Download Cell Membranes

Cell Membranes
Components
• Lipids (L) – 25-50% (includes glycolipids)
• Proteins (P) – 50-75% (includes
glycoproteins)
The plasma membrane: roles attributed to
membrane components (L=lipids; P=proteins)
• A cell’s plasma membrane provides a barrier to large,
polar or ionic solutes– L
• The membrane has passageways through which
specific ions flow, nutrients enter and wastes leave – P.
• The plasma membrane is equipped with sensors, to
detect and initiate responses to the environment –P
• The membrane exterior surface bears markers that
reflect the cell’s function and identity – P
• The membrane has pumps that, by doing work on
solutes, maintain the osmotic pressure and ionic
composition of the internal environment – P.
• The membrane has specialized areas that stabilize the
cell’s shape and relationship with its neighbors – L,P.
• The extensions of the plasma membrane function in
motility as they cover cilia, flagella and pseudopodia that
allow many cells to move themselves or the
environment L, P.
Membranes in the animal cell include:
• 1. plasma membrane
• 2. the double membrane of the nucleus
• 3. the endoplasmic reticulum and Golgi
apparatus
• 4. The membranes of organelles like lysosomes
and peroxisomes
• 5. the membranes of the mitochondrion
• 6. others…
Q:
Where do the cell’s various
membranes come from?
A: All membranes come from pre-existing
membranes, i.e., they are made by
expansion of membranes that are already
present, either by synthesis of new
phospholipid or by transfer of membrane
from another part of the cell. Most
membrane synthesis is associated with the
smooth endoplasmic reticulum (ER) and
delivery in vesicles from the Golgi apparatus.
Smooth ER
cytoplasm
Synthesis of membrane
phospholipids
Synthesis is accomplished by
enzymes present in the cytosol
side of the smooth ER membrane:
In the first steps, the enzymes
activate a fatty acid by attaching a
molecule of Coenzyme A (CoA),
unite the activated fatty acid with
a glycerol-3-phosphate, and then
add a second activated fatty acid.
A phosphatase then
removes the
phosphate group from
the glycerol backbone,
leaving a
diacylglycerol.
In the last step, a
phosphorylated amine
(phosphoethanolamine
in this example) is
activated by cytosine
triphosphate and then
is attached to the 3rd
carbon of the glycerol
backbone.
What is this coenzyme A thing,anyway?
Hey, this is ADP!
This is pantothenic acid – Vitamin B5
But…the components and the synthetic enzymes
are present only in the cytosol and inner
membrane leaflet…Do you see a problem?
The solution?
• a protein class known as flippases allows new membrane
lipids to move to the outer leaflet of the plasma lipid bilayer
or inner layer of the organelle membranes.
From Flippin' lipids
Marcus R Clark
Nature Immunology
12, 373–375
(2011)
Detailed caption for the previous slide
(a) Comparison of the functions of flippases, floppases and scramblases in the
plasma membrane. Flippases (left) use ATP to move the
aminophospholipids PS and, to a lesser extent, phosphatidylethanolamine
(PE), from the outer leaflet to the inner leaflet of the PM against a
concentration gradient. Floppases (middle) use ATP to transport substrates
such as phosphatidylcholine (PC), sphingolipid (SL) and cholesterol
against concentration gradients in the opposite direction. Scramblases
(right) are ATP independent and less substrate specific and facilitate the
movement of lipids along concentration gradients.
(b) (b) Some PS functions in cells. When cells undergo apoptosis (left), the
activation of scramblases allows the rapid appearance of PS on the outer
leaflet of the plasma membrane, where it provides an 'eat me' signal. On
the PM inner leaflet (middle), PS helps organize lipid rafts and can serve to
recruit members of the protein kinase C (PKC) family through their C2
domain, as well as signaling molecules containing hydrophobic side chains,
such as Ras, Rho and Src. PS can induce local membrane curvature (right)
and recruit specific effector complexes that facilitate receptor endocytosis,
vesicle formation and endocytic trafficking.
How do these differences in membrane
lipid composition arise?
The lipids and proteins needed in different membrane domains are targeted to
those sites after their synthesis, by incorporation into labeled vesicles. In these
epithelial cells , there are apical and basolateral membrane domains that are
characterized by specific proteins.
surfaces.
* Note that in epithelial cells tight junctions both join adjacent cells and restrict the
movement of the different kinds of membrane components between the apical (red)
and basal (green) membrane domains within the same cell
Tight junctions are sites where proteins extending
from adjacent cells link the cells to form a sheet
that is relatively impenetrable
Looking at the membrane: Membrane proteins –
the mosaic part of the membrane
• Method of visualization: Freeze-fracture microscopy
Actual fractured membranes: the frozen membrane fractures along its path of
least resistance, which is the nonpolar interior. The membrane leaflets are
coated with a deposit of gold or platinum, often followed by carbon. The
biological material is digested off and the delicate replica is viewed with the
electron microscope.
View of an erythrocyte: the outer surface is smoother than the view
revealed by fracturing off half of the bilayer to reveal the inner surface
or inner leaflet and proteins that extend between the two bilayers.
An Almost-up-to-date Membrane Cartoon
Structural features of membrane-protein
interaction
1. Proteins anchored
by attachment to
lipids
2. Transmembrane alpha helices:
hydrophobic regions of the protein
3. Beta sheets (hydrophobic
regions) forming a beta barrel, found
in bacteria, mitochondria and
chloroplasts
Hydropathy plots use a protein’s amino acid sequence to predict whether a protein
will be an intrinsic membrane protein and which parts of the protein will be the
intramembrane domains. For an intramembrane domain, there must be a run of
about 20 hydrophobic amino acids in the sequence.
