chapter 8 cell membrane

8/18/2019 Chapter 8 Cell Membrane

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Gaticales CMB Reviewer

Chapter 8: Cellular Membranes

Membrane Functions

1. Compartmentalization

• continuous unbroken sheets, enclosecompartments

• enclose entire cell

• allow specialized activities to occur withoutexternal interference

• regulate cellular activities

2. Scaffold for Biochemical Activities

• compartment themselves

• reactants present, relative position notstabilized; interaction dependent on random

collisions

• membrane provide cell with extensiveframework or scaffolding -> use for effectiveinteraction

3. Selectively permeable barrier

• restrict exchange of molecules from oneside to other

• means of communication bet. compartmentsthey separate

• like moat around castle with gated “bridges”promote movement of select elements in/outof cell

4. Transport Solutes

• machinery for physical transport ingsubstances

• from region of low concentration of solute toregion of higher concentration

• accumulate substances (sugars, aminoacids, macromolecules)

• transport specific ions -> ionic gradients

5. Respond to External stimuli

• signal transduction

• receptors combine with ligands (specificmolecules) or other stimuli (light, mechanicaltension)

• diff cells with diff receptors -> capable ofrecognizing/ respond to diff. environmentalstimuli

• interaction of membrane and ext. stimuli ->generate signal stimulates or inhibits internal

activity

6. Intercellular interaction

• mediate interaction between a cell andneighbor

• recognize to adhere and to exchangematerials and information

• proteins may facilitate interaction bet.extracellular and intracellular cytoskeleton

7. Energy Transduction

• energy transduction

• photosynthesis

• light absorbed by membraned boundpigments-> chemical energy stored incarbohydrates

• transfer of chemical energy to ATP

• in mitochondria/chloroplasts

Brief History of Plasma Membranes

- Ernst Overton (1890)• non polar solutes readily dissolve in

nonpolar solvents

• test permeability -> more lipid-soluble solute,more rapid entrance the root hair cells

- E. Gorter and F. Grendel (1925) proposed thatcell membrane contain lipid bilayer

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• extract lipid from RBC measure ant ofsurface area of lipid

• plasma membrane only lipid containingstructure

• contained bimolecular layer of lipid -> lipidbilayer

- polar head group face cytoplasm

- hyrdrophobic fatty acyl chains areprotected from contact with H2O

- decrease in surface tension -> presence ofPROTEINS

• Hugh Davson & James Danielli (1935)->plasma membrane lipid bilayer + lined byglobular proteins; (1950) revised: has outer

and inner protein layers and proteinpenetrate the membrane (provide polarsolutes and ions entrance to cell)

- Jonathan Singer & Garth Nicolson(1972) Fluid-mosaic model

• “central dogma”

• focused on physical state of lipid

• presented in fluid state; lipid molecules movelaterally within plane of membrane

• “mosaic” of discontinuous particles penetratelipid sheet

• dynamic structures-> mobile and can engagein va r ious t ypes o f t r ans ien t o r

semipermanent interactions

Chemical Composition of Membranes

- held together by non-covalent bonds

- lipid bilayer (structural backbone, preventrandom movement of water-soluble materials)

- protein (specific functions)

- ratio of lipid-protein depends on type of cellularmembrane, organism, cell; in other words,depends on the function of membrane in

certain organelle

• mitochondrial membrane - high protein/lipidratio for protein carriers of ETC

• myelin sheath- low concentration of protein,more lipid for electrical insulation

MEMBRANE LIPIDS

- amphipathic both hydrophilic and hydrophobic

- 3 types

1. Phosphoglycerides

• phospholipids (contain phosphate grp)

• b u i l t w i t h g l y c e r o l b a c k b o n e(phosphoglycerides)

• diglycerides (2 hydroxyl grps of glycerolesterified to fatty acid; 3rd is esterfied tohydrophilic P group) -> phosphatidic

acid

• a d d i t i o n a l g r o u p : c h o l i n e (phosphatidylcholine, PC), ethanolamine(PE), serine (PS), inositol (PI)

• small and hydrophilic + negativecharge P = highly water-soluble at oneend (head group)

• @ Physiological pH PS and PI (-); PC andPE (neutral)

• fatty acyl chains are hydrophobic,

unbranched hydrocarbons (16-22carbons)

• membrane FA may be fully saturated (nodouble bonds); monounsaturated (one

double bond); polyunsaturated (>1double)

• contain 1 unsaturated and 1 saturatedfatty acyl chain

• EPA and DHA (2 highly unsat. FA) contain5and 6 double bonds in PE and PC (brainor retina); omega-3 FA

2. Sphingolipids

• less abundant membrane lipid

• sphingosine - amino alcohol with longHC chain

• sphingosine + FA by its amino group -ceramide

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• sphingomyelin (phosphoryl choline)

• glycolipid - substitiution is a carbohydrate

• simple sugar -> cerebroside

• includes silica acid ->ganglioside

• amphiphatic

• FA chains are longer and highly saturated

• glycolipids- Nervous system, myelin sheath(galactocerebroside)

3. Cholesterol

• 50% of lipid molecules

• plant cells contain cholesterol like sterols

• small hydrophilic hydroxyl grip towardmembrane surface

• hydrophobic rings are flat and rigid ->interfere with FA tails of phospholipids

