6.1 IAA, Taxis/Kinesis, Pacinian, Retina, Heart Rate Control Flashcards
Define stimulus
a detectable change in the internal & external environment
Chain of events when an organism responds to change
stimulus → receptor → coordinator → effector → response
Two types of coordinators
- nervous system
- hormones (chemical)
All tropisms are…
a GROWTH RESPONSE (in response to DIRECTIONAL stimuli)
Tropisms are coordinated by…
plant growth factors
What effect does IAA have on cells in the shoots?
STIMULATES cell elongation
What effect does IAA have on cells in the roots?
INHIBITS elongation
Phototropism: shoots vs roots
shoots show positive phototropism
roots show negative phototropism
Effect of IAA in even (non-directional) light
IAA is produced at the TIP of the shoot and diffuses equally down both sides of the plant.
There is equal cell elongation on either side.
Effect of IAA in directional light
IAA diffuses and accumulates on the SHADED SIDE of the plant.
This causes cell elongation on the side of the shoot away from the light, causing the shoot to bend towards the light.
When is gravitropism important?
germination!
How does IAA work? (part 1: H+)
It activates a proton pump in the plasma membrane, which causes the secretion of H+ into the cell wall.
pH decreases, so microfibrils within the cellulose cell walls loosen & become more plastic.
This makes cells more likely to be stretched.
How does IAA work? (part 2: K+)
K+ channels are stimulated to open at the same time as the H+ pumps.
The cell absorbs water via osmosis, which is stored in the vacuole.
This increases the internal pressure of the cell, causing the cell wall to stretch.
The cell elongates.
Spec statement for growth of flowering plants
In flowering plants, specific growth factors move from the growing regions to other tissues, where they regulate the growth in response to a directional stimulus
What are amyloplasts, and what do they do?
Columellar cells near the root tip have heavy organelles: amyloplasts.
Amyloplasts are densely packed with starch, so the sink to the bottom of the cell.
When a root moves from the vertical to horizontal plane, these organelles fall to where the bottom of the cell is.
Relationship between IAA and amyloplasts
IAA is actively transported to the region in the root tip where the amyloplasts have sunk.
Larger [IAA] at lower side of root inhibits cell elongation.
Lower side grows at slower rate than upper side.
This causes the root to bend downwards.
Define taxis
A directional response to a stimulus.
Can be positive or negative if the movement is towards or away from the stimulus.
Example of taxis
Euglena (single-celled photosynthetic organism) shows positive phototaxis
Define kinesis
A non-directional response to a stimulus.
Organisms changes the speed at which it moves, and the rate at which it changes direction.
Kinesis: the rate of movement of an organism is affected by…
the INTENSITY of the stimulus
How does kinesis help an organism survive?
Favourable conditions: slow movement so organism remains in favourable conditions.
Unfavourable conditions: rapid movement so organism more likely to leave conditions.
Autonomic vs somatic nervous system
Autonomic: involuntary (e.g. peristalsis), controls heart rate
Somatic: voluntary (mainly skeletal muscles)
Both the autonomic and somatic nervous systems are part of the…
Motor neurones
Sympathetic vs parasympathetic nervous system
Sympathetic: fight or flight
Parasympathetic: rest & digest
Why are reflexes useful?
They protect the body from harm/prevent damage to the body!
What is the relationship between a receptor and a stimulus?
All receptors are SPECIFIC to ONE TYPE of stimulus!
What do receptors act as (in terms of stimuli)?
Transducers: they convert the stimulus into a nervous impulse (action potential)
What type of receptors are Pacinian corpuscles, and where are they found?
Pressure receptors in the SKIN
Spec point: the stimulation of the Pacinian corpuscle leads to…
the establishment of a GENERATOR POTENTIAL
Where are Pacinian corpuscles found?
At the ends of sensory neurone axons
Describe the structure of a Pacinian corpuscle
- made of layers of membrane separated by a viscous gel (like an onion)
- the gel contains Na+
- the membrane surrounding the sensory neurone contains stretch-mediated Na+ gates
What do stretch-mediated Na+ gates allow Na+ to do?
Move into the neurone via facilitated diffusion
What is a generator potential (in the context of the Pacinian corpuscle?)
An electrical p.d. across the axon membrane
Describe what happens when the Pacinian corpuscle is compressed (i.e. when pressure is detected)
- the layers of membrane are deformed: stretch-mediated Na+ channels open
- Na+ enters the axons via facilitated diffusion
- influx of ions changes electrical p.d. across membrane
- this leads to depolarisation, which establishes a generator potential, which triggers action potential that travel along the sensory neurone
Where are the Na+ usually when the Pacinian corpuscle isn’t stimulated?
There is usually an excess of Na+ SURROUNDING THE AXON
What are rod cells and cone cells sensitive to?
Rod cells: light intensity
Cone cells: different wavelengths of visible light (colour)
Where are the rod and cone cells found?
