Membrane Structure
and Function
Chs. 8 and 11
Cell Membrane – Introduction
 Separates the living cell from its
nonliving surroundings
 8 nm thick
 Controls traffic into and out of the cell
(selectively permeable)
 Composed of lipids (phospholipids) and
proteins, but include some
carbohydrates
Cell Membrane – Introduction
 Phospholipids and most other
membrane constituents are amphipathic
molecules
 Amphipathic molecules have both
hydrophobic and hydrophilic regions
 Described by the fluid mosaic model
Membrane Model
Development
 1895 – Charles Overton; hypothesized
membranes made of lipids
 Observed lipid-soluble substances
move across membrane easier than
lipid-nonsoluble substances
 1917 – Irving Langmuir;
 Dissolved
phospholipids in benzene
and mixed with water
 When benzene evaporated,
phospholipid film formed on water
 1925 – E. Gorter and F. Grendel
concluded membrane must be bilayer of
phospholipids
 Polar phosphorous head interacts
with polar water (hydrophilic)
 Nonpolar fatty
acid tails are
sheltered from
the water
(hydrophobic)
 Experiments showed real membranes
attract water stronger than artificial ones
 Hypothesis: proteins aid in water
attraction
 1935 – H. Davson
and J. Danielli
propose sandwich
model: bilayer
between layers
of proteins
 Davson-Danielli model considered
dominant, even after EM images
 Two problems:
 Membranes differed in size,
composition, and stained appearance
 Membrane proteins are amphipathic;
can’t be on surface only
Fluid Mosaic Model
 1972 – S.J. Singer and G. Nicolson
present revised model; hypothesize
proteins are
distributed
throughout and
among the
bilayer
Membranes are fluid
 Membrane molecules are not held
together by bonds; they can slip/move
past/around each other
 Evidence: when human and mouse
cells are fused together, membrane
proteins don’t stay separated.
 Most membrane molecules can move
laterally; rarely do they flip-flop
 Some proteins can’t move; bound to the
cytoskeleton
 Fluidity influenced by two factors:
 Temp:
As temp decreases, lipids
pack closer together – become more
solid
 Saturation: unsaturated fatty acid tails
make the membrane more
fluid
 Cholesterol is wedged in the plasma
membrane
 Warm temps: it restrains the movement
of phospholipids and reduces fluidity
 Cool temps: it maintains fluidity by
preventing tight packing
Membranes are mosaics
Membranes are mosaics
 Membranes each have a unique
collections of proteins
 Membrane functions determined mostly
by proteins
 Two types of membrane proteins:
 Peripheral proteins: not embedded in
lipid bilayer
 Integral proteins: penetrate the
hydrophobic core of lipid bilayer, often
completely spanning the membrane
(transmembrane protein)
Membranes are mosaics
 Membranes have
distinctive inside and
outside faces
The outer surface
has carbohydrates
This asymmetrical
orientation begins
during synthesis of
new membrane
in the endoplasmic
reticulum
Membranes are mosaics
 Membrane protein functions:
Cell-Cell Recognition
Ability of a cell to distinguish one type of
neighboring cell from another
The membrane plays the key role in
cell-cell recognition
Cells recognize other cells from surface
molecules, often carbs, on membrane
 Glycolipids
 Glycoproteins (more common)
Cell-Cell Recognition
Carbs on external side of membrane
vary from species to species, individual
to individual, and even from cell type to
cell type within the same individual
Variation marks each cell type as distinct
The four human blood groups (A, B, AB,
and O) differ in the external carbohydrates
on red blood cells
It is also the basis for rejection of foreign
cells by the immune system
This attribute is important in cell sorting
and organization as tissues and organs in
development
Transport
 Membranes act as gatekeepers
(selectively permeable)
 Select based on size and charge
 Small, uncharged atoms/molecules
don’t have problems
 Large and/or charged
atoms/molecules do have problems
 Proteins can help transport
Transport Proteins
 Each transport protein is specific as to
the substances that it will translocate
 Some act like a channel or tunnel
through the membrane
 Others bind to their specific molecules
and physically carry them across the
membrane
Passive Transport
 No E required
 Requires gradient (separation of
concentrations)
 Movement from areas of Hi to Low
(down, along, or with) concentrations
 Movement continues even after
equilibrium is reached
 Rate of diffusion depends on size and
charge of molecules (interaction with
the membrane)
Passive Transport
 Simple Diffusion: movement of
molecules from Hi to Low
concentrations
