Stimuli and Response (receptors) - A2 Flashcards
What are plant growth factors and where are they produced?
- chemicals that regulate plant growth response to directional stimuli
- produced in plant growing regions (apical meristems)
- diffuse from cell to cell / phloem mass transport
Explain why shoots show positive phototropism?
- Indoleacetic acid (IAA) diffuses to shade side of root tip.
- as IAA diffuses to shaded side, it causes active transport of H+ ions into cell wall.
- disruption to H bonds between cellulose molecules and action of expansions make cell more permeable to water (acid growth hypothesis)
- 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 the root
- IAA inhibits elongation of root cells
- cells on upper side of the root elongate faster, so root tip bends downwards.
Define taxes and kinesis. Explain their advantage.
Taxes - directional movement in response to external stimulus.
Kinesis - non-directional response to presence and intensity of external stimulus
- Maintain mobile organism in optimum environment eg. to prevent dessicasion
Many organisms respond to temperature and humidity via kinesis rather than taxes. Explain why.
- less directional stimuli
- often no clear gradient from one extreme to another
How could a student recognise kinesis in an organisms movement?
- organism crosses sharp division between favourable and unfavourable environment - turning increases
- if organism moves considerable distance into unfavourable environment - turning slowly decreases - begins to turn in long straight lines (sharper turns lead organisms to new environment)
Outline what happens in a simple reflex arc.
receptor detects stimulus - sensory neuron -
relay neuron in CNS coordinate response - motor neuron - response by effector
Give the advantage of a simple reflex.
- rapid response to a potentially dangerous stimuli since only 3 neuron involved
- instinctive
What features are common to all sensory receptors?
- act as energy transducers which establish a generator potential
- respond to specific stimuli
Describe the basic structure of a Pacinion corpuscle.
- single nerve fibre surrounded by layers of connective tissues which are separated by viscous gel and are contained by a capsule
- stretch mediated Na+ channels on plasma membrane
- capillary runs along base layer of tissue
What stimulus does a Pacinian corpuscle respond to and 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
Name the 2 types of photoreceptor cell located in the retina.
- cone cells
- rod cells
Where are the rod and cone cells located in the retina?
cone - mainly central fovea, no photoreceptors at blind spot
rod - evenly distributed around periphery but not in central fovea
Outline the pathway of light from a photoreceptor to the brain.
photoreceptor - bipolar neuron - ganglion cell of optic nerve - brain
Define myogenic.
Contraction of heart is initiated within the muscle itself rather than by nerve impulses.
State the name and location of the 2 nodes involved in heart contraction.
Sinoatrial node (SAN) - within the wall of the right atrium
Atrioventricular node (AVN) - near lower end of right atrium in the wall that separates the 2 atria
Describe how heartbeats are initiated and coordinated.
- SAN initiated wave of depolarisation (WOD)
- WOD spreads across both atria = atrial systole
- layer of fibrous, non-conducting tissue delays impulse whilst ventricles fill and valves close
- AVN conveys WOD down septum via Bundle of His, which branches into Purkinje fibres along ventricles
- causes ventricles to contract from apex upwards
What is the autonomic nervous system?
- system that controls involuntary actions of glands and muscles
- 2 subdivisions: sympathetic and parasympathetic
State the difference between the sympathetic and parasympathetic nervous system.
Sympathetic involved in fight or flight response - stimulates effectors to speed up activity
Parasympathetic involved in normal resting conditions - inhibits effectors to slow down activity
Name the receptors involved in changing heart rate and state their location.
Baroreceptors - detect change in blood pressure: carotid body
Chemoreceptors - detect change in PH eg. due to increase in CO2 concentration: carotid body and aortic body
How does the body respond to an increase in blood pressure?
- Baroreceptors send more impulses to cardioinhibitory centre in the medulla oblongata
- more impulses to SAN down vagus nerve via parasympathetic nervous system
- stimulates release of acetylcholine, which decreases heart rate
How does the body respond to a decrease in blood pressure?
- Baroreceptors send more impulses to cardio-acceleratory centre in the medulla oblongata
- more impulses to SAN via sympathetic nervous system
- stimulates release of noradrenaline, which increases heart rate and strength of contraction
How does the body respond to an 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 and ventilation increases
