Download REading Assignment: Chapter 12 Membrane Transport pgs. 389

Biol 205 Spring 2008 Week 3 Lecture 2
REading Assignment:
Chapter 12 Membrane Transport
pgs. 389-393 & Figure 12-7 on
pg 394
The Lives of the Cell
by Lewis Thomas
“ … It takes a membrane to make sense out of
disorder in biology. You have to be able to
catch energy and hold it, … releasing it in
small shares……. To stay alive, you have to be
able to hold out against equilibrium, maintain
imbalance, bank against entropy and you can
only transact this business with membranes in
our kind of world…..”
1
Maintaining an orderd structure: membranes, of
course, define the cell itself and the subcellular
compartments found in a eukaryotic cell
Identify each of these subcellular structures
Answers on last page of lecture notes
10 = garbage disposal/recycler
7 = network of protein filaments
(Ignore 4 & 12)
2
Many activities of living cells take
place on or across membranes:
• Defining and maintaining an ordered
compartment
• Receiving and transducing environmental
signals: known as Signal transduction
• Mediating Cell-cell interactions
• Capturing & transforming energy
• Storing potential energy in the form of
ion or solute asymmetries established and
maintained by membrane associated
proteins
3
A preview of the plasma membrane
All biological membranes have a common general structure:
a thin film of lipid and protein molecules held together by
noncovalent interactions
 cell membranes are dynamic and fluid and most molecules
can move about in the plane of the membrane
4
LIPIDS:
• large biological molecues that have little affinity
for water
• are NOT polymers
• consist mostly of hydrocarbons
• include fats, steroids and phospholipids
5
Phospholipids are the most abundant lipid in the
membrane
Phospholipids are amphipathic:
• hydrophilic region and a hydrophobic region:
Figure 4.4a
Phospholipid
Formula
Schematic
Space-filling
Icon
NH3+
HC COOH
Polar head
(hydrophilic)
CH2
O–
O
P O–
H O
C CH2
O O
Nonpolar tail
(hydrophobic)
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
Glycerol
OC O
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
Fatty acid
C
CH2
CH2
CH2
Phosphate
Fatty acid
H2C
Serine
What happens when phospholipids are mixed with
water?
6
Would a bilayer form is there was no hydrophilic
head on the molecule?
7
Proteins are imbedded in membranes
Nature 438: 578 2005 Membranes are more mosaic than fluid
8
Proteins are imbedded in membranes
9
 THE TRANSACTIONS OF THE CELL MEMBRANE
 Controls molecular traffic into and out of cell: which
molecules and particles enter and exit the cell
 Transduces signals from outside to inside of the cell
 Mediates the physical interactions between animal cells
 Mediates many enzymatic reactions including those
involving energy transformation in the chloroplast and
mitochondria
10
 Features of the plasma membrane
 the thin film of lipid and protein molecules
held together by noncovalent interactions
 membranes are dynamic and fluid and most
molecules can move about in the plane of the
membrane
Membrane fluidity is important for many
reasons:
• enables membrane proteins to diffuse rapidly
in the plane of the bilayer and to interact with
one another
• provides a simple means of distributing lipids
and proteins by diffusion from sites where
they are added to the bilayer
• allows membranes to fuse with one another
and mix their molecules -- important for many
cellular events
11
What factors might affect membrane fluidity?
12
Membrane fluidity is determined by:
• temperature
• the nature of the hydrocarbon tails on
phospholipids
• presence of steroid molecules such as
cholesterol (present in animals and absent in
plants, yeast and bacteria)
13
Vegetable Oil versus Butter ?
What’s the difference?
14
15
The kink resulting from the
double bond is exaggerated for emphasis
16
The fluidity of a membrane
• __________ with increasing amounts of
saturated hydrocarbon tails: these can
pack tightly together and are ____ fluid as
a result
• _________with increasing amount of
unsaturated hydrocarbons: kinks in the
tail introduced by C=C _______ close
packing of the hydrocarbon tails
The fluidity of a membrane
• varies ____________ with the length of the
hydrocarbon tail
• decreases with increasing amounts of
cholesterol: short, rigid molecule which
modulates the fluidity of the bilayer
17
Cholesterol
• common component of animal cell membranes
• cholesterol is a type of steroid
• steroids are lipids consisting of four fused rings
18
19
• cholesterol decreases membrane fluidity because its
rigid steroid ring system interferes with the motions
of the hydrocarbon chains in the phospholipids
• but it also inhibits the close packing of the
hydrocarbon chains
animations showing effect of cholesterol
http://telstar.ote.cmu.edu/biology/MembranePage/index2.html
20
Proteins can associate with the plasma
membrane
Various ways in which membrane proteins can associate with
the lipid bilayer. Most transmembrane proteins are thought to
extend aacross the bilyaer as a (1) single alpha helix, (2) as
multiples alphas helices, or (3) as a rolled-up beta sheet.
