Download Q1: Imagine a spherical organism. We can define its

BIOL 220, Summer 2014: Study Questions for Week 1 (June 23-26)
Q1: Imagine a spherical organism. We can define its radius as r, its surface area as 4*pi*r2, and its
volume as (4/3)*pi*r3. How will the surface area/volume ratio change as r increases?
A1: The surface area/volume ratio will be 3/r, so as r increases, the ratio will decrease. Thus, the trend
of a decreasing SA/V with increasing size holds true for hypothetical spherical organisms as well as for
hypothetical cubic ones (discussed in lecture).
Q2: An animal's surface area and volume are (crude) measures of the animal's capacity for what and
what, respectively?
A2: Surface area represents the animal's ability to exchange nutrients, wastes, water, and heat with the
environment. Volume represents the animal's ability to metabolize (more volume => more cells using and
producing ATP, etc.).
Q3: Consider the figure below (from S. Brody, Bioenergetics and Growth, 1945). The Y axis shows an
indicator of metabolic rate; the X axis shows body size. Does this figure support, conflict with, or not
directly address the general trend that mass-specific metabolic rate decreases as size increases?
A3: This figure shows that larger organisms have a higher TOTAL metabolic rate, e.g., elephants can
generate more power than mice. However, the general trend is that metabolic rate PER UNIT MASS
decreases as size increases. Thus, this curve does not contradict the general trend. In fact, the slope
shows that metabolic rate is proportional to mass to the 0.734 power – that is, metabolic rate rises more
slowly than mass. (If the two were directly proportional, metabolic rate would be proportional to
mass1.0.) Thus, the figure is consistent with the general trend.
Q4: Imagine that a certain species of monkey aims to keep its internal body temperature at 35 degrees
Celsius. It has thermoreceptors that sense the actual body temperature and integrator cells that (1)
compare the actual temperature to the setpoint of 35 and release hormones in proportion to how far
above or below the setpoint the actual temperature is. Does this constitute a negative feedback
system? Briefly explain.
A4: Negative feedback systems include an effector that moves the regulated parameter (temperature, in
this case) back toward the setpoint (see Figure 42.13). Thus, the system described is a negative feedback
system only if the hormones have the effect of reducing the discrepancy between the setpoint and the
actual temperature.
BIOL 220, Summer 2014: Study Questions for Week 1 (June 23-26)
Q5: Imagine a neuron whose intracellular calcium concentration is 0.0001 mM and whose extracellular
calcium concentration is 1 mM. Over what range of voltages will calcium flow into the cell? Over what
range of voltages will calcium flow out of the cell? Explain.
(
)
A5:
(
)
. Since 116 mV is the equilibrium
potential, calcium will flow inward at membrane potentials of less than +116 mV, and calcium will flow
outward at membrane potentials of greater than +116 mV.
Q6: Consider the following membrane potentials: -30 mV, 0 mV, +30 mV. At which of these membrane
potentials will the net inward flow of calcium ions be HIGHEST? Briefly explain.
A6: -30 mV is the answer. One way of explaining it is that, at -30 mV, both the chemical gradient AND
the electrical gradient favor the inward flow of calcium. This is not true at 0 mV or at +30 mV.
Q7: Sodium ion channels in the axon membrane have a selectivity filter, a voltage sensor, and a
tetrodotoxin binding site. Briefly explain the physiological significance of each of these components.
A7: The selectivity filter ensures that only sodium ions (not other ions or molecules) pass through the
channel. The voltage sensor determines the membrane potential and triggers the opening of the channel
when a voltage threshold (often around 10 mV above the resting potential) is reached. Tetrodotoxin is a
molecule that can bind to the sodium channels and block the passage of sodium – thus preventing action
potentials, if the concentration is high enough.
Q8: If low concentrations of tetrodotoxin were applied to an axon, show and explain how the action
potential waveform – that is, the changes in membrane potential over time – would be affected. Plot
membrane potential (Y axis) versus time (X axis).
