Topic 6.1 - Stimuli And Responsep Flashcards
What is a stimulus?
A change in an organism’s internal or external environment
Why is it important important that organisms can respond to stimuli?
Organisms increase their chance of survival by responding to stimuli
What is a tropism? (2)
1) Growth of a plant in response to a directional stimulus
2) Positive tropism = towards a stimulus, negative stimulus = away from stimulus
Summarise the role of growth factors in flowering plants (2)
1) Specific growth factors (hormone-like growth substances) e.g. Auxins (such as IAA) move (via phloem or diffusion) from growing regions e.g. shoot / root tips where they’re produced
2) To other tissues where they regulate growth in response to directional stimuli
Describe how indoleactic acid (IAA) affects cells in roots and shoots (2)
1) In shoots, high concentrations of IAA stimulates cell elongation
2) In roots, high concentrations of IAA inhibits cell elongation
Explain gravitropism in flowering plants (5)
1) Cells in tip of shoot / root produce IAA
2) IAA diffuses down shoot / root
3) IAA moves to lower side of shoot / root (so concentration increases)
4) In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
5) So shoots bend away from gravity whereas roots bend towards gravity
Explain phototropism in flowering plants (5)
1) Cells in tip of shoot / root produce IAA
2) IAA diffuses down shoot / root
3) IAA moves to shaded side of shoot / root (so concentration increases)
4) In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
5) So shoots bend towards the light whereas roots bend away from light
Describe the simple responses that can maintain a mobile organism in a favourable environment (1+2 / 1+3)
1) Taxes (tactic response)
- Directional response
- Movement towards / away from a directional stimulus
2) Kinsesis (kinetic response)
- Non-directional response
- Speed of movement or rate of direction change
- Depending on intensity of stimulus
Give a real life example of taxis
Woodlice moving away from light to avoid predators
Give a real life example of kinesis
Woodlice moving faster in drier environments to increase their chance of moving in an area with higher humidity to prevent drying out
Explain the protective effect of a simple (e.g. 3 neurone) reflex (3)
1) Rapid as only 3 neurones and a few synapses
2) Autonomic so doesn’t have to be learnt
3) Protects from harmful stimuli e.g. escape predators / prevent damage to body tissues
Key features of a Pacinian corpuscle (6)
1) Lamellae (layers of connective tissue)
2) Sensory neurone ending
3) Sensory neurone axon
4) Myelin sheath
5) Stretch mediated sodium ion channel
6) Gel
Describe how a generator potential is established in a Pacinian corpuscle (3)
1) Mechanical stimulus e.g. pressure deforms lamellae and stretch mediated sodium ion channels
2) So Na+ (sodium) channels in membrane open and Na+ diffuse into sensory neurone
- Greater pressure causes more Na+ channels to open and more Na+ to enter
3) This causes depolarisation, leading to a generator potential
- If generator potential reaches threshold it triggers an action potential
Explain what the Pacinian corpuscle illustrates (2)
1) Receptors respond only to specific stimuli
- Pacinian corpuscle only responds to mechanical pressure
2) 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 3 key differences in sensitivity to light for rods and cones in the retina
1) Rods are more sensitive to light
2) Several rods connected to a single neurone
3) Spatial summation to reach / overcome threshold
1) Cones are less sensitive to light
2) Each cone connected to a single neurone
3) No spatial summation
Explain the 3 key differences in visual acuity for rods and cones in the retina
1) Rods give lower visual acuity
2) Several rods connected to a single neurone
3) So several rods send a single set of impulses to the brain (so can’t distinguish between separate sources of light)
1) Cones give higher visual acuity
2) Each cone connected to a single neurone
3) Cones send separate sets of impulses to brain (so can distinguish between 2 separate sources of light)
Explain the 2 key differences in sensitivity to colour for rods and cones in the retina
1) Rods allow monochromatic vision
2) 1 type of rod / pigment
1) Cones allow colour vision
2) 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
Cardiac muscle is myogenic. What does this mean?
It can contract and relax without receiving electrical impulses from nerves
Describe the myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity (4)
1) Sino-atrial node (SAN) acts as a pacemaker -> releases regular waves of electrical activity across atria -> causing atria to contract simultaneously
2) Non-conducting tissue between atria / ventricles prevents impulse passing directly to ventricles
- Preventing immediate contraction of ventricles
3) Waves of electrical activity reach atrioventricular node (AVN) which delays impulse
- Allowing atria to fully contract and empty before ventricles contract
4) AVN sends wave of electrical activity down bundle of HIS, conducting wave between ventricles to apex where it branches into Purkyne tissue
- Causing ventricles to contract simultaneously from the base up
Where are chemoreceptors and pressure receptors located?
In the aorta and carotid arteries
Describe the roles of chemoreceptors, pressure receptors, the autonomic nervous system and effectors in controlling heart rate (6)
1) Baroreceptors detect (fall / RISE) in blood pressure and / or chemoreceptors detect blood (rise / FALL) in blood CO2 concentration of (fall / RISE) in blood pH
2) Send impulses to medulla / cardiac control centre
3) Which send more frequent impulses to SAN along (sympathetic / PARASYMPATHETIC) neurones
4) So (more / LESS) frequent impulses sent from SAN and to / from AVN
5) So cardiac muscle contracts (more / LESS) frequently
6) So heart rate (increases / DECREASES)
*Use the lowercase / left words in brackets for a fall in blood pressure OR rise in blood CO2 concentration / fall in blood pH
Use the UPPERCASE / right words in brackets for a rise in blood pressure OR fall in blood CO2 concentration / rise in blood pH
