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8/6/2019 Copy of Cell Biology 2
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Bio-membranes
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Eukaryotes.. Eukaryotic cell has an efficient membrane functions due to increase in the surface
area. This is attributed to the presence of many internal membranes with
numerous convolutions and infoldings.
Membranes surround the nucleus, mitochondria, and (in plant cells) the
chloroplasts as well as Endoplasmic Reticulum. Membranes form stacks of sacs
with Golgi apparatus, surround lysosomes responsible for intracellular digestion,
peroxisomes as well as form small vesicles and, in plants, a large liquid-filledvacuole.
Each organelle is surrounded by one or more biomembranes, with unique set of
proteins specific for its characteristic function.
All these membrane-bounded structures correspond to distinct internal
compartments within the cytoplasm. In a typical animal cell these organelles
occupy nearly half the total cell volume and the remaining is the cytosol.
Keeping all the organelles in proper shape and in a controlled movement
eukaryotic cells have a tough cytoskeleton.
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Functions ofbiomembranes
Cell membranes are crucial to the life of the cell.
The plasma membrane encloses the cell, defines its boundaries, andmaintains the essential differences between the cytosol and the extracellular
environment.
The membranes of the ER, Golgi apparatus, mitochondria, and other
membrane-bounded organelles in eukaryotic cells maintain the characteristicdifferences between the contents of each organelle and the cytosol.
Responsible for maintaining ion gradients across membranes (function of
specialized membrane proteins) which can be used to synthesize ATP, to drive
the transport of selected solutes, or, in production and transmission of
electrical signals in nerve and muscle cells.
Plasma membrane also contains proteins that act as sensors of external
signals, allowing the cell to change its behavior in response to environmental
changes; These are called receptors , that transfer information rather than
ions or molecules across the membrane.
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Membrane structure
Inspite of diverse functions, all the cell membranes have a common
basic molecular structure.
Each membrane is a thin film of lipid and protein molecules, held
together mainly by non-covalent interactions. Carbohydrates are
present in covalent linkage with lipids or proteins.
The lipid molecules are arranged as a continuous double layer (~5 nm
thick). This lipid bilayerprovides the basic structure of the membrane
and is a relatively impermeable barrier to the passage of most water-
soluble molecules.
Cell membranes are asymmetrical structures:
The lipid and protein compositions of the outside and inside faces
differ from one another in ways that reflect the different functions
performed at the two surfaces of the membrane.
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(a) Electron micrograph of plasma membrane (b, c) Schematic representations of lipid bilayer
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The Lipid bilayer-Composition
The lipid bilayer has been firmly established as the universal basisfor cell-membrane structure.
It is easily seen by ordinary electron microscopy, although
specialized techniques, such as x-ray diffraction and freeze-fracture
electron microscopy, are needed to reveal the details of itsorganization.
The bilayer structure is attributed to the special properties of the
lipid molecules i.e. the amphipathic nature, due to which it
assembles spontaneously to form bilayers.
The most abundant are the phospholipids consist of two long-chain,
nonpolar fatty acid groups linked (usually by an ester bond) to small,
highly polar groups, including a phosphate.
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The tails are usually fatty acids which may vary in length (14-24 C atoms).
One tail usually has one or more cis-double bonds (that is , it is unsaturated),
while the other tail does not (that is, it is saturated). Each double bondcreates
a small kinkin the tail.
Differences in the length and saturation of the fatty acid tails influence the
ability of phospholipid molecules to pack against one another, and for this
reason they affect the fluidity of the membrane.
Major phosphoglyceride, phopshatidylcholine
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Types of fatty acids in membrane
14:0 myristic acid
16:0 palmitic acid
18:0 stearic acid
18:1 cis 9 oleic acid
18:2 cis 9,12 linoleic acid
18:3 cis 9,12,15 linonenic acid
20:4 cis 5,8,11,14 arachidonic acid 20:5 cis 5,8,11,14,17 eicosapentaenoic acid
(an omega-3 fatty acid because of
double bond 3 C from distal end)
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Three Classes of Lipids Are Foundin Biomembranes
A typical biomembrane is assembled from phosphoglycerides or
glycerophospholipids (major class), sphingolipids, and steroids.
Phosphoglycerides contain two fatty acyl chains esterified with glycerol-3-P and a polar head group
attached to the P group; nature of the polar group decides the classification. For eg. In
Phosphatidylcholine head group consists of choline, a positively charged alcohol, esterified to the
negatively charged phosphate.
Esterificn of C1 and
C2 hydroxylswith fatty acids and
C3 hydroxyl with
phosphate
Phosphate esterified to an
alcohol of a polar headgrp: choline, Inositol,
ethanolamine, serine
X=OR
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Sphingolipids andCholesterol are other two classes of membrane lipids
Sphingolipids are derivatives
of sphingosine (red), anamino alcohol with a long
hydrocarbon chain.
Various fatty acyl chains are
connected to sphingosine byan amide bond to yield
ceramide inolved in forming
complex sphingolipids.
The sphingomyelins (SM),which contain a phospho-
choline or phospho-
ethanolamine head group.
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Cholesterol.Like other membrane lipids, the steroid
cholesterol is amphipathic. Its single
hydroxyl group is equivalent to the polar
head group in other lipids; the
conjugated ring and short hydrocarbon
chain form the hydrophobic tail.
Other sphingolipids are glycosphingolipids
with single sugar residue or branched
oligosaccharide attached to the sphingosine
backbone. eg A cerebroside is a
sphingolipid (ceramide) with a
monosaccharide such as glucose or
galactose as polar head group.
A ganglioside is a ceramide with a polar
head group that is a complexoligosaccharide, including the acidic sugar
derivative sialic acid
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In aqueous solution, when lipid molecules are surrounded on all sides by
water, they tend to aggregate so that their hydrophobic tails are buried in the
interior and their hydrophilic heads are exposed to water.
Depending on their shape, they can do this in either of two ways: they form
spherical micelles, with the tails inward, or they can form bimolecular sheets,
or bilayers , with the hydrophobic tails sandwiched between the hydrophilic
head groups.
The hydrophobic effect and van der Waalsinteractions, cause the tail groups to self-
associate into a bilayer with the polar head
groups oriented toward water.
Because of their cylindrical shape,
membrane phospholipid moleculesspontaneously form bilayers in aqueous
environments and tend to seal on
themselves to form compartments.
This is one property that makes it an
ideal structure to form membranes.
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Liposomes: artificial micelles ofphospholipids