Fluid Mosaic Model of The Structure of Cell Membrane

Wenny Pintalitna Tarigan

Pend.Biologi2013

Background

Biological membranes play a crucial role in almost all cellular phenomena

Organization of proteins and lipids of membranes can be discerned

Generalizations about protein structure – understanding the properties and functions of protein molecules

Detailed structure

Objectives

Thermodynamics and Membrane Structure

Properties of proteins and lipids of functional membrane

The fluid mosaic model in detail

Experimental evidence in terms of the model

The fluid mosaic model suggests new ways of thinking about membrane functions and membrane phenomena

Thermodynamics and Membrane Structure

Two kinds of non covalent interactions are hydrophobic and hydrophilic

Hydrophybic – non polar groups, away from water, requires energy

Hydrophylic – polar groups for aqueous environment

Thermodynamics and Membrane Structure

Ratio of proteins to lipids = 1.5 – 4

Cell membrane recognition sites activate enzyme in membrane which begins "cascade" of events which activate other enzymes. This is called signal transduction

(pg 155 Campbell)

Properties of Proteins

Peripheral Proteins - mild treatment: increase the ionic strength - dissociate them intact from the membrane - weak non covalent – not strongly associated with membrane lipid (free of lipid) - spread out as monolayer

Integral/Transmembrane Proteins - drastric treatment (using many reagents) - remain associated with lipid - globular in shape, no on the surface – prevent membrane thickness larger than 75-90 A

Classes of amino acidsWhat do these amino acids have in common?What do these amino acids have in common?

nonpolar & hydrophobicnonpolar & hydrophobic

Classes of amino acidsWhat do these amino acids have in common?What do these amino acids have in common?

polar & hydrophilicpolar & hydrophilic

Proteins domains anchor molecule

Within membrane nonpolar amino acids

hydrophobic anchors protein

into membrane On outer surfaces of

membrane polar amino acids

hydrophilic extend into extracellular

fluid & into cytosol

Polar areasof protein

Nonpolar areas of protein

Properties of Proteins

Figure 7.9

N-terminus

helix

C-terminus

EXTRACELLULARSIDE

CYTOPLASMICSIDE

Membrane Proteins

peripheral proteins loosely bound to surface of

membranecell surface identity marker (antigens)

integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane proteintransport proteins

channels, permeases (pumps)

NH2

H+

COOH

Cytoplasm

Retinalchromophore

Nonpolar(hydrophobic)a-helices in thecell membrane H+

Porin monomer

b-pleated sheets

Bacterialoutermembrane

proton pump channel in photosynthetic bacteria

water channel in bacteria

function through conformational change = shape change

function through conformational change = shape change

Examples

Many Functions of Membrane ProteinsOutside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzymeactivity

Cell surface identity marker

Attachment to thecytoskeleton

Cell adhesion

Stronger electron microscopes would show that the cell membrane was not covered in protein, but rather had protein embedded in it.

Proteins Embedded on Cell Membrane

Knife

Plasma membrane Cytoplasmic layer

Proteins

Extracellularlayer

Inside of extracellular layer Inside of cytoplasmic layer

TECHNIQUE

RESULTS

Properties of Lipid

Membrane phospholipids form a bilayerPhospholipids

Have a hydrophilic head and two hydrophobic tails

Are the main structural components of membranes

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH

CH

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH2

CH2

CH3

CH3

CH3N+

O

O O–P

O

CH2CHCH2

C O C O

O O

Phosphategroup

Symbol

Hydrophilic head

Hydrophobic tails

Properties of Lipid

Figure 7.2

Hydrophilichead

Hydrophobictail

WATER

WATER

Properties of Lipid

Figure 7.6

Lateral movement occurs107 times per second.

Flip-flopping across the membraneis rare ( once per month).

Properties of Lipid

Figure 7.8

Fluid

Unsaturated hydrocarbontails

Viscous

Saturated hydrocarbon tails

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol within the animal cell membrane

Cholesterol

Properties of Lipid

Properties of Lipid

FUNCTIONS: Cell protection and/or insulation Receptor sites for binding of other molecules - signaling Attachment of cells to one another = tissues Carbohydrate Chains – Cell ID – autoimmunity

Carbohydrate

Mosaic: something made of small pieces

Hydrophilic regionof protein

Hydrophobic region of protein

Phospholipidbilayer

Mosaic: Proteins dispersed among phospholipids in membrane

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

Membrane fat composition varies Fat composition affects flexibility

membrane must be fluid & flexible about as fluid as thick salad oil

% unsaturated fatty acids in phospholipids keep membrane less viscous cold-adapted organisms, like winter wheat

increase % in autumn cholesterol in membrane

Fluid Mosaic Model

Permeability to polar molecules?

Membrane becomes semi-permeable via protein channels specific channels allow specific material across

cell membrane

outside cell

inside cell sugaraaH2O

saltNH3

Experimental Evidence

Integral protein - a globular molecule - embedded in the membrane.

In this technique, a frozen specimen is fractured with a microtome knife; some of the frozen water is sub-limed (etched) from the fractured surface if desired;

The surface is then shadow cast with metal, and the surface replica is examined in the electron microscope.

By this method the topography of the cleaved surface is revealed.

A characteristic feature of the exposed surface of most functional membranes examined by this technique, including plasma lemma, vacuolar, nuclear, chloroplast, mitochondrial, and bacterial membranes, is a mosaic-like structure consisting of a smooth matrix interrupted by a large number of particles.

New Ways of Thinking

Model restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules (amphipathic structure)

The bulk of the phospholipid is organized as a fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins.

The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent.

Evidence stated that all of which are consistent with, and add much detail to, the fluid mosaic model.

Any Questions??

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