Passive Transport
 Each substance diffuses down its own
concentration gradient, independent of
the concentration gradients of other
substances
Passive Transport
 Osmosis: diffusion of water across a
semi-permeable membrane
 Osmosis continues until the solutions
are isotonic
 When two solutions are isotonic, water
molecules move
at equal rates
from one to the
other, with no
net osmosis
Passive Transport
 A solution with a higher concentration of
solutes is hypertonic
 A solution with a lower concentration of
solutes is hypotonic
 These are comparative terms
 Tap water is hypertonic compared to
distilled water but hypotonic when
compared to sea water
 Solutions with equal solute
concentrations of solute are isotonic
Passive Transport
Passive Transport
 Paramecia have contractile vacuoles to
expel excess water
Passive Transport
 Facilitated diffusion: diffusion using
“helper” molecules
 Those atoms and molecules that
were too big or charged can still move
down their concentration gradient (hi
to low)
Passive Transport:
Facilitated diffusion
 Some proteins (channel) act like
corridors
 Allow for fast, bulk flow
 Ex: aquiporins
Passive Transport:
Facilitated diffusion
Some channel proteins (gated
channels) open or close depending on
the presence or absence of a physical
or chemical stimulus
The chemical stimulus is usually different
from the transported molecule
Ex: when neurotransmitters bind to specific
gated channels on the receiving neuron,
these channels open
This allows sodium ions into a nerve cell
When the neurotransmitters are not present,
the channels are closed
Passive Transport:
Facilitated diffusion
 Some proteins change shape to
physically translocate the molecules
 These shape changes could be
triggered by the binding and release of
the transported molecule
 Transport proteins are much like
enzymes
 They may have specific binding sites
for the solute
 Transport proteins can become
saturated when they are translocating
passengers as fast as they can
 Transport proteins can be inhibited by
molecules that resemble the normal
“substrate”
 When these bind to the transport
proteins, they outcompete the normal
substrate for transport
Active Transport
 Requires E (ATP)
 Movement of molecules against or up
their concentration gradients
 Low to Hi
 Performed by receptor proteins
Active Transport
 The sodium-potassium pump actively
maintains the gradient of sodium (Na+)
and potassium ions (K+) across the
membrane
 Typically, an animal cell has higher
concentrations of K+ and lower
concentrations of Na+ inside the cell
 The sodium-potassium pump uses
the E of one ATP to pump three Na+
out and two K+ in
Ions keep separate charges
across a membrane
 Membrane potential: voltage difference
across the membrane
 Electrochemical gradient
 Gradient due to concentrations of
ions
 Gradient due to membrane potential
 electrogenic pumps generate voltage
gradient
Ions keep separate charges
across a membrane
 In plants, bacteria, and fungi, a proton
pump is the major electrogenic pump,
actively transporting H+ out of the cell
 Proton pumps in the cristae of mitochondria
and the thylaloids of chloroplasts,
concentrate H+ behind membranes
 These electrogenic
pumps store energy
that can be accessed
for cellular work.
Cotransport
 A single ATP-powered pump that
transports one solute can indirectly
drive the active transport of several
other solutes through cotransport via a
different protein
 As the solute that has been actively
transported diffuses back passively
through a transport protein, its
movement can be coupled with the
active transport of another substance
against its concentration gradient
 Plants commonly use the gradient of
H+
that is generated by proton pumps to
drive the active transport of amino acids,
sugars, and other nutrients into the cell
 The high concentration of H+ on one
side of the membrane, created by the
proton pump, leads to the facilitated
diffusion of protons
back, but only
if another molecule,
like sucrose, travels
with the H+
Endo- vs. Exocytosis
 Both move large molecules into/out of
the cell
 Both use vesicles
 Reverse processes of each other
Endocytosis
 A small area of the plasma membrane
sinks inward to form a pocket
 The pocket deepens, pinches in, and
forms a vesicle containing the material
that had been outside the cell
Endocytosis
 Two types:
 Phagocytosis:
cell eating
 Pinocytosis:
cell drinking
 Receptor
mediated
endocytosis
Receptor mediated
Endocytosis
 Triggered when extracellular
substances bind to special receptors,
ligands, on membrane surface,
especially near coated pits