copy of cell biology 2

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

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