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Biology 240
Name: ____________________________
Homework #3: Introduction to Organisms (25 points)
1. For each of the statements below relating to action potentials, fill in the blank
with the correct ion: Ca++, K+, Na+, or Cl-. (5 points)
At resting potential, the equilibrium voltage is maintained in part by chloride ions and
other anions inside the cell and by the action of the _____ – ______ pump. Because of
the action of the pump, there is a higher concentration of _________ions inside the cell
and a higher concentration of ___________ions outside the cell. A small depolarization
of the cell results in rapid opening of voltage-gated _______ channels while the voltagegated __________channels stay closed. ________ flows into the axon, causing further
depolarization. The inactivation loop closes the voltage-gated __________ channels,
and the voltage-gated _______ channels finally open. Now, ________ions flow out of
the cell. At the end of the axon, calcium ions flow into the nerve cell to trigger fusion of
vesicles containing neurotransmitters to enter the synapse.
2. Fill in the blanks in the paragraph below with the correct ion to describe how a
muscle fiber contracts. (5 points)
When a motor neuron triggers a muscle fiber to contract, _________ion is released
from the __________________________________. This ion binds to the
________________, which pushes the tropomyosin aside, exposing the
___________________________on the actin filaments. If ____________is present, it
will bind to a myosin head and release energy, causing a change in shape (the highenergy configuration). This myosin head then forms a ___________________with the
actin filament. ___________and __________ are released, and the myosin head
returns to its normal shape (_________________________), which makes the actin
filament slide with respect to the _________ filament. With many of these events
happening along the muscle fiber, an overall muscle contraction is achieved.
3. Suppose that vampires actually exist and that the condition is caused by a prion
disease that eliminates the ability to manufacture red blood cells (RBC) by changing
their body’s DNA throughout the bone marrow. They need to drink blood in order to
replenish their RBC count. RBC are normally produced in the bone marrow and
destroyed in the spleen once they have existed for about 4 months. Sketch a diagram
Biology 240
of the regulatory pathways, including feedback loops, described here: The
number of red blood cells (RBC count) in the body is the variable being regulated. The
kidneys have nerve endings that detect blood O2 levels as a measure of RBC count and
release the hormone erythropoietin (EPO) to stimulate release of RBC from the bone
marrow. However, in vampires the bone marrow cannot make more cells. They have
evolved a mechanism where the release of EPO triggers a diversion of blood flow away
from the spleen to slow the rate of RBC destruction. Also, detection of low RBC count
triggers the brain to release the hormone vampyrine which causes the sensation of
hunger (thirst?). If the vampire feeds, levels of RBC go up and the sensation of hunger
is turned off. Besides the passage of time since the last meal, the other factor which
decreases RBC is exposure to sunlight. Sunlight activates vitamin D, which in vampires
dramatically raises the Ca++ concentration in plasma, triggering apoptosis (cell death)
of RBC. (5 points)
4. Read the paper “Exercise Controls Gene Expression” from American Scientist posted
as.pdf file in the Unit III Blackboard folder. Answer the questions about it in the
Blackboard assessment. (10 points)

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