l3 cell membrane final 2013

Cell Surface Membrane

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Membrane History

Charles Overton 1890Charles Overton 1890Langmuir 1917Langmuir 1917Gorter & Grendel 1925Gorter & Grendel 1925Davson & Daneili 1935Davson & Daneili 1935David Robertson 1957David Robertson 1957Singer & Nicholson 1972Singer & Nicholson 1972Karnovsky 1982Karnovsky 1982Unwinn & Henderson 1984Unwinn & Henderson 1984Simmons & van Meer Simmons & van Meer 19881988

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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,

glycolipid and cholesterol Variation in lipid: protein content Viscosity of membrane depends on lipid

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Learning outcomes:Role of membrane

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

Passage of Passage of hydrophilic c and and hydrophobic material across the material across the membranemembrane

Pores


Where does our picture of the cell membrane come from?

Charles Ernest Overton (1865-1933)First indications that lipids are importantObserved lipid soluble substances past through membrane more easily than othersConclusion large part of the membrane must be lipid



Observations on the behaviour of cell Observations on the behaviour of cell surface membranessurface membranes

Most membranes seal themselves Most membranes seal themselves when punctured by a fine needlewhen punctured by a fine needleLed to the idea that membranes are Led to the idea that membranes are fluidfluid



Evert Gorter and F GrendalMeasured the total size of the monolayer film formed by lipid from human red blood cellsFound measured area of monolayer was twice the estimated surface area of a red blood cellConclusion cell membrane was a lipid bilayer

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Gorter and Grendel, 1925


Basic unit membrane structure Basic unit membrane structure under Electron microscopeunder Electron microscope


Where does our picture of the cell membrane come from?Hugh Davson and James Danielli 1935Hugh Davson and James Danielli 1935Produced model with lipid centre coated on each side with protein

James Robertson Electron microscope work showed three layered structure – two distinct lines with a gap in the middle



Singer and Nicholson (1972)Proposed the fluid mosaic modelA dynamic structure in which much of the protein floats about although some is anchored to organelles within the cellLipid also moves about


Units of size used in biology

1 1 centimetrecentimetre (cm) 10 (cm) 10-2-2metre (1/100)metre (1/100) 1 1 millimetre millimetre (mm) 10(mm) 10-3-3metre (1/1000)metre (1/1000) 1 1 micrometremicrometre ( (µµm) 10m) 10-6-6metre (1/000,000)metre (1/000,000) 1 1 nanometrenanometre (nm) 10 (nm) 10-9-9metre (1/000,000,000)metre (1/000,000,000) 1 1 picometrepicometre (pm) 10 (pm) 10-12-12metre metre

(1/000,000,000,000)(1/000,000,000,000)


Cell Surface Membrane Cell Surface Membrane StructureStructure Under the electron microscope Under the electron microscope

bilayer structure is revealedbilayer structure is revealed Two distinct lines 7nm wide (1nm Two distinct lines 7nm wide (1nm

=10=10-9-9metre)metre) Basic structure is 2 layers of Basic structure is 2 layers of

phospholipidsphospholipids


Phospholipids Lipid molecule three fatty acid Lipid molecule three fatty acid

molecules and a glycerolmolecules and a glycerol Phospholipid only two fatty acids, Phospholipid only two fatty acids,

a negatively charged phosphate a negatively charged phosphate group replaces the third fatty acidgroup replaces the third fatty acid


Lipid molecule


Phospholipid


Phospholipid bilayer bilayer Phophate head of the

molecule is polar; one end is slightly positive and the rest slightly negative

This makes the phosphate head attract other molecules , like water and is therefore hydrophilic (water loving)


Phospholipid bilayer 2 Fats and water don’t mixFats and water don’t mix When added to water When added to water

phospholipids arrange themselves phospholipids arrange themselves to avoid contact with between to avoid contact with between hydrophobic tails and the waterhydrophobic tails and the water


Phospholipid bilayer 3 They form a layer on the surface They form a layer on the surface

with their hydrophobic tails with their hydrophobic tails directed out of the water, arrange directed out of the water, arrange themselves into spherical cluster themselves into spherical cluster ((micellesmicelles) or form a bilayer) or form a bilayer


Phospholipids in water Phospholipids in water form a monolayer on the form a monolayer on the surface or spherical surface or spherical micellesmicelles


