5.5 Plant And Animal Responses Flashcards
Auxins
A group f hormones produced by the tip of a shoot or root that regulate plat growth
Auxins in shoots
Stimulates cell elongation and thus high concentration of auxin promote growth
Auxins in roots
Inhibits cell elongation and thus high concentration of auxin limit growth
How does auxin cause cell elongation
- Auxin activates a proton pump in the plasma membrane
- This causes the secretion of H+ ions into the cell wall
- The resultant decrease in pH of the cellulose cell wall is the optimum pH for the enzyme expanase
- Expanase breaks the bonds in the cellulose fibres
- Cell wall becomes more flexible
- The cell can now take in more water and become more turgid
- The elongates the cell
Why do plants need to defend themselves
Avoid predation- some plants produce toxic substances
Avoid abiotic stress- carrots produce antifreeze to bind to ice crystals and lower freezing point of water
Survive- to reproduce
Why do plants need to respond to their environment
To cope with environmental conditions
Avoid abiotic stress
Maximise photosynthesis
Avoid bherbivory (biotic stress)
Tannins
Very bitter taste
Toxic to microorganisms and insects
Alkaloids
Chemicals with bitter tastes, noxious smells or poisonous characteristics that deter or kill herbivores
Eg tobacco plants produce the alkaloid nicotine in response to tissue damage. Nicotine is highly poisonous to many insects
Pheromones
Chemicals which are released by one individual, which can affect behaviour and physiology of another member of the same species
Nastic response
A non-directional response to stimuli
Tropism
A directional growth response in which the direction of the response is determined by the direction of the external stimulus
Positive tropism
A growth response towards the stimulus eg. Light
Negative Tropism
A growth response away from the stimulus eg. Gravity
Types of plant tropism
Phototropism-growth response to light
Geotropism- growth response to gravity
Chemotropism- growth response to chemicals
Thigmotropism- growth response to touch
Hydrotropism-growth response to water
Ethene
Stimulates leaf loss
How does ethene stimulate leaf loss
As leaf gets older, more ethene produced
A layer of cells develops at the bottom of leaf stalk and the abscission layer separates leaf from the plant. Ethene stimulates cells in abscission layer to expand, breaking the cell walls and causing the leaf to fall off
How does abscisic acid (ABA) trigger stomata closure
- Abscisic acid (ABA) binds to specific receptors on the guard cell membranes
- ABA causes calcium ions to move into the cytoplasm of the guard cells
- The increased concentration of calcium ions causes other ion channels to open
- Ions leave the guard cell increasing the water potential inside the guard cell
- Water leaves the guard cells by osmosis
- The guard cells become flaccid causing the stoma to close
Role of gibberellin in the germination of a seed
- Water absorbed
- Gibberelins synthesised by embryo
- gibberellin stimulates aleurone layer cells to synthesise amylase
- Amylase hydrolyses starch to form maltose
- Maltose converted to glucose
- Glucose transported to embryo providing energy for growth
Auxins negative geotropism in shoot
Auxin moves to lower side
The cells of the shoot grow more on the side with most auxin, so it stimulates cells to grow more here
This makes the shoot bend and grow away from the ground
Apical dominance
The central stem of the plant is dominant and grows more strongly than the side branches
Inhibition of lateral bud growth due to hormones produced by the apical buds
Cytokinins
Promote bud growth
