organisms respond to changes in their internal and external environments 3.6 Flashcards
aqa alevel biology
when does action potential get generated?
when neurones voltage increases beyond threshold from resting potential
what is the structure of a neurone?
- cell body
- dendrites (towards cell body)
- axon (away from cell body)
- myelin sheath
- nodes of ranvier
how is resting potential set up in the neurone membrane?
- 3Na+ are actively transported out of the axon 2K+ are transported into axon using the Na+/K+ pump and ATP
- this creates an electrochemical gradient where K+ has a higher concentration inside axon and theres a higher concentration of Na+ outside axon
- differential membrane permeability is formed as it is permeable to K+ (channels open) and not permeable to Na+ (channels closed)
what is depolarisation of membrane?
- the specific stimulus arrives and Na+ channels open
- membrane permeability to Na+ increases and Na+ diffuses into axon down the gradient leading to depolarisation (less negative)
- if threshold potential reached an action potential is generated
- more voltage gated Na+ channels open so more Na+ diffuse into axon
what is repolarisation of the membrane?
- voltage gated Na+ channels close
- voltage gated K+ channels open
- K+ ions diffuse out of membrane
what is hyper polarisation? (refractory period)
- K+ gated channels to slow to close so theres a slight overshoot (to many K+ ions diffuse out of axon)
- this is restored by Na+/K+ pump
what is the all or nothing principle?
- for an action potential to be produced depolarisation must exceed threshold potential
- AP produced are always the same magnitude (size) potential
- but if bigger stimuli registered an increase in frequency of AP
what are the differences between unmyelinated axon and myelinated axon?
- unmyelinated has no myelin sheath whereas myelinated axon does
- unmyelinated sheath has no nodes of ranvier but myelinated does
- unmyelinated has slower speed of transmission but myelinated has faster speed of transmission
- depolarisation spreads across the whole axon membrane whereas depolarisation only occurs at the nodes of ranvier (saltatory conduction)
what is saltatory conduction of a nerve impulse?
action potential jumps from nodes of ranvier to the next because nerve impulse cant travel along myelin sheath
how does myelin sheath affect speed of nerve impulses?
- insulates axon
- prevents action potential forming at parts of axon where myelin sheath is
- depolarisation only occurs in nodes of ranvier
how does diameter of axon affect speed of nerve impulses?
- greater the diameter of the axon
- the faster the speed of conduction
- less leakage of ions from larger axon
how does temperature effect speed of nerve impulses?
- increases rate of diffusion of ions as more kinetic energy so leads to faster transmission
- affects the Na+/K+ pump which requires enzymes and ATP
- can denature proteins in plasma membrane (voltage gated channels etc) if temperature to high
what is the refractory period?
- following an action potential when another action potential cant be generated
- membrane has to repolarises before aother action potential can be generated
what is the importance of refractory period?
- impulses are discrete so can travel in one direction and cant travel backwards
- limits frequency of action potential (important in neuronal signalling)
what is a stimulus?
detecable change in the environment detected by receptors
how do organisms increase their chances of survival?
by responding to changes in their environment
what are tropisms?
growth of a plant in response to a directional stimulus (can be negative or positive)
what is positive tropism?
grow towards stimulus
what is negative tropism?
grow away from stimulus
what is gravitropism?
growth of plant in response to gravity
what is phototropism?
growth of plant in response to light
what are tropisms controlled by?
growth factors for example IAA
where is IAA produced?
tips or shoots of plants
what is the role of IAA?
controls cell elongation
what is the effect of IAA in plant shoots? (positive phototropism)
- cells in shoot tip produce IAA
- diffuses down evenly initally
- IAA moves to shades side so concentration increases
- high concentration of IAA stimulates cell elongation
- shoots bend towards the light
what is the effect of IAA in plant roots? (negative phototropism)
- cells in root tips produce IAA
- diffuses down evenly initally
- IAA moves to more shades side so concentration increases
- high concentration of IAA inhibits cell elongation
- roots bend away from light
what is the effect of IAA in plant shoots? (negative gravitropism)
- IAA will diffuse from upper to lower side
- if plant verticle causes cell elongation and plants grow vertically and up
- if plants on its side shoots bend upwards
what is the effect of IAA in plant roots? (positive gravitropism)
- IAA moves to lower side of root
- so upper side elongates
- roots bend down towards gravity and anchors plant in
what is taxis?
when an organism moves its entire body towards (favourable) or away from (unfavourable) a stimulus
what is positive taxis?
organism moves towards stimulus
what is negative taxis?
organism moves away from stimulus
why do earthworms show negative phototaxis?
move away from light to dark to avoid dehydration and predators and find food
why do bacteria show positive chemotaxis?
move towards certain chemicals to aid survival
what is kinesis?
when an organism changes the speed of movement and the rate it changes direction
when do organisms increase the rate it changes direction?
when it crosses the boundary between favourable and unfavourable stimuli so will move around more to quickly get back into favourable conditions
when do organisms decrease the rate it changes direction?
when in an area surrounded by negative stimuli will walk in a relative straight line to increase chances of finding favourable conditions again
what is the order of the reflex arc?
1) stimulus
2) receptor
3) sensory neurone
4) intermediate neurone (passes spinal cord)
5) motor neurone
6) effector (muscle)
what is the structure of motor neurone?
