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The Plasma Membrane
Membrane Transport
Figure 5.1
The Fluid Mosaic Model
• Phospholipids- the main “fabric”
– amphipathic= they have both hydrophilic AND a
hydrophobic regions
• Proteins- embedded in the phospholipid
membrane
– Also amphipathic
– Determine the function of the membrane
– Proteins are not distributed randomly or evenly,
but rather according to function
How fluid is fluid?
• The membrane is held together my
hydrophobic interactions-weaker than
covalent bonds
• Constant lateral movement
• Proteins larger than lipids therefore move
more slowly
Viscosity
• A measure of a fluid’s resistance to flow; how
“thick” or “sticky” it is
• Due to molecular makeup and internal friction
• Honey is more viscous than water
What determines a membrane’s
viscosity?
• Hydrocarbon tails on its phospholipids
– Saturated- more viscous
– Unsaturated- less viscous, more fluid
• Temperature
– Decrease in temp more viscous; may eventually
solidify
– Increase in temp less viscous; too fluid, cannot
support proteins and their function
What determines a membrane’s
viscosity?
• Cholesterol- helps membranes resist changes
in fluidity with changes in temperature
– High temps- restricts movement of phospholipids
– Low temps- prevents phospholipids from packing
together
• Evolution
– Membrane composition evolves to meet specific
environmental needs
• Cold
Figure 5.5
Fluid
Unsaturated tails prevent
packing.
Viscous
Saturated tails pack
together.
(a) Unsaturated versus saturated hydrocarbon tails
(b) Cholesterol reduces
membrane fluidity at
moderate temperatures,
but at low temperatures
hinders solidification.
Cholesterol
What determines a membrane’s
viscosity?
• Evolution
– Membrane composition evolves to meet specific
environmental needs
• Cold water fish
• Archea that live at 90°C (194°F)
• Some alter their composition seasonally
Membrane Proteins and Their
Functions
• The proteins within the phospholipid bilayer
determine the function of the membrane.
• Different cells  different membrane proteins
• Different organelles with a specific cell 
different membrane proteins
Two Major Types of Proteins
• Integral
• Peripheral
• Can you see the difference?
Figure 5.2
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
Integral
• Penetrates the membrane
– Transmembrane- through to both surfaces
– Partially embedded- only exposed on one surface
• The embedded portions have hydrophobic
amino acids, often in an α helix
• Some have hydrophilic channels through them
to allow for passage of substances through the
membrane
Figure 5.6
N-terminus
 helix
C-terminus
EXTRACELLULAR
SIDE
CYTOPLASMIC
SIDE
Peripheral
• Not embedded
• Bound to either surface
– Extracellular matrix (outside)
– Cytoskeletal elements (inside)
• Provide extra support for the membrane
Figure 5.2
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
6 Major Functions of Plasma
Membrane Proteins
1.
2.
3.
4.
5.
6.
Transport
Enzymatic activity
Attachment to the cytoskeleton and ECM
Cell-cell recognition
Intercellular joining
Signal transduction
Figure 5.7
Enzymes
ATP
(a) Transport
(b) Enzymatic activity
(c) Attachment to the
cytoskeleton and extracellular matrix (ECM)
Signaling
molecule
Receptor
Glycoprotein
(d) Cell-cell recognition
(e) Intercellular joining
(f) Signal transduction
Membrane Carbohydrates
• Cell-cell recognition
• Can be covalent bound to either lipids or
proteins on the extracellular
side of the membrane
– Glycoproteins
– Glycolipids
• Act as markers to
cells
– Ex. ABO blood types
distinguish
Membrane Synthesis
• Proteins and lipids- ER
• Carbohydrates added –Golgi
Selective permeability
• 2 aspects of “selectivity”
– The membrane takes up some small ions and
molecules, but not others
– Substances that are allowed through, do so at
different rates
• How does the membrane accomplish this
selectivity?
Figure 5.3
Form Follows Function
Hydrophilic
head
WATER
WATER
Hydrophobic
tail
• Nonpolar substances= hydrophobic
– Cross easily
– Ex. Hydrocarbons, CO2 ,O2
• Ions & polar substances= hydrophilic
– Hard to pass
– Ex. Glucose, H2O, Na+, Cl– Ions especially have a hard time as they tend to be
surrounded by a “shell” of water molecules
Transport Proteins
• Channel proteins vs. carrier proteins
– Channel proteins create a channel through which
hydrophilic substances may pass. Ex. Aquaporins
– Carrier proteins hold onto substances, change
shape and redeposit them on the other side
Figure 5.14
EXTRACELLULAR
FLUID
1
CYTOPLASM
[Na] high
[K] low
[Na] low
[K] high
2
6
3
5
4
ADP
Directionality of transport
• Controlled by
– Passive transport
• Diffusion
• Osmosis
• Facilitated diffusion
– Active transport
• Ion pumps, membrane potential
• Cotransport
– Bulk transport
• Exocytosis
• Endocytosis
Active transport
• Moves substances against their gradient; from
an area of low concentration to one of high
concentration
• Requires energy- supplied by ATP
• Allows cells to maintain a different
environment inside vs. outside the cell
An example is the sodiumpotassium pump
Figure 5.14a
EXTRACELLULAR [Na] high
FLUID
[K] low
CYTOPLASM
[Na] low
[K] high
1 Cytoplasmic Na binds
to the sodium-potassium
pump. The affinity for Na
is high when the protein
has this shape.
ADP
2 Na binding stimulates
phosphorylation by ATP.
Figure 5.14b
3 Phosphorylation leads
to a change in protein
shape, reducing its affinity
for Na, which is released
outside.
4 The new shape has a
high affinity for K, which
binds on the extracellular
side and triggers release
of the phosphate group.
Figure 5.14c
5 Loss of the phosphate
group restores the protein’s
original shape, which has a
lower affinity for K.
6 K is released; affinity
for Na is high again, and
the cycle repeats.
Ion pumps maintain voltage across
membranes
• Membrane potential= the voltage across a
membrane
• Cytoplasmic side relatively negative
• Creates electrical potential energy that drives
passive transport of cations into the cell and
anions out
• Electrochemical gradient= chemical
(concentration gradient) and electrical forces
that drive diffusion across membranes
Main electrogenic pumps
• Animals– Sodium-potassium pump
• Plants– Proton pump
Figure 5.16
EXTRACELLULAR
FLUID
Proton pump
CYTOPLASM
Cotransport
• A process by which one protein transports 2
molecules or ions at a time. It uses the
diffusion of solute to force the other against
it’s gradient.
• It does not use ATP directly, but often is
coupled with an ion pump that does use ATP
Figure 5.17
Proton pump
Sucrose-H
cotransporter
Diffusion of H
Sucrose
Sucrose
Bulk Transport
• Exocytosis
• Endocytosis
– Phagocytosis
– Pinocytosis
– Receptor-mediated endocytosis
Figure 5.18
Phagocytosis
Pinocytosis
Receptor-Mediated
Endocytosis
EXTRACELLULAR
FLUID
Solutes
Pseudopodium
Plasma
membrane
Coat
protein
“Food”
or other
particle
Food
vacuole
CYTOPLASM
Coated
pit
Coated
vesicle
Receptor