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Fundamental of Cell Biology & PhysiologyBiological Membranes
Cell Membranes
❖ Membranes are essential for living cells
❖ Define boundaries of cell
❖ Maintain differences between cytosol and extracellular environment
❖ Eukaryotic cells have membrane enclosed organelles- Golgi apparatus, Mitochondria
and Endoplasmic Reticulum- maintain characteristic differences between contents of
each organelle and cytosol
❖ Allow for ionic gradients- established by activities of specialised membrane proteins- can be used to
synthesise ATP and drive transport of selected movement of molecules and solutes (in nerve cells or muscle
cells)
❖ Generation of electrical signals- protein sensors/ receptors transfer information across membrane
❖ Proteins embedded in membrane sense changes in the external environment- generate signals affecting
cellular behaviour
All biological membranes have common
general structure- each is very thin film of
lipid and protein molecules- held together
by non-covalent interactions
Types of membrane
Cell Membrane
❖ Delimits semi-autonomous functional unit- cell
❖ Controls movement of ions and molecules into and out of cell, catalysis of membrane- associated reactions-
ATP synthesis
❖ Protection from external environment
❖ Provides attachment site
❖ Functions in cell signalling
In plasma membrane- some transmembrane proteins serve structural links that connect cytoskeleton through lipid
bilayer to either extracellular matrix or adjacent cell
While others serve as receptors to detect and transduce chemical signals in cell’s environment
Endomembrane System
❖ Internal membranes delimiting organelles
Cell membrane
Cell membranes are dynamic, fluid structures and most of their molecules move about in plane of membrane. Lipid
molecules arranged as continuous double layer.
Lipid bilayer provides- basic fluid structure of all cell membranes
- impermeable barrier to passage of most water-soluble molecules
❖ Fatty acid tails usually 14-12 Carbons in length
❖ One tail usually has one/more cis-double bonds- UNSATURATED
, ❖ Double binds create kink in tail
❖ Tail length and degree of saturation affects lipid packaging- influences fluidity of membrane
Lipids
Constitute around 50% of membrane mass.
Small animal cell has around 1 billion (109) lipid
molecules organised in bilayer
Lipids are self-organising
Lipid bilayer embedded/ attached proteins held together via non- covalent
interactions.
All of lipid molecules in cell membranes are AMPHIPHILIC:
- Hydrophilic (water loving) polar hear group
-Hydrophobic (water fearing) non-polar tail
Most abundant membrane lipids are phospholipids:
- Polar head containing phosphate group
- Two hydrophobic hydrocarbon tails
❖ Main phospholipids in most animal cell membranes are phosphoglycerides- have three- Carbon glycerol
backbone
Teo long-chain fatty acids are linked through ester bonds to adjacent Carbon atoms of glycerol. Third Carbon
atom of glycerol is attached to phosphate group
Major phospholipids in mammalian plasma membranes:
❖ A-C- All phosphoglycerides derived from glycerol with Ethanolamine, Serine or Choline
(Phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine)- most abundant
❖ D- Sphingolipid called sphingomyelin derived from sphingosine (long acyl chain with amino group (NH2) and
two hydroxy groups (OH))
❖ Phosphatidylserine is only one to carry negative charge
Together phospholipids, Phosphatidylcholine, Phosphatidylethanolamine, Phosphatidylserine and sphingomyelin
constitute more than half the mass of lipid in most mammalian cell membranes
Lipid bilayers contain phospholipids, glycolipids and cholesterol
- Phospholipids- fall into two subclasses- phosphoglycerides and sphingolipids
- Glycolipids- resemble Sphingolipids – instead of phosphate-linked head group, they have sugars attached
-Cholesterol- is sterol, contains rigid ring structure attached to polar OH group and short nonpolar hydrocarbon-
orient themselves in bilayer with OH group close to polar head of adjacent phospholipid molecules
Phospholipids forming bilayers
Shape and amphiphilic nature of phospholipid molecules cause them to form
BILAYERS in aqueous environment.
- Hydrophilic molecules dissolve readily in water- contain charged groups/ uncharged polar groups
- Hydrophobic molecules are insoluble in water- uncharged and nonpolar
In bilayer, free edge is created with water- energetically unfavourable- lipids tend to rearrange themselves to
eliminate free edge
The only way for bilayer to avoid having edges is by closing itself and forming sealed compartment
,Small tear in bilayer creates free edge with water- energetically unfavourable- lipids rearrange to eliminate free
edge. (In eukaryotic plasma membranes- fusion of intracellular vesicles repair large tears)
Phospholipids forming Spheres-
Polar nature of phospholipid bilayer- energetically favours forming sealed
compartment
- either form electrostatic attractions or hydrogen bonds with water molecules
Spontaneous closure of phospholipid bilayer forms SEALED COMPARTMENT-
stable- avoids exposure of hydrophobic hydrocarbon tails to water-
energetically unfavourable
Form cage structures- more ordered than surrounding water- formation
increases free energy
Free energy cost is minimised- if hydrophobic molecules cluster together- smallest number of water molecules are
affected
❖ Phospholipid bilayer forms enclosed cell- hydrophobic tails in interior (shielded from water) and expose their
hydrophilic heads to water
❖ Lipid molecules can diffuse freely within plane of lipid bilayer- Artificial liposomes can be created with
phospholipids in aqueous solution
Liposomes don’t fuse spontaneously with one another when suspended in water- polar lipid head groups
bind with water molecules that need to be displaced for bilayers of two different liposomes to fuse.
