MEMBRANE FUNCTION AND
TRANSPORT DYNAMICS
MEMBRANE
STRUCTURE
Plasma membranes are
made up of
phospholipids
Contains both
hydrophilic (waterloving) and
hydrophobic (waterfearing) regions.
Hydrophilic regions will
contain and react with
molecules that are
polar or carry a charge
(ions).
Hydrophobic regions
are made up of
hydrocarbons and thus
will have no charged
regions.
We call the
membranes
“amphipathic” because
of these two regions.
Hydrophilic head
Hydrophobic tails
Choline
Hydrophilic
head
Phosphate
Hydrophobic
tails
Glycerol
(c) Phospholipid symbol
Fatty acids
Generic cell
membranes are
approx. 4 nm in
width.
Works out to
1:500 when
comparing cell
width to width
of the cell
Membrane
Kink due to cis
double bond
(a) Structural formula
(b) Space-filling model
(d) Phospholipid bilayer
Analagous to the
Alumimum skin
of a Jetliner and
the width of the
fuselage.
Cell Biology by the numbers. Milo and
Phillips. 1st Ed. , pp 54-55
DYNAMICS INSIDE THE PLASMA
MEMBRANE
Outer Leaflet (Faces outside of the cell)
Inner Leaflet (Faces inside of the cell)
Outer Leaflet
Outside of Cell
Phospholipids can move both laterally inside the
membrane and can “flip-flop”
Uncatalyzed movement
Lipids can move from one leaflet to the other in
process called “Transbilayer Diffusion
Very slow process without catalysis
Nicknamed the process “Flip-flop”
Lateral movement occurs most often
Happens very quickly
Inside Cell
Inner Leaflet
(Cambridge Coaching)
Catalyzed Movement of Lipids
Catalyzed by ATP hydrolysis
Transbilayer Action
Flippase
Moves lipids from Outer Leaflet to
Inner Leaflet
Floppase
Moves lipid from Inner Leaflet to
Outer Leaflet
Both of these happen very rapidly since
they are catalyzed.
Scramblase
Catalyzed but not with ATP
Moves Lipids both ways across the
membrane
Outer Leaflet
Inner Leaflet
FLUID MOSAIC MODEL
• The Fluid Mosaic Model states that a membrane is a fluid structure
with a “mosaic” of various proteins embedded in it
• Also embedded are cholesterol rafts, and glycoproteins, glycolipids
which operate mostly in cell signaling.
TYPES OF PROTEINS IN THE CELL
MEMBRANE
 Transport Proteins
 Transports molecules across the cell membrane
 With Concentration Gradient
 Against Concentration Gradient
 Enzymes
 Signal Transduction proteins
Transport
Enzymes
Signaling
Cell to Cell
Tissue
formation
ECM
attachment
 Cell to cell recognition proteins
 Tissue joining proteins
 ECM (extracellular matrix) attachment proteins
PASSIVE TRANSPORT & FACILITATED
DIFFUSION
Why do we need transport
proteins?
IF the molecules aren’t
charged and are small (H2),
they are able to move
down their concentration
gradient through the
membrane unaided
• Remember: The Plasma
Membrane is “selectively
permeable.”
• Only certain things can cross the
membrane
• Hydrophobic tails make up the
inside of the membrane. So only
hydrophobic (non-charged
molecules) can pass through
unaided.
• No charged molecules can pass
unaided.
Passive transport proteins
allow charged molecule to
diffuse DOWN their
concentration gradient
• No energy input is needed.
• Other Channel proteins provide a
route through the membrane
• Channel proteins are specific for
the molecule they transport.
• The channel contains residues
that are charged oppositely to
the charge on the molecule
itself.
OSMOSIS
(DIFFUSION FOR H 2 O)
 Aquaporins
 Channel proteins specifically designed to move H2O (a molecule with partial charges) through the plasma
membrane
 Movement will take place in order to equalize solute (particle) concentration on both sides of the membrane.
 Water Balance of Cells Without Cell Walls
 Tonicity is the ability of a surrounding solution to cause a cell
to gain or lose water
TONICITY
 Isotonic solution: Solute concentration is the same as that
inside the cell; no net water movement across the plasma
membrane
 Hypertonic solution: Solute concentration is greater than
that inside the cell; cell loses water
 Hypotonic solution: Solute concentration is less than that
inside the cell; cell gains water
High Solute Conc.
Inside Cell
Hypotonic
Isotonic
(a) Animal cell
H2O
H2O
Lysed
Hypertonic
H2O
Shriveled
Normal
Cell
wall
H2O
H2O
H2O
Plasma
membrane
H2O
(b) Plant cell
Plasma
membrane
H2O
Low Solute Conc.
Inside Cell
Solute Conc.
Equal
Turgid (normal)
(Campbell)
Flaccid
Plasmolyzed
 Active Transport occurs when solute particles need to
move AGAINST their concentration gradient.
 requires energy, usually in the form of ATP
 performed by specific proteins embedded in the membranes
ACTIVE TRANSPORT
 allows cells to maintain concentration gradients that differ
from their surroundings
 sodium-potassium pump is one type of active transport system
Proteins of the ETC function as proton channels




Moves protons against concentration gradient
Powered by the movement of electrons as they are passed from protein to protein in the ETC
The protons will gather in the intermembrane space of the mitochondria
The will eventually move back down their concentration gradient by moving through the ATP synthase protein.
CO-TRANSPORT
Occurs when active transport of a solute indirectly drives transport of
other substances
Plants commonly use the gradient of hydrogen ions generated by proton
pumps to drive active transport of nutrients into the cell
ION PUMPS MAINTAIN MEMBRANE
POTENTIAL
Membrane potential is
the voltage difference
across a membrane
An electrogenic pump is
a transport protein that
generates voltage across a
membrane
• Differences in positive/negative ion distribution across membrane causes VOLTAGE
• Two combined forces, collectively called the electrochemical gradient, drive the
diffusion of ions across a membrane
• A chemical force (the ion’s concentration gradient)
• An electrical force (the effect of the membrane potential on the ion’s movement)
• The sodium-potassium pump is the major electrogenic pump of animal cells
• The main electrogenic pump of plants, fungi, and bacteria is a proton pump
• Electrogenic pumps help store energy that can be used for cellular work
The Sodium-Potassium Pump
 Responsible for nerve
and muscle signal
conduction.
 Build the electrochemical
gradient that transmits
signals along the neurons
 The signal will then cause
muscles to move (or not)
 All of this together makes the selectively
permeable plasma membrane a crucial
portion of a cell
 providing a way to regulate intracellular traffic,
 feeding of the cell through endocytosis
 Allowing proteins and other molecules to be
excreted from the cell.
 The cell membrane is the first line of
defense to keep a cell healthy.
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