Organisms respond to changes in their internal and external environments Flashcards
what is sensitivity
ability of a living organism to detect changes in the environment (stimuli), and respond appropriately to them. many of these are automatic, fast and innate. these are reflexes
the order of the reflex arc
stimulus - receptor - sensory neurone - cell body - synapse - relay neurone - motor neurone - effector (muscle/gland)
what is a tropism
a growth response in a plant. they are controlled by specific growth in factors e.g. auxins
phototropism
plant shoots are +ve trophic (grow towards light). the growth is controlled by the auxin-indoleacetic acid which is made in the shoot tip and moved down into the growing region of the shoot
how does phototropism work
Auxin moves to the dark side of the shoot. the auxin promotes growth by interfering with the hydrogen bonds in the cell wall. his makes the cellulose more flexible, allowing elongation and division of the cell
gravitropsim/ geotropism
roots are +ve gravitropic - they grow towards gravity. this is due to the presence of dense organelles called amyloplasts
how deos gravitropiam work
as amyloplasts move to the bottom of the roots, they take the IAA with them. In the roots, the IAA inhibits growth and division
define taxis
a directional movement response. a positive taxis, movement towards stimulus
define kinesis
a non-directional movement. the rate of movement and frequency of turns increases as the stimulus is less favourable
what is the Pacinian corpuscle
pressure receptor in the skin of mammals. it is a series of membranes (lamellae) around the end of an axon (nerve cell)
what are within the membranes of the Pacinian corpuscle
stretch-mediated channel protein, which opens when pressure is applied. this allows facilitated diffusion of the Na+ between the lamellae and into the axon
what does the movement of ions across the membrane change
the membrane potential or potential difference
the effect of the change in potential difference
If sufficient Na+ cross the membrane and create a big enough change in potential difference to pass a threshold. then, a generator potential is achieved and a nerve impulse (action potential) is produced.
what are the two types of photoreceptors in the retina
- cone cells
- rod cells
what do cone cells provide
colour vision with high visual acuity. however, they only function in higher light intensity
what are the three types of cone cells
- red sensitive
- blue sensitive
- green sensitive
what is the trichromate theory of colour vision
red, blue, green-sensitive cone cells detect and respond to different wavelengths of light. the combination gives us colour perception
what are rod cells
detect low light intensity but only provide black-and-white vision with low visual acuity
where do we get our highest visual acuity
our fovea only has cone cells and they connect to just one neurone which means that the brain knows exactly where on the retina the light is focused.
where do we get low visual acuity
in rod cells, as many rod cells connect to one neurone so our brain does not know exactly where the light is focused
what happens in low light intensity with rod cells
the generator potentials produced in rod cells can summate, enabling the threshold to be passed and an action potential produced
cells respiring
when cells respire they produce CO2, which diffuses into blood plasma where it forms a carbonic acid
H2O + CO2 <=> H2CO3
this reaction is catalysed by carbonic anhydrase
H2CO3 dissociates into
H+ and HCO3-. The H+ diffuse into red blood cells and binds to haemoglobin, making haemoglobinic acid. this breaks H and ionic bonds temporarily altering the tertiary structure of haemoglobin, forcing oxygen to dissociate from it - bohr effect
what are the two parts of the nervous system
central nervous system
- brain
- spinal cord
peripheral nervous system
- neurones
receptor
peripheral nervous system
- somatic (voluntary)
- autonomic (involuntary)
-sympathetic - fight & flight
-parasympathetic - rest and digest
chemoreceptors
detect changes in blood pH are found in the aorta, carotid artery and the medulla (brain)
what happens when blood pH falls
chemoreceptors generate more frequent action potentials, that go to the cardioregulatory centre in the medulla (brain). this sends more frequent action potentials along sympathetic nerves that release excitatory neurotransmitters onto the sino-atrial node, increasing heart rate
what happens when blood pH increases
chemoreceptors send less frequent action potentials to the cardioregulatory centre. this sends more frequent action potential along parasympathetic nerves, releasing inhibitory neurotransmitters onto the sino atrial node, decreasing heart rate
what are baroreceptors
these detect blood pressure in the aorta and medulla. when blood pressure is too high they slow the heart rate, and vice versa
features of motor neurone
- dendrites
- nucleus
- cell body
- Schwann cells
- axon
- myelin sheath
- node of Ranvier
- motor end plate
what is a resting potential
when a neurone is “at rest” and is not transmitting an action potential
the resting potential for all neurones
- NA+/K+ pump actively transports 3x Na+ out of the axon while actively transporting 2x K+ into it.
