Cell membrane, transport

Prof. Gábor Szabó, 2017

This lecture:

• Essential Cell Biology: chapters 11-12

All the pages are required, except:

• ion channels, the pumps and mechanisms relevant to

Ca, osmo- and pH regulation, also included in this

chapter of the book, will be discussed in other

lectures

• Text briefly describing the various membrane

transporters discussed in this lecture is

downloadable from our website! Together with the

- marked lecture slides, that text is

required material for the tests and exam!

Keywords

• amphiphylic (amphipathic)

compounds

• lipid bilayer

• electrochemical gradient

• lipid-water partitioning

coefficient

• Henderson-Hasselbach equation

and its meaning

• facilitated diffusion

• carrier mediated passive

transport

• passive and active transport

• coupled transport (secondary

active) transport, examples of

(Na/glucose and Na/Ca)

• Na/K pump,

• glucose uniport

• P-type, V-type transporters

Lipid

bilayer

5 nm

Functions of the

plasmamembrane:

barrier

transport

signal transduction

Composition:

40-60 % lipid

30-50 % protein

10 % carbohydrate

Glycocalix

Glycolipids,

glycoproteins

Actin cell cortex

Membrane proteins

RBC: ~50 membrane

proteins

(„Gram-negative”) bacteria have two cell membranes

outer membrane: porins render it permeable

inner membrane: transporters

Lodish, Fig.

15-15

Alberts, Fig.

11-17

Lipid synthesis and steady-state composition of cell membranes

Mitochondria

have bacterial

lipids.

Low cholesterol

in Mit/ER/NE

Nat. Rev. Mol. Cell. Biol. 2008

Major

differences

between

organelles

site of synthesis

Asymmetric

lipid

composition

1. lipid incorporation into cytoplasmic leaflet

2. selective retention

3. flippases

/floppases

curvature

Asymmetric lipid

composition

scramblases

PS exposure on the surface of dying

cells: eat-me signal for the phagocytes

Lipid shape and supramolecular organization (polymorphism).

Kai Simons, and Julio L. Sampaio Cold Spring Harb

Perspect Biol 2011;3:a004697

©2011 by Cold Spring Harbor Laboratory Press

curvature

Cytosolic and exoplasmic surface

Mobility may be restricted by clustering and molecular fences

Examples of heterogenous distribution of proteins in the plasma

membrane of mammalian cells. (PMC5040727/figure/F4/)

Cell 2015

Transbilayer lipid

interactions

mediate

nanoclustering of

lipid-anchored

proteins*.

*

!

Intimate relationship with the cell cortex

RBC

Intracellular side

Figure 4. Red cell morphology. Hereditary

spherocytosis (HS; top panel); nonhemolytic

hereditary elliptocytosis (HE; middle panel);

elliptocytes, poikilocytes, and fragmented red cells in

hemolytic HE (bottom panel). BLOOD, 2008, 112(10)

mutation

altered morphology

Cell shape is determined by the

cell cortex cytoplasm

Defects that interrupt the vertical spectrin interactions

are the biochemical and molecular basis for hereditary

spherocytosis (HS), whereas defects in horizontal

interactions cause hereditary elliptocytosis (ES).

BLOOD, 2008, 112(10)

ES

HS

rso

isohypotony normotony

ghostRBC

Membrane fluidity - permeability, deformability,

regenerative capacity

Transport

Categories of membrane transport, according to…

→ Energetics

– passive

– active

→ Transported

substance

solubility

– hydrophylic

– hydrophobic

→ Membrane structure

– lipid bilayer

– complete biological

membrane

→ Number of different

transported

substances/direction of

transport

– uniport

– symport, antiport

1C

C

aq

m

>=R

5-8

nm

Lipid bilayers (arteficial membranes)

Vesicles and MBLs

Hundertwasser,

Vienna

Real membranes: passive and active transport

Carrier-mediated - passive - transport

uniport ……. mechanism resembles that of

the ionophore Valinomycin

K+

Main categories of active transport

symport antiport

P V F ABC

Na/K* Pgp,etc

vacuolar

mitochondrial

Secondary active

transporters, carriers

* The Na/K-ATPase is also

antiport by directionality, but it is

not a coupled/secondary active

transporter; it is a P-type pump.

Intestinal glucose

transport

What do you expect to

occur as a result of

inhibiting the Na/K-

pump?

Why is intestinal glucose transport unidirectional (rectified)?

Why do coupled transport processes depend on transmembrane Na+

gradient?

http://academic.brooklyn.cuny.edu/biology/bio4fv/page/sympo.htm

~ 30% of total energy consumption

in the cell!

A rise in the intracellular Ca2+ concentration causes muscle cells

to contract. In addition to an ATPdriven Ca2+ pump, muscle cells

that contract quickly and regularly, such as those of the heart,

have an additional type of Ca2+ pump—an antiport that

exchanges Ca2+ for extracellular Na+ across the plasma

membrane. The majority of the Ca2+ ions that have entered the

cell during contraction are rapidly pumped back out of the cell by

this antiport, thus allowing the cell to relax. Ouabain and digitalis

are used for treating patients with heart disease because they

make heart muscle cells contract more strongly. Both drugs

function by partially inhibiting the Na+ pump in the plasma

membrane of these cells.

Can you propose an explanation for the effects of the drugs in the

patients? What will happen if too much of either drug is taken?

Categories of transport according to

solubility of

„S”:

A. hydrophylic substrates

B. hydrophobic substrates

1C

C

aq

m

>=R

Transport of hydrophobic molecules

• Passive

– R (lipid / water partition coeff.)

– i.c. partition - Henderson-Hasselbach eq.

• Active

– ABC (ATP binding casette) transporters

– Other transporters

pH = pK + log(M/M+)

~lipid-water partition coeff.

~ m

inim

al

eff

ecti

ve c

c.

1. lipid-water partition

coeff. determines the efficiency

of general anaesthetics

P

Conformation of proteins is sensitive to the

distribution of lateral pressure across the membrane

ion

channels

R-NH2

lysosome

pH ≈ 5cytoplasm

pH ≈ 7

R-NH2

R-NH3+

R-NH3+

The lipid-water partition coefficient may

be pH-dependent

Daunorubicin distribution in cytoplasts

Selwyn J. Hurwitz et al. Blood 1997;89:3745-3754

©1997 by American Society of Hematology

Live cells incubated simultaneously with

MitoTracker® Deep Red FM,

LysoTracker® Green DND-26, and

Hoechst 33342 (InvitrogenTM)