Schedule of the present Lecture
1. The Plasma Membrane, the Lipid Bilayer
❖ Chemical structure of cell membranes
❖ Membrane proteins
2. Principles of Membrane Transport
❖ Transporters, passive and active membrane
transport
❖ Ion channels and the electrical properties of
membranes
2. Principles of Membrane Transport
Transporters, passive and active
membrane transport
Membrane transport function
Because of its hydrophobic interior, the lipid
bilayer of cell membranes prevents the
passage of most polar molecules
This barrier function allows the cell to maintain
concentrations of solutes in its cytosol that
differ from those in the extracellular fluid and in
each of the intracellular enclosed
compartments
The ion concentrations in and out
Typical mammalian cell
Diffusion
Only for
❖ molecules dissolvable in lipids
❖ small molecules without any charge
Any other small organic molecule = some
membrane transport molecules must be used
➢ the transporters are proteins
➢ collection of the protein differ according to the
membrane type
Relative permeability
➢ the smaller the molecule
and, more importantly,
the less strongly it
associates with water,
the more rapidly the
molecule diffuses across
the bilayer
Membrane transport proteins
❖ cross membranes (plasmatic or intracellular) and
form channels (narrow hydrophilic pore) allowing
transport of specific molecules
❖ Are responsible for presence of certain molecules in
cell or in its compartments
There are two main classes of MTP
➢ transporters (carriers, permeases)
➢ channels (for inorganic ions)
Examples
Each membrane has its own set of transporter
molecules
nucleotide sugar AA Na+
plasma membrane
lysosome mitochondrion
pyruvate
Channel proteins
They form
❖ Porins and aquaporins (permanently open, wider,
only for water molecules)
❖ Ion channels (narrow channels, may be closed, ion
selective = ions are coming through according to
their size and electric charge)
If channel is open only small molecules with
suitable charge is coming through …
… ions are transported very quickly (more than
1 million ions per second)
Structure of aquaporins
In the membrane,
aquaporins form
tetramers
monomer
Each monomer
contains a pore in its
centre (not shown)
Space-filling model
(cut and opened as a
book)
Transporters
➢ highly selective
➢ mostly transport only one type of molecule
➢ select molecules which fit to its binding site
Transport speed is about 1 000 molecules per
second
Three types of transporter movement
UNIPORT SYMPORT ANTIPORT
Passive and active transport
Require energy
- it is against
electrochemical gradient
Does not require energy
- simple diffusion or
facilitated diffusion
Passive transport
Does not require an input of metabolic energy
➢ simple diffusion through the lipid bilayer
➢ facilitated diffusion through channels and
passive transporters
➢ molecules come across the membrane
thanks their electrochemical gradients
Active transport
➢ actively pump certain molecules across the
membrane against their electrochemical
gradients
➢ transporters are named pumps
➢ transport is coupled with ATP hydrolysis
Requires an input of metabolic energy
Three types of active transport
❖ Coupled transport
One molecule is transported by ion gradient (down
the hill) and another against it (up the hill)
❖ ATP-driven pump
The energy for up the hill transport is coupled with
ATP hydrolysis
❖ Light-driven pump (only halobacteria)
Three types of active transport
Examples of coupled transport
❖ Animal cells
use to support the active transport
Na+ ions gradient
❖ Plant cells, bacteria, fungi, and yeast
use to support the active transport H+ ions
gradient
Examples of protein transporters
Glucose can be transported by active or passive transport
Transporter Energy source Function
glc transporter Na+ gradient Active transport of glc
Na+ and H+ antiport ATP hydrolysis
Active export of H+, pH
regulation
Na-K ATPase ATP hydrolysis
Active export of Na+ and
import K+, osmotic balancy
Ca ATPase ATP hydrolysis
Active export Ca2+ from
cytosole
H ATPase ATP hydrolysis Active export of H+ from cell
bacteriorhodopsin light Active export of H+ from cell
Transcellular transport of glucose
Glucose transporter can be driven by a
Na+ gradient
Transporter oscillates between two alternate states
state A: the protein is open to extracellular space
state B: it is open to the cytosol
Binding Na+ and glc is cooperative – ligand induces the
conformational change that increase the protein's
affinity for the other ligands
Glucose transporter can be driven by a
Na+ gradient
Since Na+ concentration is much higher in the
extracellular space …..
