Plasma membrane structure   ,Fluid mosaic model of plasma membrane     طchemical composition      طFluidity of cell membrane     ط                                           D4 180 -196 

IntroductionThe cell membrane have a trilaminar appearance fig5.1, an outer dense line, a thicker inner line and spaceThe plasma (cellular) membrane function is to protects cell and adjust its shape.Its asymmetric structure with its chemical content control selectively transport of components from side to side.The intracellular membranes are usually thinner than the plasma membrane.Lipids and proteins are the major components of all membranes.fig5.2, protein from 20% in myelin up to 70% in inner mitochondrial membrane.Intracellular membrane have higher protein %, because of the enzymes activity.

Plasma Membrane Chemical Composition1.  Lipids, are the major component:  A.  Phospholipids, are most abundant lipids:

i) Glycerophospholipids (glycerol, phosphate, lipids), are most common lipids

    ii) Sphingolipids, amino alcohol:  B.  Cholesterol: 3rd major lipid in memb2.  Proteins:  A.  Inetgral (intrinsic):

i) Lipoproteins: present in many memb in particular myelin (50%)ii) Glycoproteins: contain carbohydrates

  B.  Peripheral (extrinsic):3.  Carbohydrates: Present a glycoprotein or as lesser amount as

glycolipids

Plasma Membrane Chemical Composition1.  Lipids, are the major component:  A.  Phospholipids, are most abundant lipids:

i) Glycerophospholipids (glycerol, phosphate, lipids), are most common lipids:    structure base is phosphatitic acid fig5.3,    amphypathic: polar head (ester) at C3     and hydrophobic tails (sat FA at C1 & unsat FA at C2) fig 5.4    Examples:· lecithin, cephalin fig5.6 (in most memb),· cardiolipin fig5.7 (inner mitoch memb)· inisitol fig5.8 (plasma memb)· plasmalogen fig5.10 has ether instead of sat FA at C1 (nervous tissue &

heart)    NOTE: FA content can very depending on physiological & pathological status of

tissue

Plasma Membrane Chemical Composition1.  Lipids, are the major component:  A.  Phospholipids, are most abundant lipids:

ii) Sphingolipids, amino alcohol:    structure base is amino alcohol (sphingosine & dihydroxysphingosine)

fig5.11,    sat FA or unsat FA on the amine at C2 and ester at C3 (ceramide) fig

5.12    Examples:·         Sphingomyeline fig5.13 (in most abundant sphingolipid, myeline),·         Glycosphingolipids fig5.14 (no phosphate with sugar attached at C1)    NOTE: Various substitution at C1

Plasma Membrane Chemical Composition1.  Lipids, are the major component:   B.  Cholesterol:

3rd major lipid in membHas polar hydroxyl group at C3 fig5.16Chol content is affected by diet

  Quantity of lipids in different cell membrane fig5.17:· Myelin memb is rich in sphingolipids (high proportion of glycosphingolipids)· Plasma memb has greatest variation because of chol (highest % in sphingolipids)· Intracellular memb primary contain glycerophospholipids (little sphingolipids & chol)o mitoch, nuclei & rough end reticulum memb are similaro golgi memb is in between other intracellular memb and plasma memb

·  phsphotidylcholine is predominant phospholipid·  phsphotidylethanolamine is 2nd major phospholipid·  cardiolipin found exclusively in inner mitoch memb

  Composition of various lipids in membrane indicates the relationship between lipid and membrane function

Plasma Membrane Chemical Composition2.  Proteins:-Have a role in transmembrane of molecules and as receptors for hormones -Proteins in neurons and RBCs memb have a role in maintenance of shape -High protein content in a membrane correlates with the complexity and variety of membrane function

Plasma Membrane Chemical Composition2.  Proteins:

A.  Inetgral (intrinsic): amino-terminal, transmembrane sequence, carboxyl-terminalContain tightly bound lipid (sequence of hydrophobic AAs)

i) Lipoproteins: present in many memb in particular myelin (50%)    Example: Lipophilin, in barin myelin

ii) Glycoproteins: contain carbohydrates Example fig5.22:a) Glycophoryn: transmembrane sequence (one loop across the membrane)    found in RBCs, contain 60% carbohydrates at extracellular side of membb) Anion Channel: transmembrane sequence (multi loop across the membrane)    found in RBCs, can anchor cytoskeleton and other cytosolic proteins    protein acts as enzyme (DHBDH) found in inner mitoch memb

Plasma Membrane Chemical Composition2.  Proteins:

B.  Peripheral (extrinsic):Protein location on the surface of the membrane Many are enzymes (water soluble) Example fig5.22: c) Ankyrin: bind to anion channel d) Electrostatin: Electrostatic Binding (– charge head of lipoproteins bind to + charged portion of protein) e) Cytochrome b5: found in endoplasmic reticulum, attach to hydrophobic sequence of AAs at one end f) Glycan: AAs bound covalently together and to membrane (contain carbohydrates)

  Although proteins are randomly distributed throughout the membrane, there is a high degree of functional organization on localization of some proteins.

Plasma Membrane Chemical Composition3.  Carbohydrates:Present a glycoprotein

or as lesser amount as glycolipids Sugars include: glucose, galactose, mannosem fructose, fig5.18 Carbohydrate is on the exterior side of plasma memb

or at luminar side of end reticulum Role includes: cell-cell recognition, adhesion and as participate in receptor action

Structure of Biological Membrane· All biological memb are arranged so the amphypathic lipid and chol are oriented so that hydrophobic tails interact and polar head is at the interface with aqueous environment.· Some proteins (intrinsic) immersed in the lipid bilayer (e.g. micelles), while others (extrinsic) are loosely attached to the surface of memb.·  Fluid Mosaic Model fig5.21, is to indicate the movement of both lipids and proteins in the memb.·  The lipid bilayer explains many cellular protpeties as fluidity, flexibility, ability to self=seal and impermeability.

Structure of Biological Membrane1. Distribution fig5.24:    The asymetric distribution of lipids across the memb is in contrast to the random distribution of lipids between outer and inner lipid monolayer    each layer of memb lipid bilayer has a different composition of glyceropholipids and sphingolipids    specific proteins promote transverse movement (transport across membrane) of specific lipid from one side to the other    specific mechanism maintain both composition and asymetry

Structure of Biological Membrane2. Fluidity:· Interaction among different lipids and between lipids and proteins are very

complex and dynamic· There is fluidity in lipid portion of memb, where both lipids and protein move· The degree of fluidity is dependent on:

1. temp and composition of memb (gel-liquid state), phase transition temp 2. Fluidity changes with diet or physiological state (FA, chol): • chol stiff structure reduces coiling of FA chain and decrease fluidity • cis double bond (in unsaturated FA) of phospholipids prevents tight packing

leading to pockets, which increase fluidity • Motion of hydrocarbon chain of lipids produce fluidity in the hydrophobic

core 3. Movement of integral protein is slower than lipids and depend on its

attachment to matrix proteins and other memb proteins 4. drugs (anesthetic) increase fluidity

Cell Membrane is constantly changing with movement of memb proteins and lipids, but also with molecules moving into and out of the memb