21
22
How can a protein be imbedded in the
hydrophobic interior of a membrane?
23
24
 Functions of integral membrane proteins:
• Transport proteins: control molecular traffic into and out of
cell
• Enzymes: photosynthesis and respiration
• Transduce signals from outside to inside of the cell:
• transduction of polypeptide hormone signals
• transduction of nerve impulses between cells
• many other processes
• Mediate the physical interactions between animal cells (cell
adhesion): form intercellular junctions
• Attach to the cytoskeleton
25
MEMBRANES: So WHAT?
Woe to that child which when kissed on the
forehead tastes salty. He is bewitched and soon
must die. This adage, from northern European
folklore, is an early reference to the common genetic
disease recognized today as cystic fibrosis. As the
saying implies, the disorder once routinely killed
children in infancy and is often identifiable by
excessive salt in sweat..
(Scientific American Dec. 1995)
cystic fibrosis: most common severe recessive
monogenic disorder affecting people of European
descent
Info about cystic fibrosis
http://www.nlm.nih.gov/medlineplus/cysticfibrosis.html
http://ghr.nlm.nih.gov/condition=cysticfibrosis
26
The “cystic fibrosis” gene codes for a transmembrane protein
involved in chloride transport (note gene is named for its mutant
phenotype and not for the protein that it specifies)
These figures
illustrate the
domain structure
of the protein
CFTR= cystic fibrosis transmembrane conductance regulator
CFTR= a chloride channel
PREVIEW: This gene is 270 kb long but the mRNA is only 6129 bp long
27
To understand the significance of this defect we need
to first consider membrane permeability
28
The plasma membrane lies at the interface of the
controlled interior and fluctuating exterior of a cell
Membrane controls two distinct and crucial
activities:
• Selective Permeability: select among different
molecular species, slowing down the
permeation of some while allowing others to
pass unimpeded
• Active Transport: brings about the transport of
material, either inward (accumulation) or
outward (excretion, secretion) against a
concentration gradient
Permeate: to diffuse through or penetrate
something
29
What factors control whether an atom or
molecule can cross the plasma membrane?
Lipid composition of the membrane:
 a high % of cholesterol makes a membrane less
permeable to small molecules
 a high % of unsaturated hydrocarbon chains
increases permeability
Characteristics of the molecule itself SUCH AS?
Presence of transporter proteins
30
The relative permeability of a synthetic lipid bilayer to
different classes of molecules. The smaller the molecule
and, more importantly, the less strongly it associates
with water, the more rapidly the molecule diffuses
across the bilayer
31
Permeability coefficients for the passage of various
molecules through synthetic lipid bilayers
What is the rate at which a given molecule will cross a protein-less lipid bilayer?
• the smaller the molecule the more readily it will cross the membrane
• more hydrophobic, the more readily it will cross the membrane
32
Nice animations on diffusion and transport
http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.swf
Small nonpolar molecules
 O2: the covalent bond between the two atoms is
nonpolar because the electrons are shared equally
 It is a small molecule and therefore it will readily
dissolve in a lipid bilayer and diffuse across it
 CO2 also readily diffuses
Small uncharged polar molecules
 if small enough, uncharged polar molecules such as
water and ethanol will diffuse rapidly across a bilayer
 glycerol diffuses less rapidly
Large uncharged polar molecules
 glucose: hardly at all
Charged molecules - large or small lipid bilayers are highly impermeable to any charged
molecule, no matter how small
 Charge and degree of hydration of such molecules
prevents them from entering the hydrocarbon phase of the
bilayer
33
MEMBRANE TRANSPORT PROTEINS
The passage of large, polar or charged
molecules requires special membrane proteins
Two major classes of membrane transport
proteins:
Carrier proteins and channel proteins
• Both form continuous protein pathways across
the lipid bilayer
• All membrane transport proteins are relatively
specific with respect to what molecules they
transport
34
 Channel Proteins
 Forms a hydrophilic water-filled pores that
extends across the lipid bilayer
 Specific for inorganic ions of the appropriate
size and charge allowing them to cross the
membrane
 Transport through channel proteins occurs at
a much faster rate than transport through carrier
proteins (discussed below)
35
 Channel Proteins Pores can be open or closed
Allosteric Regulation of a cation channel:
Top and lateral representations of a cation channel
• The channel is made of four identical blue subunits.
