#1 Membrane Transport

#1 Membrane Transport

1. Driving Forces

   • Chemical Forces (ΔC)
     • ΔC = the difference in the concentration gradient
     • Spontaneous movement is DOWN a concentration gradient (HIGH to LOW)
     • Size of ΔC (the concentration gradient) affects the rate
     • Specific ΔC for each substance
   • Electrical Forces (Vm)
     • AKA Membrane Potential
     • Vm = the difference in VOLTAGE across a membrane, measured in millivolts (mV)
     • The direction of Vm depends on the sign of Vm and the sign of ions.
       • INSIDE the cell is more NEGATIVE OUTSIDE the cell is more POSITIVE
       • The sign of Vm is the charge INSIDE the cell RELATIVE to the outside
         • Most commonly, Vm = -70 mV this attracts cations into the cell and pulls anions out of the cell
   • Electrochemical Forces
     • Electrochemical Force = COMBINATION of ΔC and Vm
     • Direction of the electrochemical force
       • If both the chemical and electrical forces are the SAME, then the electrochemical force is in the SAME direction.
       • If the chemical and electrical forces are OPPOSITE, then the electrochemical force goes in the direction of the LARGER force.
         • 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).

2. Rate of Transport

3. Types of Transport

   • PASSIVE TRANSPORT
     • Simple Diffusion
       • PASSIVE TRANSPORT through membrane
       • Movement is due to Thermal Motion
       • DOWN the concentration gradient: from HIGH to LOW
       • Rate is affected by: lipid solubility, size and shape of molecules, temperature, thickness of membrane
     • Facilitated Diffusion
       • Passive Transport using membrane proteins
         • 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
         • 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
       • Rate is affected by:
         • Whether its via carrier or channel
         • The amount of carriers or channels, and if the carriers become saturated
         • If the cell upregulates or downregulates the number of carriers or channels
         • Drugs or hormones: calcium channel blockers, insulin
     • Osmosis
       • PASSIVE TRANSPORT of water
       • Water moves DOWN the concentration gradient: from HIGH to LOW concentration
       • The concentration gradient is determined by the amount of solutes "STUFF" present
         • More "STUFF" Less H2O
         • Less "STUFF" More H2O
       • Osmolarity = the total solute concentraion 1 MOLE solute = 1 Osm
         • 0.1M glucose and 0.1M sucrose = 0.2 Osm
         • 0.15M NaCl = 0.15M Na+ + 0.15M Cl- = 0.30 Osm
         • HYPER-osmotic Solution: the solution has more solutes (a higher osmolarity) than the cell.
           • The cell will shrink (crenate)
         • HYPO-osmotic Solution: the solution has less solutes than the cell.
           • The cell will swell (lysis)
   • ACTIVE TRANSPORT
     • Primary Active Transport
       • DIRECTLY uses ATP to transport molecules
       • Uses a pump, similar to protein carriers, but has an enzyme
       • EXAMPLE: Na/K/ATPase Pump
         • This pump transports Na+ and K+ in opposite directions
           • 3 Na+ move OUT
           • 2 K+ move IN
         • ATP hydrolysis is necessary to phosphorylate the pump. The phosphorylation changes the conformation of the pump, changing the binding site
         • 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)
     • Secondary Active Transport
       • Uses energy stored in ion gradient
       • Cotransport - Symport
         • Two substances move in the SAME DIRECTION
         • EXAMPLE: Sodium-Linked Glucose Transport
           • Sodium moving IN causes the gradient that provides the energy for glucose to move IN
       • Countertransport
         • Two substances move in OPPOSITE DIRECTIONS
         • EXAMPLE: Sodium-Proton Exchanger
           • Sodium moving IN causes the gradient that provides the energy for protons to move OUT
     • Movement against the concentration gradient: from LOW to HIGH concentrations
     • Requires energy