Cell membrane and membrane transport
Dr. Goda Katalin
Key words: phospholipds, cholesterol, membrene fluidity, glycocalyx, active transport,
Passive transport, facilitated diffusion, secondary active transport, uniport, symport,
antiport, Na2+/K2+-ATPase, P-glycoprotein, CFTR, Multidrug resistance, ion channel
Suggested readings:
Essential Cell Biology 3rd eddition: pages 363-374, 387-405
PLASMA MEMBRANE
Functions: • barrier
• transport
• receiving information
hydrophilic parts hydrophobic parts
proteinslipids
lipid
bilayer
STRUCTURE OF THE
BIOLOGICAL MEMBRANES
40-60 % of the membrane mass is lipid,
30-50 % is protein and approx. 10 % is carbohydrate
carbohydrate
Transmembrane and peripheral proteins (extrinsic or intrinsic)
Two of the –OH groups of are linked to fatty acids while the third –OH group is linked to phosphoric acid.
The phosphate is linked to small polar molecules:
• ethanolamine• serine• choline
Phospholipids
Fa
tty a
cid
Fa
tty a
cid
Fa
tty a
cid
Fa
tty a
cid
Fa
tty a
cid
Fa
tty a
cid
Phosphatidyl-
etanolamine
Phosphatidyl-
serine
Phosphatidyl-
choline
Properties of lipids
• amphipathic molecules (hydrophilic head and hydrophobic hydrocarbon tail)• they form bilayer or micelle in water• fluidity depends on the composition of membrane
phospholipid micelle
water
phospholipid bilayer
Phases of the membrane
T<Tm, gel phase,
tighter packing of the
lipids, limited molecular
motions and diffusion
T>Tm, liquid phase
looser packing of the lipids,
more intense molecular motions
and faster diffusion
Tm : phase transition temperature
What factors influence membrane fluidity?
length of fatty acyl chains: shorter chains – weaker
interaction – Tm
amount of unsaturated fatty acids: double bond –
bend in the chain – weaker interaction - Tm
amount of cholesterol (a steroid lipid, essential part
of membranes)
dual effect: increases fluidity below Tm, decreases
fluidity above Tm, stabilizes the membrane
CHOLESTEROL
• important constituent of membranes – fills the spaces
between neighbouring phospholipid molecules
• stabilizes the membrane
• decreases the membrane fluidity and permeability
Rigid planar
steroid ring
structure
Polar head group
Nonpolar
hydrocarbon tail
What are “lipid rafts” (DIG-microdomains)?
Membrane domains of special composition (high sphingolipid,
glycolipid and cholesterol content) that also include proteins
(Detergent Insoluble Glycolipid microdomain)
p56lck
CD
4/C
D8
TCR/
CD3
hDlg
Kv1.3
ZIP-1/2
1 inte
grin
Kv2
PKC
What is the function of lipid rafts?
Lateral organization of membrane proteins, keeping essential
elements of certain signaling events in each other’s vicinity to
enable their interaction
• Typical for biological membranes
• Generated by phospholipid translocases in the ER
THE LIPID BILAYER IS ASYMMETRICAL
Outer leaflet
The glycocalyx of a eucaryotic cell
Passive transmembrane diffusion
The rate of diffusion of molecules across the lipid membrane is
inversely proportional to their size, polarity and charge.
Many of the molecules that
the cell uses as nutrients are
too large and polar to pass
through a pure lipid bilayer.
Membrane transport
• Energetically:
– passive
– active
• Transported
substrate
- hydrophobic
- hydrophilic
1C
CK
aq
m
passivetoward lower concentration
no energy required
diffusion
facilitated diffusion
Primary active
secondary activedirect use of ATP:e.g. Na+/K+-pump, ABC transporters,
Ca++-pumps
transport protein aiding the
passage of the target moleculee.g. glucose uniport, ion channels
indirect use of ATP: transport of the
target molecule using energy from the
gradient of another moleculeeg. glucose-Na+symport, aminoacid-Na+-
symport
How can transport mechanisms be classified?
toward higher concentration
requires energy
active
Facilitated diffusion
e.g.. glucose uniport
Classification of transporters according the direction
of transport
Secondary active transport
symport: Na+-glucose
Na+-amino acid
antiport: Na+/Ca2+ exchanger (3:1)
uniport symport antiport
carrier molecule helps transport toward
the lower concentration
selective, saturable,
can be selectively inhibited
Glucose uptake from the gut
Intestinal lumenGlucose ,
High Na+
Epithelial cell Low Na+
High K+
BloodHigh Na+
Low K+
Velocity of transport
Co-transport
The Main Properties of Ion Channels
hydrophilic channel through
the membrane=transmembrane
aqueous pore
driving force + open pore ion flow(electrochemical
potential gradient)
• they are not simple holes
• ion selectivity
• they fluctuate between closed
and open conformations
Classification of ion channels according to their
gating mechanism
Voltage gated
channels= controlled by
the mem-brane potential
change
(e.g. K+ and Na+ channels
of nerve cells).
Ligand gated = a
ligand binding at
the extracellular
side (e.g.acetil-
choline receptor).
Intracellular
massenger gated = a
ligand binding at the
intracellular side
(e.g. Ca2+ activated
K+ channels).
Membrane stretch
activated (“stretch
gated”) channel =
controlled by a mec-
hanical force applied
to the channel (e.g.
Cl– channels involved
in volume regulation of
lymphocytes)
Active pumps
• P-type ATPases: Na+/K+-pump, Ca++-pump
• V-type ATPases: H+-ATPase (e.g. in the
lysosome membrane)
• F-type ATPases: F0F1-ATPase (mitochondria)
• ABC-transporters
Na+/K+ pump
maintenance of Na+ and K+ gradients:
• membrane potential (electrogenic: 3 Na+/2 K+, source
of diffusion potential
• decrease of osmotic pressure
• energy for secondary active transport
ABC transporters
• Bacterial ABC transporters
• Transporters of lower eucaryotes (a)
• P-glycoprotein (a)
• MRP1 (a)
• ABCG2 (a)
• CFTR (b,c)
• SUR (c)
a., pump
b., channel
c., channel
regulator
A P-glycoprotein (MDR1)
P-glycoprotein (Pgp, ABCB1)
-Structure: 2x6 transmembrane alpha helix,
2 ATP-binding site (ABC motívum)
-Expression: small intestine, pancreas, adrenal glandsblood-brain barrier, tumor cells
-Substrates:
endogén: ?
exogén: xenobiotikumok, gyógyszerek
-Mechanism: from cell membrane → to extracellularspace
-significance: tumor, HIV chemotherapy, drug absorption
ABC pumps
decrease the absorption of toxic
substances from the intestine
„Second line defense against toxic
substances
Multidrug transporters, especially MDR1/Pgp
and ABCG2, are expressed in the apical membrane
(facing the blood compartment) of the endothelial cells
and extrude a variety of hydrophobic drugs and toxins.
CFTR:
(cystic fibrosis transzmembrane
conductance regulator)
ATP-dependent chloride ion
channel and channel regulator
thick mucus in the
bronchi and bronchioli
(genetic disease)
SUR1: involved in the blood glucose level dependent
regulation of insulin secretion in the beta cells of
pancreas
Hetero-
octamer
K+
Increased glucose level in the blood → elevated glucose uptake into the
beta cells→ increased intracellelar ATP/ADP ratio →ATP binding to
SUR1 → conformational change → closure of Kir6.2 K+-channel →
depolarisation → elevated intracellular Ca++ level → insulin secretion