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The Cell Surface and the Extracellular Matrix

Membranes: Their Structure,Function and Chemistry

The Functions of Membranes

1. Membranes define boundaries and serve as permeability barriersa. Plasma(cell) membraneb. Intracellular membranes

2. Membranes are sites of specificfunctions

3. Membranes regulate the transport of solutes

4. Membranes detect and transmit electrical and chemical signals

5. Membranes mediate cell-to-cell communication

Overview of Membrane Function

1. Compartmentalization2. Scaffold for biochemical activities3. Providing a selectively permeable barrier4. Transporting solutes5. Responding to external signalsThe plasma membrane plays a critical role in the response of a cell to external stimuli, a process known as signal transduction.Membranes possess receptors that combine with specific molecules(or ligands) having a complementary structure.6. Intercellular Interaction7. Energy transduction

Models of Membrane Structurea) Lipid Nature of membrane

b) Lipid monolayer

c) Lipid bilayer

d) Lipid bilayer plus proteinlamellae

e) Unit membrane

f) Fluid-Mosaic model

g) Membrane protein structure alpha helix1880190019201940196019802000

Cell Membrane

-separate the interior of the cell from its environment-structure & function of cells are critically dependent on membranes-formation of biological membranes is based on the properties of lipids-all CMs share a common structural organization: bilayers of phospholipids with associated proteins-exhibits a fluid-mosaic model (by Singer and Nicholson)-semi-permeable

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Membrane Lipids PHOSPHOLIPIDS -fundamental building blocks of all CM -amphipathic molecules: a) 2 hydrophobic fatty acidchains linked to a b) phosphate contg hydrophilic head group- form bilayers in aqueous soln which forms stable barrier

Plasma membrane- 50% lipid and 50% proteinInner membrane of mitochondria- about 75% protein (reflecting theabundance of protein complexes involved in electrontransport and oxidative phosphorylation)

Plasma membrane of E. coli- predominantly phosphatidylethanolamine (80% of total lipid)

Mammalian PM- more complex contg 4 major phospholipids: phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine & sphingomyelin(together constitute 50-60% of total membrane lipid) - in addition to phospholipids, PMs of animal cells contain glycolipids & cholesterol

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Membrane LipidsThere are three main types ofmembrane lipids: phosphoglycerides, sphingolipids, andcholesterol.

The chemical structure of membrane lipids. (a) Thestructures of phosphoglycerides (see also Figure 2.22). (b) Thestructures of sphingolipids. Sphingomyelin is a phospholipid; cerebrosidesand gangliosides are glycolipids.

Sphingolipids-a less abundant class of membrane lipids-are derivatives of sphingosine, an amino alcohol that contains a long hydrocarbon chainceramide-Sphingolipids consist of sphingosine linked to a fatty acid (R of Figure 4.6b) by its amino groupsphingomyelin-the only phospholipid of the membrane that is not built with a glycerol backbone-the substitution is phosphorylcholine

glycolipid-if the substitution is a carbohydrateif the carbohydrate is a simple sugar, the glycolipid is called a cerebroside; if it is an oligosaccharide, the glycolipid is called a ganglioside.-interesting membrane components though relatively little is known about them but they play crucial roles in cell function. -HOW? -the nervous system is particularly rich inglycolipids. The myelin sheath contains a high content of a particular glycolipid, called galactocerebroside, which isformed when a galactose is added to ceramide*Mice lacking the enzyme that carries out this reaction exhibit severe muscular tremors and eventual paralysis.

-a group of fungal toxins, called fumonisins, act by inhibitingsynthesis of these membrane components.-Fumonisins disturb such diverse processes as cellgrowth, cell death, cellcell interactions, and communicationfrom the outside of the cell to the interior.-Glycolipids also play a role in certain infectious diseases*toxins that cause cholera and botulism both enter their target cell by first binding to cell-surface gangliosides, as does the influenza virus.

