CELL MEMBRANES

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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.
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THE FUNCTION OF CELL MEMBRANES

• 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.
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PHOSPHOLIPIDS The Backbone of the Membrane

• Cartoon of a phospholipid molecule

This cartoon depicts the basic amphipathic
structure common to all phospholipids
Glycerol plus polar side group
Fatty acids
both polar and non-polar regions
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WATER MOLECULES ARE POLAR

• 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-)
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Water is a good solvent for polar molecules and
ions
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Hydration Shells
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PHOSPHOLIPIDS

• Cartoon of a phospholipid molecule

Non-polar tail (Hydrophobic- hates water)
Polar head (Hydrophilic- likes water)
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OIL/WATER PARTITION THE KITCHEN EXPERIMENT
MIX OIL WATER AND TEST SUBSTANCE
CONCENTRATION OF TEST SUBSTANCE IN OIL/
CONCENTRATION OF TEST SUBSTANCE IN WATER OIL
WATER PARTITION COEFFICIENT (OWPCK) K gt 1
HYDROPHOBIC (NON-POLAR) K lt 1 HYDROPHILIC (POLAR)
WATER
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MIXING PHOSPHOLIPIDS AND WATER Spontaneous
Self-Organization
Click to next slide to see the spontaneous
self-organization
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MIXING PHOSPHOLIPIDS AND WATER Spontaneous
Self-Organization
Click ahead
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MIXING PHOSPHOLIPIDS AND WATER Spontaneous
Self-Organization
Click ahead
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MIXING PHOSPHOLIPIDS AND WATER Spontaneous
Self-Organization
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Phospholipids plus water make a self-organizing
system

• Low lipid/water ratio - micelles
• High lipid/water ratio - lamellae
• Single lamellae bilayer or sheet

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Sheet
Micelle
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The Black Lipid Membrane (BLM)

• Formed across a small hole in a teflon beaker
  using a sable artists 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

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The Black Lipid Membrane (BLM)

• Experimental Setup

Microscope
Outer Beaker
Inner Beaker
What you see
Light Source
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Permeability of the BLMmol/sec-sq. cm.
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The Black Lipid Membrane Could be used to Show
what else is in Cell Membranes

• It has too low permeability for ions- it 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

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The membrane is fluid
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The membrane is fluid
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The membrane is fluid
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The membrane is fluid
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Cholesterol sits between fatty tails
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Proteins can span the bilayer
Hydrophilic
Hydrophobic
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The fluid-mosaic model

• Lipid bilayer
• Cholesterol between tails
• Protein, Glycoprotein, Lipoprotein dissolved in
  the lipid portion

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The fluid-mosaic model
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Channels and carriers are needed to get ions
across the bilayer
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Channels and carriers are needed to get ions
across the bilayer
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Cell Membranes Become Network Elements in Tissue
Membranes

• 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

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An Epithelial Membrane in Cartoon Form
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A Network Model of Coupled Salt and Volume Flow
Through an Epithelium
CL
PL
LUMEN
AM
TJ
BL
CELL
BM
BLOOD
PB
CB
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