M5C16 - Plant responses Flashcards
What physical defences prevent plants from being eaten by herbivores?
Thorns, spikes, inedible tissues, folding in response to touch.
Describe alkaloids as a chemical defence against herbivores:
Alkaloids are a group of chemicals that are bitter tasting and often toxic.
Examples: Caffeine - toxic to insects and fungi, nicotine - released by tobacco plants in response to tissue damage..
Describe alkaloids as a chemical defence against herbivores:
Pheromones are chemicals made by organisms to affect the social behaviour of other members of the same species.
Example: When maple trees are attacked they produce pheromones which branches of other trees absorb, triggers production of chemicals used in an attack.
Describe leaf loss as a response to abiotic stress
-Plants are sensitive to day length and a lack of light in their environment.
-The responses of the plant to this is known as photoperiodism
-One example is the loss of leaves as the day length and light levels fall in autumn
How do plants prevent freezing (abiotic stress)
-Different genes are expressed/suppressed which increase the amount of sugars, polysaccharides, amino acids and proteins in the cytoplasm.
-These solutes lower the freezing point of the cytoplasm and thus act as antifreeze.
Explain the role of hormones in abscission (leaf loss)
1)In a leaf stalk there is an abscission layer where abscission will occur. It is formed of 2 layers: the separation layer and the protection layer.
2)Plant detects a reduction in light levels and auxin concentration is reduced. Leaves respond by producing the ethene (gas).
3)Ethene is involved in the regulation of genes in the abscission region. New enzymes are produced which break down the cell walls in the separation layer of the abscission region.
4)Cells in the separation layer also begin to retain more water, putting extra pressure on the outer layer.
5)Fatty deposits are laid down in the protective layer that will act as a barrier and prevent pathogens entering when abscission occurs.
6)Eventually abiotic factors such as wind, rain etc. will knock the weakened leaf off. Leaving behind the waterproof, pathogen-proof protective layer.
What is germination?
Germination in plants is the process by which a dormant seed begins to sprout and grow into a seedling under the right growing conditions.
Explain the role of gibberellins in seed germination
Seed absorbs water which triggers the production of gibberellins (plant growth hormones).
-Stimulate the production of enzymes to break down starch stores into glucose. Glucose used to produce ATP/
Energy for growth and the production of new molecules.
What is evidence for the Role of Gibberellins in Seed Germination
1)Genetically altered seeds that are unable to produce gibberellins are unable to germinate. If gibberellins applied externally then seeds can germinate.
2)Gibberellin biosynthesis inhibitors can be applied so seeds are unable to germinate. When inhibitor removed seeds can germinate.
Explain the role of ABA in Stomatal Closure
-The main hormone involved in stomatal closure is abscisic acid (ABA)
-ABA is produced in response to stress
-ABA binds to receptors on the guard cell membranes activating a pathways that result in ion channels opening and ions leaving the guard cells.
(Turgidity caused by accumulation of K+ ions in the guard cells. As K+ levels increase in guard cells, water potential decreases, and water enters the guard cells via osmosis. – guard cell turgid = stomata open.)
Explain how the loss of ions from the guard cell causes the stomata to close?
Water potential increases.
Water moves out of cell via osmosis.
Cells lose turgor and become flaccid.
Explain evidence for the role of gibberellins in stem elongation
-Gibberellins have a role in stimulating cell division and elongation causing stems to grow.
Evidence for the role of gibberellins in stem elongation:
-Plants treated with gibberellins grow very tall (known as bolting).
-Plants also treated with a gibberellin inhibiter did not bolt.
-Dwarf varieties of plants treated with gibberellins grow to normal heights.
What is IAA?
Indoleacetic Acid (IAA) is an important Auxin involved in phototropism and geotropism.
Describe and explain the process of phototropism
-IAA moves to the more shaded parts of the roots and shoots, so there is uneven growth.
-Where IAA moves to in shoots, cells elongate and the shoot bends towards the light.
-Where IAA moves to in roots, growth is inhibited so the root bends away from the light.
