stimuli, reflexes and tropisms Flashcards
What is a stimulus?
A detectable change in the internal or external environment of an organism that leads to a response in the organism
Why is it important that organisms respond to stimuli?
- to move away from harmful stimuli
- to move towards a food source
- to maintain stable internal environments (homeostasis)
Examples of external and internal stimuli
External: light, temperature (external) humidity
Internal: blood glucose, blood pressure, temperature (internal), water potential (blood)
What is taxes?
Directional response to a directional stimulus
What is kineses?
Response to a stimulus by changing the amount of activity
- speed of movement
- rate of turning
What does kineses allow?
It allows the organism to stay in a favourable environment, non-directional response to a non-directional stimulus
What is a reflex?
A rapid, involuntary response to a stimulus
What is a tropism?
A growth response to a directional stimulus resulting in growth towards (positive response) or growth away (negative response) the direction of the stimulus
Name the three types of tropism
Light = phototropism
Gravity = gravitropism
Water = hydrotropism
How do reflexes aid survival?
Involuntary = no decision making is required, frees the brain up to carry out more complex responses
Innate = do not have to be learnt, hence effective from birth at protecting body from harm
Rapid = due to short neurone pathways, very few synapses and lack of decision making
What’s the general sequence from stimulus to response?
Stimulus => receptor => co-ordinator => effector => response
What’s a stimulus?
A detectable change in the internal or external environment which produces a response
What’s a response?
Cells or organs which detect a stimulus and transform the energy into a form which can be processed by the organism
What is a co-ordinator?
Connects information from each receptor with the appropriate effector
What is an effector?
Cells or organs which carry out a response
What is a response?
The change which occurs as a result of the stimulus
What is the two main divisions of the CNS?
- the central nervous system is made up of the brain and spinal cord that are composed of relay neurones
- the peripheral nervous system is originates from either the brain or the spinal cord and is composed of sensory and motor neurones
What is IAA?
IAA controls plant cell elongation
It stimulates cell elongation in shoots and it inhibits cell elongation in roots
What is the sequence of events in a plant shoot that bring about positive phototropism?
- cells at the tip produce IAA
- it diffuses evenly throughout all regions initially, but light causes IAA to move from the light side to the shaded side
- IAA causes cell elongation in the cells on the shaded side, so the shoot tip will bend towards the light
what is the sequence of events in plant roots that bring about negative phototropism?
- the IAA causes inhibition of cell elongation in the root cells
- light causes the IAA to move to the shaded side, and cells will be unable to elongate due to inhibition
- roots will grow away from the light
what is the sequence of events in plant roots that causes positive gravitropism?
- IAA is transported evenly across the plant from the root tips
- gravity causes IAA to move from the upper side of the root to the lower side
- IAA in plant roots inhibits cell elongation, so the plant roots will grow downwards
- down towards gravity
what is the sequence of events in plant shoots that causes negative gravitropism?
- IAA is transported evenly across the plant
- gravity causes the IAA to move from the upper side of the shoot to the lower side
- IAA in plant shoots causes cell elongation, this means the plant shoot will grow upwards
- away from gravity
whats the benefits of plants having a negative gravitropism in their shoots and a positive gravitropism in their roots?
negative gravitropism: shoots will grow upwards making it more likely for sunlight to reach the leaves
positive gravitropism: roots grow downwards and are more likely to absorb nitrates and phosphates for growth
describe the IAA mechanism of action
cell elongation is stimulated by IAA binding to a receptor protein in the target cells membrane activating a H+ pump.
this pumps protons from the cytoplasm => cell walls.
the resulting decrease in pH (inc. H+), activates an enzyme breaking the microfibrils between the cellulose.
this loosens the cell wall and allows the cell to elongate under the internal turgor pressure.
