NU Cell membrane Final 2013.ppt

8/13/2019 NU Cell membrane Final 2013.ppt

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Cell Surface Membrane

11/19/2013 Pork Chop Willie cell membrane structure 1


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Membrane History

Charles Overton 1890 Langmuir 1917

Gorter & Grendel 1925

Davson & Daneili 1935 David Robertson 1957

Singer & Nicholson 1972

Karnovsky 1982 Unwinn & Henderson 1984

Simmons & van Meer 1988

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Learning outcomes:Membrane structure

Mono- and bi- layers of lipid

Integral and peripheral proteins

The fluid Mosaic model of membrane structure Raft model of membranes

Phospholipids, sphingolipid, glycoprotein,

glycolipidand cholesterol Variation in lipid: protein content

Viscosityof membrane depends on lipid content



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Learning outcomes:Role of membrane

Boundary layer but also an active part ofthe biochemical functioning of the cell

Passage of hydrophilic and hydrophobicmaterial across the membrane

Pores



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Where does our picture of thecell membrane come from?

Charles Ernest Overton (1865-1933)

First indications that lipids are important

Observed lipid soluble substances pastthrough membrane more easily thanothers

Conclusion large part of the membranemust be lipid



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Where does our picture of the cellmembrane come from?

Observations on the behaviour of cellsurface membranes

Most membranes seal themselves whenpunctured by a fine needle

Led to the idea that membranes are fluid



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Where does our picture of thecell membrane come from?

Evert Gorter and F Grendal

Measured the total size of the monolayer film

formed by lipid from human red blood cells Found measured area of monolayer was twice

the estimated surface area of a red blood cell

Conclusion cell membrane was a lipid bilayer

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Gorter and Grendel, 1925



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Where does our picture of the

cell membrane come from?Hugh Davson and James Danielli 1935

Produced model with lipid centre

coated on each side with protein

James Robertson

Electron microscope work showedthree layered structuretwo distinctlines with a gap in the middle



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Where does our picture of the

cell membrane come from?Singer and Nicholson (1972)

Proposed the fluid mosaic model

A dynamic structure in which much ofthe protein floats about although someis anchored to organelles within the cell

Lipid also moves about



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Units of size used in biology

1 centimetre(cm) 10-2metre (1/100)

1 millimetre (mm) 10-3metre (1/1000)

1 micrometre(m) 10-6metre (1/000,000) 1 nanometre(nm) 10-9metre (1/000,000,000)

1 picometre(pm) 10-12metre (1/000,000,000,000)



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Cell Surface Membrane

Structure

Under the electron microscope bilayerstructure is revealed

Two distinct lines 7nm wide (1nm=10-9metre)

Basic structure is 2 layers of

phospholipids



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Phospholipids

Lipid molecule three fatty acidmolecules and a glycerol

Phospholipid only two fatty acids, anegatively charged phosphate groupreplaces the third fatty acid



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Lipid molecule



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Phospholipid



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Phospholipid bilayer

Phophate head of themolecule is polar; one end is

slightly positive and the restslightly negative

This makes the phosphate

head attract other molecules, like water and is thereforehydrophilic(water loving)



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Phospholipid bilayer 2

Fats and water dont mix

When added to water phospholipids

arrange themselves to avoid contactwith between hydrophobic tails andthe water



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Phospholipid bilayer 3

They form a layer on the surface withtheir hydrophobic tails directed out of

the water, arrange themselves intospherical cluster (micelles) or form abilayer


h h l d f


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Phospholipids in water form amonolayer on the surface or

spherical micelles



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Phospholipid 4

Cells are filled with a watery oraqueous cytoplasm and are

surrounded by aqueous tissuefluid

The cell surface membrane

phospholipids tend to adopttheir most stable arrangement,which is a bilayer



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Phospholipid

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Phospholipid 5

This arrangement avoids thehydrophobic fatty acid tails having any

contact with water on either side ofthe membrane but ensures that thehydrophilic phosphate heads are in

contact with the water.



