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