Internal & External stimuli are detected & lead to a response Flashcards
Topic 6.1
Explain why shoots show positive phototropism
- IAA diffuses to shaded side of shoot tip
- As IAA diffuses down shaded side, it causes active transport of H+ ions into cell wall.
- Disruption to H-bonds between cellulose molecules & action of expansins make cell more permeable to water
- Cells on shaded side elongate faster due to higher turgor pressure
- Shoot bends towards light
Explain why roots show positive gravitropism
- Gravity causes IAA to accumulate on lower side of root
- IAA inhibits elongation of root cells
- Cells on upper side of root elongate faster, so root tip bends downwards
Define taxis and kinesis. State advantage.
- Taxis: directional movement in response to external stimulus
- Kinesis: non-directional response to presence and intensity of external stimulus
- Maintain mobile organism in optimum environment (e.g. to prevent dessication)
Outline what happens in a simple reflex arc
receptor detects stimulus -> sensory neuron -> relay neuron in CNS coordinates response -> motor neuron response by effector
Basic structure of Pacinian Corpuscle
- Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule
What stimulus does Pacinian corpuscle respond to? How?
- Pressure deforms membrane, causing stretch-mediated Na+ ion channels to open.
- If influx of Na+ raises membrane to threshold potential, a generator potential is produced.
- Action potential moves along sensory neuron
Where are rod and cone cells located in retina?
Rod: evenly distributed around periphery but NOT in central fovea
Cone: mainly central fovea no photoreceptors at blind spot
Outline pathway of light from photoreceptor to brain
Photoreceptor -> bipolar neuron -> ganglion cell of optic nerve -> brain
State name and location of 2 nodes involved in heart contraction
- Sinoatrial node (SAN): within wall of right atrium
- Atrioventricular node (AVN): near lower end of right atrium in wall that separates the 2 atria
Describe how heartbeats are initiated and coordinated
- SAN initiates wave of depolarisation (WOD)
- WOD spreads across both atria= atrial systole.
- Layer of fibrous, non-conducting tissue delays impulse while ventricles fill & valve close.
- AVN conveys WOD down septum via Bundle of His, which branches into Purkinje fibres along ventricles.
- Causes ventricles to contract from apex upwards
Formula of cardiac output
Cardiac output (CO) = Stroke volume (V) x heart rate (R)
State difference between sympathetic and parasympathetic nervous system
Sympathetic involved in ‘fight in flight’ response: stimulates effectors to speed up activity.
Parasympathetic involved in normal resting conditions: inhibits effectors to slow down activity
Name receptors involved in changing heart rate & state their location
Baroreceptors (detect changes in blood pressure): carotid body
Chemoreceptors (detect changes in pH e.g. due to increase in CO2 concentration): carotid body & aortic body
How does body respond to increase in blood pressure?
- Baroreceptors send more impulses to cardio-inhibitory centre in medulla oblongata.
- More impulses to SAN down vagus nerve via parasympathetic nervous system.
- Stimulates release of acetyl-choline, which decreases heart rate
How does body respond to decrease in blood pressure?
- Baroreceptors send more impulses to cardio-acceleratory centre in medulla oblongata.
- More impulses to SAN via sympathetic nervous system.
- Stimulates release of noradrenaline, which increases heart rate & strength of contraction
How does body respond to increase in CO2 concentration?
- Chemoreceptors detect pH decrease and send more impulses to cardio-acceleratory centre of medulla oblongata.
- More impulses to SAN via sympathetic nervous system.
- Heart rate increases, so rate of blood flow to lungs increases= rate of gas exchange & ventilation rate increases.
