Membrane Potential

Cellular Processes
Diffusion, channels and transporters
Cellular Membranes
Two main roles
• Allow cells to isolate themselves from the
environment, giving them control of
intracellular conditions
• Help cells organize intracellular pathways
into discrete subcellular compartment,
including organelles
Membrane Structure
Lipid bi-layer: phospholipids, primarily phosphoglycerides
Other lipids
Sphingolipids: alter electrical properties
Glycolipids: communication between cells
Cholesterol: increase fluidity while decreasing permeability
Figure 3.20
Membrane Proteins
Can be more than half of the membrane mass
Two main types
• Integral membrane proteins – tightly bound to the
membrane, either embedded in the bilayer or spanning
the entire membrane
• Peripheral proteins – weaker association with the lipid
We will discuss membrane proteins that allow for flow of ions or for transport of molecules
Membrane permeability
Lipid bilayer
Membrane proteins we will discuss
Membrane Transport
Three main types
• Passive diffusion
• Facilitated diffusion
• Active transport
Passive Diffusion
Lipid-soluble molecules (alcohol, CO2)
No specific transporters are needed
No energy is needed
Depends on concentration gradient
High  low
Steeper gradient results in higher rates
Gradients can be chemical, electrical or both
depending on the nature of the molecule
e.g., Membrane potential – electrical gradient
across a cell membrane
Facilitated Diffusion
Hydrophilic molecules
Protein transporter is needed - Uniporter
No energy is needed
Depends on concentration gradient
Examples: amino acids, nucleosides,
sugars (glucose)
Facilitated Diffusion, Cont.
Three main types of proteins
1. Ion channels – form pores, channel has to be open
a) Open/close in response to a membrane potential
b) Open via specific regulatory molecules
c) Regulated through interactions with subcellular proteins
Facilitated Diffusion, Cont.
2. Porins – like ion channels, but for larger molecules
Cool stuff: aquaporin allows water to cross the plasma
membrane – 13 billion H2O molecules per second!
But, as pointed out by T. Todd Jones that is only
0.000000000000018 ml of water.
3. Permeases – function more like an enzyme. Binds
the substrate and then undergoes a conformation
change which causes the carrier to release the
substrate to the other side. Ex. Glucose permeases
Facilitated Diffusion - Uniporter
GLUT1 – mammalian glucose transporter
Uses concentration gradient of glucose to drive transport
Can work in reverse
Used by most mammals
Electrical Gradients
All transport processes affect chemical
Some transport processes affect the
electrical gradient
Electroneutral carriers: transport uncharged
molecules or exchange an equal number of
charged particles
Electrogenic carriers: transfer a charge,
e.g., Na+/K+ ATPase  exchanges 3Na+ for
Membrane Potential
Difference in charge inside and outside the
cell ↔ electrochemical gradient
Active transporters establish this
Two main functions
Provide cell with energy for membrane transport
Allow for changes in membrane potential used by
cells in cell-to-cell signaling
Can be determined by Nernst equation and
Goldman equation
Nernst equation
Used to calculate the electrical potential at equilibrium
Recall: ΔG = RTln([Xi]/[Xo]) + zFEm
Chemical component + electrical component
At equilibrium: zFEm = RTln([Xo]/[Xi])
Equilibrium potential is:
Ex = (RT/zF) ln [Xo]/[Xi]
where R – gas constant, T = absolute temperature (Kelvin),
z = valence of ion, F – Faradays constant
Example: K+ out: 0.01 M; K+ in: 0.1 M; T = 22oC
So, EK+ = (1.9872*295)/(1*23062) ln (0.01/0.1)
= -58 mV at 22oC
Nernst equation
Each ion has a different potential given the difference in concentration gradients.
Must have pores or channels to create potential!
Nernst equation and ion concentrations
Differences in Nernst potential reflect differences in chemical gradients!
We will discuss the protein pumps that are necessary to maintain these gradients.
Active Transport
Protein transporter is needed
Energy is required
Molecules can move from low to high
Active Transport, Cont.
Two main types: distinguished by the
source of energy
Primary active transport – uses an
exergonic reaction ie ATP
Secondary active transport – couples the
movement of one molecule to the movement
of a second molecule
Primary Active Transport
Hydrolysis of ATP provides energy
Three types
• P-type: pump specific ions, e.g., Na+, K+, Ca2+
• F- and V-type: pump H+
• ABC type: carry large organic molecules, e.g.,
P-class pumps: Na+/K+ ATPase pump
 pumps 2 K+ in and 3 Na+ out
 important for many cellular functions (osmotic balance of cells)
 uses ATP as energy source
 can be blocked with poisons like ouabain or digitalis
 the potential built up in the Na+ ions will be used by many different
processes i.e. cotransporters, neuronal signaling etc.
