#1 Membrane Transport

Driving Forces
1. Chemical Forces (ΔC)
  1. ΔC = the difference in the concentration gradient
  2. Spontaneous movement is DOWN a concentration gradient (HIGH to LOW)
  3. Size of ΔC (the concentration gradient) affects the rate
  4. Specific ΔC for each substance
2. Electrical Forces (Vm)
  1. AKA Membrane Potential
  2. Vm = the difference in VOLTAGE across a membrane, measured in millivolts (mV)
  3. The direction of Vm depends on the sign of Vm and the sign of ions.
    1. INSIDE the cell is more NEGATIVE OUTSIDE the cell is more POSITIVE
    2. The sign of Vm is the charge INSIDE the cell RELATIVE to the outside
      1. Most commonly, Vm = -70 mV this attracts cations into the cell and pulls anions out of the cell
3. Electrochemical Forces
  1. Electrochemical Force = COMBINATION of ΔC and Vm
  2. Direction of the electrochemical force
    1. If both the chemical and electrical forces are the SAME, then the electrochemical force is in the SAME direction.
    2. If the chemical and electrical forces are OPPOSITE, then the electrochemical force goes in the direction of the LARGER force.
      1. This is when we need to calculate the Equlibrium Potential (E)
        THE NERNST EQUATION Equilibrium Potential (E) = (61/z) x (log(Cₒ/Cᵢ))
        z = charge of ion (abs value)
        Cₒ = concentration of ion outside cell
        Cᵢ = concentration of ion inside cell
        EXAMPLES: if E=-94 and Vm=-94 then it is at equilibrium if E=-94 and Vm=-70 then ΔC is stronger if E=-94 and Vm=-100 then Vm is stronger
        The Equilibrium Potential is the strength of the chemical gradient
        Sign of Equilibrium Potential (E).
Rate of Transport
Types of Transport
1. PASSIVE TRANSPORT
  1. Simple Diffusion
    1. PASSIVE TRANSPORT through membrane
    2. Movement is due to Thermal Motion
    3. DOWN the concentration gradient: from HIGH to LOW
    4. Rate is affected by: lipid solubility, size and shape of molecules, temperature, thickness of membrane
  2. Facilitated Diffusion
    1. Passive Transport using membrane proteins
      1. Protein Carriers
        Transmembrane proteins that bind molecules on one side and transport to other side by a CONFORMATIONAL CHANGE
        They have one or more binding sites that are specific
        The conformational change occurs randomly due to thermal agitation
        Can become saturated
      2. Protein Channels
        Also transmembrane proteins that transport molcules
        Transport via PASSAGEWAY or PORE
        Also are specific, but there is no binding site, therefore no conformational change is needed and are faster
    2. Rate is affected by:
      1. Whether its via carrier or channel
      2. The amount of carriers or channels, and if the carriers become saturated
      3. If the cell upregulates or downregulates the number of carriers or channels
      4. Drugs or hormones: calcium channel blockers, insulin
  3. Osmosis
    1. PASSIVE TRANSPORT of water
    2. Water moves DOWN the concentration gradient: from HIGH to LOW concentration
    3. The concentration gradient is determined by the amount of solutes "STUFF" present
      1. More "STUFF" Less H2O
      2. Less "STUFF" More H2O
    4. Osmolarity = the total solute concentraion 1 MOLE solute = 1 Osm
      1. 0.1M glucose and 0.1M sucrose = 0.2 Osm
      2. 0.15M NaCl = 0.15M Na+ + 0.15M Cl- = 0.30 Osm
      3. HYPER-osmotic Solution: the solution has more solutes (a higher osmolarity) than the cell.
        The cell will shrink (crenate)
      4. HYPO-osmotic Solution: the solution has less solutes than the cell.
        The cell will swell (lysis)
2. ACTIVE TRANSPORT
  1. Primary Active Transport
    1. DIRECTLY uses ATP to transport molecules
    2. Uses a pump, similar to protein carriers, but has an enzyme
    3. EXAMPLE: Na/K/ATPase Pump
      1. This pump transports Na+ and K+ in opposite directions
        3 Na+ move OUT
        2 K+ move IN
      2. ATP hydrolysis is necessary to phosphorylate the pump. The phosphorylation changes the conformation of the pump, changing the binding site
      3. Purpose of this pump: electrical signaling, absorption of glucose, difference in Na+ and K+ concentration across the membrane, helps establish the standard membrane potential (-70 mV)
  2. Secondary Active Transport
    1. Uses energy stored in ion gradient
    2. Cotransport - Symport
      1. Two substances move in the SAME DIRECTION
      2. EXAMPLE: Sodium-Linked Glucose Transport
        Sodium moving IN causes the gradient that provides the energy for glucose to move IN
    3. Countertransport
      1. Two substances move in OPPOSITE DIRECTIONS
      2. EXAMPLE: Sodium-Proton Exchanger
        Sodium moving IN causes the gradient that provides the energy for protons to move OUT
  3. Movement against the concentration gradient: from LOW to HIGH concentrations
  4. Requires energy

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#1 Membrane Transport

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	Created by Amy M over 8 years ago