Cholesterol

-another lipid component of certain membranesis the sterol cholesterol which in certain animal cells may constitute up to 50 percent of the lipid molecules in the plasma membrane. -absent from the plasma membranes of most plant and all bacterial cells. smaller than the other lipids of the membrane and less amphipathicmolecules are oriented with their small hydrophilic hydroxyl group toward the membrane surface and the remainder of the molecule embedded in the lipid bilayer the hydrophobic rings of a cholesterol molecule are flat and rigid, and they interfere with the movements of the fatty acid tails of the phospholipids

Plasma membraneLipidE. coliErthrocyteRough endoplasmic reticulumOuter mitochondrial membranesPhosphatidylcholine0175550Phosphatidylserine0632Phosphatidylethanolamine80161623Sphingomyelin01735Glycolipids0200Cholesterol045645oC) possible reasons: nerve CMs become so leaky to ions thus ion gradients cant be maintained and overall nervous function is disabled

e.g. homeotherm or warm-blooded organism- effects on humans during chilly days, fingers and toes get so cold that the membranes of sensorynerve endings cease to function, resulting in temporary numbness

How to regulate or compensate T changes?- by changing lipid composition of their membranes thru Homeoviscous adaptation ( in poikilotherms)-the main effect of this regulation is to keep the viscosity of the membrane approximatelythe same despite the changes in TExample:1. Micrococcus (transferred from high T to low T results to an increase in the proportion of 16-C rather than 18-C fain the PM thus minimizing effect of the low T.

*shorter fa chains decrease the melting T of a membrane2. E. coli (alteration in the extent of unsaturation ofmembrane fa rather than in length)-low T triggers synthesis of desaturase E that introduces double bonds into the HC chains of fa.

- HVA also occurs in yeasts, in plants (membrane fluiditydepend on the increased solubility of oxygen in the cyto-plasm at lower T)Oxygen- substrate of desaturase ETherefore: more Oxygen available at low T, more unsaturated fa synthesized at rapid rate and membrane fluidity increases

Amphibians and reptiles adapt to lower T by increasing proportion of unsaturated fa in their membrane aswell as cholesterol

Mammals or animals entering hibernation, the body T drops substantially but adapts to this change by incorporating a greater proportion of unsaturated fa into membrane phospholipids as its body T falls.

Membrane Proteins: The Mosaic Part of the Model

MEMBRANE PROTEINS-other major constituents of CM (25-75% of the mass of the various membranes of the cell)-carry out the specific functions of the different membranes of the cell some act as receptors that allow the cell to respond to external signalssome are responsible for the selective transport of molecules across the membraneothers participate in electron transport & oxidative phosphorylationcontrol the interactions between cells of multicellular organisms

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-divided into 3 general classes:(based on the nature of their attachment with the membrane)a. integral membrane proteins- embedded directly within the lipid bilayer(by the affinity of hydrophobic segments on the protein for the hydrophobic interior of the lipid bilayer)b. peripheral membrane proteins- not inserted into the lipid bilayer but are associated with the membrane indirectly, generally by inter-actions with integral proteins(hydrophilic, located on the surface of the membrane where they are linked noncovalently to the polar head groups ofphospholipids and/or to the hydrophilic parts of other membrane proteins)

c. Lipid-anchored proteins-though not a part of the original fluid mosaic model but are now included as a third class of membrane lipids. -essentially hydrophilic proteins and reside on membrane surfaces but they are covalently bound to lipid molecules that are embedded within bilayer

Integral monotopic proteins- appear to be embedded on only one of the bilayer

Singlepass proteins- transmembrane proteins that span the bilayer once

Multipass proteins - span the bilayer multiple times- may consist of either a1. single polypeptides2. several associated polypeptides (Multisubunitproteins)

Peripheral membrane proteins - too hydrophilic to penetrate into the membrane - attached to the membrane by electrostatic and H-bondsthat link them to adjacent membranes proteins or to phospholipid headgroups

Lipid-anchored membrane proteins-covalently bound to lipid molecules that are embeddedIn the lipid bilayer1. Fatty-acid or prenyl group proteins on the inner surface of the membrane 2. Glycosylphosphatidylinositol (GPI)-most common lipid anchor-proteins on the outer membrane surface

Figure 2.48. Fluid mosaic model of membrane structure

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Transport Across Membranes:Overcoming the Permeability Barrier

AquaporinTransport Processes Within a composite Eukaryotic cell

Important Transport Processes of the Erythrocyte

Overview of Membrane Transport ProteinsThree Major classes of membrane transport proteins1. ATP-powered pumps (simply pumps) ATPases that use the energy of ATP hydrolysis to move ions and small molecules across a membrane against a chemical concentration gradient or electric potentialfxns: 1.maintain the low Ca+ and Na+ ion concns inside all animal cells relative to that in the medium 2. generate the low pH inside animal-cell lysosomes , plant-cell vacuoles and the lumen of the stomach 2. Channel proteins transport water or specific types of ions down their concn or electric potential gradients - form a protein lined passageway across the membrane through which multiple water molecules or ions move simultaneously single file at every rapid rate (up to 108 per second.)