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Phospholipids



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Fluid-Mosaic Model 1

The cell surface membrane is not justa phospholipid bilayer

It also contains proteins, cholesterol,glycoproteins(protein molecule withpolysaccharide attached) and

glycolipid(lipid molecule withpolysaccharide attached)



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Fluid-Mosaic Model 2

Some of the proteins span the layer

Other proteins are found only within

the inner layer or only within the outerlayer

Membrane proteins have hydrophobic

areas and these are positioned withinthe membrane bilayer



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Fluid Mosaic Model



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Evidence for the model 1

The most widely accepted model untilthe early 1970s was a three layer

protein-lipid layer sandwich based onelectron micrographs (diagram A)

However this model does not allow the

hydrophillic head to come into contactwith water



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(A) Phospholipid sandwich model(B) in the Fluid mosaic integral protein have

polar and non polar regions



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Experiments showed that there were twotypes of protein- those that could be

dissociated easily by increasing the ionicstrength of the surrounding solution andthose that could only be removed withdetergent

This evidence indicated some proteins were looselyattached and some are fully embedded



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Several integral proteins were shown tohave regions at their ends that had polar

hydrophilic amino acids, with the middleportion composed mainly of non polarhydrophobic amino acids (diagram B)



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(A) Phospholipid sandwich model(B) in the Fluid mosaic integral protein have

polar and non polar regions



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Additional evidence for integralproteins came from freeze-fracture

electron microscope studies Freeze-fracture sections were

fractured along their weak point

between lipid layers Scanning Electron microscopy gave a

three dimensional image11/19/2013 Pork Chop Willie cell membrane structure 35


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Freeze-fracture of membrane

revealing intregral proteins



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Fusion of mouse cells with human cells

Before cells were fused a specific

membrane protein was labelled ineach cell type

Mousegreen fluorescent label

Humanred fluorescent label



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Movement of membrane Proteinswithin cell surface membranes

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Functions of Membrane

Proteins Channel Proteins:

Tubular Allow passage of molecules through membrane

Carrier Proteins:

Combine with substance to be transported Assist passage of molecules through membrane

Cell Recognition Proteins: Provides unique chemical ID for cells Help body recognize foreign substances

Receptor Proteins: Binds with messenger molecule Causes cell to respond to message

Enzymatic Proteins: Carry out metabolic reactions directly



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More unsaturated

phospholipidsmore fluid The more phospholipids containing

unsaturated fatty acids the more fluid

the membrane The kinks in the hydrocarbon tails of

the unsaturated tails prevents them

from packing closely together, so moremovement is possible



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Cholesterol

Cholesterol reduces the fluidity of themembrane by preventing movement

of the phospholipids



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Membrane Glycolipids

Glycolipidsshown as bluesugar groups projecting intothe extracellular space.

These components of themembrane may be protective,insulators, and sites of receptorbinding.

Among the molecules bound by

glycososphingolipids includecell poisons such as choleraand tetanus toxins.



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Sphingolipid

Structural lipid of which the parentstructure is sphingosine rather than

glycerol. Synthesised in the Golgi complex

Form the lipid rafts



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Raft Model



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Raft Model

Lipid rafts are possible island like structurepresent in cellular membranes.

They are enriched in cholesterolandsphingolipids.

Cellular membranes with lipid rafts have ahigher concentration of glycosphingolipidsand cholesterolthan do non-raft parts ofmembrane.



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Raft Model

The existence of lipid rafts in cellmembrane has not yet been approved

completely by all scientists, but manythink they serve as communicationhubs by recruiting proteins that need

to come together in order to transmita signal.



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Q1

According to the fluid-mosaic model for theplasma membrane, there is a___________bilayer in which proteins are scatteredthroughout the membrane.

The__________(water loving) polar headsof the phospholipids face the intracellularand extracellular fluid. The_____________

(water hating) nonpolar tails of thephospholipid molecules face each other.


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A1

According to the fluid-mosaic model for theplasma membrane, there is a phospholipidbilayer in which proteins are scatteredthroughout the membrane.

The hydrophilic(water loving) polar headsof the phospholipids face the intracellularand extracellular fluid. The hydrophobic

(water hating) nonpolar tails of thephospholipid molecules face each other.


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A2

Phospholipids have their hydrophilic

polar heads facing the intracellularandextracellularfluid. The hydrophobicnonpolar tails face each other.

The other two types of lipids present inthe plasma membrane are theglycolipidsand cholesterol.



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A3

The proteins found in the plasmamembrane may be integralproteins,which are found within the membrane,

or peripheralproteins, which occureither on the cytoplasmic side or theouter surface side of the membrane.



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Q 4

State two roles of cholesterol in themembrane (2 marks)



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A 4

State two roles of cholesterol in the

membrane (2 marks)

Regulates membrane fluidity;

Mechanical stability;

Reduces leakage of polar ions bydiffusion;



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A5 There are many types of proteins in a


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A5There are many types of proteins in amembrane. Describe the role of two (2 marks)

Channel proteins to allow facilitateddiffusion;

Carrier proteins for active transport ofmolecules in/out of ;cell;

Receptor molecules for hormones/neurotransmitters;

Recognition site; Enzymes for digestion/ respiration;


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NU Cell membrane Final 2013.ppt