Proteins can also associate
with the membrane by
electrostatic attraction to
phospholipids, integral
membrane proteins or surface
sugars.
Electrostatic attraction holds annexin (a
Ca++-binding protein important in
membrane fusion reactions) in place on
the membrane surface.
Rafts in the membrane.
•
Rafts result from preferential association of special lipids, such as
sphingomyelin; these are semisolid regions that allow concentration of
certain proteins or attachment points for the internal skeleton
How do we know about rafting and other factors that affect dynamics of
proteins in membranes? Here, a fluorescent bead visible with a light
microscope is coupled to a membrane protein, allowing it to be tracked
over time and to identify conditions that restrict movement.
Useful features of membranes:
1. enzymes
• Many enzymatic reactions take place on the
membrane surface – we will focus on
mitochondrial reactions, but there are many
more.
Useful features of the membranes:
2. receptors
• Receptors allow the cell to detect cues from the
environment. (cues can be paracrines,
hormones or neurotransmitters, the level of CO2
or glucose, the presence of a new type of cell
next door…)
• Receptors set in motion the chain of events that
coordinate cell’s responses to its environment.
(responses can be quick or prolonged: opening
a K+ channel, changing the level of an enzyme,
turning on a set of genes…)
Useful features of membranes:
3. ionic gradients
The fact that pumps in the cell’s membranes
can separate different concentrations of
ions and other substances across the
membrane (cytosol vs. extracellular,
intraorganelle vs. cytosol) can be used in
the following processes:
Processes driven by ionic gradients
• Uptake/efflux of nutrients, salts, metabolites
• Osmotic regulation (water follows salt)
• ATP synthesis in mitochondria and in prokaryote
cell membranes
• drug and toxin efflux, pH regulation
• Signal transduction (Ca++ entry, action
potentials)
• H+-driven flagellar rotation (bacterial flagella).
Formulate definitions of the
following terms
•
•
•
•
•
Gradient
Diffusion
Permeability
Passive versus active transport
Primary active transport versus
secondary active transport
Movement through the membrane: Passive = down a
free energy gradient
1. Simple Passive diffusion: small, uncharged
molecules pass through the phospholipid structure
by “dissolving” in it, so no gradient for this category
of molecules is maintained across the membrane.
2. Facilitated (passive) diffusion:
a) channel proteins form open pores through the
membrane, selecting what can pass through on the
basis of size and charge.
b) Passive carrier proteins bind to the molecule to be
transported and undergo a conformational change to
deliver it to the other side of the membrane. They
simply facilitate downhill movement.
Water channels, or
aquaporins
• Aquaporins belong to an
ancient family that has
been conserved from
bacteria to humans. The
two halves of the protein
arose by gene duplication.
The 4 subunits provide 4
separate water channels.
The presence of these
channels in red blood cells
explains why they swell
and shrink so rapidly when
exposed to hypotonic
(diluted) or hypertonic
(concentrated) solutions.
The glucose transporter (GLUT1) increases the
rate that glucose travels down its concentration
gradient: structure of the 12 transmembrane
helices and model illustrating its operation
An example of an ion channel at work
• Ion channels are designed to allow the passage of specific ions
on the basis of size and charge. Those that are open
constitutively allow a specific type of ion to pass through the
membrane freely; however, most ion channels are “gated”. An
example of a free-passage or “leak” channel is the one for
potassium ions that is open in the “resting” membrane of a
nerve cell such as the squid axon, illustrated below:
Active transport = movement of solute
against a free energy gradient
• 1. Primary active transport: the carrier
protein is also an ATPase – ATP provides
the energy to concentrate solute against a
chemical (or electrochemical) gradient.
• 2. Secondary active transport: The carrier
draws on one solute’s transmembrane
energy gradient to move another solute
against its transmembrane gradient.
Active transport driven by ATP hydrolysis
• The ABC transporters
are the largest family of
membrane transporters,
characterized by their
ATP binding cassettes.
The chloride transporter
that is defective in cystic
fibrosis belongs to this
family, as does the
multidrug resistance
gene product, MDR.
MDR normally pumps
out toxins, but its
expression can be
greatly increased by
selection pressure in
cancer cells during
chemotherapy.
How natural selection defeats chemotherapy
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Channels that apply energy to push ions against their
concentration gradient are called pumps
• Active transport driven by ATP: the Na+-K+ pump
maintains a critical difference: the concentration of K+ is
higher in the cytoplasm and the concentration of Na+ is
around 10x higher outside than in the cytoplasm.
The Na+/K+ pump
Model of the Na+K+ pump: Goal is to throw out 3 Na+’s and
get 2 K+’s – cost is 1 ATP
In some cells, secondary active transport of
glucose is driven by the Na+ gradient (which is
+/K+ pump)
maintained
by
the
Na
The
electrochemical
gradient of the Na+
ions is the force
that drives the
movement of
glucose up its
concentration
gradient.
In mammals, this
SGLT1 transporter
is found in kidney
tubules, the
intestine, and
cerebral capillaries.
Conclusions
• Membranes create the possibility for the cell to regulate
its internal environment. The membrane’s phospholipids
form a barrier and at the same time position the proteins
to mediate the cell’s interactions across the barrier. The
following functions are regulated by interactions at the
cell’s membrane: ion concentrations (and therefore
water), accumulation of molecules to fuel the cell’s
demands for growth, repair and energy, release of toxins
inadvertently taken in or produced, regulation of gene
activity and decisions about cell division (based on
molecular cues sent by neighbors and detected by
membrane receptors), and a host of specific functions
characteristic of the different membranes and different
kinds of cells…