6.1.1 Survival and response Flashcards
Define stimulus
Detectable change in the internal and external environment
Define response:
The organism’s response to that change
Give three important directional stimuli that plants respond to:
- Light
- Gravity
- Water
What is the advantage to a plant of this type of response?
- Maximising photosynthesis conditions
- Roots grow towards water in order to absorb it for use of photosynthesis
What are tropisms co-ordinated?
Plant growth factors - Indoleacetic Acid (IAA) - type of auxin
What effect does IAA have on cells:
In the shoots?
IAA stimulate cell elongation
What effect does IAA have on cells:
In the roots?
IAA inhibits cell elongation
What phototropism does shoots show?
Positive
What phototropism does roots show?
Negative
What is IAA’s effect in even light (non-directional)?
- IAA is made in the shoot tip
- IAA moves down the shoot by diffusion, where it causes cell elongation
What is IAA’s effect in directional light?
- When light is directional the IAA moves away from the light, towards the shaded side of the shoot
- This causes uneven cell elongation, and causes the shoot to bend towards the light
Where is IAA produced? How it moves and where it has its effect?
ROOT:
- Auxin inhibits cell growth/elongation
- Lower cell layer is shorter
- Turns down/away from light
STEM:
- Auxin stimulates cell growth
- Lower cell layer is longer
- Turns up/towards the light
What is gravitropism?
A response to gravity
What gravitropism in the shoots show?
Negative gravitropism
What gravitropism in the roots show?
Positive gravitropism
Why is gravitropism important?
Important during germination - seed is under-ground, there is no light
Therefore gravitropism ensures that root will grow downwards, and shoots up towards the light
What is gravitropism also controlled by?
By the distribution of IAA within the plant cells
- IAA will accumulate on the lower side of the plant in response to the force of gravity
What does positive gravitropism in the roots look like?
- high IAA concentration inhibits cell elongation causing a positive gravitropic response
What happens in the roots at high auxin concentration?
In roots, high auxin concentrations inhibit cell elongation, meaning that the lower side of the root becomes shorter and the roots turn downwards into the earth
What happens in the shoots at high auxin concentration?
In shoots, high auxin concentrations promotes cell elongation, meaning that the lower side of the shoot elongates and shoots grow away from the ground
How does IAA increase cell elongation in shoots?
- Auxin activates a proton pump in the plasma membrane which causes the secretion of H+ ions into the cell wall
- The resultant decrease in pH causes cellulose fibres (H+ disrupts hydrogen ions) within the cell wall to loosen and become more plastic (stretch and stay stretched)
- This increases the elongation possible, particularly in young cells
What are reflex arcs?
Simple, fast, involuntary responses, that protect the body from harm
What is the sequence when reflex arc happens?
Stimulus -> receptor -> sensory neurone -> coordinator - CNS/relay neurone -> motor neurone -> effector -> response
Why are reflex arcs important?
- Rapid (short pathway) because only 3 neurones and few synapses (synaptic transmission is slow)
- Autonomic because doesn’t involve passage to brain - doesn’t have to be learnt
- Protect from harmful stimuli eg/ escape from predator/prevents damage to body tissues
What is taxis?
A directional response to a particular stimulus. Can be positive or negative depending if the movement is towards (positive) or away (negative) from the stimulus
- Allows organisms to move towards favourable conditions and away from unfavourable ones
Example of taxis response?
Euglena (single cells photosynthetic organism) slows positive phototaxis
- Moves towards light therefore better conditions for photosynthesis
What is kinesis?
An organism changes the speed at which it moves, and the rate at which it changes direction. Kinesis is not a directional response
What happens in favourable conditions (kinesis)?
- Slow movement
- Organism remains in favourable conditions
What happens in unfavourable conditions (kinesis)?
- Rapid movement
- Organism is more likely to leave the conditions