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-PM of all animal cells contains potassium-specific channel proteins that are generally open and are critical to generating the normal, resting electric potential across the PM-other types of channel proteins are usually closed and open only in response to specific signals.

3. Transporters- move a wide variety of ions and molecules across CM - bind only one (or a few) substrate molecules at a time - after binding substrate molecules, the transporter undergoes a conformational change such that the bound substrate molecules (and only these molecules) are transported across the membrane.

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3 types:1. Uniporters transport 1 molecule at a time down a concn gradient - moves glucose or amino acids across the plasma membrane into mammalian cells2. Antiporters and symporters-couple the movement of 1 type of ion or molecule against its concn gradient to the movement of a different ion or molecule down its concn gradient.

-catalyze uphill movement of certain molecules(active transporters) but unlike pumps, they do not hydrolyze ATP during transport.-also known as cotransporters (referring to their ability to transport two different solutes simultaneously).

Figure 15-3. Schematic diagrams illustrating action of membrane transport proteins.

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The Movement of Substances Across Cell MembranesPassive Transport- does not require energy (eg. Simple diffusion, osmosis & facilitated diffusion)- net movement of molecules & ions across a membrane from higher to lower concentration (down a concentration gradient)

2. Active Transport- requires expenditure of metabolic energy (ATP) - involves carrier proteins - net movement across a membrane that occurs against a concentration gradient (to the region of higher concentration)

*both leads to the net flux of a particular ion or compoundNET FLUX indicates that the movement of the substances into the cell (influx) and out of the cell (efflux) is not balanced, but that one exceeds the other

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2 Categories of Transport 1. Non- carrier mediated does not require carrier proteins(simple diffusion) 2. Carrier-mediated requires specific carrier proteinsa. facilitated diffusion (uniport)b. active transport

Fig. 4 Basic mechanisms by which solute molecules move across membranes

Diffusion- spontaneous process in which a substance moves from a region of high concentration to a region of low concentration, eventually eliminating the concentration difference between the two regions.- molecules that are non-polar (lipid soluble) can easily pass thruone side of the membrane to the other (eg. O2 or steroidhormones - small molecules with polar covalent bonds but uncharged (eg. CO2, ethanol ).Two Qualifications must be met before a nonelectrolyte can diffuse passively across a membrane1. Substance must be present at high concentration on one side of the membrane than the other2. Membrane must be permeable to the substanceA membrane may be permeable to a given solute1. bec that solute can pass directly thru the lipid bilayer2. bec that solute can traverse an aqueous pore that spans the membrane and prevents the solute from coming into contact with the lipid molecules of the bilayer

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ExamplesFactorMore permeableLess PermeablePermeability Ratio*1. Size: bilayer more permeable to smaller moleculesH2O (water)H2N-CO-NH2 (urea)102:12. Polarity: bilayer more permeable to nonpolar moleculesCH3-CH2-CH2-OH(Propanol)HO-CH2-CHOH-CH2-OH (glycerol)103:1

3. Ionic: bilayer highly impermeable to ionsO2 (oxygen)OH- (Hydroxide ion)109:1

*Ratio of diffusion rate for the permeable solute to the less permeable soluteTable 2. Factors Governing Diffusion Across Lipid Bilayers

THE DIFFUSION OF WATER THRU MEMBRANES-water molecules move much more rapidly thru a CM than do dissolved ions or small polar organic solutes (CM is said to be semi-permeable)OSMOSIS- the process where water moves readily thru a semi-permeablefrom a region of lower solute concn ( water concn) to a regionof higher solute concn ( water concn).Two Requirements for Osmosis:1. there must be a difference in the concn of a solute on the twosides of a selectively permeable membrane2. the membrane must be relatively impermeable to the solute*OSMOTICALLY ACTIVE solutes that cannot freely pass thru membrane

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Transport mechanisms

Osmotic pressure is defined as the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different compositions

Figure 5. Experimental system for demonstrating osmotic pressure. Solutions A and B are separated by a membrane that is permeable to water but impermeable to all solutes. If CB (the total concentration of solutes in solution B) is greater than CA, water will tend to flow across the membrane from solution A to solution B. The osmotic pressure p between the solutions is the hydrostatic pressure that would have to be applied to solution B to prevent this water flow. From the van't Hoff equation, p=RT(CBCA).

