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Membrane Transport
Teaching Points
The boundaries of a cell are determined by the presence of a lipid bilayer known as the
cell membrane. This nonpolar structural feature enables the cell to have specific
characteristics within the intracellular compartment relative to its extracellular
environment. In order to maintain a constant environment, cells need to regulate their
internal and external environment. This can be accomplished through structures such
as channels, which allow for specific substrates to travel from an area of high
concentration to an area of low concentration. These channels are essential in the
regulation of the cellular environment.
The Nobel Prize for 2003 in Chemistry was awarded to two scientists who are exploring
channels in the cell membrane. The Nobel Prize was awarded to Dr. Peter Agre, who
examines the movement of water through the membrane channel, aquaporin, and to Dr.
Roderick MacKinnon, who explores the movement of ions across the membrane,
specifically the movement of potassium through the K+ channel.
Aquaporin
The lipid bilayer of cell membranes is impermeable to water. Movement of water needs
to be regulated in order to maintain the internal pressure of the cell. The presence of
water channels, such as aquaporin, enables the cell to regulate the water content of
cells to prevent swelling or shrinking. This feature is especially important in the filtration
system of the kidneys.
Potassium Channel
Within the nervous system, cells communicate to one another through action potentials.
These signals are dependent on the movement of ions across the cell membrane. The
movement of potassium ions is essential for these communications. The structure of the
potassium channel is specific to potassium ions through the presence of the selectivity
filter. This filter is composed of amino acids positioned in such a way that the carbonyl
oxygens can coordinate the potassium ion as it moves through the membrane.
Students can explore the different ways that substrates can be moved through the lipid
bilayer with the models in this collection. The two channels serve similar purposes, but
their structures are unique.
Models in this Collection
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Aquaporin
K+ Channel
Model Details
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Aquaporin
o α-carbon backbone
o Asparagine residues 76 and 192 colored in CPK
o Hydrophobic residues lining the channel colored yellow
o Based on PDB file 1FQY
o Model made of nylon with the SLS Machine
+
K Channel
o α-carbon backbone
o 4 K+ ions are colored purple
o The carbonyl oxygens of the selectivity filter are red
o Based on PDB file 1J95
o Model made of nylon with the SLS Machine
Documentation included
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Teaching points and inventory
How do the models fit back in the suitcase?
Membrane Channels (from Nobel Prize site)
Resources
Nobel Prize Website Information
http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/index.html
http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/public.html
Research of the Nobel Prize Laureates
http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/chemadv03.pdf
*A printed copy is included with the documentation found in this collection.
Aquaporin
See water move through aquaporin
http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/animations.html
Structure, Dynamics and Function of Aquaporin
http://www.ks.uiuc.edu/Research/aquaporins/
Potassium Channel
Roderick MacKinnon has recently crystallized another form of the potassium channel in
which there are voltage sensor paddles as a part of the protein structure. His group has
suggested that these paddles move through the lipid cell membrane in response to a
change in the membrane polarity to allow for the opening and closing of the potassium
channel.
Animation of How the Potassium Ions Can be Moved Through the Channel (need
Chime installed)
http://www.andrew.cmu.edu/course/03-231/ProtStruc/1bl8.htm
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