P-class pumps: Na+/K+ ATPase pump
Binding of phosphate from ATP drives conformation change that allows ions to be
transported to appropriate sides: → an asparate residue becomes phosphorylated
and the energy transfer changes the proteins conformational shape
Na+ binding sites switch from high affinity on inside to low affinity on outside to allow
for binding of Na+ on inside and release of Na+ ions on outside.
K+ binding sites with from high affinity on outside to low affinity on inside for the same
P-class pumps: Ca
ATPase pump
pumps 2 Ca2+ ions out for every 1 ATP molecule used
Uses ATP to drive Ca 2+ out against a very large concentration gradient
Internal Ca 2+ binding sites have a very high affinity
(in order to overcome extremely low Ca2+ concentrations inside cell)
Energy transfer from ATP to the aspartate of the Ca2+ ATPase causes
a protein conformational change and Ca2+ transported across membrane
Ca2+ binding sites on outside are low affinity and Ca2+ is released
The transfer of energy from the ATP to the pump triggers a conformational
change that moves the protein and allows the translocation of
Ca 2+ across the membrane
At the same time the Ca2+ binding sites change from high to low affinity.
P-class pumps: Ca
ATPase pump cont.
 In muscle cells the Ca2+ ATPase is the major protein found in the membrane
of the sacrcoplasmic reticulum (SR)
80% of the protein in the SR is the Ca2+ ATPase
 SR is a storage site for Ca2+ that is release to drive muscle contraction
 Ca2+ ATPase will remove excess Ca2+ from the cytoplasm and pump it into
the lumen of the SR
V-class pumps: proton pumps
 These pumps transport H+ only
 Found in lysosomes, endosomes and plant vacuoles
 Transport H+ ions to make the lumen or inside of the lysosome acidic
(pH 4.5 - 5.0)
 Many of these pumps are paired with Cl- channels to offset the electrical
gradient that is produced by pumping H+ across the membrane.
V-class pumps: proton pumps
 H+ is transported into the lysosome
 Cl- flows in to keep a balance
 If Cl- doesn't flow in then there is rapid build up of potential (charge) across
the membrane which would block the further transport of H+.
 This would occur long before the lumen becomes acidic because
not that many ions need to be transported to produce the voltage potential
Secondary Active Transport
Use energy held in the electrochemical gradient
of one molecule to drive another molecules
against its gradient
Antiport or exchanger carrier: molecules move in
opposite directions
Symport or cotransporter carrier: molecules
move in the same direction
Secondary Active Transport
Uniporter: One molecule. Amino acids, nucleosides,sugars
Symporter/cotransporter: movement in the same directions.
Na+/glucose cotransporter in the intestine
Antiporter/Exchanger: Cl-/HCO3- exchanger in the red blood cell
Example of a Cotransporter
Membrane Potential and Na+
 Animal cells are more negative
on the inside than on the outside
(~ -80 to -70 mV)
 Mostly due to K+ ions (inside >
outside) created via Na+ /K+
pump, K+ leak channels and
anions inside the cell (proteins
 Remember K+ will move down its
concentration gradient
 Nernst potential for K+ is - 80 to 70 mV.
Why is this important???
 Transport of Na+ down the
chemical gradient and the
electrical gradient. Makes Na+ a
powerful co-transporter!
Favors movement of Na+ into the cell
Membrane Potential and Co-transporters
Na+/glucose co-transporter
Used by cells in the intestine to transport glucose against
a large concentration gradient
This is a symporter: both in the same direction
ΔG for 2 Na+ is -6 kcal/mol
Membrane Potential and Co-transporters
3 Na+/Ca2+ antiporter
Important in muscle cells
Maintains the low intracellular concentration of Ca2+
Plays a role in cardiac muscle
[Ca2+]i = 0.0002 mM and [Ca2+]o = 2 mM
So ΔG = RTln (2/0.0002) = 5.5 kcal.mol
ΔG = zFEm = 2(23062)(0.070Volts) = 3.3 kcal/mol
Total = 8.8 kcal/mol
►So must transport 3 Na+ in for 1 Ca2+ out
HCO3-/Cl- antiporter
Regulate pH
Carbon dioxide from respiration:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- in the presence of
carbonic anhydrase (enzyme)
Note: ~80% of the CO2 in blood is transported as HCO3-.
This is generated by red blood cells (RBC)
RBC have a protein (AE1) and this is the
HCO3-/Cl- antiporter
Pumps 1 X 109 HCO3- every 10 msec.
Clears the CO2 and Cl- transport ensures that there isn't a
build up of electrical potential
Membrane Potential and Co-transporters
HCO3-/Cl- antiporter
Other transporters that regulate pH
Na+/H+ antiporter: Remove excess H+ when cells become
Na+HCO3-/Cl- co-transporter: HCO3- is brought into the cell to
neutralize H+ in the cytosol: HCO3 + H+ ↔ H2O + CO2 in the
presence of carbonic anhydrase. Driven by Na+: Couples
the influx of HCO3- and Na+ to an efflux of Cl-
Exchangers are regulated by internal pH and increase
their activity as the pH in the cytosol falls