Transcript Document

Plasma membrane is about 50 atoms thick and serves as a
selective barrior.
Membranes include 1. sensors which enable the cell to respond to the
environment and 2. highly selective channels and pumps. Mechanical
properties of the membranes are remarkable. Enlarges and changes
shape as needed with no loss of integrety.
Eucaryotic cells
contain many
compartments created
by intracellular
membranes
The lipid bilayer.
A. An electron micrograph
The lipids in the cell
membrane are
amphipathic.
Phosphatidylcholine is the most common type of phospholipid.
Positive
negative
Three kinds of membrane lipids, all amphipathic, incude phospholipids,
sterols, and glycolipids.
Hydrophilic heads
Hydrophilic molecules interacting with
water molecules.
Hydrophobic molecules in water. Water molecules
form a more ordered structure. See question 1.
Purely
hydrophobic
molecules
coalesce into
a single drop
in water.
Amphipathic molecules
like
phosphotidylethanolamine
form a lipid bilayer energetically most
favorable.
Self-sealing property
“free edges” are quickly
eliminated because they are
energetically unfavorable hydrophobic areas are in
contact with water.
The lipids will spontaneously
seal and will always form a
closed compartment.
A small tear will be repaired. A
larger tear may lead to the
break up of the membrane into
separate vesicles.
With water inside and out, the lipid bilayer remains intact, no
lipids leave. However, the lipids do move freely within the
bilayer. Experiments use liposomes, which form spontaneously.
Or flat bilayers formed across a hole in a partition between to aqueous
compartments. The fluidity of the lipid bilayer is crucial for the
function of the membrane. In these experimental systems
phospholipids very rarely flip from one layer to the other without
proteins to facilitate the process.
Due to thermal motions, lipid molecules within a monolayer rotate very
rapidly and diffuse rapidly through the fluid membrane. Any drop in
temperature decreases the rate of lipid movement, making the lipid
bilayer less fluid. This inhibits many functions of the cell’s membranes.
All this has been
confirmed in whole
cells.
• The fluidity of a lipid bilayer depends on its
composition.
– As temperature and environment changes, the
fluidity of the cell’s membranes must be kept
functional.
– The closer and more regular the packing of the
tails, the more viscous and less fluid the bilayer
will be
– The length and degree of saturation with
hydrogens affect their packing
• shorter tails can not interact as much - more fluid
• one of the two hydrocarbon tails often has a double
bond - unsaturated. This creates a kink - less
packing, more fluid.
Plant fats are generally unsaturated and liquid at room temperature.
Animal fats are solid at room temperature. Hydrogenated plant fats are
no longer unsaturated.
• In bacterial and yeast cells, both the lengths
and the unsaturation is constantly adjusted
to maintain the membrane at a relatively
constant fluidity.
– At higher temperatures the cell makes longer
tailed lipids with fewer double bonds.
• In animal cells, membrane fluidity is
modulated by cholesterol, which is absent in
plants, yeast and bacteria.
Cholesterol fills in the spaces left by the kinks; stiffens the bilayer and
makes it less fluid and less permeable.
• Membrane fluidity is important to a cell for many
reasons.
– 1. Enables membrane proteins to diffuse rapidly and
interact with one another - crucial in cell signaling etc.
– 2. Provides a simple means of distributing membrane
lipids and proteins by diffusion from sites of insertion.
– 3. Allows membranes to fuse with one another and mix
their molecules
– 4. Ensures that membrane molecules are distributed
evenly between daughter cells.
• Remember though, cell has control - cytoskeleton
and other interactions can limit the mobility of
specific lipids and proteins.
The lipid bilayer is asymmetrical, with the cytoplasmic side being
different from the non-cytoplasmic side. Proteins are embedded with a
specific orientation crucial for their function. Phospholipid composition
also varies.
New phospholipid molecules are synthesized in the ER by membranebound enzymes which use substrates (fatty acids) available only on one
side of the bilayer.
Flipases transfer specific phospholipid molecules selectively so that
different types become concentrated in the two halves. One sided
insertion and selective flippases create an asymmetrical membrane
In eucaryotic cells nearly all new membrane synthesis occurs in the ER.
The new membrane is exported to the golgi apparatus for modification
and export. Carbohydrate chains are added in the golgi - glycolipids.
The enzymes that add sugar groups to lipids are confined to the golgi
apparatus and sugars are added only to the non-cytoplasmic side. No
flippases exist for glycolipids. Forms a protective coat on most animal
cells.
Intracellular signal transduction
Lipids are made in the ER and transported via vesicles to their
destination. This form of transport preserves the cytoplasmic face and
the non-cytoplasmic face which is exposed to the exterior of the cell or
the interior of an organelle.
Lipid bilayers are impermeable
to solutes and ions. Rate of
diffusion varies depending on
size and solubility properties.
This has been demonstrated in
synthetic bilayers.
In this way,
cells control the
passage of
molecules
across its
membranes
Specialized transport proteins
transfer specific substrates
across the membrane
A crucial function of any cell membrane is to act as a barrior and to
control the passage of molecules across it.
The proteins in membranes serve many functions besides transporting nutrients etc.
Linkers link intracellular actin filaments to extracellular matrix proteins. Receptors
bind hormones and other signaling molecules and transmit that signal to the interior of
the cell. Enzymes catalyze specific reactions - example: flippases.
