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Ch. 3: Plasma Membrane Structure and Function
Biochemistry: The unique properties of water
δ-
δ+
Hydrogen bonding is
when the partial +
charge on Hydrogen is
attracted to the partial
– charge of another
compound.
δ+
Water molecules have polar covalent bonds.
Properties of Phospholipids
Phospholipids
P.Membrane (PM) held together by weak hydrophobic interactions
– Glycerol with Phosphate Head +
2 Fatty Acid Chains
– Amphiphilic (“Both” “lover”)
•
•
Hydrophilic headPhosphate
Hydrophobic tail
Glycerol
– Forms 2 layers in water
(negatively charged phosphate head attracted
Fatty water
Acids molecules)
to + end of polar
– Makes up cell membranes
Fluid Mosaic Model
Fluidity: (not rigid)
• P.Membrane (PM) held together by weak
hydrophobic interactions (bi-lipid tails face
each other, away from water)
• Lateral drifting ability of lipids
• Temperature Dependent
Why is this rare?
Cholesterol puts gaps between
phospholipids, increasing fluidity
Unsaturated tails prevents packed phospho-lipid rafts
“Mosaic”
PM is made up of a mosaic or “collage” of:
• Phospholipids
Hey Sugar! –
• cholesterol
Let’s “stick”
• integral and peripheral proteins
Hey Suga-!
• glycolipids
glycocalyx
• glycoproteins
Integral or
Transmembrane Proteins
• Penetrate hydrophobic core of membrane
Surface or Peripheral Proteins
• Loosely bound to surface
• Some attaches to cyto-skeleton or ECM
(Extracellular matrix)
Review: What organelles are responsible for
creating membrane proteins?
Membrane Transport
• Cells NEED to be able to:
– remove waste
– take in necessary nutrients from interstitial
fluids
– Send out signals to other cells
– Receive signals from other cells
• Transport Classified as:
– Passive
– Active
Selective Permeability
of Plasma Membrane
General rule: like dissolves like
• Non-polar, hydrophobic solutes dissolve in
lipid
• Ions, hydrophillic, or polar solutes dissolve in
water
Selective Permeability
of Plasma Membrane
Selective Permeability: some substances can pass
through lipid core or membrane more easily
than others
1. CO2, O2, non-polar molecules, and other lipids,
are hydrophobic and can pass hydrophobic
lipid membrane core easily
2. Water, sugars, charged ions, or polar
molecules cannot pass lipid core easily  so
must use hydrophillic transport proteins to pass
(ex. Aquaporins)
3. Small molecules are more permeable than
larger ones
Passive Transport
• Molecules move down [gradient] (from high to low
concentration) until equilibrium is reached
• Spontaneous process
• No ATP needed; uses Kinetic energy (KE)
or Hydrostatic Pressure as E source
• Types of Passive Transport:
– Simple Diffusion
– Facilitated Diffusion
– Osmosis
– Filtration
Simple Diffusion
• Diffusion –
molecules of any
substance moves
down [gradient],
unassisted
• Ex. O2 in blood,
CO2 in cells
Back to Types of PT
Back to Types of PT
Facilitated Diffusion
• Assisted diffusion of molecules with help
from channels or carriers
Channels specific for
particular molecule,
like sugars, amino
acids
Carriers move substances
like ions, water. Selective
by size and charge
Water always moves from
hypotonic to hypertonic
Osmosis
• Diffusion of WATER across the membrane
• Tonicity dependent
– Isotonic solution: solution in equilibrium to another
solution across the membrane
– Hypotonic: solution with less dissolved [solute], higher
[water] compared to another solution
– Hypertonic: solution with more dissolved [solute],
lower [water] compared to another solution
Back to Types of PT
Filtration
• “Forcing” of water and solutes through
membrane by hydrostatic pressure
• Selective only by SIZE
• Ex. only blood cells/proteins too large to
pass are held back
Active Transport
• Molecules move up or against [gradient] (from
low to high) to create an electrochemical
gradient
