Stimuli and Responses Flashcards
What is a stimulus
A change in an organism’s internal or external environment.
Why is it important that organisms can respond to stimuli?
Organisms increase their chance of survival by responding to stimuli.
Describe how indoleacetic acid (IAA) affects cells in roots and shoots
● In shoots, high concentrations of IAA stimulates cell elongation
● In roots, high concentrations of IAA inhibits cell elongation
What is a tropism?
● Growth of a plant in response to a directional stimulus
● Positive tropism = towards a stimulus; negative tropism = away from stimulus
Explain gravitropism in flowering plants
- Cells in tip of shoot / root produce IAA
- IAA diffuses down shoot / root (evenly initially)
- IAA moves to lower side of shoot / root (so concentration increases)
- In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
- So shoots bend away from gravity whereas roots bend towards gravity
Explain phototropism in flowering plants
- Cells in tip of shoot / root produce IAA
- IAA diffuses down shoot / root (evenly initially)
- IAA moves to shaded side of shoot / root (so conc. ↑)
- In shoots this stimulates cell elongation whereas in
roots this inhibits cell elongation - So shoots bend towards light
whereas roots bend away from light
Describe the simple responses that can maintain a mobile organism in a favourable environment
- Taxes (tactic response)
○ Directional response
○ Movement towards or away from a directional stimulus - Kinesis (kinetic responses)
○ Non-directional response
○ Speed of movement or rate of direction change changes in response to a non-directional stimulus
○ Depending on intensity of stimulus
Explain the protective effect of a simple reflex arc
● Rapid as only 3 neurones and few synapses (synaptic transmission is slow)
● Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt
● Protects from harmful stimuli eg. escape predators / prevents damage to body tissues
give an example of a reflex arc with a pin
-A pin (the stimulus) is detected by a pain receptor in the skin
-The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
-Electrical impulses are passed on to relay neurone in the spinal cord
-The relay neurone connects to the motor neurone and passes the impulses on
-The motor neurone carries the impulses to the muscle in the leg (the effector)
-The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)
Describe the basic structure of a Pacinian corpuscle
-lamelle
-sensory neurone ending
-sensory neurone axon
-myelin sheath
-gel
-stretch mediated sodium ion channels
Describe how a generator potential is established in a Pacinian corpuscle
- Mechanical stimulus (eg. pressure) deforms lamellae and stretch- mediated sodium ion (Na+) channels
- So Na+ channels in membrane open and Na+ diffuse into sensory neurone
○ Greater pressure causes more Na+ channels to open and more Na+ to enter - This causes depolarisation, leading to a generator potential
○ If generator potential reaches threshold it triggers an action potential
Explain what the Pacinian corpuscle illustrates
● Receptors respond only to specific stimuli
○ Pacinian corpuscle 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)
Explain the differences in sensitivity to light for rods & cones in the retina
Rods are more sensitive to light:
● Several rods connected to a single neurone
● Spatial summation to reach / overcome threshold (as enough neurotransmitter released) to generate an action potential
Cones are less sensitive to light:
● Each cone connected to a single neurone
● No spatial summation
Explain the differences in visual acuity for rods & cones in the retina
Rods give lower visual acuity:
● Several rods connected to a single neurone
● So several rods send a single set of impulses to brain (so can’t distinguish between separate sources of light)
Cones give higher visual acuity:
● Each cone connected to a single neurone
● Cones send separate (sets of) impulses to brain (so can distinguish between 2 separate sources of light)
Explain the differences in sensitivity to colour for rods & cones in the retina
Rods allow monochromatic vision:
● 1 type of rod / 1 pigment
Cones allow colour vision
● 3 types of cones - red-, green- and blue-sensitive
● With different optical pigments → absorb different wavelengths
● Stimulating different combinations of cones gives range of colour perception
