
What do low concentrations of auxin lead to ?
They promote growth
What do high concentration of auxin lead to ?
They inhibit root growth
What do gibberellins affect ?
They affect the length of the internodes
What are the internodes ?
They are the regions between the leaves on a stem
How were gibberellins discovered ?
They were discovered because they are produced by a fungus from the genus gibberella that affects rice
Describe the experiment that led to the founding of gibberellins
- The infected seedling grew extremely tall and thin
- Scientists investigated the rice and isolated chemicals which produce the same spindly growth in the plants
- It was then discovered that plants themselves produced the compound
If plants do not have gibberellins what are they likely to be ?
They are likely to have stems that are a lot shorter
What does a short plant stem lead to ?
It reduces waste and also makes the plant less vulnerable to damage by weather and harvesting
How do most plant hormones work ?
They work by interacting with other substances
What does hormones working together lead to ?
Very fine control over the responses of the plant that can be achieved
What is synergism ?
When different hormones work together, their effect can be larger than if they were on their own
What is antagonism ?
When two different hormones work against each other, the balance of them will determine the response of the plant
What are some examples of abiotic stresses ?
- Changes in day length
- Cold and heat
- Lack of water / excess water
- High winds
- Changes in salinity
What adaptations may plants have for a hot/dry/windy environment ?
- Thick cuticles
- Hairy leaves
- Sunken stomata
- Wilting response
What adaptations may a plant have for an aquatic environment ?
They may develop aerenchyma
How do seasonal changes affect the amount of photosynthesis possible ?
As light and temperature affect the rate of photosynthesis, seasonal changes have a big impact on the amount of photosynthesis possible.
When is leaf loss in deciduous plants as a response to abiotic stress likely to take place ?
Amount of glucose required for respiration > amount of glucose produced by photosynthesis
When do deciduous trees lose all of their leaves ?
In the winter and they remain dormant until the days lengthen and temperatures rise again in spring
What is photoperiodism ?
The response to a lack of light in the environment of plants
Which plant responses are affected by the photoperiod ?
- Breaking of the dormancy of the leaf buds so they open
- The timing of the flowering in a plant when tubers are formed in preparation for overwintering
Which pigment is responsible for sensitivity to day/dark length ?
Phytochrome
What are the two forms of phytochrome ?
- Pr
- Pfr
How do the two forms of phytochrome differ ?
Each absorbs a different type of light and the ratio of Pr to Pfr changes depending on the levels of light
What does the lengthening of the dark period trigger ?
- Abscission or leaf fall
- Period of dormancy during the winter months
What do falling light levels lead to the fall in ?
Concentrations of auxin
What do falling light levels trigger the plant to produce ?
Ethene
What is the abscission zone ?
- It is at the base of the leaf stalk
- It is made up of two layers that are sensitive to ethene.
What does ethene trigger to happen in the abscission zone ?
- It initiates gene switching in these cells resulting in the production of new enzymes
- These enzymes weaken and digest the cell walls in the outer layer of the abscission zone
What is the separation layer ?
The area that the enzymes have weaken, in the outer layer of the abscission zone
What does the separation layer cause to happen in the plant ?
- The vascular bundles which carry materials into and out of the cell are sealed off
- The fatty material is deposited in the cells on the stem side of the separation layer
What does the separation layer form when the leaf falls ?
- A waterproof protective scar
- It prevents the entry of pathogens
How does the leaf separate from the plant ?
- Cells deep in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain on the already weakened outer layer.
- Then further abiotic factors such as low temps or strong winds finish the process - the strain is too much and the leaf separates from the plant
What is another way that plants respond to abiotic stresses ?
Stomatal control
What does stomatal control refer to ?
The opening and closing of the stomata to conserve/lose water
Which hormone controls the opening and closing of the stomata ?
ABA
When do leaf cells release ABA ?
Under abiotic stress
What do scientists now believe regarding ABA ?
That the roots also provide an early warning of water stresses through ABA
What do the plant roots produce when the levels of soil water fall ?
They produce ABA
Where is ABA transported once it is produced by the plant roots ?
It is transported to the leaves where it binds to the receptors on the plasma membrane of the stomatal guard cells
What does ABA activate ?
ABA activates changes in the ionic concentration of the guard cells, reducing the water potential and therefore turgor of the cells.
What does reduced turgor lead to in the guard cells ?
As a result of reduced turgor, the guard cells close the stomata and water loss by transpiration is greatly reduced
What is herbivory ?
The process by which herbivores eat plants
What are some physical defences plants have developed to prevent herbivory ?
- Thorns
- Barbs
- Spiny leaves
- Inedible tissue
- Hairy leaves
- Stings
What are some chemical defences plants have developed to prevent herbivory ?
- Tannins
- Alkaloids
- Terpenoids
What group are tannins a part of ?
Phenols
What is the role of tannins ?
- They bind to the digestive enzymes produced in the saliva and inactivate them
- They are toxic to insects
What is the taste of tannin like ?
They have a bitter astringent taste
What are alkaloids ?
They are a large group of bitter tasting nitrogenous compounds
What do alkaloids act as ?
- They act as drugs
- Affecting the metabolism of animals that take them in and sometimes poisoning them
Give some examples of alkaloids
- Caffeine
- Morphine
- Nicotine
- Cocaine
How does caffeine protect a plant ?
- Caffeine is toxic to fungi and insects
- Caffeine produced by coffee bush seedlings spreads through the soil and prevents the
germination of the seeds of other plants
How does nicotine protect a plant ?
