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A-Level Biology > Unit 6 - Response to Stimuli > Flashcards

Unit 6 - Response to Stimuli Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

what changes in their environment do organisms respond to & what is the effect?

A

organisms detect & respond to internal & external stimuli

–> increases survival chances & increases chances of reproduction so passes on beneficial alleles

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2
Q

why is there always a strong selection pressure?

A

to avoid danger/predation
to detect prey
to avoid toxic build up e.g. CO2
to ensure effective O2 delivery by altering heart rate

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3
Q

what is the purpose of taxis & kinesis?

A

they are simple movements that can maintain a mobile organism in a favourable environment

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4
Q

describe kinesis

A

simple, non-directional movement of mobile organism
in response to unfavourable stimulus
changes the speed at which the organism moves & the rate at which it changes direction depending on conditions
in response to non-directional stimulus e.g. temperature

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5
Q

in kinesis, what happens if an organism is in favourable conditions (or has just moved from favourable to unfavourable conditions)?

A

rate of changing direction increases to increase chances of returning to favourable conditions quickly

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6
Q

in kinesis, what happens if an organism is in unfavourable conditions?

A

rate of changing direction decreases so organism moves in straighter line to increase chances of finding a location with favourable conditions (surrounded by +ve stimuli)

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7
Q

describe taxis

A

more advanced than kinesis
directional movement of mobile organism towards favourable conditions & away from unfavourable conditions
+ve taxis = towards stimulus
-ve taxis = away from stimulus
in response to directional stimulus e.g. light, chemicals, gravity etc.

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8
Q

describe tropism & example

A

plant growth response (or part of a plant)
in response to directional stimulus
enable favourable conditions for max. growth

e.g. shoots show +ve phototropism & -ve gravitropism
roots show -ve phototropism & +ve gravitropism & +ve hydrotropism

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9
Q

what causes tropism?

A

uneven distribution of IAA auxin, which causes uneven cell elongation & growth

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10
Q

what do plants produce to control growth & responses to light & gravity?

A

hormones

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11
Q

what is the benefit of phototropism?

A

to aid photosynthesis

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12
Q

what is the benefit of gravitropism?

A

to obtain water, mineral ions & better anchorage

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13
Q

what does IAA stand for?

A

indolacetic acid

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14
Q

describe the response of shoots to light from directly above? (phototropism)

A

IAA diffuses evenly to both sides of the shoot
so even cell elongation & growth on both sides
so shoot grows straight up

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15
Q

describe the response of shoots to light from one direction? (phototropism)

A

IAA diffuses to shaded side of the shoot
so cells on shaded side elongate more & grow faster than cells on sunny side
so shoot grows towards light

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16
Q

what is the effect of the force of gravity on IAA?

A

the force of gravity causes IAA to accumulate on the underside of roots & shoots

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17
Q

describe gravitropism (response to gravity) in roots

A
  1. cells in root tip produce IAA
  2. IAA diffuses & accumulates on underside of root due to the force of gravity
  3. IAA inhibits cell growth & elongation on underside of root
  4. so cells on upperside grow faster & elongate more than underside cells
    –> so roots grow downwards in the direction of gravity
    +vely gravitropic
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18
Q

describe gravitropism (response to gravity) in shoots

A
  1. cells in shoot tip produce IAA
  2. IAA diffuses & accumulates on underside of shoot due to force of gravity
  3. IAA stimulates cell growth & elongation on underside of shoot
  4. so underside cells grow faster & elongate more than upperside cells
    –> shoot grows upwards against gravity
    -vely gravitropic
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19
Q

describe the organisation of the nervous system

A

CNS: brain, spinal cord

peripheral nervous system (PNS):
sensory pathways (S neurones from receptor to CNS)

motor pathways:
somatic/voluntary NS - conscious control e.g. movement
autonomic/involuntary NS - subconscious control e.g. heart rate: sympathetic - stimulate effectors & speed up
parasympathetic - inhibits effectors & slows down

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20
Q

what is a reflex & e.g.?

A

a rapid, short-lived, localised & involuntary response to a dangerous/harmful stimulus

e.g. removing hand from hot object

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21
Q

what makes a reflex rapid?

