Membrane Transport

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Membrane Transport
Membrane transport proteins span the plasma
membrane and transport specific substances across
the membrane.
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• specific to type of solute
• specific to type of cell or organelle

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There are two major classes of membrane transport
proteins.

• Carrier proteins
• - bind with specific solute on one side
• - conformational change
• - deliver solute to other side
• Channel proteins
• - form tiny hydrophilic pores
• - solutes pass through by diffusion
• - called ion channels
• - discriminates by size and electric charge

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Which inorganic ions are the most abundant
solutes in a cells environment?
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Quantity of charge must quantity of - charge
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Lipid bilayers are impermeable to most solutes
and all ions.
In general, the smaller more hydrophobic the
solute, the faster it will diffuse across.
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Diffusion

• Movement down a concentration gradient
• From an area of greater concentration to areas of
  lesser concentration
• Due to internal kinetic energy
• No outside energy is required.

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permeable
permeable
impermeable
impermeable
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Diffusion of water across a semipermeable
membrane is osmosis.
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Passive Transport

• Motion is always DOWN the concentration gradient
• Simple Facilitated Diffusion
• Osmosis
• Does NOT require energy
• The solute concentration of intercellular fluid
  determines the direction of osmosis.

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Passive transport may require a channel or
carrier protein.
Facilitated diffusion
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Carrier proteins transfer solute molecules by
binding with them.
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A channel protein forms a hydrophilic pore across
the bilayer through which specific inorganic ions
can diffuse.
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Glucose is transported across liver cell plasma
membranes by facilitated diffusion.
The glucose-carrier protein switches reversibly
and randomly between 2 conformations.
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Active Transport

• Moves molecules (ions) UP the concentration
  gradient
• Energy is required to change conformation
• Protein carrier molecules are required.

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• There are 3 general classes of transport proteins.

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• Net driving force for ions charged molecules
  electrochemical gradient

• electrochemical gradient depends upon
  concentration gradient difference in electric
  potential energy

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Difference in electric potential across a
membrane membrane potential voltage
energy/charge
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Sodium experiences a high electrochemical
gradient because its concentration gradient and
electric potential gradient are in the same
direction.
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• Active transport moves solutes against their
  electrochemical gradients 3 main ways.

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• An ATP-driven pump transports Na out of the cell
  and K into the cell Na - K pump

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• The Na - K pump transports ions in a cyclic
  manner.

conformation changes
Release of P causes conformation to change
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At top speed, the Na- K antiport can pump 450
Na out and 300 K in per second.
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• The glucose-Na symport uses the energy from
  falling Na ions to move glucose up its
  electrochemical gradient.

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The Na - K antiport and the glucose Na
symport are both examples of active transport.
In primary active transport, the energy released
by ATP hydrolysis drives solute movement against
an electrochemical gradient.
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In secondary active transport, a gradient of ion
X (often Na) has been established by primary
active transport. Movement of X down its
electrochemical gradient now provides the energy
to drive cotransport of a second solute (S) up
its electrochemical gradient.
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The Na - K pump helps maintain the osmotic
balance in animal cells.

• Movement of water from a region of low solute
  concentration to a region of high solute
  concentration is osmosis.

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• The driving force for the movement of water
  molecules is equivalent to the difference in
  water pressure on the two sides of the membrane
  osmotic pressure.

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Types of Solutions

• Isotonic solutions having the same
  concentration of water and solutes
• Hypotonic A solution with a greater
  concentration of water a lesser concentration
  of solute than another
• Hypertonic A solution with a lesser
  concentration of water and a greater
  concentration of solute than another

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Solutions surrounding the cells - Isotonic
Hypertonic Hypotonic
Which way will water molecules move?
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Solutions surrounding the cells - Isotonic
Hypertonic Hypotonic
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• In animal cells, it is the Na - K pump that
  primarily functions to maintain osmotic balance,
  solute on both sides of membrane
• The Na - K pump moves out the Na which
  naturally moves into cells down its EC grad.

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What happens to plant cells in different
solutions?
Plasmolysis
Turgor pressure
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Turgor pressure makes plant tissues crisp.
Wilting is a loss of turgor pressure.
Sauté vegetables until wilted.
Man with kelp
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Protozoans living in fresh water eject excess
water through a contractile vacuole.
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