Transport Processes

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Transport Processes

• AP 151

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Membrane Transport

• Plasma membrane is selectively permeable
• Impermeable membrane - membrane though which
  nothing can pass
• Freely permeable membrane - any substance can
  pass through it
• Selectively permeable membrane - permits free
  passage of some materials and restricts passage
  of others
• Distinction may be based on size, electrical
  charge, molecular shape, lipid solubility
• Cells differ in their permeabilities depending
  on
• what lipids and proteins are present in the
  membrane and
• how these components are arranged.

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Membrane Permeability

• Passage across the membrane is either passive or
  active
• Passive transport requires no ATP
• movement down concentration gradient
• filtration and simple diffusion
• Active transport requires ATP
• movement against concentration gradient
• carrier mediated
• vesicular transport

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Types of Transport Processes

• Diffusion
• results from random motion of particles (ions,
  molec.)
• is a passive process
• Carrier-mediated transport
• Requires the presence of specialized integral
  proteins
• Can be passive or active
• Vesicular transport
• Movment of materials with small membranous sacs,
  or vesicles
• Always an active process

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Membrane Permeability

• Diffusion through lipid bilayer
• Nonpolar, hydrophobic substances diffuse through
  lipid layer these are lipid soluble or
  lipophilic (fat-loving) substances
• Diffusion through channel proteins
• water and charged hydrophilic solutes diffuse
  through channel proteins these are lipid
  insoluble or lipophobic (fat fearing) substances
• Cells control permeability by regulating number
  of channel proteins

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Simple Diffusion

• Net movement of particles from area of high
  concentration to area of low concentration
• due to their constant, random motion
• Difference between the high and low
  concentrations is a concentration gradient
• Diffusion tends to eliminate the gradient
• Also known as movement down the concentra- tion
  gradient

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Diffusion

• Examples
• Scent of fresh flowers, drop of ink coloring a
  glass of water, movement of oxygen and CO2
  through cell membranes
• Simple diffusion nonpolar and lipid-soluble
  substances
• Diffuse directly through the lipid bilayer
• Diffuse through channel proteins

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Factors that Influence Diffusion Rates

• Distance -
• The shorter the distance, the more quickly
  gradients are eliminated
• Few cells are father than 125 microns from a
  blood vessel
• Molecular Size
• Ions and small molecules diffuse more rapidly
• Temperature -
• ? temp., ? motion of particles
• Steepness of concentrated gradient -
• The larger the gradient, the faster
  diffusion proceeds
• Membrane surface area -
• The larger the area, the faster diffusion proceed

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Diffusion Across Membranes

• Simple Diffusion
• Lipophilic substances can enter cells easily
  because they diffuse through the lipid portion of
  the membrane
• Examples are fatty acids, steroids, alcohol,
  oxygen, carbon dioxide, and urea,
• Channel-Mediated Diffusion
• Membrane channels are transmembrane proteins
• Only 0.8 nm in diameter
• Used by ions, very small water-soluble compounds
• Much more complex than simple diffusion
• Are there enough channels available?
• Size and charge of the ion affects which channels
  it can pass through

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Diffusion Through the Plasma Membrane
Figure 3.7
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Effect of Membrane Permeability on Diffusion
Figure 3.8a
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Osmosis A Special Case of Diffusion

• Each solute in the intra- and extracellular
  fluids diffuses as if it were the only material
  in solution.
• From more to less, i.e., down the gradient
• Some into the cytosol, others out of the cytosol
• Yet, total concentration of ions and molecules on
  either side of the membrane stays the same
• This equilibrium persists because a typical cell
  membrane is freely permeable to water.
• Whenever a solute concentration gradient exist, a
  concentration gradient for water also exists.
• Thus, the higher the solute concentration, the
  lower the water concentration.

