Response To Stimuli 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
Diffuse from cell to cell/ phloem mass transport
Explain why shoots show positive phototropism
- Indoleacetic acid (IAA) diffuses to shaded side of shoot tip
- As IAA diffuses down shaded side it causes active transport of H+ ions into the cell wall
- Disruption to H-bonds between cellulose molecules make cells more permeable to water
- Cells on shaded side elongate faster due to higher turgor pressure
- Shoot bends towards the light
Explain why roots show positive gravitropism
- Gravity causes IAA to accumulate on the lower side of the root
- IAA inhibits elongation of root cells
- Cells on the upper side of the root elongate faster so the tip bends downwards
Define taxis
Directional movement in response to external stimulus
Define Kinesis
Non directional response to presence and intensity of external stimulus
What is the advantage of taxis and kinesis?
Maintain mobile organism in optimum environment
Why do many organisms respond to temperature and humidity via kinesis rather than taxis?
Less directional stimuli; often no clear gradient from one extreme to the other
How could a student recognise kinesis in an organisms movement?
- The organism crosses sharp division between favourable and unfavourable environment and turning increases, in favourable environment
- If an organism is in the unfavourable environment turning decreases and it begins to move in long straight lines
Outline what happens in a simple reflex arc
Receptor detects stimulus - Sensory neurone - Relay neurone - Motor neurone - response by effector
Give 2 advantages of a simple reflex
Rapid response to potentially dangerous stimuli
Instinctive
What features are common to all sensory receptors?
Act as energy transducers which establish a generator potential
Respond to specific stimuli
Describe the structure of a Pacinian corpuscle
Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and 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 the influx of Na+ raises the membrane to the threshold a generator potential is produced
- Action potential moves along the sensory neurone
Name the 2 types of photoreceptor cell located in the retina
Cone cells and rod cells
Where are rod and cone cells located in the retina?
Rod- Evenly distributed around the periphery but not in the central fovea
Cone- Mainly central fovea, no photoreceptors at blind spot
Give 3 differences between rod and cone cells
Rod has rhodopsin and cones have 3 types of iodopsin
Rods have a low visual acuity- many attach to one bipolar neurone and cones have a high visual acuity- one attaches to one bipolar neurone
Rods are very sensitive to light- spatial summation (impulses combine from each cell to exceed threshold) and cones are less sensitive to light
Outline the pathway of light from a photoreceptor to the brain
Photoreceptor - bipolar neurone - ganglion cell of optic nerve - brain
What does myogenic mean?
Contraction of heart is initiated within the muscle itself rather than by nerve impulses
What nodes are involved in heart contraction?
Sinoatrial node (SAN)- in the wall of the right atrium
Atroventricular node (AVN) near lower end of right atrium in the wall that separates the 2 atria
How are heartbeats 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 and valves close
- AVN conveys WOD down septum via the Bundle of His, which branches into punkinje fibres along ventricles
- Causes ventricles to contract from apex upwards
What is the formula for cardiac output?
Cardiac output = stroke volume x heart rate
What is the autonomic nervous system?
System that controls involuntary actions of glands and muscles
Divides into the sympathetic and parasympathetic
What is the difference between the sympathetic and parasympathetic nervous system?
Sympathetic - Involved in fight or flight, speeds up activity
Parasympathetic - Involved in normal resting conditions, slows down activity
What are the receptors involved in changing heart rate?
Baroreceptors, detect changes in blood pressure - in carotid body
Chemoreceptors, detect changes in pH e.g due to increase in CO2 concentration - in 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 cardioacceleratory centre in the medulla oblongata
- More impulses to SAN via sympathetic nervous system
- Stimulate 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 cardioacceleratory centre of medulla oblongata
- More impulses to SAN via sympathetic nervous system]
- Heart rate increases so the rate of blood flow to the lungs increases = rate of gas exchange and ventilation rate increase
