CELL MEMBRANES

The Fluid-Mosaic
Model
This is an introduction to the structure
and function of cell membranes. It is
designed to be a self-contained unit with
links to other, related material.
THE FUNCTION OF
CELL MEMBRANES
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Compartmentalization of tissues
Regulation of cell contents
Provides surface for enzymes,
receptors, recognition, etc.
It is thought by some that the spontaneous
organization of membranes played an important role
in the evolution of life.
PHOSPHOLIPIDS: The “Backbone” of
the Membrane
Cartoon of a phospholipid
molecule
Glycerol
plus
polar side
group
This cartoon depicts the basic
amphipathic* structure common
to all phospholipids
Fatty acids
* both polar and non-polar regions
WATER MOLECULES
ARE POLAR
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Structure of water and the Cartoon
version
Water is a dipole
d/2 +
H
d
+
O
d/2 +
H
Oxygen pulls electrons towards itself
causing a charge imballance (d-)
Water is a good solvent
for polar molecules and
ions
Hydration Shells
-
+
PHOSPHOLIPIDS

Cartoon of a phospholipid molecule
OIL/WATER PARTITION:
THE “KITCHEN”
EXPERIMENT
MIX OIL WATER AND TEST SUBSTANCE
OIL
WAIT
CONCENTRATION OF TEST SUBSTANCE IN
WATER
OIL/ CONCENTRATION OF TEST SUBSTANCE
IN WATER = OIL WATER PARTITION COEFFICIENT (OWPC=K)
K > 1 HYDROPHOBIC (NON-POLAR)
K < 1 HYDROPHILIC (POLAR)
MIXING PHOSPHOLIPIDS AND WATER:
Spontaneous Self-Organization
Click to next slide to see the spontaneous self-organization
MIXING PHOSPHOLIPIDS
AND WATER: Spontaneous
Self-Organization
Click ahead
MIXING PHOSPHOLIPIDS AND
WATER: Spontaneous SelfOrganization
Click ahead
MIXING PHOSPHOLIPIDS AND
WATER: Spontaneous SelfOrganization
Phospholipids plus
water make a selforganizing system
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Low lipid/water ratio - micelles
High lipid/water ratio - lamellae
Single lamellae = bilayer or sheet
Sheet
Micelle
The Black Lipid
Membrane (BLM)
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Formed across a small hole in a
teflon beaker using a sable artist’s
brush
Viewed with an optical system
Looks like a soap bubble with large
patches of black where the bilayer
forms
Used as a laboratory model of the
cell membrane
The Black Lipid
Membrane (BLM)
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Experimental Setup
Microscope
Outer Beaker
Inner Beaker
Light Source
Permeability of the BLM
mol/sec-sq. cm.
The Black Lipid Membrane
Could be used to Show what
else is in Cell Membranes
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It has too low permeability for ionsit needs protein and polypeptide to
help them get through (channels
and carriers)
It needs cholesterol for proper
“fluidity”
It needs carbohydrate to provides
cell recogtnition properties
The membrane is fluid
The membrane is fluid
The membrane is fluid
The membrane is fluid
Cholesterol sits between
fatty tails
Proteins can span the
bilayer
Hydrophilic
Hydrophobic
The fluid-mosaic model
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Lipid bilayer
Cholesterol between tails
Protein, Glycoprotein, Lipoprotein
“dissolved” in the lipid portion
The fluid-mosaic model
Channels and carriers
are needed to get ions
across the bilayer
Channels and carriers
are needed to get ions
across the bilayer
Cell Membranes Become
Network Elements in Tissue
Membranes
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Epithelia are tissue membranes
made up of cells
Network Thermodynamics
provides a way of modeling these
composite membranes
Often more than one flow goes
through the tissue
An Epithelial Membrane
in Cartoon Form:
A Network Model of Coupled Salt
and Volume Flow Through an
Epithelium
CL PL
LUMEN
AM
TJ
BL
CELL
BM
BLOOD
CB
PB
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