MEMBRANE STRUCTURE AND
FUNCTION
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STRUCTURE
1. Membrane models.
2. Fluid mosaic of lipids, proteins, and
carbohydrates.
3. Selective permeability.
4. Passive transport. Osmosis.
5. Active transport.
6. Exocytosis and endocytosis
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The cell membrane
The plasma membrane is the boundary that separates the
living cell from its nonliving surroundings.
Membranes are of crucial importance to life, because a cell
must separate itself from the outside environment for
two major reasons.
1. It must keep its molecules of life ( DNA , RNA , and its
assortment of proteins ) from dissipating away.
2. It must keep out foreign molecules that damage or
destroy the cells components and molecules.
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1. Membrane models
1895, Charles Overton – membranes are made of lipids;
1917, Irving Langmuir – artificial membrane;
1925, E. Gorter and F. Grendel – phospholipid bilayer two
molecules sick;
1935, H. Davson and J. Danielli – sandwich model: a
phospholipid layer between two layers of globular protein;
1950, electron microscopy of cells confirmed the “sandwich
model”;
1972, S. Singer and G. Nicolson – fluid mosaic model.
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Two generations of membrane models
(a) The Davson-Danielli model, proposed in 1935,
sandwiched the phospholipid bilayer between two protein
layers.
With later modifications, this model was widely
accepted until about 1970.
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Two generations of membrane models
(b) The fluid mosaic model disperses the proteins and
immerses them in the phospholipid bilayer, which is in a
fluid state.
Shown here in simplified form, this is our present
working model of the membrane.
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Singer and Nicolson proposed
Proteins are individually embedded in the phospholipid
bilayer, rather than forming a solid coat spread upon the
surface.
Hydrophilic portions of both proteins and phopholipids are
maximally exposed to water resulting in a stable membrane
structure
Hydrophobic portions of proteins and phospholipids are in
the nonaqueous environment inside the bilayer.
Membrane is a mosaic of proteins inserted in a fluid bilayer
of phospholipids.
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The cell membrane
The cell membrane functions as a semi-permeable barrier,
allowing a very few molecules across it while fencing the
majority of organically produced chemicals inside the cell.
The most common molecule in the model is the
phospholipid, which has a polar (hydrophilic) head and two
nonpolar (hydrophobic) tails.
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Diagram of a phospholipid bilayer
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Artificial membranes
(cross sections)
(a) Water can be coated
with a single layer of
phospholipid molecules.
The hydrophilic heads of
the phospholipids are
immersed in water,
and the hydrophobic tails
are excluded from water.
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Artificial
membranes
(b) A bilayer of
phospholipids forms a
stable boundary between
two aqueous
compartments.
This arrangement exposes
the hydrophilic parts of the
molecules to water
and shields the
hydrophobic parts from
water.
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Cell membrane EM
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2. The fluid mosaic of lipids, proteins
and carbohydrates
A membrane is held together primarily by hydrophobic
interactions, which are much weaker than covalent
bonds.
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Movement of phospholipids
Most of the lipids and some of the proteins can drift in the plane of
the membrane but not from one layer to another.
Lateral movement
(frequent)
Flip-flop
(rare)
Phospholipids move quickly along the membrane’s plane
averaging 2 microM per second. Proteins drift more slowly.
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Evidence for the drifting of membrane
proteins
When researchers fuse a human cell with a mouse cell, it
takes less than an hour for the membrane proteins of the
two species to completely mix in the membrane of the
hybrid cell.
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Membrane fluidity
Fluid
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated
hydrocarbon tails
Tails with kinks are keeping molecules from packing
together, enhancing membrane fluidity.
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Cholesterol within the membrane
Cholesterol reduces membrane fluidity by reducing
phospholipid movement at moderate temperatures and also
hinders solidification at low temperature: it makes the
membrane less fluid at warm t and more fluid at lower t.
Cholesterol
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Cell membrane
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Sidedness of the plasma membrane
The membrane has distinct
cytoplasmic and extracellular
sides.
This bifacial quality is
determined when the
membrane is first
synthesized and modified by
the ER and Golgi.
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Sidedness of the plasma membrane
The diagram color-codes the
two sides of the membranes
of the endomembrane
system,
to illustrate that the side
facing the inside of the ER,
Golgi, and vesicles
is topologically equivalent to
the extracellular surface of
the plasma membrane.
