The cell membrane

BIOMEDICAL IMPORTANCEPermit cellular individuality,form close

compartments.Plasma membrane has selective

permeabilities and act as a barrier,thereby maintainig differences in the composition between the inside and outside of the cell.

Membranes establish and maintain the concentration of electrolytes,nonelectrolytes and water between the exterior and interior of the cell.

Exchange mateerials with the extracellular environment,exocytosis and endocytosis

Signal transduction. Membranes form specialized

compartments(organelles) in the cell.Membranes localize enzymes.Membranes possess antigenic properties.

CHEMICAL COMPOSITION OF MEMBRANELIPIDS Phospholipids Glycolipids CholesterolThese lipids are amphipathic in nature,having

polar and non polar regions.Lipid content and composition of various

membranes is different.

PROTEINS Transmembrane,integeral,peripheral proteins Glycoproteins. Cytoskeletal filamentsCarbohydrate chains

Structure of the Plasma Membrane

PhospholipidsMajpr lipids in the

membrane(50-90%).Mainly lecithin and

cephalinForms the lipid

bilayer structure

GLYCOLIPIDSLeast abundant(5%),Sphingophospholipids

and Glycosphingolipids Cerebrosides,contain only one sugar

residue(glucose or galactose) Gangliosides,contain a branch chain of as

many seven residues.

CHOLESTERL2-25% of the membrane.more concentrated

in the outer leaflet of the plasma membrane.It regulates the membrane fluidity.

CARBOHYDRATES.These are either in the form of glycolipids or

glycoproteins.External to the plasma membrane, all animal

cells have glycocalyx. This consists of the carbohydrate moieties of membrane glycolipids and glycoproteins.

Functions(Glycocalyx)In digestive tractProtectionImmunity to infectionDefense against cancerTransplant compatibilityCell adhesionFertilization: Enables sperm to recognize and

bind to eggsEmbryonic development

PROTEINS Proteins may be globular or fibrous in nature.Integral proteins( may be fibrous) are

themselves amphipathic in nature,and are bound tightly to the membrane.They generally span the entire lipid bilayer.

Peripheral proteins(globular) are weakly and reversibly bound to the hydrophilic regions of the specific integral proteins.

e.g Ankyrin(peripheral protein) is bound to band 3 protein(integral) in the RBCs membrane.

Glycophorin(rich in Sialic acid) is peripheral protein in human erytherocytes.

Integral proteinsor transmembrane proteins

Span the membrane and have a hydrophilic cytosolic domain, which interacts with internal molecules, a hydrophobic membrane-spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules. The hydrophobic domain consists of one, multiple, or a combination of α-helices and β sheet protein motifs.

Ion channels, proton pumps, G protein-coupled receptor

Lipid anchored proteinsCovalently-bound to single or multiple lipid

molecules; hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane.

G proteins

Peripheral proteinsAttached to integral membrane proteins, or

associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and, once reacted the molecule, dissociates to carry on its work in the cytoplasm.

Some enzymes, some hormones

) Transport Proteins2) Receptor Proteins3) Enzymatic Proteins4) Cell Recognition Proteins5) Attachment Proteins 6) Intercellular Junction Proteins

1) Transport ProteinsChannel 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

2) Receptor Proteins– Bind to chemical

messengers (Ex. hormones) which sends a message into the cell causing cellular reaction

3) Enzymatic Proteins– Carry out enzymatic

reactions right at the membrane when a substrate binds to the active site

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

5) Attachment Proteins- Attach to cytoskeleton (to

maintain cell shape and stabilize proteins) and/or the extracellular matrix (integrins connect to both).

6) Intercellular Junction Proteins– Bind cells together

Tight junctionsGap junctions

• Different membranes have different protein/lipid ratio (3.6 for mitochondria, 1.1 for erythrocytes plasma membrane, 0.25 for myelin)

• This property can be used to isolate various membrane fractions.

The type of the membranes is identified by the presence of specific protein markers.

For example: Na,K-ATPase is a marker of plasma membrane, while succinate dehydrogenase ,ATP synthetase is a mitochondria marker,Glucose 6 phosphatase is a marker for ER.

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FLUIDFLUID- because individual phospholipids and - because individual phospholipids and proteins can move side-to-side within the proteins can move side-to-side within the layer, like it’s a liquid.layer, like it’s a liquid.

MOSAICMOSAIC- - because of the pattern produced by because of the pattern produced by the scattered protein molecules when the the scattered protein molecules when the membrane is viewed from above.membrane is viewed from above.

FLUID MOSAIC MODELFLUID MOSAIC MODEL

copyright cmassengale

http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/LIPIDS/diff.html

Membranes are fluid structures

Movement can be: Lateral Flip flop

Membrane fluidity is important for:

•fusion of membranes, i.e. fusion of vesicles with organelles

•diffusion of new lipids and new proteins laterally, so they are equally distributed

•diffusion of proteins and other molecules laterally across the membrane in signaling/reactions

•proper separation of membranes during cell division

Factors that affect membrane fluidity:

Length of the fatty acyl chains

Degree of saturation of the fatty acyl chains

Presence of cholesterol Cholesterol affects fluidity

ROLE OF MEMBRANES IN DISEASE.AchondroplasiaFamilial hypercholesterolemiaCystic fibrosisWilson’s diseaseHereditary spherocytosisParoxysmal nocturnal

hemoglobinuria etc.