- Nicotine is a toxin produced in the roots of tobacco plants
- It is transported to the leaves and stored in vacuoles to be released
when the leaf is eaten
What are terpenoids ?
They are toxins to insect and fungi that might attack the plant
What is pyrethrin ?
It acts as an insect neurotoxin that interferes with the nervous system
What is citronella ?
It is a terpenoid that acts as an insect repellent
What is a pheromone ?
A pheromone is a chemical made by an organism which affects the social behaviour of other members of the same species
What are VOC’s ?
- They act like pheromones between themselves and other organisms, particularly insect.
- They diffuse through the air in and around the plant
What can VOC’s do ?
They can alert neighbouring plants to produce VOC’s before they are attacked
When are VOCs made ?
They are usually only made when the plant detects attack by an insect pest through chemicals in the saliva of the insect
What may VOC’s elicit ?
Gene switching
What are tropisms ?
Plant growth responses to stimuli
What is phototropism ?
The growth of plants in response to light which comes from one direction
What is geotropism ?
The response to gravity
What is chemotropism ?
The response to chemicals
What is thigmotropism ?
The response to touch
What is the direction of the response related to ?
It is related to the direction from which the stimulus comes
What is used to research tropisms ?
Germinating seeds
Why are germinating seeds used to research tropisms ?
This is because they are easy to work with and manipulate as they are growing and responding rapidly, this means that any changes will show up quickly
What are coleoptiles ?
The seedlings of monocotyledonous plants
What causes phototropism ?
The movement of auxins across the shoot or root if it is exposed to light that is stronger on one side than another
What happens to the plant if it is grown in bright, all round light in normal conditions of gravity ?
They grow upwards
What will happen if the plant grows in an even but low light ?
- They grow upwards
- They will grow faster and taller than in bright lights
What will happen if plants are exposed to light which is right on one side than another ?
The shoot of the plant will grow towards that light and the roots, if exposed, will grow away from it
What are shoots said to be (phototropism) ?
- Positively phototropic
- They receive as much light as possible, allowing as much photosynthesis to take place
What are roots said to be (phototropism) ?
Negatively phototropic
Where does auxin move in unilateral light ?
It moves laterally across the tip of the shoot away from the light
What does a higher concentration of auxin on the non illuminated side cause ?
It stimulates cell elongation and growth on the dark side, resulting in observed growth towards the light
What happens when you remove the unilateral stimulus, once the shoot is growing towards the light ?
The transport of auxin stops and the shoot then goes straight towards the light
What hormone is responsible for the elongation of the internodal plant in the dark ?
Gibberellins
When do levels of gibberellins fall ?
Once the stem is exposed to light
What is etiolation ?
The rapid upward growth which tales place in a plant grown in the dark
What are etiolated plants like ?
- They are thin and pale
- This is because the plant is deprived of the chlorophyll developed in the leaves
What are shoots like (geotropism) ?
- Negatively geotropic
- Shoot grows upwards
What are roots like (geotropism) ?
- Positively geotropic
- Shoot grows downwards
What are geotropism’s also know as ?
Gravitropism’s
What hormone is involved in the ripening of climacteric fruit ?
Ethene
What are climacteric fruit ?
Fruits that continue to ripen after they have been harvested
Give some examples of climacteric fruit
- Bananas
- Tomatoes
- Mangoes
- Avocados
What are non climacteric fruit ?
Fruits that do not produce large amounts of ethene and do not ripen much after picking
What is the ripening linked to ?
A peak of ethene production which triggers a series of chemical reactions including a greatly increased respiration rate
How can you see the effect of ethene on climacteric fruit ?
- It can easily be seen if a bunch of green bananas is put in a bag with a single ripe banana.
- The bunch with the ripe banana with ripen faster, even if the temperature is exactly the same in both cases.
When are climacteric fruit harvested ?
They are harvested when they are fully formed but long before they are right
What is the unripe fruit like ?
It is hard and much less easily damaged during transport compared to the ripe versions
What is done when the fruit is needed for sale ?
- They are exposed to ethene gas under controlled conditions
- This ensures that each batch of fruit ripe at the same stage to be put on sale
What does the careful control of the ripening prevent ?
It prevents a lot of wastage of fruit during transport and increases the time available for them to be sold
What can the application of auxin to shoots lead to ?
- It stimulates the production of roots
- This makes it much easier to propagate new plants from plant cuttings
What is a cutting ?
It is a small piece of the stem of a plant, usually with some leaves on
What can happen when you place a cutting in hormone rooting powder ?
It increases the chances of roots forming and of successful propagation taking place
What is micropropagation ?
Micropropagation is when thousands of new plants are grown from a few cells of the original plant.
What do plant hormones control in micropropagation ?
They control the production of the mass of new cells and then the differentiation of the clones into tiny new plants
What are weeds ?
Weeds are plant that grow where they are not wanted
What do weeds interfere with ?
Weeds interfere with crop plants, competing for life, space, water and minerals
What have scientists developed as an effective weed killer ?
Scientists have developed synthetic dicot auxins which acts as a very effective weed killer.
How do synthetic dicot auxins kill weeds ?
- If synthetic dicot auxins are applied as weedkiller, they are absorbed by the broadleaved plants and affect their metabolism.
- The growth rate increases and becomes unsustainable, so they die.
What else can auxin be used for ?
Auxin can be used in the production of seedless fruit
What else can ethene be used for ?
- Ethene is used to promote fruit dropping in plants such as cotton, walnuts, and cherries