Chapter 8. Membrane Structure and Function
Membrane models have
evolved to fit new data.
Membranes are Fluid.
Membranes are mosaics of
structure and function.
are important for cell-cell interactions.
Passive transport is
diffusion across a membrane.
Osmosis is the passive
transport of water.
Cell survival depends on
balancing water uptake and loss.
Specific proteins can
facilitate passive transport.
Active transport is
pumping of solutes against their concentration gradient.
Some ion pumps generate
voltage across membranes.
In cotransport, a membrane
protein couples transport of two solutes.
endocytosis transport large molecules.
The plasma membrane is
the edge of life and like all membranes it is selectively permeable, that is it
allows some substances to cross more easily than others.
Staple ingredients of a
membrane are lipids (phospholipids) and proteins.
are amphipathic meaning it has a hydrophobic portion and a hydrophilic
lipids interact in the fluid mosaic model.
are abundant and essential.
cellular environments from the outside and from each other.
hydrophobic bilayer makes them good barriers (Fig 8.1).
more than define compartments they process material energy and information.
Including communication with the extracellular
environment and other cells.
models have evolved to fit new data.
Data from many experimenters have shown us that
membranes are a bilayer (Fig 8.1) allowing for them to be barriers and that
proteins are embedded in the membrane (Fig 8.3).
All of the
information has led us to our current fluid mosaic model.
The proteins have
hydrophobic domains that pass through the membrane and hydrophilic regions on
Membranes are fluid.
are not static sheets locked rigidly into place.
proteins can flow laterally quite easily even occasionally flipping (Fig 8.4).
Why would flipping only occur rarely?
Protein movement was demonstrated by a cell fusion
experiment (Fig 8.5).
fatty acids in a membrane decrease its ability to packed easily (by temp) where
saturated hydrocarbons pack more easily.
is an important part of animal membranes it keeps them fluid at low
temperatures and yet provides some rigidity.
Helps counter saturated hydrocarbons.
Two cells are fused (one from a human one from a
mouse) in this experiment and the membrane proteins are analyzed to see if they
will mix with the proteins of the other cell.
Some proteins do not move about the membrane because
they are anchored to the cytoskeleton.
Other proteins seem to have directed movement
depending on chemical signals.
are mosaic of structure and function.
is a collage of different proteins embedded in the fluid matrix of the lipid
two types of membrane proteins: integral and peripheral.
Integral membrane proteins penetrate the phospholipid
bilayer, most are transmembrane meaning they pass from one side of the
membrane to the other.
Peripheral membrane proteins are not embedded in the
bilayer, but are attached to exposed parts of integral membrane proteins or
Example: Internal membranes of the mitochondria and
chloroplast have specialized proteins for energy production.
surfaces of a membrane have different properties due to the asymmetric
distribution of transmembrane proteins.
The different domains of a protein have different
properties and transmembrane proteins can have different domains on either side
of the membrane.
Peripheral proteins are either on one side or another
of the membrane.
also regional differences.
if a membrane protein is peripheral or integral?
Amino acid side chains have different properties: some
are hydrophobic or hydrophilic.
Integral membrane proteins often have long a-helical sections that are hydrophobic.
These areas are perfect
for lying in a membrane.
hydrophobic forces keep the integral membrane protein in place.
How does the integral membrane protein get in the
during translation signal for the protein to be inserted in the membrane.
Some peripheral membrane
proteins have specific sequences for the addition of specialized lipids that
get inserted into the membrane to keep the protein peripheral to the membrane.
carbohydrates are important for cell to cell recognition.
blood cell plasma membrane consists of (by weight) 40% lipid, 52 percent
protein and 8 percent carbohydrates.
carbohydrates are on the outer surface of the membrane and serve as recognition
can recognize specific extracellular compounds.
These carbohydrates are attached to a protein or
The carbohydrates protrude from the lipid or protein
into the extracellular environment.
are bound to lipids and proteins as glycolipids and glycoproteins
are important in recognizing cancerous cells for destruction by the body.
They change their components and are recognized by
white blood cells for phagocytosis.
Most of the
carbohydrate is bound to proteins.
They are bound as oligosaccharide side chains.
Added to the proteins inside the ER and modified in
Great variety of carbohydrates due to the variety of
membranes molecular organization results in selective permeability.
maintain a constant movement of material back and forth across the membrane.
things move across the membrane equally, selective permeability.
molecules (CO2 and O2) easily pass through the membrane.
ions or polar molecules (hydrophilic) is tightly regulated.
proteins (transport proteins) play a vital role in controlling the
movement of that which cannot pass the hydrophobic barrier.
Transport proteins may actively help molecules across
the membrane or form a simple pore for things to diffuse through.
through the membrane can be an active or passive process.
transport is diffusion across a membrane.
is the tendency for molecules to move from an area of high concentration to an
area of low concentration.
This is considered down the concentration gradient.