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Sidedness of the plasma membrane
The other side always faces
the cytosol,
from the time the membrane
is made by the ER
to the time it is added to the
plasma membrane by fusion
of a vesicle.
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Sidedness of the plasma membrane
The small green "trees"
represent membrane
carbohydrates
that are synthesized in the
ER and modified in the
Golgi.
Vesicle fusion with the
plasma membrane is also
responsible for secretion of
cell products (purple).
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Cell recognition by membrane carbohydrates
Cell-cell recognition – the ability of a cell to determine if
other cells it encounters are alike or different from itself.
Cell-cell recognition is crucial in the functioning of an
organism. It is the basis for:
•Sorting of an animal embryo’s cells into tissues and
organs
•Rejection of foreign cells by the immune system
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Cell recognition by membrane carbohydrates
Because of their diversity and location, likely candidates
are membrane carbohydrates:
•Branched oligosaccharides
•Some covalently bonded to lipids (glycolipids)
•Most covalently bonded to proteins (glycoproteins)
•Vary from species to species, between individuals of
the same species and among cells in the same
individual
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Mosaics of Structure and Function
The plasma membrane and the organelles membranes
each have unique collections of proteins.
Integral proteins are transmembrane proteins with
hydrophobic regions that completely span the hydrophobic
interior of the membrane.
The hydrophobic regions consist of one or more stretches
of nonpolar amino acids.
The hydrophilic ends are exposed to aqueous solutions on
either side of the membrane.
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Mosaics of Structure and Function
Peripheral proteins are not embedded in the lipid bilayer;
they are either bound to the surface of the membrane or to
the integral protein.
To give an animal cell a stronger external framework,
frequently membrane proteins are attached either to
cytoskeleton from cytoplasmic side or to the fibers of the
extracellular matrix from the exterior side.
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The structure of a transmembrane
protein
This ribbon model
highlights the alphahelical secondary
structure of the
hydrophobic parts of
the protein,
which lie mostly
within the
hydrophobic core of
the membrane.
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The structure of a transmembrane protein
This particular
protein,
bacteriorhodopsin,
has seven
transmembrane
helices (outlined with
cylinders for
emphasis).
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The structure of a transmembrane protein
Joining the helices are
hydrophilic polypeptide
segments that together
form the parts of the
protein in contact with
the aqueous solutions on
either side of the
membrane.
Bacteriorhodopsin is a
specialized transport
protein found in certain
bacteria.
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Transport
(a) A protein that spans
the membrane may
provide a selective
hydrophilic channel across
the membrane.
(b) Some transport
proteins hydrolyze ATP as
an energy source to
actively pump substances
across the membrane.
(a)
(b)
ATP
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Enzymatic activity
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Membrane protein may be
an enzyme. Often several
enzymes are ordered as a
team that carries out
sequential steps of a
metabolic pathway.
Signal transduction
A membrane protein may
have a binding site with a
specific shape that fits the
shape of a chemical
messenger, such as
hormone.
The external messenger
(signal) may cause a
conformational change in
the protein that relays the
message to the inside of
the cell.
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Intercellular
joining
Membrane proteins of
adjacent cells may be
hooked together in
various kinds of junctions
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Cell-cell
recognition
Some glycoproteins
(proteins with short
chains of sugars) serve
as identification tags that
are specifically
recognised by other cells
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Attachment to cytoskeleton and extracellular
matrix (ECM)
Mycrofilaments may be
bonded to membrane
proteins, a function that
helps maintain cell shape
and fixes the location of
certain membrane
proteins.
Proteins that adhere to the
ECM can coordinate
extracellular and
intracellular changes.
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The diffusion of solutes
across membranes
(a) A substance will diffuse from
where it is more concentrated to
where it is less concentrated.
The membrane, viewed here in
cross section, has pores large
enough for molecules of dye to
pass.
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The diffusion of solutes
across membranes
Diffusion down the concentration
gradient leads to a dynamic
equilibrium; the solute molecules
continue to cross the membrane,
but at equal rates in both
directions.
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The diffusion of solutes
across membranes
(b) In this case, solutions of two
different dyes are separated by
a membrane that is permeable
to both dyes.
Each dye diffuses down its own
concentration gradient.