Phospholipid 4 Cells are filled with a watery or Cells are filled with a watery or

aqueous cytoplasm and are aqueous cytoplasm and are surrounded by aqueous tissue surrounded by aqueous tissue fluidfluid

The cell surface membrane The cell surface membrane phospholipids tend to adopt phospholipids tend to adopt their most stable arrangement, their most stable arrangement, which is a bilayerwhich is a bilayer


Phospholipid

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Phospholipid 5 This arrangement avoids the

hydrophobic fatty acid tails having any contact with water on either side of the membrane but ensures that the hydrophilic phosphate heads are in contact with the water.


Phospholipids


Fluid-Mosaic Model 1

The cell surface membrane is not just a phospholipid bilayer

It also contains proteins, cholesterol, glycoproteins (protein molecule with polysaccharide attached) and glycolipid (lipid molecule with polysaccharide attached)



Some of the proteins span the layer Other proteins are found only

within the inner layer or only within the outer layer

Membrane proteins have hydrophobic areas and these are positioned within the membrane bilayer



It is thought that some of the It is thought that some of the proteins are fixed within the proteins are fixed within the membrane and others are not membrane and others are not and can move in the fluid and can move in the fluid phospholipid bilayer.phospholipid bilayer.

This arrangement is known as the This arrangement is known as the fluid Mosaic Model of membrane fluid Mosaic Model of membrane structurestructure


Fluid Mosaic Model


Evidence for the model 1 The most widely accepted model The most widely accepted model

until the early 1970s was a three until the early 1970s was a three layer protein-lipid layer sandwich layer protein-lipid layer sandwich based on electron micrographs based on electron micrographs (diagram A)(diagram A)

However this model does not However this model does not allow the hydrophillic head to allow the hydrophillic head to come into contact with watercome into contact with water


(A) Phospholipid sandwich model (B) in the Fluid mosaic integral protein have polar and non polar regions


Evidence for the model 2 Experiments showed that there were

two types of protein- those that could be dissociated easily by increasing the ionic strength of the surrounding solution and those that could only be removed with detergent

This evidence indicated some proteins were loosely attached and some are fully embedded


Evidence for the model 3 Several integral proteins were

shown to have regions at their ends that had polar hydrophilic amino acids, with the middle portion composed mainly of non polar hydrophobic amino acids (diagram B)


(A) Phospholipid sandwich model (B) in the Fluid mosaic integral protein have polar and non polar regions


Evidence for the model 4 Additional evidence for integral Additional evidence for integral

proteins came from freeze-fracture proteins came from freeze-fracture electron microscope studieselectron microscope studies

Freeze-fracture sections were Freeze-fracture sections were fractured along their weak point fractured along their weak point between lipid layersbetween lipid layers

Scanning Electron microscopy Scanning Electron microscopy gave a gave a three dimensional imagethree dimensional image


Freeze-fracture of membrane revealing intregral proteins


Evidence for the model 5 Fusion of mouse cells with human Fusion of mouse cells with human

cellscells Before cells were fused a specific Before cells were fused a specific

membrane protein was labelled in membrane protein was labelled in each cell typeeach cell type

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Movement of membrane Proteins within cell surface membranes

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Membrane Protein Diversity

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Functions of Membrane Proteins Channel ProteinsChannel Proteins::

TubularTubular Allow passage of molecules through membraneAllow passage of molecules through membrane

Carrier ProteinsCarrier Proteins:: Combine with substance to be transportedCombine with substance to be transported Assist passage of molecules through membraneAssist passage of molecules through membrane

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

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

Enzymatic Enzymatic Proteins:Proteins: Carry out metabolic reactions directlyCarry out metabolic reactions directly


More unsaturated phospholipids – more fluid The more phospholipids The more phospholipids

containing unsaturated fatty acids containing unsaturated fatty acids the more fluid the membranethe more fluid the membrane

The ‘kinks’ in the hydrocarbon The ‘kinks’ in the hydrocarbon tails of the unsaturated tails tails of the unsaturated tails prevents them from packing prevents them from packing closely together, so more closely together, so more movement is possiblemovement is possible


Cholesterol Cholesterol reduces the fluidity of Cholesterol reduces the fluidity of

the membrane by preventing the membrane by preventing movement of the phospholipidsmovement of the phospholipids


Membrane Glycolipids

GlycolipidsGlycolipids shown as blue shown as blue sugar groups projecting sugar groups projecting into the extracellular space. into the extracellular space.

