Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
ii The threshold receptor potential is the
smallest receptor potential at which an
action potential is generated.
iii Neurones either send impulses or they
do not. For example, no impulses are
sent by a sensory neurone below the
receptor threshold. Neurones do not have
graded impulses with action potentials of
different sizes.
b Increasing the strength of the stimulus
produces an increasing receptor potential. At
a low strength, a small increase results in a
relatively large increase in receptor potential.
At higher strengths, the increase in receptor
potential is less. (The functional significance of
this pattern, which is found in most receptors,
could be discussed; it results in a relatively
high level of sensitivity to low-level stimuli as
long as they are above the critical threshold).
c The greater the strength of the stimulus
applied, the greater the frequency of action
potentials generated.
d the length of the refractory period
e Receptors can adjust to a constant strength
of stimulus and reduce the frequency with
which they send impulses to the CNS. This
saves energy in ion pumping that results
from sending many impulses along a sensory
neurone. This gives the CNS information
about changes in environmental and
internal conditions.
5 a exocytosis
b Acetylcholinesterase catalyses the
hydrolysis of ACh so it does not remain in the
synaptic cleft continuously stimulating the
postsynaptic neurone. If it did, muscle cells
would be continuously stimulated to contract
– a condition known as tetany.
6 a At the synapse, vesicles of transmitter
substance are only present in the presynaptic
neurone, not in the postsynaptic neurone.
b Repeated action potentials may cause the
release of transmitter substance into the cleft
at a greater rate than it can be replaced in the
presynaptic neurone. Effectively, the neurone
has no more transmitter substance until
enough is resynthesised.
7 a and b
⎫ ⎬⎭
I band
thick filament
A band
Z line
thin filament
c Measure the length of the sarcomere (Z line
to Z line) in millimetres.
Multiply this by 1000 to give the distance in
micrometres.
To calculate the magnification, divide this
distance in micrometres by 2.25.
8 similarities:
■■
arrival of an impulse at the end of the
presynaptic neurone stimulates the
opening of voltage-gated calcium ion
channels and the entry of calcium ions into
the synaptic bulb
■■
calcium ions stimulate the movement of
vesicles to the presynaptic membrane
■■
neurotransmitter is released by exocytosis
■■
both release neurotransmitter to transmit
impulses across a gap
■■
neurotransmitter binds to receptor on
postsynaptic membrane
■■
chemically gated sodium ion channels open
to allow sodium ions to diffuse through
the postsynaptic membrane (membrane
of dendrite of neurone or sarcolemma of
muscle fibre) to cause depolarisation
differences:
■■
■■
■■
■■
interneuronal synapses release a range
of transmitter substances, including
acetylcholine, GABA and dopamine;
acetylcholine is the usual neurotransmitter
at neuromuscular junctions
sarcolemma is postsynaptic membrane at
neuromuscular junctions
sarcolemma is folded to give a large
surface area, postsynaptic neuronal
membranes are not folded
at the neuromuscular junction an action
potential passes along the sarcolemma
and down T-tubules; nothing equivalent
Cambridge International AS and A Level Biology © Cambridge University Press 2014