Membrane proteins associate with the lipid bilayer in three main ways.
1.
2.
3.
All membrane proteins have a unique orientation - a particular section
always facing the cytosol. This is a consequence of how they are made.
Integral membrane proteins (transmembrane and lipid-linked) can be
isolated from the lipid membrane only by harsh treatment (detergent)
whereas peripheral membrane proteins can be released by relatively
gentle extraction methods.
Membrane proteins have a unique orientation. Always has the
same region of the protein facing the cytosol. This depends on
how it was made.
Transmembrane proteins usually cross the bilayer as alphahelices. Sometimes as beta-barrels
Transmembrane portions are composed largely of amino acids with
hydrophobic side chains. However, the peptide back bone (peptide
bonds) is hydrophilic. Therefore, a helical structure is the most
energetically favorable.
The peptide bonds are
hydrogen bonded to each
other in the interior while the
hydrophobic amino acid side
chains contact the lipid
chains.
Many transmembrane proteins cross the membrane only once. Many
receptors for extracellular signals include an extracellular portion which
binds the signal molecule (hormone etc.). Binding of the signal molecule
induces a change in shape in the cytoplasmic part, which then signals to
the cell’s interior.
Other transmembrane proteins form aqueous pores that allow watersoluble molecules to cross the membrane. These are more complicated,
often cross the bilayer a number of times as alpha-helices or as betabarrels
In these cases alphhelices contain both
hydrophobic and
hydrophilic amino acid
side chains, with the
hydrophobic side chains
on one side and
hydrophilic on the other
side.
Although the alpha-helix is the most common, transmembrane portions
of a protein can be beta-barrels (two beta-sheets connected by a disulfide
bond. The loop areas often form the active site or binding site.
Beta-barrels
are less
versatile since
the can form
only wide
channels.
To study proteins,
they can be isolated
from the lipid bilayer
by solubilization
using detergents.
Detergents are small,
amphipathic, lipidlike
molecules with only a
single hydrophobic
tail. The proteindetergent complexes
can then be separated
by SDS
polyacrylamide gel
elctrophoresis
(Chapter 5).
The complete structure of membrane proteins is difficult. They do not
crystallize well for X-ray crysallography (Chapter 5). This is
bacteriorhodopsin.
A small protein
which acts as a
membrane
transport protein
that pumps H+ out
of the bacterium.
It gets its energy
from light, which
is absorbed by
retinal.Retinal
changes shape
The structure of a
bacterial
photosynthetic
reaction center
includes four
protein
molecules.
The plasma membrane must be strengthened by the cell cortex. This is a
framework of proteins attached to the membrane via transmembrane
proteins.The shape and mechanical properties of the plasma membrane is
determined by a meshwork of fibrous proteins - the cell cortex.
Red blood cells
are very simple,
allowing study
of the cell
cortex in simple
form.
Genetic
abnormalities in
spectrin structure
result in anemia,
spherical, fragile
rbcs
The spectrin-based cell cortex of human red blood cells. Much simpler
than other cells.
Dystrophin in muscular
dystrophy
Most of the proteins in the plasma membrane have short chains of sugars
(oligosaccharides) linked to them - glycoproteins. Others have longer
polysaccharide chains - proteoglycans. All the glycoproteins,
proteoglycans, and glycolipids are found on the noncytosolic side of the
lipid membrane. They form a sugar coating called the glycocalyx.
Glycocalyx halps
to protect the cell
surface from
mechanical and
chemical damage,
absorb water and
give the cell a
slimy surface to
help cells squeeze
through narrow
spaces and prevent
them from sticking
to each other or the
walls of blood
vessels.
Besides protection and lubrication, the glycocalyx is important in cellcell recognition and adhesion. Some proteins (lectins) recognize
particular oligosaccharide side chains and bind to them. The short
oligosaccharides are enormously diverse, joined in different ways,
branched, very complex and hard to study. Ex. Recognition of an egg
by a sperm
Specific carbohydrate
chains on the surface of
neutrophils binds a lectin
on the cells of the blood
vessels at the site of
infection. Allowing them
to stick transiently and
then bind more strongly to
other adhesion molecules.
In this way the phagocytes
enter and ingest the
bacteria.
The lipid bilayer is a two-dimensional fluid. Many lipids and proteins
move freely within the plane. This is demonstrated by staining mouse
cells with rhodamine and human cells with fluorescein. These two cells
are then fused. Within 30 minutes the proteins from the mouse and
human cells have diffused and are intermixed.
However, lipids and proteins do not all float freely in the membrane.
The cell controls the movement of many proteins. Cells have ways of
confining particular plasma membrane proteins to localized areas,
creating membrane domains which are functionally specialized.
Proteins are
moved together
when signaled by
receptors like
adhesion
molecules.
Tethered to the
cell cortex
Bound by the
extracellular matrix
Stopped by
diffusion
barriors.
Held by proteins on another cell
In epithelial cells that line the gut, uptake of nutrients from the gut is
confined to the apical surface while proteins involved in the transport of
solutes out into the tissues and bloodstream is confined to the basal and
lateral surfaces. This asymmetric distribution is maintained by abarrier
formed by tight junctions - seals between adjacent cells.