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Different Cells Have Various Mechanisms for Controlling Cell Volume

Figure 6-A. Response of animal cells to the osmotic strength of the surrounding

Figure 6-B. The contractile vacuole in Paramecium caudatum, a typical ciliated protozoan, as revealed by Nomarski microscopy of a live organism.

Figure 6-C.Water relations in a plant cell.PlasmolysisTurgidity

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Water Channels Are Necessary for Bulk Flow of Water across Cell MembranesAQUAPORINS- class of integral proteins that allow the passive movement of H2O from one side of the PM to the other - special water channels that allow water to move more rapidly -in its functional form, aquaporin is a tetramer of identical 28-kDa subunits, each of which contains six transmembrane helices that form three pairs of homologs in an unusual orientation (Fig. 7A) -the channel through which water moves is thought to be lined by eight transmembrane helices, two from each subunit (Fig. 7B) -Billions of water molecules-moving in single file- can passthru each channel every secondOsmosis- important factor in a multitude of bodily functions (eg. digestive tract secretes several liters of fluid daily which is reabsorbed osmotically by the cells that line the intestineConsequence: if fluid werent reabsorbed (in cases of extreme diarrhea) rapid dehydration occurs

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-expressed in abundance in plant roots, erythrocytes and in other cells (e.g., the kidney cells that resorb water from the urine) that exhibit high permeability for water.the hormone VASOPRESSIN (stimulates H2O retention by the collecting ductsof the kidney acts by way of these channels). -some cases of the inherited disorder CONGENITAL NEPHROGENICDIABETES INSIPIDUS have been traced to mutations in this aquaporinchannel. (persons suffering from this disease excrete huge quantitiesof urine bec their kidneys do not respond to vasopressin)Rate of Diffusion depends on:1. the magnitude of the concentration difference across the membrane2. permeability of the membrane to the diffusing substances 3. the temperature of the solution4. the surface area of the membrane thru which substances are diffusing

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Channel proteins-facilitate diffusion by forming hydrophilic trans-membrane channels

3 Kinds:1. Ion channels- transmembrane proteins that allow rapid passage of specific ions (remarkably selective)- single channel can conduct almost a million ions per second!-most ion channels are gated (opened and closedby conformational changes in the protein regulating theflow of ions thru the channel)3 gated channels: 1. Voltage-gated = open and close in responseto changes in membrane potential 2. Ligand-gated = triggered by the binding ofspecific substances to the channel protein

3. Mechanosensitive = respond to mechanical forces that act on membrane2. Porins- transmembrane proteins that allow rapid passageof various solutes -pores found in the outer membranes of mitochondria,chloroplasts and bacteria-larger & much less specific-formed by multipass transmembrane proteins-made of closed cylindrical sheet called barrel -inside pore (water-filled) is lined by polar chains while outside that of nonpolar side chains-pore allows passage of various hydrophilic solutes with size depending on the pore size of the particular porin

3. Aquaporins (AQPs)-transmembrane that allow rapid passage of water

Active Transport

-ATP-powered pumps that transport ions and various small molecules against their concn gradient

Three Major functions in cells and organelles:

It makes possible the uptake of essential nutrientsfrom the environment or surrounding fluid,even when the their concns in the environmentare much lower than inside the cell

it allows various substances (secretory prodts andwaste matls) to be removed from the cell or orga-nelle, even when the concn outside is > than theinside

it enables the cell to maintain constant, nonequi-librium intracellular concentrations of specific inorganic ions such a K+, Na+, Ca+ and H+

2 types:-based on the energy source1. Direct active transport-also called primary active transport -accumulation of solute molecules or ions on one side of the membrane coupled directlyto an exergonic reaction particularly hydrolysisof ATP.-transport proteins driven directly by ATP hydrolysis are called ATPases or ATPase pumps.

2. Indirect active transport-also called secondary active transport -depends on the cotransport of two solutes withthe movement of 1 solute down its gradient driving the movement of the other solute up itsgradient.