A

very few synapses (chemical message is slower than electrical impulse)
short neurone pathway
does not go to conscious part of brain

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22
Q

why are reflexes important? classic exam Q

A

to decrease or avoid damage - give e.g. related to Q
to escape from predators
to maintain balance/posture
role in homeostasis

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23
Q

describe the reflex arc (in exam, relate to Q)

A
  1. stimulus e.g. sharp pin
  2. receptor - pressure/mechanoreceptors in skin detect stimulus & generate potential in sensory neurone
  3. sensory neurone transmits action potential to spinal cord in CNS
  4. relay/intermediate neurone links sensory neurone to motor neurone
  5. motor neurone transmits action potential from spinal cord (CNS) to effector = muscle or gland e.g. muscles on finger/arm
  6. effector - muscle contracts/gland secretes e.g. finger/arm muscle contracts
  7. response e.g. pull finger/hand away from sharp object
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24
Q

from Seneca: function of dendrites, axon & cell body

A

dendrites carry impulse towards cell body
axon - away
cell body - where nucleus is located

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25
Q

what is the structure of a myelinated motor neurone?

A

one long axon

many dendrites - large SA for receiving action potentials (APs) from relay neurone

cell body - contains organelles, lots of RER & mitochondria for protein synthesis (channel proteins) & neurotransmitters

Schwann cells - wrap around axon, provide protection & electrical insulation & contains myelin sheath

nodes of Ranvier - gaps b/w Schwann cells where there is no myelin sheath

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26
Q

what are 3 functions of Schwann cells?

A

electrical insulation
phagocytosis
nerve regeneration

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27
Q

how does an AP travel along a neurone? (general structure)

A

by saltatory conduction
from one node of Ranvier to the adjacent node

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28
Q

what makes neurones excitable?

A

have resting potential & 3 protein transporters:
1. sodium-potassium pump
works all the time
all over the neurone

  1. open Na+ & K+ channels all over the neurone
    there are more K+ channels than Na+ channels - membrane is more permeable to K+
  2. voltage-gated channels
    sensitive to charge around them
    all over axon
    lots on axon hillock (mainly VgNa+)
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29
Q

describe the neurone when it has no membrane potential (theoretical)

A
  1. equal conc. K+ & Na+ inside & outside of axon
    no membrane potential: 0mV
    no diffusion of K+ & Na+
30
Q

describe the neurone when decreasing membrane potential

A

sodium-potassium pump uses active transport to move 3 Na+ out & 2K+ into axon
–> increase conc. K+ & decrease conc. Na+ in axon
no K+ & Na+ diffusion

so overall decrease in # of positively charged ions in membrane –> decrease in membrane potential: -10mV

31
Q

describe the neurone when creating & maintaining a resting membrane potential

A

sodium-potassium pump uses active transport to move 3 Na+ out & 2K+ into axon

K+ diffuses out of axon by fac. dif. via open channel proteins down electrochemical gradient

Na+ diffuses into axon by fac. dif. down electrochemical gradient

axon membrane is more permeable to K+ than Na+ so conc. of positive ions inside axon decreases to -65mV = resting potential

32
Q

what are the essential factors for creating & maintaining resting potential?

A
  1. sodium-potassium pump actively transports 3Na+ out & 2K+ into axon using ATP
  2. axon membrane is more permeable to K+ (bc it has more K+ channel proteins) so more K+ diffuses out of axon than Na+ diffuses in
33
Q

how is resting potential established (2 marker)?

A
  1. membrane is more permeable to K+ than Na+ bc it has more K+ channels
  2. sodium-potassium pump actively transports 3Na+ out & 2K+ into axon
    establishes electrochemical gradient
34
Q

what is a generator potential?

A

a small depolarisation of the neurone’s membrane potential, causing a deviation from the resting potential at -65mV

35
Q

define depolarisation

A

the neurone’s membrane becomes less negative due to an influx of Na+ ions

36
Q

where do generator potentials occur?

A

at receptor cells or sensory nerve endings e.g. in Pacinian corpuscle

37
Q

what causes generator potentials?

A

energy transduction, where a receptor detects a stimulus in an energy form (as a result of an energy change)
this energy is used to open VgNa+

38
Q

how is an AP caused (linked to generator potentials)?

A

if generator potential causes a large enough depolarisation of membrane (above -50mV) due to sufficient diffusion of Na+ into axon, AP triggered

39
Q

what is the all or nothing law?