These components of the These components of the membrane may be membrane may be protective, insulators, and protective, insulators, and sites of receptorsites of receptor binding. binding.

Among the molecules Among the molecules bound by bound by glycososphingolipids glycososphingolipids include cell poisons such as include cell poisons such as choleracholera and and tetanus toxinstetanus toxins. .


Sphingolipid

Structural lipid of which the Structural lipid of which the parent structure is sphingosine parent structure is sphingosine rather than glycerol.rather than glycerol.

Synthesised in the Golgi complexSynthesised in the Golgi complex Form the lipid rafts


Raft Model


Raft Model Lipid rafts are possible island like Lipid rafts are possible island like

structure present in cellular structure present in cellular membranes. membranes.

They are enriched in They are enriched in cholesterol and and sphingolipids.

Cellular membranes with lipid rafts have Cellular membranes with lipid rafts have a higher concentration of a higher concentration of glycosphingolipids and and cholesterolcholesterol than than do non-raft parts of membrane. do non-raft parts of membrane.


Raft Model The existence of lipid rafts in cell The existence of lipid rafts in cell

membrane has not yet been membrane has not yet been approved completely by all approved completely by all scientists, but many think they scientists, but many think they serve as serve as communication hubs by hubs by recruiting proteins that need to recruiting proteins that need to come together in order to come together in order to transmit a signal.transmit a signal.



Q1According to the fluid-mosaic model for the plasma membrane, there is a ___________ bilayer in which proteins are scattered throughout the membrane. The __________ (water loving) polar heads of the phospholipids face the intracellular and extracellular fluid. The _____________ (water hating) nonpolar tails of the phospholipid molecules face each other.


A1According to the fluid-mosaic model for the plasma membrane, there is a phospholipid bilayer in which proteins are scattered throughout the membrane. The hydrophilic (water loving) polar heads of the phospholipids face the intracellular and extracellular fluid. The hydrophobic (water hating) nonpolar tails of the phospholipid molecules face each other.

Q2Q2Phospholipids have their Phospholipids have their hydrophilic polar heads facing the hydrophilic polar heads facing the ____________________ and and _____________fluid. fluid. The hydrophobic nonpolar tails face The hydrophobic nonpolar tails face each other. each other. The other two types of lipids The other two types of lipids present in the plasma membrane present in the plasma membrane are the are the _____________and and ______________.______________.


A2Phospholipids have their Phospholipids have their hydrophilic polar heads facing the hydrophilic polar heads facing the intracellularintracellular and and extracellular fluid. fluid. The hydrophobic nonpolar tails face The hydrophobic nonpolar tails face each other. each other. The other two types of lipids The other two types of lipids present in the plasma membrane present in the plasma membrane are the are the glycolipidslipids and and cholesterol.cholesterol.


Q3The proteins found in the plasma membrane may be _________ proteins, which are found within the membrane,or ____________ proteins, which occur either on the cytoplasmic side or the outer surface side of the surface side of the membrane.membrane.


A3The proteins found in the plasma membrane may be integral proteins, which are found within the membrane,or peripheral proteins, which occur either on the cytoplasmic side or the outer surface side of the surface side of the membrane.membrane.


Q 4Q 4State two roles of cholesterol in the State two roles of cholesterol in the membrane (2 marks)membrane (2 marks)


A 4State two roles of cholesterol in the State two roles of cholesterol in the membrane (2 marks)membrane (2 marks)Regulates membrane fluidity;Mechanical stability;Reduces leakage of polar ions by diffusion;


Q5Q5There are many types of proteins in There are many types of proteins in a membrane. Describe the role of a membrane. Describe the role of two (2 marks)two (2 marks)


A5 There are many types of proteins in a There are many types of proteins in a membrane. Describe the role of two (2 membrane. Describe the role of two (2 marks)marks)Channel proteins to allow facilitated diffusion;Carrier proteins for active transport of molecules in/out of ;cell;Receptor molecules for hormones/ neurotransmitters;Recognition site;Enzymes for digestion/ respiration;


l3 cell membrane final 2013

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