PART 1 STRUCTURE AND MODELS OF BIOLOGICAL MEMBRANES
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Membrane Structure and Function Membrane Structure Lipids and
proteins are the main components of the membranes, although
carbohydrates are also important. The most abundant lipids in most
membranes are phospholipids Phospholipids and most of proteins of
membrane are amphipathic molecules. Amphipathic molecules : A
molecule that has both hydrophilic region and a hydrophobic
regions. The membrane ia a fluid mosaic : The membrane is a fluid
structure with various protein embedded in or attached to a double
layer (bilayer)of phospholipids.
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Membrane lipids are organized in a bilayer The proteins are
scattered throughout the bilayer (perform most membrane
functions)
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Structure of the Cell Membrane Outside of cell Inside of cell
(cytoplasm ) Lipid Bilayer
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Fluid Mosaic Model of the cell membrane
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History of the Plasma Membrane 1665: Robert Hooke 1895: Charles
Overton - composed of lipids 1900-1920s: must be a phospholipid
1925: E. Gorter and G. Grendel - phospholipid bilayer 1935: J.R.
Danielli and H. Davson proteins also part, proposed the Sandwich
Model 1950s: J.D. Robertson proposed the Unit Membrane Model 1972:
S.J. Singer and G.L. Nicolson proposed Fluid Mosaic Model
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Plasma Membrane Models Overton model (1895) the layers
surrounding cells lipoids made from lipids and cholesterol.
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Plasma Membrane is made of Phospholipids Gorter + Grendel Red
Blood Cells analyzed cell membranes are made of two opposing thin
lipid double layers polar head groups pointing toward the aqueous
environment (Polar heads face out and Nonpolar tails face in) fails
to account for the manifold of functions attributed to cell
membranes. (Does not explain why some nonlipids are permeable)
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Plasma Membrane Models Sandwich Model (Danielli + Davson)
Earlist model for the biomembrane structure including proteins. The
proteins possess hydrophobic interiors and a water-containing outer
layer. Proteins are adsorbed to the lipophilic layers surrounding
cells The water-containing regions of protein layers adsorped on
lipid layers are permeable for charged solutes
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Plasma Membrane Models Sandwich Model (Danielli + Davson) 2
layers of globular proteins with phospholipid inside to make a
layer and then join 2 layers together to make a channel for
molecules to pass
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Plasma Membrane Models Sandwich Model (Danielli + Davson)
Divalent cations as calcium form complexes with lipids or proteins
that reduce their interaction with water. Therefore membranes
containing calcium are less permeable for ions. Arguments did not
exclude the possibility that the proteins may span the membrane
such that a mosaic of protein-rich and lipid-rich regions is formed
due to the lack of experimental evidence.
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Plasma Membrane Models Unit Membrane Model (Robertson) Outer
layer of protein with phospholipid bilayer inside, believed all
cells same composition, does not explain how some molecules pass
through or the use of proteins with nonpolar parts, used
transmission electron microscopy
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Plasma Membrane Models Fluid Mosaic Model (Singer + Nicolson)
Phospholipid bilayer with proteins partially or fully imbedded,
electron micrographs of freeze-fractured membrane
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Which membrane model is correct? 1) Rapidly freeze specimen 2)
Use special knife to cut membrane in half 3) Apply a carbon +
platinum coating to the surface 4) Use scanning electron microscope
to see the surface According to the electron micrograph which
membrane model is correct? Why? Fluid-Mosaic M odel
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Mosaic means an object comprised of bits and pieces embedded in
a supporting structure. (1) membrane lipids form the supporting
structure. (2) membrane proteins provide the bits and pieces. (3)
both lipids and proteins may be mobile or 'fluid'
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Fluid-Mosaic Model Fluid the plasma membrane is the consistency
of olive oil at body temperature, due to unsaturated phospholipids.
(cells differ in the amount of unsaturated to saturated fatty acid
tails) Most of the lipids and some proteins drift laterally on
either side. Phospholipids do not switch from one layer to the
next. (Both proteins and lipid bilayer move in plane)
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Structure of the Plasma Membrane
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Membranes are mosaics of structure and function A membrane is a
mosaic of different proteins embedded and dispersed in the
phospholipid bilayer. These proteins vary in both structure and
function, and they occur in two spatial arrangements 1- Integral
Proteins 2- Peripheral proteins
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1- In tegral proteins, which are inserted into the membrane:
penetrate the hydrophobic core of the lipid bilayer, often
completely spanning the membrane (a transmembrane protein). Their
hydrophilic ends are exposed on both sides of the membrane. cannot
easily be separated from the lipids. form the major fraction of
membrane proteins.
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2- Peripheral proteins, which are not embedded in the lipid
bilayer but attached to the membrane surface: May be attached to
integral proteins or held by fibers of the extracellular matrix. On
the cytoplasmic side, may be held by filaments of cytoskeleton are
only loosely attached to the membrane surface can easily be
separated from the membrane by mild treatment
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Carbohydrate Polymers form: Glycolipids when attach to
Phospholipid Molecules Glycoproteins when they attach to proteins
act as Cell Receptor Sites involved in Cell Signalling in the
Immune System.
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Cholesterol effect on Membrane Fluidity At Warm Temp. making
the membrane Less fluid by restraining the phospholipid movement At
Cold Temp. making the bilayer more fluid at lower (cool)
temperatures by preventing close packing of phospholipids.
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Structure of the Plasma Membrane Phospholipid bilayer
Phospholipid Hydrophilic head Hydrophobic tails Cholesterol
Proteins Transmembrane/ Intrinsic/Integral Peripheral/Extrinsic
Cytoskeletal filaments Carbohydrate chain Glycoproteins
Glycolipids
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Proteins of the Plasma Membrane Provide 6 Membrane Functions:
1) Transport Proteins 2) Receptor Proteins 3) Enzymatic Proteins 4)
Cell Recognition Proteins 5) Attachment Proteins 6) Intercellular
Junction Proteins
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1) Transport Proteins Channel Proteins channel for lipid
insoluble molecules and ions to pass freely through Carrier
Proteins bind to a substance and carry it across membrane, change
shape in process
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2) Receptor Proteins Bind to chemical messengers (Ex. hormones)
which sends a message into the cell causing cellular reaction
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3) Enzymatic Proteins Carry out enzymatic reactions right at
the membrane when a substrate binds to the active site
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4) Cell Recognition Proteins Glycoproteins (and glycolipids) on
extracellular surface serve as ID tags (which species, type of
cell, individual). Carbohydrates are short branched chains of less
than 15 sugars
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5) Attachment Proteins Attach to cytoskeleton (to maintain cell
shape and stabilize proteins) and/or the extracellular matrix
(integrins connect to both). -Extracellular Matrix protein fibers
and carbohydrates secreted by cells and fills the spaces between
cells and supports cells in a tissue. -Extracellular matrix can
influence activity inside the cell and coordinate the behavior of
all the cells in a tissue.
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6) Intercellular Junction Proteins Bind cells together Tight
junctions Gap junctions