A

any stimulus that causes the membrane potential to reach/exceed the threshold value triggers an AP

all APs have the same magnitude

generator potentials below the threshold value of -50mV will not trigger an AP

40
Q

describe the movement of Na+ ions when the stimulus is sub-threshold

A
  1. receptor detects a small energy change/stimulus
  2. some Vg Na+ channels open –> some Na+ diffuses into axon, down electrochemical gradient by fac. dif. –> membrane potential slightly less negative but does not reach threshold value of -50mV
  3. other VgNa+ channels do NOT open so no AP triggered
41
Q

describe the movement of Na+ ions when the stimulus is above threshold

A
  1. receptor detects large energy change/stimulus
  2. many VgNa+ channels open –> lots of Na+ ions diffuse into axon down EC gradient –> large depolarisation of membrane
  3. this causes positive feedback = more VgNa+ channels to open –> greater influx of Na+ –> this reaches/exceeds the threshold value of -50mV so AP is triggered
42
Q

all APs are the same amplitude no matter how large the initiating stimulus

A
43
Q

what are the stages of the action potential?

A

resting potential
depolarisation
repolarisation
hyperpolarisation
restoring resting potential

44
Q

what happens during resting potential?

A

-65 mV
sodium-potassium pump moving 3Na+ ions out & 2K+ ions into axon
VgNa+ & VgK+ channels closed

45
Q

what happens during depolarisation?

A

generator potential up to -50mV
fac. diff. Na+ ions into cell down electrochemical gradient
membrane potential becomes more positive
at -50mV threshold, Vg Na+ channels open, causing influx of Na+
this causes positive feedback so more Na+ channels open
membrane potential increases to +40mV

46
Q

what happens at +40mV?

A

Na+ equilibrium is reached at +40mV
VgNa+ channels close & VgK+ channels open

47
Q

what happens during repolarisation?

A

VgNa+ channels close & VgK+ channels open
fac. diff. of K+ ions out of axon down electrochemical gradient
membrane potential becomes more -ve

48
Q

what happens during hyperpolarisation?

A

when K+ ions diffuse out, membrane potential becomes more -ve than resting potential (as VgK+ channels are slow to close)
VgNa+ & VgK+ channels close
K+ equilibrium is reached at-90mV

49
Q

what is the importance of the refractory period?

A

no AP can be generated in hyperpolarised parts of membrane
promotes separate impulses
ensures unidirectional impulse
creates a time delay b/w APs
limits frequency of AP

50
Q

describe the passage of AP in an unmyelinated axon

A
  1. stimulus causes influx of Na+ ions so first section of membrane depolarises
  2. localised currents occur
  3. which causes VgNa+ channels further along membrane to open so neighbouring regions of membrane depolarise
  4. meanwhile, the previous region of membrane repolarises & is hyperpolarised then resting potential restored
51
Q

describe the passage of AP in a myelinated axon

A

axon surrounded by myelin sheath produced by Schwann cells wrapping around axon
myelin is a mixture of lipids & acts as insulation
there are not VgNa+ or VgK+ channels in the axon membrane underneath the myelin
APs can only happen at Nodes of Ranvier, which are gaps b/w the myelin sheath
localised currents stretch b/w Nodes of Ranvier, speeding up the transmission of the impulse by 3 times
the impulse jumps b/w Nodes of Ranvier, which is saltatory conductance

52
Q

what factors affect the speed of conductance of AP?

A

the myelin sheath
axon diameter
temperature

53
Q

how does the myelin sheath affect the speed of conductance?

A

causes nerve impulses to jump from one Node of Ranvier to another, called saltatory conduction
this increases the speed of transmission

54
Q

how does the diameter of the axon affect the speed of conductance?

A

the greater the diameter, the faster the speed of impulse
bc less leakage of ions from larger axons
so easier to maintain membrane potential

55
Q

how does temperature affect the speed of conductance?

A

the higher the temperature, the faster the speed of impulse up to a point
increased temp. = increased rate of diffusion of Na+ & K+ ions bc increased KE
increased temp. = increased rate respiration so increased atp production for sodium-potassium pump

56
Q

greater strength of stimulus

A

= greater frequency of APs

57
Q

what is the function of a synapse?