This requires no work and increases entropy (random
the movement is random the net movement of particles is directional until
equilibrium is reached.
a substance across a biological membrane is called passive diffusion.
transport includes: Simple diffusion and facilitated diffusion
Osmosis is passive transport (diffusion) of water.
of water depends on the amount of solute particles present (not the type).
potential is based in large part on solute concentration (Fig 8.11).
solutions have identical osmotic potentials they are isostonic no matter
their chemical composition.
If the two
solutions are not isosmotic then the solution with a high solute concentration
is hypertonic to the other solution which is hypotonic.
Hyper/hypoosmotic solutions can cause cells to shrink
or expand (sometimes burst) respectively (Fig 8.12).
survival depends on balancing water uptake and loss.
water across a membrane and the balance between water and the environment is
without cell walls have the ability to control water balance, osmoregulation.
cell walls have an ability to withstand bursting when placed in a hypoosmotic
solution due to turgor pressure.
Driving force for growth in plant cells.
can go from turgid (normal) all the way to flaccid (limp) (Isotonic) and still
survive due to their cell wall.
die in a hypertonic solution due to their PM coming apart from the cell wall, plasmolysis.
proteins facilitate passive transport of water and selected solutes.
molecules need help in order to diffuse.
diffusion with the help of a transport protein is called facilitated
are channel proteins involved in helping water (highly polar) across the
proteins form simple pores for polar molecules
to move through while others act as gated channels.
Gated channels respond to a stimulus to open and close
(no energy directly involved).
In fact binding of a solute molecule may cause the
channel to change conformation allowing the solute to pass (8.14 b)).
diffusion can saturate, when there is more solute than available
Membrane Transport is the pumping of solutes against their concentration
transport always moves molecules from areas of high concentration to areas of
transport can move molecules from low ΰ high (uphill).
specific membrane transporters as well as systems involved in membrane fusion.
important in keeping cellular concentrations of small molecules (ions)
different inside the cell than outside.
example is the sodium-potassium pump which expels sodium and brings potassium
inside the cell (Fig 8.15).
pump, an integral membrane glycoprotein found in all animal cells.
The breakdown of ATP into ADP and Phosphate (Pi)
is coupled to the movement of 3Na+ ions out of the cell and 2K+
ions into the cell.
The movement of Na+ out and K+
in by this protein classifies it as an antiporter.
Different pumps are responsible for the transport of
several other ions but only cations are transported by primary active
Other solutes are transported by secondary active
ion pumps generate voltage across membranes.
anions and cations set up a membrane potential across the membrane.
The inside of the cell is negatively charged when
compared to the outside and supplies a potential of 50 to 200 millivolts
(negative because the inside of the cell is negative).
cytoplasm is negative cations are more likely to diffuse into the cell and
anions out of the cell.
are two forces acting on an ion the concentration gradient and the chemical
gradient this combination is called the electrochemical gradient.
pumps are those involved in generating voltage across a membrane (Ex: Na/K
Cotransport results from a membrane protein coupling the transport of two solutes.
ATP-powered pump that transports a specific solute can indirectly drive the
active transport of several other solutes.
Analogous to water being
pumped up a hill then doing work as it flows back down.
Ex: Proton pump that
drives sucrose across a membrane as it moves back across the membrane.
and endocytosis transport large molecules.
is the process by which materials packaged in vesicles are secreted from the
The vesicle membrane fuses with the PM (from the
Golgi) the phospholipids of the two membranes merge and opening to the
extracellular environment develops.
Used for the secretion of cell waste, enzymes,
neurotransmitters and other cellular secretions.
three forms of endocytosis: phagocytosis, pinocytosis and receptor-mediated
All three involve invagination (infolding) of the
plasma-membrane making a small pocket.
deepening forms a vesicle containing contents from the extracellular
is a feeding process found in unicellular protists and white blood cells (often
the engulfing of an entire cell.
The phagosome that is
formed often fuses with a lysosome.
(cell drinking) uses smaller vesicles and brings in dissolved particles.
endocytosis involves specialized reactions that take place at the membrane
and trigger uptake.
Receptor-mediated endocytosis is highly specific.
Similar to other
endocytosis events except receptors on the PM bind to specific substances in
the extracellular environment.
These coated pits are depressions of the PM with
receptors on the extracellular surface and coated on the cytoplasmic surface
with the protein clathrin.
After the receptor(s) bind the appropriate substrate
the coated pit invaginates and forms a clathrin coated vesicle.
Clathrin may act to strengthen and stabilize the vesicle.
Once inside the cell the vesicle may lose its coat and
fuse with other vesicles.
Receptor-mediated endocytosis is much more rapid and
efficient method of taking up substances out of the cellular environment.
of receptor-mediated endocytosis is how cholesterol gets into mammalian cells.
Cholesterol is synthesized in the liver and
transported in the blood as a lipoprotein.
This low-density lipoprotein or LDL is taken up into
cells by attachment to a specific receptor in coated pits.
These LDL particles are engulfed, the receptor is
recycled and the LDL containing vesicle fused to a lysosome.
The LDL particle is digested and the cholesterol is
made available to the cell.
deficient receptor for LDL leads to dangerously high levels of cholesterol in
the blood, hypercholesterolemia.