- membrane is differentially permeable - no Na+ channels are open, K+ channels are. K+ moves by facilitated diffusion out of the axon
- there are more +Ve ions outside the cells, than inside. this has a membrane potential of -70Mv
the action potential
- due to a stimulus, Na+ channels opens. Na+ moves by facilitated diffusion into the axon. the bigger the stimulus the more channels open
what happens when the threshold is met in an action potential (describing the graph)
- many more voltage-gated Na+ channels open, allowing Na+ into the axon by facilitated diffusion
- membrane is depolarized and the membrane potential is +40Mv (AP)
- Na+ channels close and K+ open. K+ moves out the axon by facilitated diffusion, repolarising the membrane
- K+ channels are slow to close, more K+ leaves the axon than necessary. membrane is hyperpolarised before the Na+/K+ pump can restore the resting potential - aka refractory period
what is the refractory period
the brief time of hyperpolarisation ensures that the action potential remains discreet and unidirectional
The all-or-nothing principle
this states that an action potential is approximately 40mV, it doesn’t change according to the strength of the stimulus
effect of a stronger stimulus
stronger stimulus will generate a higher frequency of action potential, than a weaker stimulus
what is the speed of conduction of an impulse
refers to how quickly the impulse is transmitted along a neurone
What factors are the speed of conduction affected by
- presence/ absence of myelin sheath (acts as insulation)
- diameter of the axon
- Temperature
speed of conduction in unmyelinated neurones
the speed of conduction is very slow, as depolarisation must occur along the whole membrane of the axon
why the speed of conduction is faster in myelinated neurones
myelin increases the speed which action potentials travel:
- myelin sheath is formed from Schwann cells
- parts of the axon that are surrounded by myelin sheath, depolarisation and action potentials can’t occur, as myelin sheath stops the diffusion of Na+ and K+
- Action potentials only occur at the nodes of Ranvier (small uninsulated sections of the axon)
how are the nodes of ranvier involved in the speed of action potential
- local circuits of current that trigger depolarization in the next section of the axon membrane exist between the nodes of Ranvier
- Schwann cells means action potentials ‘jump’ from one node to the next (aka saltatory conduction)
- Saltatory conduction allows the impulse to travel much faster (up to 50x) than in an unmyelinated axon
the different nodes involved in myelination
- node at refractory period
- node at action potential
- node becomes depolarised
- node at resting potential
node at refractory period
- membrane becoming repolarised
- Na+ channel proteins closed
- K+ channel proteins open
node at action potential
- membrane fully repolarised (+30mV)
- all Na+ channel proteins open
- K+ channel proteins closed
node becoming depolarised
- membrane potential moving towards threshold level
- Na+ channels starting to open but many are still closed
- K+ channel proteins closed
node at resting potential
- membrane potentia aournd -70mV
- Na+ channel proteins closed
- K+ channel proteins closed
diameter effect on the conduction of speed
impulse will be conducted at a higher speed along neurones with thicker axons than with thinner axons
reasons why thicker axons have a high speed of conductions
- Thicker axon membrane has a greater surface area, which the diffusion of ions can occur
- increases the rate of diffusion of Na+ and K+ through protein channels, which increases the rate that depolarisation and action potentials occur
- also have a greater volume of cytoplasm (contains ions). This reduces their electrical resistance so action potentials push into the next section faster
temperature
as temperature increases so does the kinetic energy, increasing the rate of facilatated diffusion of K+ and Na+ during an action potential
features of cholinergic synapse
- pre-synaptic knob
- axon
- Ca2+ carrier proteins
- vesicles with acetycholine
- synaptic cleft
- acetycholine receptors
- post synaptic membrane
The cholinergic synapse
Transmission of action potential
- action potential arrives at the synaptic knob and opens Ca2+ channels
- Ca2+ moves by facilitated diffusion into the synaptic knob
- Ca2+ stimulates vesicle to move to the membrane, fuse and release acetylcholine (ACH) by exocytosis
- ACH diffuses across the synaptic cleft, it binds to specific complimentary receptors
- Receptors are part of ligand-gated Na+ channels that open when ACH binds
- Na+ enters the post-synaptic neurone by facilitated diffusion, depolarising the membrane
the cholinergic synapse
how the membrane is repolarised
- ACH is hydrolysed by acetylcholinesterase into acetate and choline
- acetate and choline are actively transported back into the pre-synaptic knob and reformed into ACH
- Ca2+ in the synaptic knob is actively transported out
what do synapses do
Synapses allow control of the nervous system and can form in several different arrangements
spacial summation
where all presynaptic knobs release ACH together is the threshold passed post synaptic membrane.
e.g. rod cells in the eyes
temporal summation
These synapses only trigger an action potential on the post synaptic membrane if the frequency of action potential is high enough in the presynaptic knob.
e.g. in cone cells