A

electrical impulse cannot travel over junction b/w neurones
neurotransmitters send impulses b/w neurones & to effectors
new impulses can be initiated in several different neurones for simultaneous responses

58
Q

define synapse

A

the gap b/w 2 neurones
the point where one neurone communicates with another neurone or w an effector

59
Q

describe the structure of a cholinergic synapse

A

presynaptic neurone:
presynaptic knob:
lots of mitochondria
SER
VgCa2+ ion channels
synaptic vesicles containing neurotransmitter/acetylecholine (ACh)

synaptic cleft:
gap b/w neurones

postsynaptic neurone:
receptors complementary to neurotransmitter/ACh
ligand-gated Na+ channels

60
Q

how does an impulse travel across a cholinergic synapse?

A
  1. When AP arrives, Na+ enters through VgNa+ channel. depolarisation of membrane causes VgCa2+ channels to open.
  2. Ca2+ ions enter via fac. dif., causing vesicles containing ACh to move towards presynaptic membrane (requires ATP)
  3. vesicles fuse with presynaptic membrane & release ACh into synaptic cleft (exocytosis)
  4. ACh diffuses across synaptic cleft towards post-synaptic membrane
  5. ACh binds to complementary receptors on ligand-gated Na+ channels on post-synaptic membrane. ligand-gated Na+ channels open so Na+ diffuses in
  6. this causes depolarisation of post-synaptic membrane so VgNa+ channels open so Na+ diffuses into axon so new AP initiated
61
Q

how does a synapse ensure unidirectionality of impulse?

A

1- neurotransmitter only produced in presynaptic neurone

2- ligand-gated Na+ channels are only in post-synaptic membrane

so impulse always goes from presynaptic to postsynaptic neurone

62
Q

what happens when ACh binds to ligand-gated Na+ channels?

A

ACH binds to receptor site
which causes conformational change in protein (3 structure changes)
so Na+ enters

63
Q

why is ACh recycled?

A

too slow & energy costly to produce new ACh every time

64
Q

describe how neurotransmitter/ACh is recycled

A
  1. enzyme acetylcholinesterase (AChE) binds to ACh & hydrolyses ACh into acetyl + choline so it is released from receptors & ligand-gated Na+ channels close

prevents overstimulation of skeletal muscle cells

  1. choline is reabsorbed into presynaptic knob & recombined with acetyl in SER (requires ATP)
  2. ACh is packaged into vesicles for future use
65
Q

define summation & name the 2 types

A

neurotransmitter from several sub-threshold impulses accumulate to generate an AP

temporal summation
spatial summation

66
Q

define & describe the process of spatial summation

A

several simultaneous APs from different presynaptic neurones cause neurotransmitter release & converge onto one postsynaptic neurone

sub-threshold: if AP from only one presynaptic neurone, insufficient neurotransmitter is released

above threshold: if AP from more than one presynaptic neurone, sufficient neurotransmitter is released so AP triggered in postsynaptic neurone

67
Q

define & describe the process of temporal summation

A

one presynaptic neurone has a high frequency of APs so releases neurotransmitter several times quickly

sub-threshold: no AP in postsynaptic neurone bc insufficient neurotransmitter released

above threshold: AP produced in postsynaptic neurone bc sufficient neurotransmitter released

68
Q

what type of synapse is a cholinergic synapse?

A

excitatory

69
Q

describe excitatory synapse

A

AP in presynaptic neurone increases chance of AP occurring in postsynaptic neurone

70
Q

describe inhibitory synapse

A

AP in presynaptic neurone decreases the chance of AP occurring in postsynaptic neurone
1. neurotransmitter binds to & opens Cl- channels in postsynaptic membrane & K+ channels open
2. Cl- moves in & K+ moves out by fac. dif.
3. membrane potential becomes more -ve = hyperpolarised
4. reaching -50mV threshold for AP is less likely bc more excitatory neurotransmitter is needed

71
Q

by what mechanisms do drugs increase & decrease synaptic transmission?

A

increase: inhibit AChE
mimic shape of neurotransmitter & bind to receptor site on ligand-gated Na+ channel so it opens

decrease: inhibit release of NT
decrease permeability of postsynaptic neurone to ions
hyperpolarise postsynpatic membrane

A-Level Biology (9 decks)

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