….. glucose is more likely to bind to
the transporter in the A state
Glucose transporter can be driven by a
Na+ gradient
Therefore, both Na+ and glc enter the cell (via A→B)
much more often than they leave it (via B→A)
The overall result is the net transport of both Na+ and glc
into the cell
Glucose transporter can be driven by a
Na+ gradient
Because the binding is cooperative, if one of the
two solutes is missing, the other fails to bind to
the transporter
Thus, the transporter undergoes a
conformational switch between the two states
only if both solutes or neither are bound
ATP-driven pumps
This transporter actively pumps Na+ out of and K+ into a
cell against their electrochemical gradients
ATP-driven pumps
1) Binding intracellular
Na+ and
phosphorylation by
ATP induce
conformational
changeintra
extra
intra
2) Transfer Na+ across
the membrane out
3) Binding of K+ and
dephosphorylation
return the protein to
original stage
4) Transfer of K+ into
cytosol
Three classes of ATP-driven pumps
1) P-type pumps
2) F-type pumps
3) ABC transporters
❖ the source of energy for the active
transport is hydrolysis of ATP
❖ ATP-driven pumps can transport through
membrane
➢ions
➢small molecules
P-type pumps
Structurally and functionally related multipass
transmembrane proteins
➢ „P-type“, because
phosphorylate themselves
during the pumping cycle
➢ This class include many of
the ion pumps that are
responsible for setting up
and maintaining gradients
of Na+, K+, H+, and Ca2+
across cell membrane
Na-K ATPase pump
Important for membrane potential and osmotic balance
Ca2+ ATPase pump
Strict regulating of Ca2+ concentration in cell
Growing concentration of Ca2+ ions is a trigger a lot of
biological processes (mediator secretion to synaptic
aperture, contraction of muscles, secretion, etc.)
The effects of calcium ions are indirect and are
mediated by binding to Ca2+ dependent protein (for
example calmodulin which is activated by
proteinkinase C, CaM kinase, etc.)
F-type pumps
Turbine-like proteins constructed from multiple
different subunits
➢ Are found in the plasma
membrane of bacteria, the
inner membrane of
mitochondria, and the
thylakoid membrane of
chloroplasts
➢ Called ATP synthases
➢ Structurally related are V-type ATPases that normally
pump H+ rather than synthesise ATP
➢ They pump H+ to organelles to acidify their interior
F-type pumps
To acidify organelles
interior?
WHY?
F-type pumps
Acidify interior of
lysosomes,
synaptic vesicles,
and plant vacuoles
ABC transporters
Primarily pump small molecules across cell
membranes, in contrast to P-type and F- or V-type
ATPases, which exclusively transport ions
➢ ABC transporters are also
on ER membrane where
help to transport molecules
from cytosol to ER lumen
➢ Includes also MDR proteins
(multidrug resistance) or
CFTR (cystic fibrosis
transmembrane regulator)
proteins
ABC in prokaryotes and eukaryotes
Multiple domains, two hydrophobic (six membranespanning
segments) which provide substrate specificity
and two ATPase domains protruding into cytosol
➢ Prokaryotic are used for
export and import
➢ Eukaryotic only for export
➢ Eukaryotic are able to
pump hydrophobic drugs
out of the cytosole
(resistency to anticancer
drugs)
2. Principles of Membrane Transport
Ion channels and the electrical properties
of membranes
Ion channels
Ions transport through membrane has crucial importance
in biology. It helps to keep internal ion composition, which
is different from extracellular space.
Ion channels are closable. They have two or three
conformations (closed – open, inactivated).
