Stimuli, both internal and external, are detected and lead to a response Flashcards
Stimulus
Detectable change in the internal/external environment of an organism that leads to a response
Receptor
Detects stimulus, specific to one type of stimulus
Coordinator
Formulates a suitable response to a stimulus e.g nervous system/hormonal system
Effector
Produces a response to a stimulus e.g. muscles/glands
Reflex arc order
Stimulus → receptor → sensory neurone → coordinator (CNS/relay neurone) → motor neurone → effector → response
Reflex arc importance
Rapid (short pathway) because only 3 neurones and few synapses (synaptic transmission is slow)
Autonomic as it doesn’t involve passage to the brain - does not have to be learnt
Protects from harmful stimuli e.g. escape from predator/prevents damage to body tissues
Taxes
Directional responses by simple mobile organisms who move towards a favourable stimulus (positive taxis) or away from an unfavourable one (negative taxis)
e.g. woodlice show a tactic response to light. Move away from light → keeps concealed under stones during day away from predators, and in damp conditions which reduces water loss → improves chances of survival
Kineses
Non-directional responses by simple mobile organisms who change the speed of movement or the rate of direction change, in response to a non-directional stimulus
e.g. woodlice show a kinetic response to humidity
Positive and negative tropism in flowering plants
Tropism is the growth of a plant in response to a directional stimulus
Positive tropism is growth towards a stimulus
Negative response is growth away from the stimulus
Growth factors in flowering plants
A plant’s responses to external stimuli involves growth factors/hormone-like growth substances
Growth factors move from growing regions e.g. shoot tips/leaves where they are produced, to other tissues, where they regulate growth in response to directional stimuli e.g. auxins (such as IAA)
Indoleacetic acid (IAA)
Auxin
In roots, IAA inhibits cell elongation
In shoots, IAA promotes cell elongation
How IAA results in phototropism in shoots
Cells in tip of shoot produce IAA which is transported down the shoot (evenly initially)
IAA concentration increases on the shaded side and promotes cell elongation
Shoot bends towards light
How IAA results in gravitropism in roots
Cells in tip of root produce IAA which is transported down the root (evenly initially)
IAA concentration increases on the lower side of the root and inhibits cell elongation
Root curves downwards towards gravity
How does the Pacinian corpuscle function
In its normal state, the neurone Pacinian corpuscle has a resting potential as the stretch-mediated sodium channels of the neurone membrane are too narrow to allow sodium ions to pass along them.
Mechanical stimulus e.g. pressure deforms lamellae and stretch-mediated sodium ion channel
The sodium ion channels open and sodium ions diffuse into the sensory neurone
Greater pressure causes more channels to open and more sodium ions to enter, causes depolarisation which leads to a generator potential
If generator potential reaches threshold it triggers an action potential (nerve impulse)
What does the Pacinian corpuscle illustrate
Receptors respond only to specific stimuli – only responds to mechanical pressure
Stimulation of a receptor leads to the establishment of a generator potential. When threshold is reached, action potential sent, all-or-nothing principle.