Describe and explain the process of gravitropism
-IAA moves to the undersides of the roots and shoots, so there is uneven growth.
-Where IAA moves to in shoots, cells elongate and the shoot bends towards the light so shoot grows upwards.
-Where IAA moves to in roots, growth is inhibited so root grows downwards.
Describe Darwin’s experiment
Investigated which part of the shoot/plant is sensitive to light with variations of a coleoptile plant with/without tip.
What were the conclusions of Darwin’s experiment?
-A growth stimulus is produced in the tip of the coleoptile (but it is the lower regions which actually respond)
-Growth stimulus is transmitted to the zone of elongation
-Cells on the shaded side of the coleoptile elongate more than the cells on the other side.
Describe Boysen-Jensen’s experiment
-Cut the tip of the plant off and placed a block of gelatin between tip and plant.
-Inserted a sheet of impermeable Mica through shaded and illuminated sides of plant.
What do the results from Boysen-Jensen’s experiment tell us?
A chemical is travelling down from the tip. This can diffuse through the permeable gelatine but Mica blocks the chemical so it can’t travel to the rest of the shoot.
The messenger chemical seems to encourage growth on the dark side of the stem.
Describe what the response of Paal’s experiment show (half placed tip):
To see if he could use the tips chemical messenger to bend the stem even when there’s equal amounts of light in all directions. The results suggest that the whole tip makes this chemical and its unequal distribution causes the stem to bend.
Describe and explain Went’s experiment
Plant tips placed on agar gel for some time, blocks cut from it and placed on plant with tip removed in the dark.
The gelatine block makes the stem bend so it seems to have the chemical messenger in it. The chemical must have diffused from the tip into the block.
What is Apical Dominance?
Apical dominance is the phenomenon whereby the main, central stem of the plant is dominant (grows more) over other side stems.
Explain the role of auxin in stimulating the growth of the main apical shoot: (Apical dominance)
1)Auxins are synthesised in the tip of the stem – in the meristematic cells.
2)When bound to receptors in the cell membrane they affect the transport of ions through the cell membrane, causing a build up of hydrogen ions in the cell walls.
3)This reduces the pH of the cell wall to about 5, the optimum pH for enzymes involved in keeping the cell walls flexible.
4)As the cell matures the auxin is destroyed and these enzymes stop working. This means the cell walls become rigid and so these cells can no longer expand and grow.
Explain the role of auxin in suppressing the growth of lateral shoots: (Apical dominance)
Further down the stem the concentration of auxin is lower so the lateral shoots can grow more, up to a given concentration.
The auxin is produced by the root tip and small amounts travel down from the growing root shoots.
Large concentrations of auxin inhibit root growth!
Explain the role of ethene in fruit ripening
Fruit repining of climatic fruits (fruits that continue to ripen after they are picked) has been linked to ethene.
When exposed to ethene the fruits will ripen.
-The unripe fruit is hard and less easily damaged.
-When the fruit is ready for sale they are exposed to ethene gas – this ensures they all ripen at the same point.
-Ethene is also used to promote fruit dropping in plants.
What is a cotyledon?
An embryonic leaf in seed-bearing plants, one or more of which are the first leaves to appear from a germinating seed.
Explain the role of auxins in weed killers
-Many of the main staple foods are narrow leaved monocot plants (e.g. rice, maize and wheat)
-Many weeds are broad leaf dicots
-Synthetic dicot auxins can be applied as a weed killers. The interruption to their metabolism leads to an unsustainable growth rate which eventually leads to them dying.
What are the benefits of weed killers?
Specific auxins to target / kill weed species
Crops have less competition
Higher yields
More food for growing population
More profit for farmers
Explain rooting powders
Scientists have found that applying auxin to a cut stem stimulates the production of new roots, making it easier to propagate new plants from cuttings.
What are some other commercial uses of auxins?
Sprayed onto developing fruits to prevent abscission
Sprayed onto flowers to initiate fruit growth without fertilisation