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Osmosis - By Definition

• Movement of water
• Across a selectively permeable membrane
• Down its concentration gradient (from high to low
  concentration)
• Toward the solution containing the higher solute
  concentration
• This solution has a lower water concentration
• Continues until water concentrations and solute
  concen-trations are the same on either side of
  the membrane

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Effect of Membrane Permeability on Diffusion and
Osmosis
Figure 3.8b
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Osmolarity and Tonicity

• Mole - the gram molecular weight of a substance
• 1 mole of Glucose 180 1 mole of NaCl 58.5
• Molarity - the number of moles of solute per
  liter of solution
• 1.0 M glucose contains 180 g/L 1.0 M NaCl
  contains 58.5 g/L
• Most body fluids are less concentrated than 1 M
  use mM (millimolar) or µM (micromolar)
  concentrations --10-3 and 10-6, respectively.
• Osmolarity the total solute concentration in an
  aqueous solution
• Osmolarity molarity (mol/L) x of particles in
  solutions
• A 1 M Glucose solution 1 Osmolar (Osm)
• But a 1 M NaCl soln 2 Osmolar because NaCl
  dissociates into 2 particles (Na and Cl) whereas
  Glucose does not
• A 1 M MgCl2 solution what osmolarity????
  __________
• Physiological solutions are expressed in
  milliosmoles per liter (mOsm/L)
• blood plasma 300 mOsm/L or 0.3 Osm/L

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Tonicity

• Tonicity - ability of a solution to affect fluid
  volume and pressure within a cell
• depends on concentration and permeability of
  solute
• Isotonic solution
• solution with the same solute concentration as
  that of the cytosol normal saline
• Hypotonic solution
• lower concentration of nonpermeating solutes
  than that of the cytosol (high water
  concentration)
• cells absorb water, swell and may burst (lyse)
• Hypertonic solution
• has higher concentration of nonpermeating solutes
  than that of the cytosol (low water
  concentration)
• cells lose water shrivel (crenate)

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Osmosis and Cells

• Important because large volume changes caused by
  water movement disrupt normal cell function
• Cell shrinkage or swelling
• Isotonic cell neither shrinks nor swells
• Hypertonic cell shrinks (crenation)
• Hypotonic cell swells (lysis)

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Effects of Tonicity on RBCs
Hypotonic, isotonic and hypertonic solutions
affect the fluid volume of a red blood cell.
Notice the crenated and swollen cells.
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Filtration

• Cell membrane works like a sieve
• Depends on pressure difference on either side of
  a partition
• Moves from side of greater pressure to lower
• Water and small molecules move through the pores
  of the membrane while large molecules dont.
• Example urine formation in the kidneys.

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Carrier Mediated Transport

• Many molecules cannot enter or leave cell by
  diffusion
• CMT utilizes proteins to carry solutes across
  cell membrane
• Characteristics of mediated transport
• Specificity - each transport protein binds to and
  transports only a single type of molecule or ion
• Competition - results from similar molecules
  binding to the same protein.
• Saturation - rate of movement of molecules is
  limited by the number of available transport
  proteins

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Membrane Carriers

• Uniporter
• carries only one solute at a time
• Symport
• carries 2 or more solutes simultaneously in same
  direction (cotransport)
• Antiport
• carries 2 or more solutes in opposite directions
  (countertransport)
• sodium-potassium pump brings in K and removes
  Na from cell
• Any carrier type can use either facilitated
  diffusion or active transport

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Saturation of a Carrier Protein

• When the concentration of x molecules outside the
  cell is low, the transport rate is low because it
  is limited by the number of molecules available
  to be transported.
• When more molecules are present outside the cell,
  as long as enough carrier proteins are available,
  more molecules can be transported thus, the
  transport rate increases.
• The transport rate is limited by the number of
  carrier proteins and the rate at which each
  carrier protein can transport solutes. When the
  number of molecules outside the cell is so large
  that the carrier proteins are all occupied, the
  system is saturated and the transport rate cannot
  increase.

