Download hhhhh - Lectures For UG-5

Electrical Properties of Human
Cells
22.1.13
• Non polar regions of phospholipid molecules in each
layer face core of bilayer and their polar head groups
face outward, interacting with aqueous phase on
either side.
Mechanisms of Transport across
Cell Membrane
• Facilitated diffusion
Transmembrane proteins create a water-filled pore
through which ions and some small hydrophilic
molecules can pass by diffusion. The channels can be
opened (or closed) according to the needs of the cell.
• Active transport
Transmembrane proteins, called transporters, use
the energy of ATP to force ions or small molecules
through the membrane against their concentration
gradient
Protein Channels in Cell Membrane
• Protein channels can either be gated or non-gated (leakage
channels)
• The transmembrane channels that permit facilitated diffusion
can be opened or closed. They are said to be "gated“. Gated
channels can be opened or closed like gates. Gates are gate
like extensions of channel proteins which change their
conformation under specific conditions
• These channels can be
Voltage gated
Mechanically gated
Chemically/Ligand gated
Electrical Properties of Membranes
• Pure phospholipid bilayers are quite good insulators (there
are no free ions in the membrane so there are no carriers to
transport charges)
• Membranes act as barriers to the free diffusion of various
substances including ions. Phospholipid bilayer is itself highly
impermeable to ions
• The high conductance of biological membranes even at rest
(i.e., without synaptic influences etc) is mainly due to protein
channels which penetrates the membrane and allow ion
currents to flow across the membrane
Charge Carriers Across the
Membrane (ions)
• Mainly Na+, K+ and Cl• Other ions include Mg++, Ca++,HCO3-, NH4+ or
phosphate ions
• Ions pass through protein channels that are selective
for that specific ion
Na+ equilibrium potential:
K+ in Compartment 2,
Na+ in Compartment 1;
BUT only Na+ can move.
Ion movement:
Na+ crosses into
Compartment 2;
but K+ stays in
Compartment 2.
At the sodium
equilibrium potential:
buildup of positive charge in Compartment 2
produces an electrical potential that exactly
offsets the Na+ chemical concentration gradient.
K+ equilibrium potential:
K+ in Compartment 2,
Na+ in Compartment 1;
BUT only K+ can move.
Ion movement:
K+ crosses into
Compartment 1;
Na+ stays in
Compartment 1.
At the potassium
equilibrium potential:
buildup of positive charge
in Compartment 1 produces an electrical potential that
exactly offsets the K+ chemical concentration gradient.
Resting Membrane Potential
• Under resting or unstimulated condition,
there exists an electrical potential difference
across the cell membrane with the inside
negative relative to the exterior.
• Typically -70 mV= -0.07V (-40 to -90 mV)
– House current 120 Volts
– AA battery 1.5 Volts
– So – very slight potential
• membrane is said to be polarized
ECF and ICF are electrically neutral
• The cell membrane separates interstitial fluid
from intracellular fluid (ICF). The two fluids
(inside and outside) are both electrically neutral
but they contain many different kinds of ions
• The cell interior tends to have a much higher
concentration of K+ than exists outside the cell.
These positive ions are electrically balanced by
large negatively charged proteins ((membrane
is impermeable to them, A-)
• Outside the cell, there is a much higher
concentration of Na+ than inside. These
positive ions are electrically balanced by
negatively charged chloride ions (Cl-) and small
amount of other anions
Sources of Resting Membrane
Potential
• Nature of the membrane: Selective
permeability
• Gibbs–Donnan effect
• Na+/K+ pump
Role of Na+/K+ pump in establishing
resting membrane potential
• A major contribution to establishing the membrane potential
is made by the sodium-potassium exchange pump.
• On each cycle, the pump exchanges three Na+ ions from the
intracellular space for two K+ ions from the extracellular
space.
• If the numbers of each type of ion were equal, the membrane
would be electrically neutral, but, because of the three-fortwo exchange, it gives a net movement of one positive charge
from intracellular to extracellular for each cycle, thereby
contributing to a positive voltage difference.
Role of Na+/K+ pump in establishing
resting membrane potential
• The pump has three effects:
(1) it makes the sodium concentration high in
the extracellular space and low in the
intracellular space
(2) it makes the potassium concentration high
in the intracellular space and low in the
extracellular space;
(3) it gives the extracellular space a positive
voltage with respect to the intracellular space.
Nature of the membrane: Selective
permeability
• Cell at rest is 30 folds more
permeable to K+ than to Na+ through
passive nongated leakage channels
• There is a greater tendency of K+ ions
to move out of the cell than of Na+ to
move into the cell
• When more positive ions move
outward than inward, the inside of
the membrane becomes electrically
negative than the outside. Thus an
electric potential difference develops
across the membrane
Gibbs–Donnan effect
• The Gibbs–Donnan effect (also known as
the Donnan effect) is a name for the
behavior of charged particles near a semipermeable membrane which fail to
distribute evenly across the two sides of
the membrane
• The usual cause is the presence of a
different charged substance that is unable
to pass through the membrane and thus
creates an uneven electrical charge. For
example, the large anionic proteins in
intracellular fluid are not permeable to cell
membrane. They attract positive ions
across the membrane which cal also not
penetrate membrane
Resting Membrane Potential
• When the cell is at rest, gated channels are closed and it is
mainly K+ ions that flow across the membrane (through leakage
channels). By convention, the ion that determines the so-called
"resting" membrane potential of a cell, is potassium, although
other ions do contribute in more minor ways.
• By convention, the electric potential of the outside of the
membrane is normally taken as the reference point (or zero).
The sign of the membrane potential is designated as the voltage
inside relative to ground outside the cell.
• This means that the potential inside is negative. In the
equilibrium state or the resting state the membrane potential is
about -70mV, although it varies for different types of cells from
about -40 to -90mV
Concentration and electric gradients
• An electric force develops due to the presence of membrane
potential which acts on the ions
• The flow of ions across the cell membrane occurs as a result
of these two gradients: concentration gradient and electric
gradients. Concentration gradient moves K+ out of cell while
electric gradient moves it inside the cell
• These two components to the motion of ions are passive
transport because they need no input of energy
Membrane resistance
• Since electric current in an aqueous solution is a flow of ions,
the relatively low mobility of ions through the lipid bilayer is
equivalent to a high electric resistance. Membrane resistance
is a measure of ease with which ions move across the
membrane under the influence of a potential difference
• Ion channels acts as resistors however, since they are
characterized as being open or closed, thus they are variable
resistors
Equivalent circuit model for Plasma Membrane
• Protein channels acts as variable
resistors
• The cytoplasm and interstitial
fluid
are
the
electrical
conductors and they are
separated by a lipid bilayer of the
membrane which has an
insulating property. This feature
can be modeled as a capacitor.
Capacitance for a cell membrane
is about 1µF/cm2
• Membrane resting potential is
indicated as a voltage source