Membrane Transport of Small Molecules'

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Chapter 11
Lecture 26 pages 615-631

• Membrane Transport of Small Molecules.

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The phospholipid bilayer is a barrier that
controls the transport of molecules in and out of
the cell.
Gases diffuse freely, no proteins required.
Water diffuses fast enough that proteins arent
required for transport
Sugars diffuse very slowly so proteins are
involved in transport.
Charged molecules are virtually impermeable.
Studies of synthetic lipid bilayers help define
which types of transport will require the
activity of a protein. Hence, transport of an ion
should require a protein.
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Why charged compounds dont permeate the bilayer?
They are strongly attracted to the water.
Panel 2-2, page 112
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Transport of molecules that are impermeable to
the lipid bilayer is achieved by two classes of
membrane transport proteins.
Conformational change carries the solute across
the membrane.
Aqueous pore provides passage way so solute can
diffuse through the membrane.
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Two types of transport are defined by whether
metabolic energy is expended to move a solute
across the membrane.

• Passive transport no metabolic energy is needed
  because the solute is moving down its
  concentration gradient.
• In the case of an uncharged solute, the
  concentration of the solute on each side of the
  membrane dictates the direction of passive
  transport.
• Active transport metabolic energy is used to
  transport a solute from the side of low
  concentration to the side of high concentration.

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When considering an uncharged solute, one only
considers the concentration gradient. If the
solute is charged, movement across the membrane
is affected by both the chemical gradient and the
electrical gradient. The combination of the the
two is called the electrochemical gradient.
Passive transport occurs when a solute moves down
the electrochemical gradient. Active transport
occurs when a charged solute moves up the
electrochemical gradient.
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Possible scenarios for a charged solute Passive
transport charged solute moving from region of
high concentration to low concentration and
moving towards the side of the membrane with
opposite charge. Active transport charged
solute moving towards the side of the membrane
having the same charge. Sometimes the electrical
potential acts in the opposite direction of the
concentration gradient. Whether transport is
active or passive depends on which dominates.
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Carrier proteins and active membrane transport.
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A carrier protein binds solute on one side of a
membrane, undergoes a conformational change, and
releases the solute on the other side of the
membrane.
As illustrated, is this active or passive
transport?
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A carrier protein resembles an enzyme.
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By coupling the conformational change to a source
of energy, a carrier protein can perform active
transport.
Provided that the illustration indicates an
electrochemical gradient, what does this imply
about the yellow solute?
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Depending on how many solute molecules are
transported and in what direction, carrier
proteins are dubbed uniporters, symporters, or
antiporters.
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Three carrier proteins, appropriately positioned
in the plasma membrane, function to transport
glucose across the intestinal epithelium.
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Na-K pump (aka Na-K ATPase) in the plasma
membrane is an antiporter that performs active
transport.
This protein establishes a concentration
gradients with Na low inside the cell and high
outside, and K high inside the cell and low
outside.
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The Na gradient generated by the Na - K ATPase
powers the transport of glucose into the cell by
a Na -driven glucose symporter.
The energetically favorable movement of Na down
its electrochemical gradient is coupled to the
energetically unfavorable transport of glucose up
its concentration gradient. Hence, glucose is
being subjected to active transport.
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Glucose uniporter binds glucose inside the cell,
undergoes a conformational change, and releases
the glucose into the fluid where it then enters
the blood. Since glucose is moving from high to
low concentration, this is passive transport.
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The Na -K pump helps stabilize the volume of
the cell.
Observation inhibiting the Na-K pump with
toxin (ouabain) causes cells to swell and
burst. Explanation The high concentration of
solutes inside the cell causes water to move into
the cell. This is called osmosis. The Na-K
pump maintains a high concentration Na and,
indirectly Cl- outside the cell to counteract the
flow of water into the cell.
Regulating the osmolarity of the fluid is an
important consideration when working with cells.
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Concentration gradients of Na, K, and Ca in
animal cells are maintained by structurally
similar proteins the Na - K ATPase and the
Ca ATPase.
Maintaining the low concentration of Ca inside
the cell is critical for controlling many
cellular processes because the responses are
trigger by sudden increases in intracellular
Ca. The Na gradient is essential for uptake
of many metabolites. As you will learn shortly,
the Na and K gradients are also essential for
generating nerve impulses.
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ABC transporters constitute a large family of
proteins that have members of medical
significance. Cycles of ATP binding and
hydrolysis somehow power transport of a variety
of molecules including peptides, hydrophobic
drugs, and ions.
Increased expression of the multidrug resistance
protein in cancer cells renders the cells
resistant to anti-cancer drugs because protein
pumps the drugs out of the cells. Cystic
fibrosis results from the malfunctioning of an
ABC transporter that regulates chloride transport.