- motor nerve endings
- dendrons with dendrites carry nerve impulse to cell body
- schwann cells wrap around the axon and insulate forming myelin sheath
- node of ranvier (gap between schwann cells)
- axon takes impulse away from cell body to muscle
describe factors of hormal system
- communication by hormones
- tranmission by blood system
- slow transmission
- travels around whole body but only target cells respond
- response widespread
- slow response
- long lasting response
- effect may be permanent and irreversible
describe factors of nervous system
- communication by nerve impulse
- transmission by neurones
- rapid transmission
- impulses travel to specifc parts of the body
- response localised
- rapid response
- short lived response
- effect is temporary and reversible
what is the function of a synapse?
where one neurone communicates with another or with the effector
what is the structure of a neurone?
- synaptic knob
- synaptic vesicle (contains neurotransmitter acetylcholine)
- lots of mitochondria
- Ca2+ channels
- pre synaptic membrane
- synaptic cleft
- post synaptic membrane
- Na+ channel proteins
- receptors on post synapse
what happens in the pre synaptic neurone for a synaptic transmission?
- depolarisation of pre synaptic membrane causes openings of voltage gated Ca2+ channels
- Ca2+ diffuse into neurone
- causes vesicles containing acetylecholine to move and fuse with pre synaptic membrane
- this releases ACH into synaptic cleft via exocytosis
what happens in post synaptic neurone during a synaptic transmission?
- ACH diffuses across synaptic cleft to bind to specific receptors on post synaptic membrane
- this causes Na+ channels to open and diffuse into post synaptic knob cause depolarisation
- if threshold met an action potential generate
what happens to acetylcholine after a synaptic transmission?
- ACH is hydrolysed by acetylcholinerase
- the products (choline and ethanoic acid) reabsorbed by pre synaptic neurone to stop over stimulation
what would happen if choline and ethanoic acid wasnt removed from post synaptic knob?
aceytlcholine would keep binding to receptors causing depolarisation
what is temporal summation?
- when a single pre synaptic neurone releases neurotransmitters many times over in a short period of time
- if conc of neurotransmitter exceed threshold value of post synaptic neurone then new action potential triggered
what is spatial summation?
- many presynaptic neurones share one post synaptic cleft
- collectivelu release sufficient neurotransmitters to reach threshold level to trigger new action potential
what are inhibitory synapses?
- post synaptic receptor have Cl- channels
- Cl- channels open so Cl- ions diffuse in causing hyperpolarisation
- membrane potential more negative than resting potential
- so not enough Na+ ions to reach threshold level to generate new action potential in post synaptic neurone
what is an example of inhibitory synapses?
GABA
what are receptors?
cells that detect and respond to specific stimuli
what is the structure of a pacinian corpuscle?
- lamellae
- axon
- sensory neurone
how does a pacinian corpuscle respond to pressure?
- mechanical stimulus deforms lamellae and stretch mediated channels change)
- so Na+ channels in membrane open and Na+ diffuse into sensory neurone
- causes depolarisation leading to a generator potential
- if generator potential reaches threshold it triggers action potential across sensory neurone
how many cone cells are connected to a bipolar neurone?
one
how many rod cells are connected to a bipolar neurone?
three
what does visual sensitivity to light?
how well you can see in different light intensity (mainly low)
what is the visual sensitivity to light for rods and cones?
rods - high visual sensitivity
cones - low visual sensitivity
what is the visual sensitivity to colour for rods and cones?
rods - low (black/white vision)
cones - high (coloured vision)
what is the visual acuity for rods and cones?
rods - low
cones - high
what is the name of the pigment in rods?
rhodopsin
what is the pigment in cones?
iodopsin
how does rhodopsin affect rods sensitvity to light?
- breaks down easily in low light intensities
- threshold level reached and generator potential in bipolar neurone
- an action potential passes to optic nerve
how does iodopsin affect cones sensitivity to light?
needs high light inetnsity to be broken down
what are the three types of wavelengths cones are sensitive to?
blue light
red light
green light
how do pupils constrict?
circular muscle contract and radial muscle relax
how do pupils dilate?
circular muscle relax and radial muscle contracts
what does myogenic stimulation mean?
the contracts is initated from within the muscle itself rather than a nerve impulse
how is heart rate initiated?
- SAN sends electrical impulse across atria causing atria to contract simulataenously (atriol systole)
- non conductive tissue between atria/ventricles prevent impulse passing directly to ventricles (prevents immediate contraction of ventricles)
- waves of electrical activity reach AVN which delays impulse (allows atria to fully contract + empty)
- AVN sends wave of electrical activity down bundle of HIs conducting wave between ventricle to apex where it branches into puncyne tissue causing ventricles to contract
what is the parasympathetic route to heart rate being controlled?
- pressure revceptors in aorta and carotoid detect high BP
- sends nerve impulse along sensory neurone to the medulla oblongata that decreases heart rate
- sends more impulses along parasympathetic pathway
- aceytlcholine relased
- slows down heart rate and BP returns back to normal
what is the sympathetic route to heart rate being controlled?
- pressyre receptors in aorta and carotoid detect low BP
- sends nervse impulse along sensory neurone to medulla oblongata to increase heart rate
- sends more impulses along sympathetic pathway
- noradrenaline released
- speeds up heart rate and BP back to normal