• Nonspontaneous
• Requires ATP as E source
• Types:
– Primary Active Transport (T)
– Secondary Active (T)
– Clathrin-coated Vesicular (T)
• Endocytosis
• Exocytosis
Active Transport generates an electrochemical
gradient: charge difference (disequilibrium)
between both sides of the membrane Back to Types of AT
Back to Types of AT
Primary Active Transport
Uses ATP E directly
Ex 1: Sodium-Potassium Pump 3-D overview
– Pump keeps Na+ moving out of the cell, against its
gradient, building its concentration (disequilibrium)
– Pump keeps K+ moving into the cell against its gradient,
building its concentration
– Na+:K+ ratio 3 out : 2 in
– Na+:K+ pump uses high concentration gradients to store
PE for future cellular work or for secondary AT
• Ex 2: Pumping H+ ions into lysosome to create
acidic env’t for cellular digestion
Back to Types of AT
Secondary Active Transport
(Coupled Transport)
Symport:
twothe
• Involves
transported
transport of a
substances
move
substance
in against
the samea
direction
concentration
Ex. Hydrogen gradient creating PE
ATP
ADP
+ Pi
H+
H+
gradient
H+
Antiport:
two
H+
H+
powered
transported
H+
indirectlymove
by
substances
in an
opposite
ATP
direction
(“wave”
powered
to pump
each other) Sucrose transported against gradient into cell, using KE
stored from H gradient, falling back down gradient
Clathrin: protein coat on PM helps with 1) specifying cargo 2)
membrane deformation
Endocytosis
• The engulfing of substances by
pseudopods extensions of the plasma
membrane
• Three types:
– Phagocytosis (cell eating – lg. particles
engulfed)
– Pinocytosis (cell drinking – sm. ions and
liquids engulfed)
– Receptor Mediated Endocytosis (use of
surface
proteins
tothat
engulf
specific
substrate)
Often
hijacked
by pathogens
mimic aaneeded
substance
by the cell
Exocytosis
Fusing of vesicles to the plama membrane,
thus releasing its contents
Function of Membrane Proteins
TRANSPORT
SIGNAL TRANSDUCTION
INTERCELLULAR
JOINING
•protein channels
or carriers
•substrates bind to
protein
for passive
transport
•Ex.
Gap
Junctions,
Tight Junctions,
surface  sendsDesmosome
a signal
• protein pumps for active
within the cell to start atransport
chemical chain reaction or
•Clathrin-lined
membrane for
cell response
CELL to CELL
vesicular transport
RECOGNITION
ENZYMATIC
CYTOSKELETON and ECM
•Sugar on glycoproteins or
•Catalysis of Chemical Reactions
ATTACHMENT
glycolipids
at the Membrane
Surfaceact as “name
•Maintenance of Cell Shapetags” for cells. Recognition of
invaders, helps with cell communication
and coordination
End of Slide Show
Signal Transduction
3 Stages of Signal Transduction
1) Reception: A ligand or substrate binds to
receptor protein. Receptor proteins can be on the cell
surface, but not always. Receptor protein
changes shape
2) Transduction: Amplifies and sends the
signal through chemical relay
3) Cell Response: Specific response is
triggered
Examples of Signal Transduction
Back to Function of
Membrane Proteins
Why and
is this
hormone-receptor
Steroids
Hormones
are types of
protein
found
the surface
lipids,
whichnot
can
passon
through
of the plasma
membrane?
phospholipid
membranes
easily.
Back to Function of
Membrane Proteins
Cell Junctions
Tight Junctions:
Gap Junctions:
Desmosomes:
-“anchoring” junctions
-Intermediate
filaments extend from
disc-shaped plaque to
reduce tearing due
mechanical stress to
prevent separation
-Integral proteins
communicating
of-Disc-shaped
neighboring
cells
fusew/
junction
plaque
Transports ions,
linkersugars,
protein
simple
-prevents
Hollow,
fibers
small
molecules
leakage
btwn
transmembrane
“zipping”
cells
into
Abundant
in ionprotein
cylinders,
tissues
extracellular
dependent
together
to
connexons,
that
space
(ex.
excitable
cells
prevent
provide
Digestive tract)
(ex.
neurons)
separation
cytoplasmic
channels btwn cells