Cell Surface Membrane

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, glycolipid and cholesterol
Variation in lipid: protein content
Viscosity of membrane depends on lipid content
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Learning outcomes:
Role of membrane
Boundary layer but also an active part of the biochemical functioning of the cell
Passage of hydrophilic and hydrophobic material across the membrane
Pores
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Where does our picture of the cell membrane come from?
Charles Ernest Overton (1865-1933)
First indications that lipids are important
Observed lipid soluble substances past through membrane more easily than others
Conclusion large part of the membrane must be lipid
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Where does our picture of the cell membrane come from?
Observations on the behaviour of cell surface membranes
Most membranes seal themselves when punctured by a fine needle
Led to the idea that membranes are fluid
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Where does our picture of the
cell 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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Basic unit membrane structure under Electron microscope
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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 showed three layered structure – two distinct lines 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 of the protein floats about although some is 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 (µm) 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 bilayer structure 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 acid molecules and a glycerol
Phospholipid only two fatty acids, a negatively charged phosphate group replaces the third fatty acid
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Lipid molecule
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Phospholipid
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Phospholipid 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)
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Phospholipid bilayer 2
Fats and water don’t mix
When added to water phospholipids arrange themselves to avoid contact with between hydrophobic tails and the water
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Phospholipid bilayer 3
They form a layer on the surface with their hydrophobic tails directed out of the water, arrange themselves into spherical cluster (micelles) or form a bilayer
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Phospholipids in water form a monolayer on the surface or spherical micelles
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Phospholipid 4
Cells are filled with a watery or aqueous cytoplasm and are surrounded by aqueous tissue fluid
The cell surface membrane phospholipids tend to adopt their most stable arrangement, which is a bilayer
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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.
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Phospholipids
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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)
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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 outer layer
Membrane proteins have hydrophobic areas and these are positioned within the membrane bilayer
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Fluid-Mosaic Model 3
It is thought that some of the proteins are fixed within the membrane and others are not and can move in the fluid phospholipid bilayer.
This arrangement is known as the fluid Mosaic Model of membrane structure
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Fluid Mosaic Model
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Evidence for the model 1
The most widely accepted model until the early 1970s was a three layer protein-lipid layer sandwich based on electron micrographs (diagram A)
However this model does not allow the hydrophillic head to come into contact with 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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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
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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)
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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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Evidence for the model 4
Additional evidence for integral proteins 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 image
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Freeze-fracture of membrane revealing intregral proteins
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Evidence for the model 5
Fusion of mouse cells with human cells
Before cells were fused a specific membrane protein was labelled in each cell type
Mouse – green fluorescent label
Human – red fluorescent label
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Movement of membrane Proteins within cell surface membranes
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Membrane Protein Diversity

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 phospholipids – more 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 more movement is possible
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Cholesterol
Cholesterol reduces the fluidity of the membrane by preventing movement of the phospholipids
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Membrane Glycolipids
Glycolipids shown as blue sugar groups projecting into the extracellular space.
These components of the membrane may be protective, insulators, and sites of receptor binding.
Among the molecules bound by glycososphingolipids include cell poisons such as cholera and tetanus toxins.
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Sphingolipid
Structural lipid of which the parent structure 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 structure present in cellular membranes.
They are enriched in cholesterol and sphingolipids.
Cellular membranes with lipid rafts have a higher concentration of glycosphingolipids and cholesterol than do non-raft parts of membrane.
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Raft Model
The existence of lipid rafts in cell membrane has not yet been approved completely by all scientists, but many think they serve as communication hubs by recruiting proteins that need to come together in order to transmit a signal.
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Q1
According 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.

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A1
According 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.

Q2
Phospholipids have their hydrophilic polar heads facing the __________ and _____________fluid. The hydrophobic nonpolar tails face each other.
The other two types of lipids present in the plasma membrane are the _____________and ______________.
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A2
Phospholipids have their hydrophilic polar heads facing the intracellular and extracellular fluid. The hydrophobic nonpolar tails face each other.
The other two types of lipids present in the plasma membrane are the glycolipids and cholesterol.
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Q3
The 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 membrane.
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A3
The 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 membrane.
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Q 4
State two roles of cholesterol in the membrane (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 by diffusion;
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Q5
There are many types of proteins in a membrane. Describe the role of two (2 marks)
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