There will be a net diffusion of
the green dye toward the left,
even though the total solute
concentration was initially
greater on the left side.
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Selective permeability:
traffic across membranes
Sugars, amino acids, and other nutrients enter the cell;
metabolic waste products leave the cell.
The cell takes in oxygen for cellular respiration and expels
carbon dioxide.
It regulates its concentrations of inorganic ions (Na+, K+,
Ca2+, Cl-) by shuttling them across the membrane.
To be able to accomplish all this functions the cell membrane
have to be selectively permeable – not to allow substances
to cross the barrier indiscriminately.
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Selective permeability: traffic across
membranes
Hydrophilic molecules such as ions and polar molecules have
problems to travel across the hydrophobic core of the membrane
themselves.
In contrast, hydrophobic molecules: hydrocarbons, CO2, and O2,
can dissolve in the membrane and cross it with ease.
The key role in the regulation of the transport across the
membrane belongs to transport proteins.
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Selective permeability: traffic across
membranes
Hydropobic molecules avoid the transport proteins, whereas
hydrophilic have to interact with them.
Transport proteins either build hydrophilic channel which allows
travelling through the hydrophobic part
or serve as carrier by binding to passengers and physically
moving them across.
In both cases the protein is very specific for the substances it
moves.
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Passive transport is diffusion across a
membrane
The tendency to spread out into available space - diffusion is the
result of thermal motion (intrinsic kinetic energy). It is a
movement of a substance down a concentration gradient.
Diffusion continues until the dynamic equilibrium concentration is
reached.
In order to reach equilibrium any substance will diffuse down its
concentration gradient – regular, graded concentration change
over a distance in a particular direction
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Passive transport is diffusion across a
membrane
Since any system has a tendency to entropy, diffusion of a solute
in water is a spontaneous process which requires no energy.
Oxygen is constantly diffusing into the cell during cellular
respiration.
A substance diffuses down its own concentration gradient and is
not affected by the gradients of other substances.
The diffusion of a substance across a biological membrane is
called passive transport (the cell requires no energy).
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Passive transport is diffusion across a
membrane
Spontaneous process which is a function of a concentration
gradient when a substance is more concentrated on one side of
the membrane.
Passive process which does not require the cell to expend
energy. It is the potential energy stored in a concentration
gradient that drives diffusion.
Rate of diffusion is regulated by the permeability of the
membrane, so some molecules diffuse more freely than others.
Water diffuses freely (although not easily) across most cell
membranes.
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Cells and Diffusion
Water, carbon dioxide, and oxygen
are among the few simple molecules
that can cross the cell membrane by diffusion (or a type of
diffusion known as osmosis).
Diffusion is one principle method of movement of
substances within cells, as well as the method for essential
small molecules to cross the cell membrane.
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Cells and Diffusion
Carbon dioxide is produced by all cells as a result of cellular
metabolic processes.
Since the source is inside the cell, the concentration
gradient is constantly being replenished/re-elevated, thus
the net flow of CO2 is out of the cell.
Metabolic processes in animals and plants usually require
oxygen, which is in greater concentration outside the cell,
thus the net flow of oxygen is into the cell.
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Osmosis
Osmosis is the passive transport of water across a semipermeable membrane.
Hypertonic
solution – a
solution with a
greater solute
concentration
than that
inside a cell.
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Osmosis
Hypotonic solutions – a solution with a lower solute concentration
compared to that inside a cell
Isotonic solutions
have equal (iso-)
concentrations of
substances (as
compared to that
inside a cell).
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Osmosis
Water potentials are
thus equal, although
there will still be
equal amounts of
water movement in
and out of the cell,
the net flow is zero.
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The water balance of living cells
Water diffuses down its concentration gradient:
If two solutions of different concentrations are separated by
a selectively permeable membrane that is permeable to
water but not to solute, water will diffuse from the
hypoosmotic solution to the hyperosmotic.
Direction of osmosis is determined by the difference in total
solute concentration, regardless of the type or diversity of
solutes in the solutions.
Osmotic concentration – total solute concentration of a
solution
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The water balance of living cells
How living cells react to changes in the solute concentrations of
their environments depends on whether or not they have cell
walls.
Animal cells do not have cell walls; plant cells do.
