Cell Membranes

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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http://views.vcu.edu/~mikuleck/courses/clm/cmref.htm

http://views.vcu.edu/complex/mikulecky/mikuleck/cmref.htm

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.


http://views.vcu.edu/~mikuleck/courses/clm/evl.htm

PHOSPHOLIPIDS: The “Backbone” of the Membrane

Cartoon of a phospholipid molecule

This cartoon depicts the basic This cartoon depicts the basic amphipathic* amphipathic* structure common structure common to all phospholipidsto all phospholipids

* both polar and non-polar regions

Fatty acids

Glycerol pluspolar side group


WATER MOLECULES ARE POLAR■ Structure of water and the Cartoon

version■ Water is a dipole

O

H

H

+

+

d

d/2

d/2

+

Oxygen pulls electrons towards itselfcausing a charge imballance (d-)www.freelivedoctor.com

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

WAIT

OIL

WATERCONCENTRATION OF TEST SUBSTANCE IN 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



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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


Micelle

Sheet


The Black Lipid Membrane (BLM)■ 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)

■ Experimental Setup

Light Source

Microscope Outer Beaker

Inner Beaker


Permeability of the BLMmol/sec-sq. cm.


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


The membrane is fluid


Cholesterol sits between fatty tails


Proteins can span the bilayer

Hydrophilic

Hydrophobic


The fluid-mosaic model

■ 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


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


http://views.vcu.edu/~mikuleck/courses/nt.htm

An Epithelial Membrane in Cartoon Form:


A Network Model of Coupled Salt and Volume Flow Through an Epithelium

AM TJ

BM

BL

CL PL

CB PB

CELL

LUMEN

BLOODwww.freelivedoctor.com

What to do now?

■ Please contact me to tell me what your reaction to the presentation is.

■ If you wish to see how well you understood the presentation, try the SELF TEST.


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Cell Membranes

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