Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
Answers to SAQs
Chapter 15
1
Feature Motor neurone Sensory neurone
Position of
cell body
Location of
cell body
Direction of
impulses
Function
at one end of the cell
grey matter of CNS
CNS to effector
stimulates muscle
to contract; some
glands to secrete
(e.g. adrenaline from
adrenal glands)
part way along the
axon
dorsal root ganglion
/ near receptor
receptor cells to
CNS
transmits
information about
internal or external
environment to CNS
2 A wide variety of answers is possible, some of
which are suggested in the table below.
Stimulus Receptor Effector Response
sudden
loud
sound
smell
of food
cooking
sharp tap
on knee
hair cells in
cochlea of ear
chemoreceptors
in nose
stretch receptors
various
muscles
especially
in legs
salivary
glands
thigh
muscle
rapid
contraction
producing
movement
secretion of
saliva
contraction,
causing
lower leg to
be raised
3 a Resting potentials vary between –60 and
–70 mV, so the line could be drawn anywhere
between these two figures.
b The inside of the axon has a charge of –70 mV
compared with the outside.
c By active transport using ATP to provide
energy for sodium–potassium pumps in the
membrane of the axon. Both sodium and
potassium ions are positively charged. Three
sodium ions are pumped out for every two
potassium ions pumped in, so this builds up a
positive charge outside compared with inside.
There are also many negatively charged
organic compounds (e.g. proteins) inside the
axon that contribute to the resting potential.
d i It is called depolarisation because the axon
was polarised so that it had a negative
charge inside and a positive charge outside.
Now this is altered and there is a negative
charge outside and a positive charge inside.
ii Annotations for the rising phase of the
action potential could be:
Voltage-gated sodium ion channels in the
membrane open and allow sodium ions
to diffuse in down their electrochemical
gradient. As they enter the axon, their
positive charge causes the negative charge
inside the axon to be brought to zero and
become more positive until it reaches
+30 mV. There is a positive feedback at
this time as depolarisation stimulates
more and more voltage-gated sodium ion
channels to open.
e Annotations could be:
Between 1 ms and 2 ms, the axon is
repolarised. The voltage-gated sodium ion
channels close (and cannot open). Voltagegated
potassium ion channels open and
potassium ions diffuse out to restore the
resting potential.
f about 4 ms
g The refractory period is the period of time
after the axon membrane has been stimulated
and an action potential begun until the
membrane can be stimulated again. You can
show it on the graph between the time when
the potential difference reaches –50 mV (the
threshold for an action potential) and the time
when the resting potential is restored.
4 a i A receptor potential is an electrical
potential generated in a receptor such as
a chemoreceptor. It is produced by the
inflow of sodium ions through channel
proteins. This causes a depolarisation
of the receptor to give a less negative
potential inside the cell.
Cambridge International AS and A Level Biology © Cambridge University Press 2014