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CMT Facilitated Diffusion

• Glucose and amino acids are insoluble in lipids
  and too large to fit through membrane channels
• Passive process, i.e. no ATP used
• Solute binds to receptor on carrier protein
• Latter changes shape then releases solute on
  other side of membrane
• Substance moved down its concentration gradient

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CMT Active Transport

• Uses ATP to move solutes across a membrane
• It is not dependent on a gradient
• Can move substances against their gradients
  - i.e. from lower to higher concentrations! Wow!
• Allows for greater accumulation of a substance on
  one side of the membrane than on the other.
• Carrier proteins utilized called ion or exchange
  pumps.
• Ion pumps actively transport Na, K, Ca, Cl-
• Exchange pumps Na-K pump

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Types of Active Transport
Figure 3.11
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Sodium-Potassium Pump
Extracellular fluid
K is released and Na sites are ready to bind
Na again the cycle repeats.
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Binding of cytoplasmic Na to the pump protein
stimulates phosphorylation by ATP.
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Cytoplasm
2
Phosphorylation causes the protein to change its
shape.
Concentration gradients of K and Na
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The shape change expels Na to the outside, and
extracellular K binds.
Loss of phosphate restores the original
conformation of the pump protein.
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K binding triggers release of the phosphate
group.
Figure 3.10
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Functions of Na -K Pump

• Regulation of cell volume
• fixed anions attract cations causing osmosis
• cell swelling stimulates the Na- K pump to ?
  ion concentration, ? osmolarity and cell swelling
  
• Heat production (thyroid hormone increase of
  pumps heat a by-product)
• Maintenance of a membrane potential in all cells
• pump keeps inside negative, outside positive
• Secondary active transport (No ATP used)
• steep concentration gradient of Na and K
  maintained across the cell membrane
• carriers move Na with 2nd solute easily into
  cell
• SGLT saves glucose in kidney

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Secondary Active Transport

• Ions or molecules move in same (symport) or
  different (antiport) direction.
• Is the movement of glucose a symporter example or
  an antiporter example?
• This example shows cotransport of Na and
  glucose.
• A sodium-potassium exchange pump maintains a
  concentration of Na that is higher outside the
  cell than inside. Active transport.
• Na moves back into the cell by a carrier protein
  that also moves glucose. The concentration
  gradient for Na provides the energy required to
  move glucose against its concentration gradient.

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Vesicular Transport

• Transport large particles or fluid droplets
  through membrane in vesicles
• uses ATP
• Exocytosis transport out of cell
• Endocytosis transport into cell
• phagocytosis engulfing large particles
• pinocytosis taking in fluid droplets
• receptor mediated endocytosis taking in
  specific molecules bound to receptors

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Vesicular TransportEndocytosis

• Packaging of extracellular materials in vesicles
  at the cell surface
• Involves relatively large volumes of
  extracellular material
• Requires energy in the form of ATP
• Three major types
• Receptor-mediated endocytosis
• Pinocytosis
• Phagocytosis

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Receptor Mediated Endocytosis

• A selective process
• Involves formation of vesicles at surface of
  membrane
• Vesicles contain receptors on their membrane
• Vesicles contain specific target molecule in high
  concentration
• Clathrin-coated vesicle in cytoplasm
• uptake of LDL from bloodstream
• If receptors are lacking, LDLs accumulate and
  hypercholesterolemia develops

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Receptor Mediated Endocytosis
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Vesicular TransportPinocytosis or
Cell-Drinking

• Taking in droplets of ECF
• occurs in all human cells
• Not as selective as receptor-mediated
  endocytosis
• Membrane caves in, then pinches off into the
  cytoplasm as pinocytotic vesicle

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Vesicular TransportPhagocytosis or Cell-Eating
Keeps tissues free of debris and infectious
microorganisms.
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Vesicular Transport Exocytosis

• Secreting material or replacement of plasma
  membrane

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Passive Membrane Transport Review -
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Active Membrane Transport Review