Ion channels
When the membrane is at rest (highly polarized), the
closed conformation has the lowest free energy and is
therefore most stable
When the membrane is depolarized, the energy of the
open conformation is lower, so the channel has a high
probability of opening
Ion channels
However in the depolarized membrane the inactivated
conformation is the most probably state
So after a variable period spent in the open state the
channel becomes inactivated
The red arrows
= sequences
that follows a
sudden
depolarization
The black
arrow
= return to
the original
conformation
as the lowest
energy state
The types of ion channels
membrane
depolarisation
Voltage-gated ion channels
This type of the ion channels play the main role
in the tissues which are excitable (neurons,
muscle and heart cells)
The membrane potential is directed by permeability of
membrane for specific ions
They are regulated by membrane potential
Voltage-dated ion channels are Na+ channels, K+
channels, Ca2+ channels of N type (Neuron) and T type
(Transient)
Ligand-gated ion channels
The channel opening is regulated by binding of
some ligand molecule into receptor
(extra or intracellular)
They are usually part of membrane receptors
This channels mediate very quick responses to
stimuli = in milliseconds
➢ Acetylcholine-nicotine receptor connected to Na+/K+ or
Ca2+ channel (neuro-muscle plate or vegetative ganglia)
➢ Glutamate receptor connected to Na+/K+ or Ca2+ channel
➢ GABAA –receptor and Glycine receptor connected to Cl-
channel
➢ Muscarine receptor M2 connected to K+ channel
➢ Serotonin receptor – part of cations channel
Na+ channel with acetycholine
The channel opening is regulated by binding of
acetylcholine molecule into receptor
(extra or intracellular)
global structure closed opened
binding site to
acetylcholine
lipid
bilayer
acetylcholine
cytosol
Voltage and ligand-gated
They are open by membrane depolarisation, but
probability of opening depends on ligand
binding
Examples are so named slow channels for Na+
(type L) and Ca2+ (type L) in the cell of myocard
Mechanically-gated ion channels
Channel opening is regulated by mechanical
force, which affects the channel
Example: K+ channels in the vestibular apparatus
The vibration of sound mechanically open the K+
channels in hair cells of internal ear which generate
neuronal excitement
Mechanical connection between the channel and
membrane is mediated by microfibril, which open
the channel only than the membrane is taut
The electricity of membrane
The plasma membrane of all electrically excitable
cells contains voltage-gated cation channels
They are responsible for generating the action
potential
Na+
Cl Cl
-
Na+K+
K+
PO4
-
HCO3
Negatively
charged
organic molecules
Na+
Ions concentration
Comparison of ions concentration inside and
outside a typical mammalian cell
Polarization of membrane
A difference between electrical potential on
external and internal side of membrane exist =
the membrane potential. It is NEGATIVE
Na+
Cl Cl
-
Na+K+
K+
PO4
-
HCO3
Negatively
charged
organic molecules
Na+
+
Animal
cells
from -20 to -200 mV
-
THE
MEMBRANE IS POLARIZED
Description of membrane polarization
Inside the cell there are in the high concentration of
negative ions of organic molecules
This negative charge is balanced by K+ ions
that are the most important ions in the cell
● Cell actively pump K+ inside (against gradient
of concentration)
● K+ ions escape by channels out of the cell
➢ The ions imbalance is produced by this process
➢ The negative charge stay inside, which the cell is not
able to equilibrate
➢ Steady state is set up = RESTING MEMBRANE
POTENTIAL
This potential protect another K+ movement out of the
cell (electrochemical gradient is ZERO)
Two forces
The force which drives ion through the
membrane has two parts
resting membrane potential varies in mammalian cells
from –50 to –90mV according to the cell type. It has
negative charge because intracellular space is
negative in regard to extracellular environment
(negative ions are in the cell in excess to positive
ions)
➢ electrical potential the membrane
➢ concentration gradient of this ion
Action potential
Any change in membrane permeability for ions
(especially cations) activates change of ion
potential and can disrupt the resting membrane
potential and cause action potential
This is used in electrical signalisation of cells in
excitable tissues (neuronal, muscle or heart
cells)
Activation of action potential – 1/2
1. Sudden supply of positive ions to the cell →
change of resting membrane potential →
MEMBRANE DEPOLARIZATION
2. Sufficiently large depolarization leads to opening
of voltage-gated Na+ channels
Local depolarization of plasma membrane started
the ACTION POTENTIAL
Activation of action potential – 2/2
3. Na+ ions enter very quickly to the cell, the
depolarisation is intensify, and polarity is
convert to +30 up to +40 mV = MEMBRANE
TRANSPOLARIZATION. In this moment the
ion gradient for Na+ is equilibrated. No more
ions enter to the cell.
4. After it the leaving K+ channels are open and
Na/K pump returns the ions ration to initial
stage (Na + ionts out, K + ionts in). MEMBRANE
REPOLARISATION is coming (return to resting
membrane potential)
Activation of action potential
REPOLARIZATION
TRANSPOLARIZATION
Resting
membrane
potential
Action potential
Cell communication by electric
signalisation – 1/2
Action potential is able to depolarise also
neighbouring parts of the membrane and extend itself
as a wave (tight junctions)
Gradually more and more Na+ channels are opened and
whole process is accelerated
During several miliseconds the resting membrane
potential changes from –60mV to +40mV and again
back
Cell communication by electric
signalisation – 2/2
The action potentials contribute for quick cell
communication on long distance
They administer in neurons during neuronal signals
transportation, during muscle cells contractions,
including myocardium
The electric activity of the cells is usually mediated by
voltage-gated Na+ channels (in myocardium Ca2+ ions
are used)