A8: Low concentrations of tetrodotoxin will block some but not all sodium channels. Thus, sodium can
still enter the cell upon depolarization of the membrane, but its rate of entry will be less than normal.
Also recall that sodium channels close automatically after being open for a short period of time (on the
BIOL 220, Summer 2014: Study Questions for Week 1 (June 23-26)
order of 1-2 milliseconds), and that they then enter a refractory period during which they cannot reopen.
Thus, tetrodotoxin lowers the number of open channels through which sodium can enter the cell, but
does not appreciably affect the time window during which sodium can enter. Upon initiation of an action
potential, the membrane potential will climb more slowly than normal for a millisecond or two, reaching
a lower peak voltage than the usual value of +40 mV. The repolarization phase of the action potential
should not be affected much, since that phase depends mostly on potassium.
Q9: In a laboratory experiment on an isolated squid axon, enough sodium to trigger an action potential
is artificially added into the cytoplasm (via micropipette) exactly halfway down the length of the axon.
Graph the membrane potential at the axon hillock and at the axon terminus following this artificial
addition.
A9: In this artificial situation, with no sodium channels in a refractory period, the action potential will
propagate in BOTH directions from the middle of the axon. The action potential will thus arrive at the
axon hillock and axon terminus simultaneously. It will have the same "standard" shape and magnitude
in both locations.
Q10: What is wrong with the following sentence? "Glutamate affects the membrane potentials of
different neurons by binding to ionotropic and metabotropic ion channels in the postsynaptic
membranes."
A10: The term "metabotropic ion channel" is incorrect. "Ionotropic" and "metabotropic" are types of
RECEPTORS; the iontropic receptors also function as ion channels, whereas the metabotropic receptors
are not ion channels, though they can influence ion channels via second messengers.
Q11: How did Otto Loewi establish the existence of neurotransmitters? What kind of a control would be
needed for Loewi's experiment?
A11: Loewi first showed that stimulation of a vagus nerve slowed the beating of the heart to which it was
attached. He then collected the solution around the end of the nerve, added that solution to a different
heart, and found that the second heart slowed down as well. This showed that the vagus nerve must
release a chemical (which we call a neurotransmitter) that affects the heart, i.e., that the effect on the
heart was not strictly electrical. An appropriate experiment would be to collect the solution around the
end of a non-stimulated nerve and show that this solution does not slow the beating of a heart.
Q12: We have seen how cocaine affects dopamine-releasing neurons. One approach to cocaine
addiction is to reduce the euphoric sensations associated with cocaine use, so that the user no longer
craves cocaine so much. Baclofen, a GABA-like molecule that can stimulate GABA receptors, reduces
this craving. Speculate on why it has this effect, referring to EPSPs and/or IPSPs.
A12: GABA generally opens chloride channels. According to chloride's equilibrium potential relative to
the membrane potential, chloride ions often flow into the neurons, causing IPSPs, taking these neurons
away from their threshold. Therefore GABA imitators (GABA agonists) can decrease the frequency of
action potentials in the dopamine reward pathway that cocaine stimulates. Less firing of these neurons
will correspond to less pleasure and less craving for more.
Q13: Below is a picture of the axons of neurons A and B forming synapses with neuron C.
BIOL 220, Summer 2014: Study Questions for Week 1 (June 23-26)
(i)
(ii)
Use asterisks (*) to label all regions on all appropriate neurons where voltage-gated calcium channels
are located.
When neurons A and B both release neurotransmitter simultaneously, neuron C's membrane
potential does not change. Explain the lack of a change.
A13:
(i)
(ii)
Asterisks should be at the ends of axons of neurons A and B, where vesicles fuse with
membrane.
Most likely, one of the two neurons (A or B) causes EPSPs in the postsynaptic neuron (C),
while the other causes IPSPs, so that the two cancel each other out and there is no net effect.
Alternatively, if the postsynaptic neuron's membrane potential happens to be at the
equilibrium potential of an ion whose channels are opened by both A and B, that too could
account for the observed lack of change.