e.g. Techniques have been used to measure motion of individual lipid molecules and their components
- Construct lipid molecule with fluorescent dye/ small gold particle attached to its polar head group and follow
with diffusion of even individual molecules in membrane
- Modify lipid head group to carry ‘spin label’- such as NITROXIDE GROUP (N=O)- Contains unpaired electron
whose spin creates paramagnetic signal detected ESR (ELECTRON SPIN RESONANCE) SPECTROSCOPY
Motion and orientation of spin-labelled lipid in bilayer can be deduced from ESR Spectrum
Such studies show that phospholipid molecules in synthetic bilayers rarely migrate from monolayer (leaflet) on
one side to another
Cholesterol - Important component of
Eukaryotic membranes
❖ Polar hydroxyl head group of cholesterol
inserts close to polar group of membrane
phospholipids
❖ Plant cells don’t have cholesterol- instead they have related STEROL compounds
❖ Bacterial cells don’t have cholesterol
Membrane Fluidity
Membrane isn’t static structure
Bilayer made form single type of phospholipid changes from liquid state to a two-dimensional rigid crystalline state
at characteristic temperature.
Phase transition- change of state, temperature in which it occurs is lower- membranes becomes more difficult to
free, if hydrocarbon chains are short or have double bonds
, Shorter chain length reduces tendency of hydrocarbon chains to interact with one another- in both same and
opposite monolayer
Cis double bonds produce kinks in chains- makes more difficult to pack together- membrane remains fluid at lower
temperatures
Bacteria. Yeasts, and other organisms whose temperature fluctuates with that of their environment adjust fatty acid
composition of their membrane lipids to maintain constant fluidity
- As temperature falls- cells of those organisms synthesise fatty acids with more cis double bonds- avoid
decrease in bilayer fluidity
Temperature
❖ At low temperature- reduced energy- phospholipids move less and pack together tighter
-Saturated hydrocarbons allow for closer packing
- CRYSTALLINE state is forms
❖ At high temperature - phospholipids have increased kinetic energy- move more and pack less tightly
-Unsaturated hydrocarbons create greater spacing- INCREASE fluidity
Influence of Cholesterol on Membrane fluidity
Cholesterol places itself in lipid bilayer- it enhances permeability barrier properties of lipid bilayer
- inserts itself with its OH group closer to polar head groups of phospholipids- rigid, platelike steroid rings interact
with, and partially immobilise, those regions of hydrocarbon chains closest to polar head groups
By decreasing mobility of first few CH2 groups of chains of phospholipid molecules- cholesterol makes lipid bilayer
less deformable in this region- decreases permeability of bilayer to small water-soluble molecule
Cholesterol tightens packing of lipids in bilayer- doesn’t make membranes any less fluid
❖ At high cholesterol concentration in most eukaryotic plasma membranes- prevents hydrocarbon from
coming together and crystallising
❖ Low temperatures- Cholesterol INCREASES spacing between hydrocarbons- INCREASE fluidity
❖ High temperatures- Cholesterol PULLS hydrocarbon tails TOGETHER- DECREASES fluidity (STABILISES)
Lipid composition
Lipid composition varies between different membranes- not
all membranes are same
❖ Bacterial plasma membranes- has one main type of
phospholipid and contains no cholesterol
❖ Archaea- lipids usually contain 20-25 carbon- long prenyl chains instead of fatty acid
Prenyl and fatty acid chains are similarly hydrophobic and flexible
❖ In thermophilic archaea- longest lipid chains span both leaflets- make membrane stable to heat
Plasma membranes of most eukaryotic cells are more varied than of prokaryotes and archaea- not only in
containing large amounts of cholesterol but also in containing mixture of different phospholipids
❖ Lipid composition of typical eukaryotic cell membrane is much more complex- combinatorial variation in head
groups, hydrocarbon chain lengths and desaturation of major phospholipid classes
❖ E.g. Inositol phospholipids (minor class of phospholipids withing cytosolic leaflet) - present in small
quantities in animal cell membranes
- functions- guiding membrane traffic, cell signalling their local synthesis and destruction are
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