Download Membranes & Channels PPT

Cell Membranes
• Animal cells have a cell membrane that separates them from the environment
• Cell membranes are phospholipid bilayers with associated proteins
• Cell membranes may allow some substances to pass from one side to the other
Cell Membranes: Phospholipid Bilayer
• Phospholipid bilayers are made of phospholipids
• Phosphate head is polar (= charged)
• Fatty acid tails are nonpolar (= not charged)
Cell Membranes: Phospholipid Bilayer
• Phospholipid molecules naturally align
themselves with their fatty acid tails joining
together to form the middle of the
membrane
• The polar heads face outwards towards body
fluids (water), and form hydrogen bonds with
water molecules
Cell Membranes: Membrane Components
• Proteins and other molecules are bound to the cell membrane
• Peripheral proteins are bound only to one side of the membrane
• Integral proteins pass completely through the membrane
 Integral proteins often form ion channels
Cell Membranes: Integral & Peripheral Proteins
Calcium Channel
Calcium Channel
Potassium Channels
• Strings of amino acids corkscrew through
the membrane and fold up to form ion
channels
Channel Units and Subunits
• Get used to the many different ways to draw a cartoon of an ion channel
Cell Membranes: Ion Channels
•
In living cells, a flow of
ions occurs through ion
channels in the cell
membrane
•
This creates a
difference in electrical
potential between the
two sides of the
membrane
•
Neurons are electrically
excitable due to the
voltage difference
across the membrane
Membrane Channels: Ion Channels
• Ion channels allow
ions to pass from
one side of the
membrane to the
other
• Ion channels can
have selectivity
mechanisms,
which allow them
to let some ions
pass through
while excluding
other ions
• An ion channel that allows anions to cross, but
excludes cations
Ions
•
Ions are charged
particles in
solution
•
Many ionic
compounds exist
as crystals when
not in solution
(e.g. table salt)
Ions
•
Ionic compounds
dissociate in
solution, and
individual ions
exist as charged
particles
•
Because water
carries both
partial positive
and partial
negative charges,
ions are usually
surrounded by
water molecules
Diffusion
• Solutes, including ions, diffuse in solution, until they reach equilibrium
Crossing Cell Membranes
• Passive Diffusion
 Wanders downhill across
the membrane
• Passive Transport
 Downhill on an electrical or
chemical gradient
 Carrier Mediated
• Primary Active Transport
 Uphill against the gradient
 Requires ATP
• Secondary Active Transport
 Uphill against the gradient
 Hitches a ride with an ion
going downhill
Crossing Membranes: Passive Transport
• Some membrane
channels are always
open
• Some membrane
channels change
conformation when
a solute binds, and
this allows the
solute to pass from
one side of a
membrane to the
other
Crossing Membranes: Active Transport
• The sodium/potassium pump
(Na+/K+/ATPase) which moves 3 Na+ out as
it moves 2 K+ in is an example of active
transport
• It burns an ATP for each exchange
• It is electrogenic
• Helps create the
concentration & electrical
gradients for the action
potential
Concentration Gradients
•
Concentration of ions
is different inside &
outside the cell
membrane
 Extracellular fluid
rich in Na+ and Cl Cytosol full of K+,
organic phosphate
& amino acids
•
The result is a
concentration
gradient
•
Created in part by
the sodium/
potassium pump
Electrical Gradients
•
Negative ions line
the inside of cell
membrane &
positive ions line
the outside
 Potential energy
difference at
rest is -70 mV
 Cell is polarized
•
The result is an
electrical gradient
•
Created in part by
the sodium/
potassium pump
Resting Membrane Potential
•
The overall concentration
of positive and negative
ions in the axoplasm is
roughly equal
•
Positive ions line up on
the outside of the
axolemma
•
Negative ions line up on
the inside of the
axolemma
Resting Membrane Potential : The Big Picture
•
•
The inside of the
•
membrane is lined mostly
with K+ and negatively
charged protein anions
The outside of the
membrane is lined mostly
with Na+ and Cl-
•
The inside of the
membrane is slightly
negative relative to the
outside (-70mV)
•
Where do the electrical
and concentration
gradients push K+?
Where do the electrical
and concentration
gradients push Na+?
Leakage Ion Channels
•
Leakage (nongated) channels are always open




Nerve cells have more K+ than Na+ leakage channels
As a result, membrane permeability to K+ is higher
This explains the resting membrane potential of -70mV in most nerve tissue
The resting membrane is basically a “K+ membrane”
Gated Ion Channels
•
Gated channels open and close in response to a stimulus

•
Results in neuron excitability, and a change in membrane potential
There are three types of gated channels



Voltage-gated channels respond to a direct change in the membrane potential
Ligand-gated channels respond to the binding of a chemical stimulus (e.g. a neurotransmitter)
Mechanically gated channels respond to mechanical vibration or pressure
Voltage Gated Ion Channels
•
Voltage-gated channels respond to a direct change in the membrane potential
•
In particular, many voltage gated channels open as a result of a depolarization of
the membrane
Ligand Gated Ion Channels
•
Ligand gated ion channels are one of the three types of gated channels
 Ligand-gated channels respond to a specific chemical stimulus
 In particular, when a neurotransmitter binds to a ligand gated channel, it often
opens or facilitates the opening of the ion channel