• The orange triangles are small molecules (called
cyclic nucleotides) that can bind to the subunits of the
channel.
36
CFTR= cystic fibrosis transmembrane conductance regulator
CFTR= a chloride channel
http://www.genet.sickkids.on.ca/cftr/CftrDomainPage.html?domainName=MSD9
37
NOTE: the
elegant
secondary and
tertiary
structure
All channel proteins allow the solute to cross the
membane passively: down the concentration
gradient
This is known as passive transport or facilitated
diffusion
38
Diffusion:
• net drift of molecules down a concentration
gradient due to random thermal movement
• net random drift of molecules in the direction
of lower concentration
Fun web site on diffusion and osmosis:
http://physioweb.med.uvm.edu/diffusion/tocpage.htm
39
A conformational change in the transporter mediates the passive movement
of a solute.
Carrier Proteins (aka Transporters):
 have a specific binding pocket for the solute to be
transported; only solute molecules that fit into the binding
pocket can be transported – conferring a level of specificity
similar to that of the substrate-active site interaction seen with
enzymes
 The transporter shown can exist in two conformational states:
in state A, the binding sites for solute are exposed on the outside
of the lipid bilayer; in state B, the same sites are exposed on the
other side of the bilayer. The transition between the two states
can occur randomly. It is completely reversible and does not
depend on whether the solute binding site is occupied. Therefore,
if the solute concentration is higher on the outside of the bilayer, more solute binds
to the transporter in the A conformation than in the B conformation, and there is a
net transport of solute down its concentration gradient (or, if the solute is an ion,
down its electrochemical gradient).
40
Transport animations
http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.swf
passive transport or facilitated diffusion:
 channel or carrier-mediated diffusion down
the a concentration gradient
active transport:
 carrier mediated
 active pumping of certain solutes across
membrane against their concentration gradient
 active transport is tightly coupled to a source
of metabolic energy
41
Three ways of driving active transport.
The actively transported molecule is shown in
yellow, and the energy source is shown in red.
Coupled transport uses potential energy in the form
of one solute gradient (see next page) to drive the
movement of another solute against its
concentration gradient
42
Ion Transport across cell membranes is of central
importance in biology:
• Cells maintain an internal ion concentration that is
different from that of the fluid around them
• These ion gradients are critical to a number of
cellular processes
Comparison of Ion Concentrations Inside and Outside of a
Typical Mammalian Cell
Component
Cations:
Na+
K+
Mg++
Ca++
H+
Intracellular
Concentration (mM)
Extracellular
Concentration (mM)
5-15
140
0.5*
10-7*
7 X10-5
[10-7.2M = pH 7.2]
145
5
1-2
1-2
4 X 10-5
[10-7.4 or pH 7.4]
Anions
Cl5-15
110
Fixed Anions**
high
0
* concentrations of free ions in the cell; the total cellular concentration is
much higher
** negatively charged small and large organic molecules that are trapped
inside the cell unable to cross the plasma membrane
43
OPTIONAL STUFF
HOW TO LET A BIG ION THROUGH WITHOUT LETTING A
SMALLER ION OF THE SAME CHARGE THROUGH
K+ ions are attracted to the selectivity filter of the Potassium Channel by the
electrostatic potential set up by amino acids with a negative charge. Focused around
the Channel's entrance, the negatively charged amino acids also repels anions,
therefore increasing the concentration of cations (K+) at the pore's entrance.
Around the channel water molecules surround the cations, keeping them stable. The
selectivity filter is not big enough to accommodate an ion and its water molecules.
Therefore, before any ion can enter the selectivity filter it must lose all of its water
molecules. However, to enter the selectivity filter the ion must remain stable.
Oxygen atoms in the backbone act as surrogate water for K+ ions, keeping them
stable. The smaller Na+ ion is not big enough for the oxygen atoms to act as
surrogate water and stabilize it. Therefore, only K+ ions can enter the selectivity
filter and remain stable.
44
45
46