Direct Active Transport Depends on Four Types of TransportATPases-transport ATPases or pumps are responsible for mostdirect active transport in both prokaryotic and eukaryo-tic cells.1. P-type2. V-type3. F-type4. ABC-type

Figure 15-10. The four classes of ATP-powered transport proteins. P-class pumps are composed of two different polypeptides, and , and become phosphorylated as part of the transport cycle. The sequence around the phosphorylated residue, located in the larger subunits, is homologous among different pumps. F-class and V-class pumps do not form phosphoprotein intermediates. Their structures are similar and contain similar proteins, but none of their subunits are related to those of P-class pumps. All members of the large ABC superfamily of proteins contain four core domains: two transmembrane (T) domains and two cytosolic ATP-binding (A) domains that couple ATP hydrolysis to solute movement. These core domains are present as separate subunits in some ABC proteins (depicted here), but are fused into a single polypeptide in other ABC proteins. [Adapted from C. H. Higgins, 1995, Cell 82:693; P. Zhang et al., 1998, Nature 392:835; Y. Zhou, T. Duncan, and R. Cross, 1997, Proc. Nat'l. Acad. Sci. USA 94:10583; and T. Elston, H. Wang, and G. Oster, 1998, Nature 391:510.]

Solutes TransportedKind of MembraneKind of OrganismsFunction of ATPaseP-type ATPases (P for phosphorylation)Na+ and K+Plasma membraneAnimalsKeeps [Na+] low and [K+] high within cell; maintains membrane potentialH+Plasma membranePlants, fungiPumps protons out of cell; generates membrane potentialCa2+Plasma membraneEukaryotesPumps Ca2+ out of cell;keeps [Ca2+] low in cytosolV-type ATPases (V for vesicleH+Lysosomes;secre-tory vesiclesAnimalsKeep pH in organelle low, which activates hydrolytic enzymesH+Vacuolar membranePlants, fungiKeeps pH in vacuole low,which activates E

Table 2.1 Main Types of Transport ATPases (Pumps)

F-Type ATPases (F for factor); also called ATP synthasesH+Inner mitochondrial membraneEukaryotesGenerates H+ gradient that drives ATP synthesisH+Thylakoid membranePlantsGenerates H+ gradient that drives ATP synthesisH+Plasma membraneProkaryotesGenerates H+ gradient that drives ATP synthesisABC ATPases (ABC for ATP-binding cassette)A variety of solutes*Plasma membrane, Organellar membranesProkaryotes, eukaryotesNutrient uptake; protein export; possibly also transport into and out of organellesAntitumor drugs**Plasma membraneAnimal tumor cellsRemoves hydrophobic drugs(and hydrophobic natural prodts from cell)

*Solutes include ions, sugars, amino acids, carbohydrates, peptides and proteins**Drugs include colchicine, taxol, vinblastine, actinomycin D, and puromycine

Transport mechanisms

Active Transport

Figure 15-13. Models for the structure and function of the Na+/K+ ATPase in the plasma membrane. (a) This P-class pump comprises two copies each of a small glycosylated subunit and a large subunit, which performs ion transport. Hydrolysis of one molecule of ATP to ADP and Pi is coupled to export of three Na+ ions (blue circles) and import of two K+ ions (dark red triangles) against their concentration gradients (large triangles). It is not known whether only one subunit, or both, in a single ATPase molecule transports ions. (b) Ion pumping by the Na+/K+ ATPase involves a high-energy acyl phosphate intermediate (E1~P) and conformational changes, similar to transport by the muscle Ca2+ ATPase. In this case, hydrolysis of the E2P intermediate powers transport of a second ion (K+) inward. Na+ ions are indicated by blue circles; K+ ions, by red triangles. See text for details. [Adapted from P. Luger, 1991, Electrogenic Ion Pumps, Sinauer Associates, p. 178.]Na+/K+ ATPase maintains the Intracellular Na+ & K+ concns in Animal Cells

Indirect Active Transport: Sodium Symport Drives the Uptake of Glucose

Figure 15-19. Proposed model for operation of the two-Na+/one-glucose symporter. The simultaneous binding of Na+ and glucose to sites on the exoplasmic surface induces a conformational change, generating a transmembrane pore or tunnel that allows both bound Na+ and glucose to move through the protein to binding sites on the cytosolic domain and then to pass into the cytosol. After this passage, the protein reverts to its original conformation. [See E. Wright, K. Hager, and E. Turk, 1992, Curr. Opin. Cell Biol. 4:696 for details on the structure and function of this and related transporters.]

Endocytosis

Exocytosis

Transport mechanisms

FIGURE 4.55 The sequence of events during synaptictransmission with acetylcholine as the neurotransmitter.

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