Plant and Animal Responses Flashcards
Types of stimuli
- biotic
- abiotic
Examples of abiotic stress
- drought
- freezing
- increased soil salinity
Plant response to abiotic stress examples
Drought- shut stomata to reduce water loss through transpiration, or dropping leaves
Freezing- some plants can produce antifreeze chemicals in their cells which decreases the formation of ice crystals
Responses to herbivory
Biotic stress
Produce chemicals:
1. Alkaloids- bitter tasting/toxic
2. Pheromones- signal to nearby plants that they are under attack
Some respond to touch as a response to herbivory or to reduce transpiration
Tropism
Directional growth in response to a stimulus
What are the 5 different tropisms
- Phototropism
- Geotropism
- Thigmotropism
- Hydrotropism
- Chemotropism
Phototropism
In response to Light
- abiotic
Advantage: maximise photosynthesis
Geotropism
In response to gravity
- abiotic
Advantage: ensures shots/roots grow in the right direction
Thigmotropism
In response to touch
- both
Advantage: useful in climbing plants to access more light
Hydrotropism
In response to moisture
- abiotic
Advantage: increases access to water
Chemotropism
In response to chemicals
- both
Advantage: pollen tubes grow down towards the ovules
Positive phototropism
Growth towards light
What is ‘roots positively geotropic’
Roots grow downwards
Role of auxins (IAA)
Light is even
The IAA molecules are distributed
Light is uneven
The IAA molecules move towards the shaded side
IAA stimulates cell elongation, causing the shaded side to grow at a faster rate
What do auxins do
- IAA is moved around the plant by diffusion and active transport (the phloem over long distances)
- IAA diffuses from the tip/meristem to the zone of elongation
- when IAA binds to a receptor this causes acidification of the cell wall. This breaks bonds within cellulose so they become less rigid and can expand
- ion channels open causing a change in concentration, leading to water moving into the cell by osmosis causing elongation
Darwin’s (1880): phototropism
- he discovered the removing the tip of a coleoptile stopped the phototropic response to a unidirectional light source from occurring
- he covered the tip of a coleoptile with an opaque cover/cap to block out the light. This stopped the phototropic response from occurring, showing that the tip of the coleoptile was responsible for detecting light
Boysen-Jensen (1913): phototropism
- he found that if he replaced the cut top back on top of the coleoptile and inserted a gelatin block as a barrier in between, the phototropic response was restored
- this showed that the stimulus for growth was a chemical (hormone), which was able to travel through the gelatin block
- he then inserted a mica barrier halfway through the coleoptile just below the tip, first on the lit side and then the shaded
- when the mica barrier was inserted on the lit side, phototropic response occurred
- when the mica barrier was inserted on the shaded side, phototropic response didn’t occur
This confirmed that the stimulus for growth was a hormone and showed it was produced at the tip before travelling down the coleoptile on the side opposite to the stimulus
What is the advantage of deciduous plants losing their leaves?
- less light so less photosynthesis
- less transpiration to reduce water loss
Leaf loss in deciduous plants
- a layer of cells known as the abscission layer develops at the base of the leaf stalk. These have thin walls, making them weak and easy to break
- the plant ethene stimulates the breakdown of cell walls in this abscission layer, causing the leaf to drop off
- auxins play a role in leaf loss. Usually, auxins inhibit leaf loss and are produced in young leaves, making the leaf stalks insensitive to ethene.
The concentration of auxins in leaves decreases as they age until leaf loss can once again occur in response to ethene
Why do plants close their stomata
- to prevent water loss
Turgid guard cell, water moving into guard cells by osmosis
Flaccid guard cell, water moves out by osmosis
Stomatal closure
- Abscisic acid (ABA) triggers stomata closure by binding to receptors on the guard cell membrane
- This causes calcium ion channels to open and calcium ions to enter
- Potassium ions leave the guard cell
- The water potential of the cells increases, water leaves the cells by osmosis causing the guard cells to become flaccid
Seed germination
- Water absorbed
- Gibberellins 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
Commercial uses of plant hormones
- selective weed killers
- rooting powders
- control ripening
