1

Boundary

2

Selectively Permeable

Fluid mosaic of lipids and proteins

Lipid bilayer

Contains embedded proteins

Phosphate head hydrophilic

Fatty acid tailshydrophobic

Arranged as a bilayerFatty acid

Phosphate

Aaaah, one of thosestructure–functionexamples

“repelled by water”

“attracted to water”

Amphipathic

Phospholipids in the plasma membrane Can move within the bilayer two ways

4

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Phospholipids can move laterally

hydrocarbon tails affect membrane fluidity

Unsaturated hydrocarbon tails:

(-C=C-)kinks prevents tight packing incr fluiditySaturated hydrocarbon tails:

(-C-C-) tight packing decr fluidity

5

Fluid Viscous

Unsaturated hydrocarbontails (kinks) Saturated hydrocarbon tails

Cholesterol (a steroid) affects fluidity

T fluidityT fluidity

6

Figure 7.5

Cholesterol

Transmembrane proteins embedded in phospholipid bilayer

-create semi-permeable channels

lipid bilayermembrane

protein channelsin lipid bilyer membrane

1972: Singer and Nicolsonmembrane proteins are dispersed

and individually inserted into the phospholipid bilayer

8

Phospholipidbilayer

Hydrophilic region of protein

Hydrophobic region of protein

fluid structure“mosaic” of various proteins embedded in it

when viewed from the top

9

Membrane proteinscan move side-to-side or laterally making the membrane fluid

Freeze-fracture studies support the fluid mosaic model

of membrane structure

10

A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

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Fibers of

extracellular

matrix (ECM)

Integral proteinsPenetrate the hydrophobic core of the lipid

bilayerAre often transmembrane proteins,

completely spanning the membrane

12

EXTRACELLULAR

SIDE

Peripheral proteinsAre appendages loosely bound to the

surface of the membrane

13

Cell membrane separates living cell from aqueous environmentthin barrier = 8nm thick

Controls traffic in & out of the cell-allows some substances to cross more easily than

others

-hydrophobic (nonpolar) vs. hydrophilic (polar)

Serves as a cellular barrier / border

polarhydrophilicheads

nonpolarhydrophobictails

polarhydrophilicheads

H2Osugar

lipids

salt

waste

impermeable to polar molecules

Membrane becomes semi-permeable via protein channels

◦ specific channels allow specific material across cell membrane

inside cell

outside cell

sugaraaH2O

saltNH3

2007-2008

Why areproteins the perfect molecule to build structures in the cell membrane?

Within membranenonpolar amino acids hydrophobic

anchors protein into membrane

On outer surfaces of membrane in fluidpolar amino acids

hydrophilic

extend into extracellular fluid & into cytosol

Polar areasof protein

Nonpolar areas of protein

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;Retinal chromophores (rhodopsin) in human eyes

aquaporin = water channel in bacteria, (& plants, mammals)

conformational change

protein changes shape

H2O

H2O

H+

H+

Water moves rapidly into & out of cells-evidence that there were water channels

protein channels allowing flow of water across cell membrane

1991 | 2003

Peter AgreJohn Hopkins

Roderick MacKinnonRockefeller

Proteins determine membrane’s specific

functions

◦ cell membrane & organelle membranes each have

unique collections of proteins

Classes of membrane proteins:

-peripheral proteins

loosely bound to surface of membrane

ex: cell surface identity marker (antigens)

-integral proteins

penetrate lipid bilayer, usually across whole membrane

transmembrane protein

ex: transport proteins

channels, permeases (pumps)

Outside

Plasmamembrane

Inside

Transporter Cell surfacereceptor

Enzyme

activity

Cell surface identity marker

Attachment to thecytoskeleton

Cell adhesion

“Antigen”

“Channel”

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Transporthydrophilic channel across the membrane;

may be selective for a particular solute. (left)

transport protein shuttles substance across membrane

by changing shape. (right)

Some hydrolyze ATP as an energy source

to actively pump substances across the membrane.

Enzymatic Activityenzyme built into membrane; active site

exposed to substances in the adjacent solution.

Sometimes, several enzymes in a membrane are organized

as a team that carries out sequential steps of a metabolic pathway.

Signal TransductionMembrane protein has a binding site with a specific shape

that fits the shape of a chemical mssgr, such as a hormone.

External mssgr (signal) conformational change

in the protein (receptor)

relays the message to the inside of the cell.

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Cell-Cell recognitionSome glycoproteins serve as ID tags

that are specifically recognized by other cells.

Intercellular JoiningMembrane proteins of adjacent cells may hook together

in various kinds of junctions,

(Ex. gap junctions/tight junctions)

Attachment to cytoskeleton & extracellular matrix (ECM)

Microfilaments/other cytoskeletal elements bind to membrane proteins

-maintains cell shape

-stabilizes localization of certain membrane proteins

Proteins adhered to ECM coordinate extra- & intra-cellular changes

glycoprotein

Cell-Cell Recognition:a cell’s ability to distinguish one type of neighboring cell from another

development - cells tissues organsimmune function - ID & rejection of “foreign”

Membrane Carbohydrates:Interact with the surface molecules of other cells, facilitating cell-cell recognition

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http://www.youtube.com/watch?v=hRsLnQ6MbmA&feature=mfu_in_order&list=UL

http://www.youtube.com/watch?v=2as2bsFhoqk

http://www.youtube.com/watch?NR=1&v=37KNBEfcbsA&feature=fvwp

http://www.youtube.com/watch?v=2as2bsFhoqk

http://www.youtube.com/watch?v=hRsLnQ6MbmA&feature=mfu_in_order&list=UL

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