The retina
Rods and cones: which one shares a neurone, which one gets their own?
Rods share a neurone
Cones get their own
Rods: formal term for sharing a neurone
Synaptic/retinal convergence
Several rods are connected to one sensory neurone
What pigments do rod cells and cone cells contain?
Rods: rhodopsin
Cones: iodopsin
What happens to the pigments in the rods and cones, and what happens as a result?
They are broken down, resulting in a generator potential being produced
How are rhodopsin and iodopsin broken down?
Rhodopsin broken down in dim light
Iodopsin broken down in bright light
What is the condition for a generator potential to be produced in the rods/cones?
If there is enough light to reach the threshold potential
Define light sensitivity
Amount of light required to stimulate receptor
Define visual acuity
Ability to distinguish two separate points
Rods: sensitivity & visual acuity
INCREASE sensitivity: many weak generator potentials from each rod combine to reach the threshold potential in the shared bipolar cell and ganglion cell
DECREASE visual acuity: brain cannot interpret which impulses are sent by specific rods. Multiple rod cells synapse with a single bipolar cell, and multiple bipolar cells synapse with a single ganglion cell
What are the three different cone cells?
Red-sensitive, blue-sensitive, green-sensitive
Connections between rods/cones & nervous system
Synapses connect rods & cones to bipolar neurones, which connect to ganglion cells
Cones: sensitivity & visual acuity
DECREASE sensitivity: single cone cell synapses with single bipolar cell which synapses with single ganglion cell. More light required to reach threshold potential.
INCREASE visual acuity: single cone cell synapses with single bipolar cell which synapses with single ganglion cell. If two cones are stimulated to send an impulse, the brain is able to interpret these as two different spots of light.
What is summation in terms of rods?
When a group or rods are stimulated at the same time, their combined generator potentials are sufficient to reach the threshold and stimulate the bipolar cell to conduct an action potential to the optic nerve.
Which regions of the retina are rods and cones found?
Rods: periphery/edge of retina
Cones: most in fovea, fewer around the periphery
Pros & cons of summation
Pros: enables organisms to see in much dimmer light than cones allow
Cons: produces a less sharp image
Do nocturnal animals tend to have mostly/solely rods or cones?
RODS
Define myogenic
Contractions of heart muscle are initiated from within heart muscle itself, not by nervous system
Where is the SAN?
Within the wall of the right atrium
Role of SAN
Initiates a wave of depolarisation which spreads out through atrial walls, causing atria to contract
What is in between the atria and ventricles and why is this structure important?
Annulus fibrosus: region of non-conducting tissue
Stops depolarisation spreading straight into ventricle walls
Where is the depolarisation carried to after the SAN?
AVN: region of conducting tissue between atria & ventricles
Why is important that there is a slight delay between AVN stimulation & passing impulse to bundle of His?
Ventricles contract after atria
What specifically happens to the impulse in the bundle of His?
Passes along Purkyne fibres, carrying the wave of excitation along with them.
How do the ventricles contract?
The Purkyne fibres spread around the ventricles & initiate the depolarisation fo the ventricles from the apex of the heart upwards.
Which part of the brain controls heart rate?
Medulla oblongata
Location & structure of medulla oblongata
Found at base of brain near top of spinal cord.
Acceleratory centre: causes heart to speed up, connected to SAN by sympathetic nervous system
Inhibitory centre: causes heart to slow down, connected to SAN by parasympathetic nervous system
Which branch of the nervous system do the neurones involving the medulla oblongata etc belong to?
Autonomic nervous system
Where can chemoreceptors and baroreceptors be found?
Carotid artery & aorta
Stimulus: high blood pressure
Baroreceptors in aorta & carotid arteries detect pressure change
Medulla sends impulses along parasympathetic nervous system from inhibitory centre to REDUCE heart rate
SAN reduces frequency of electrical waves it produces due to secretion of acetylcholine at the synapse with SAN
Stimulus: low blood pressure
Baroreceptors in aorta & carotid arteries detect pressure change
Medulla sends impulses along sympathetic nervous system from acceleratory centre to INCREASE heart rate
SAN increases frequency of electrical waves it produces due to secretion of noradrenaline at synapse with SAN
Stimulus: low CO2 (high pH)
Chemoreceptors in aorta, carotid arteries & medulla detect change
Medulla sends impulses along parasympathetic nervous system from inhibitory centre to REDUCE heart rate
SAN reduces frequency of electrical waves it produces due to secretion of acetylcholine at the synapse with SAN
Stimulus: high CO2 (low pH)
Chemoreceptors in aorta, carotid arteries & medulla detect change
Medulla sends impulses along sympathetic nervous system from acceleratory centre to INCREASE heart rate
SAN increases frequency of electrical waves it produces due to secretion of noradrenaline at synapse with SAN
Why might an individual have a low blood pH/high CO2 levels?
Exercise (think lactic acid)