Unless it has
special
adaptations to
offset the osmotic
uptake or loss of
water, an animal
cell fares best in
an isotonic
environment.
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The water balance of living cells
Plant cells are generally healthiest in a hypotonic environment, where
the tendency for continued uptake of water is balanced by the elastic
wall pushing back on the cell.
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One model for facilitated diffusion
The transport protein (purple) alternates between two
conformations, moving a solute across the membrane as the
shape of the protein changes.
The protein can
transport the solute
in either direction,
with the net
movement being
down the
concentration
gradient of the
solute.
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Passive Transport
Passive transport requires no energy from the cell. Examples
include the diffusion of oxygen and carbon dioxide, osmosis
of water, and facilitated diffusion.
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Active Transport
Active transport requires the cell to spend energy, usually in
the form of ATP.
Examples include transport of large molecules (non-lipid
soluble) and the sodium-potassium pump.
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Voltage across membranes
Because anions and cations are unequally distributed
across the plasma membrane, all cells have voltages
across their plasma membranes.
Membrane potential - voltage across membranes
Ranges from -50 to -200mv. As indicated by the negative
sign, the cell’s inside is negatively charged with respect to
the outside.
Affects traffic of charged substances across the membrane
Favors diffusion of cations into cell and anions out of the
cell (because of electrostatic attractions)
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The sodium-potassium pump:
This transport system pumps ions against steep
concentration gradients.
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The sodium-potassium pump:
The pump oscillates between two conformational states in a pumping
cycle that translocates three Na+ ions out of the cell for every two K+
ions pumped into the cell.
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The sodium-potassium pump:
ATP powers the changes in conformation by phosphorylating the
transport protein (that is, by transferring a phosphate group to the
protein).
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An electrogenic pump
Proton pumps are examples of membrane proteins that store
energy by generating voltage (charge separation) across
membranes. Using ATP for power, a proton pump translocates
positive charge in the form of hydrogen ions.
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An electrogenic pump
The voltage and H+ gradient represent a dual energy source that
can be tapped by the cell to drive other processes, such as the
uptake of sugar and other nutrients.
Proton pumps are the main electrogenic pumps of plants, fungi,
and bacteria.
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Cotransport
An ATP-driven pump stores energy by concentrating a
substance (H+, in this case) on one side of the membrane. As
the substance leaks back across the membrane through
specific transport proteins, it escorts other substances into the
cell.
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Cotransport
In this case, the proton pump of the membrane is indirectly
driving sucrose accumulation by a plant cell, with the help of a
protein that cotransports the two solutes.
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Carrier-assisted Transport
The transport proteins are highly selective.
Some of these proteins can move materials across the
membrane only when assisted by the concentration
gradient, a type of carrier-assisted transport known as
facilitated diffusion (glucose).
The rapid breakdown of glucose in the cell (a process
known as glycolysis) maintains the concentration gradient.
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Carrier-assisted Transport
In the case of active transport, the proteins are having to
move against the concentration gradient.
For example the sodium-potassium pump in nerve cells.
Na+ is maintained at low concentrations inside the cell and
K+ is at higher concentrations.
The reverse is the case on the outside of the cell.
When a nerve message is propagated, the ions pass
across the membrane, thus sending the message.
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Carrier-assisted Transport
After the message has passed, the ions must be actively
transported back to their "starting positions" across the
membrane.
This is analogous to setting up 100 dominoes and then
tipping over the first one.
To reset them you must pick each one up, again at an
energy cost.
Up to one-third of the ATP used by a resting animal is used
to reset the Na-K pump.
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Types of transport molecules
Uniport transports one solute at a time.
Symport transports the solute and a cotransported solute
at the same time in the same direction.
Antiport transports the solute in (or out) and the cotransported solute the opposite direction. One goes in the
other goes out or vice-versa.
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Endocytosis
Endocytosis - the incorporation of materials from outside
the cell by the formation of vesicles in the plasma
membrane.
Phagocytosis is the type of endocytosis where an entire cell
is engulfed.
Pinocytosis is when the external fluid is engulfed.
Receptor-mediated endocytosis occurs when the material to
be transported binds to certain specific molecules in the
membrane.
Examples include the transport of insulin and cholesterol
into animal cells.
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Endocytosis
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Reading
Ch. 7 (125-141)
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