NATURE IMPORTANCE OF LIPID BILAYER

- important effects on biological properties ofmembrane

• composition determine physical state ofmembrane; influence activity of mem.

proteins

• provide precursors for highly active chemicalmessengers regulate cellular function

• flexibility -> membranes are deformable,shape changee (locomotion, cell div)

• facilitate regulated fusion or budding

• maintain proper internal composition,separate charges

• self assemble (liposomes)

ASYMMETRY OF MEMBRANE LIPIDS

- lipid bilayer has 2 distinct leafletswith differentlipid composition

• lipid digesting enzyme cannot fully penetrate

• bilayer contains 2 more/less stableindependent monolayers with diff physical

and chemical properties

• glycolipids -> outer leaflet serve as receptors

• PE -> inner leaflet for curvature (for budding/ fusion)

• PS -> inner “ binding (+) lysine/arginine

• PI -> inner “ phosphorylated on inositol ring -

> phosphoinositides (role in transfer ofstimuli)

MEMBRANE CARBOHYDRATES

- 2-10% by weight (90%) covalently linked toproteins -> glycoproteins; (10%) linked to

lipids -> glycolipids

- glycosylation - addition of carbohydrates mostcomplex modifications

- glycoprotein carbohydrate - short branchedhydrophilic oligosaccharides (15 sugars/

chain)

- carbohydrate projection -> mediate interactionof cell with environment; sorting of membraneproteins to diff cellular compartments

• glycolipids of RBC determine A, B, AB, O

Structure and Function of Membrane

Proteins

- grouped into 3 distinct classes:

1. Intergral proteins

• transmembrane protein; penetrate lipid

bilayer

2. Peripheral Proteins

• entirely outside; either cytoplasmic orextracellular by noncovalent bonds

3. Lipid-anchored proteins

• outside lipid bilayer but are covalently linked to lipid molecule within bilayer

INTERGRAL MEMBRANE PROTEINS

- receptors, channels or transporters, agents thattransfer electrons (photosynthesis/ respiration)

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- amphipathic

- amino acid residues in transmembranedomains form van der Waals with fatty acyl

chains -> seal protein and anchor within bilayer

- DISTRIBUTION OF INTEGRAL PROTEINS:Freeze- Fracture Analysis

• freeze-fracture replication

- structure and properties

• difficult to isolate in soluble form

• use of detergent (SDS -> denature protein;Triton X-100 -> tertiary structure)

PERIPHERAL MEMBRANE PROTEINS

- weak electrostatic bonds

- solubilized by extraction with high conc salsolutions (weaken bond)

- best studied are cytosolic peripheral proteins ->form fibrillar network as “skeleton”

- mechanical support; anchor for integral proteins

- function as enzyme, specialized coats, transmittransmembrane signals

LIPID-ANCHORED MEMBRANE PROTEINS

- smal l , complex o l igosacch l inked tophosphatidylinositol in outer leaflet

- G P I - a n c h o r e d p r o t e i n s - g l y c o s y l -phosphatidylinositol linkagecontained byperipheral proteins

- released by phospholipase (cleave inositolphospholipid)

Membrane Lipids and membrane fluidity

- physical state described by fluidity (viscosity)

- temperature- at which change occur

• Saturated FA

- straight flexible rod

- more compact

• Cis - unsaturated FA

- crooks in chain where there are doublebonds

• greater degree of unsaturation: lowertemperature before bilayer gels

- fatty acid chain length• shorter: lower melting temperature

- by cholesterol

• disrupts close packing of fatty

• abolish sharp transition temperatures;regulate intermediate fluidity

• increase durability while decreasingpermeability of membrane

IMPORTANCE OF MEMBRANE FLUIDITY

- perfect compromise between rigid, orderedstructure (mobility is absent) and completely

fluid (mobility present)

- allow interaction; assemble proteins atparticular sites to form specialized structures(junctions, light capturing, synapses)

- membrane growth accomplished by insertion oflipids and proteins into fluid matrix

- allow cell movement, growth, division,formation of intercellular junctions, secretion,

endocytosis

-

MAINTAIN MEMBRANE FLUIDITY

- lower temperature, response is mediated byenzymes that remodel membranes make itcold resistant

• desaturating single bonds; form doublebonds (by desaturases - enzymes catalyzesingle to double bond)

• r e s h u f fl i n g c h a i n s b e t . d i f f e r e n tphospholipids to form 2 unsaturated FA(lower temp) (by phospholipases - split FAfrom glycerol backbone; acyltransferases -transfer FA)

Lipid Rafts

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- distinctive physical properties, micro domainstend to float within the more fluid and

disordered environment of artificial bilayer

Dynamic Nature of Plasma Membrane

- Diffusion of Membrane Proteins after CellFusion

• cell fusion- technique 2 different types ofcells fused to produce one cell with commoncytoplasm and single continuous plasma

membrane

• fuse by making surface “sticky” for adhesion;by addition of inactivated viruses attach tosurface; adding polyethylene glycol, or mild

electric shock

- Restrictions on Protein and Lipid Mobility

• fl u o r e s c e n c e r e c o v e r y a f t e rphotobleaching (FRAP)

- if proteins are mobile; random movementproduce gradua l appearance o ffluorescence in irradiated circle

• single-particle tracking (SPT)

• antibody coated gold particles

- Control of Membrane Mobility

- Membrane Lipid Mobility

• lipid diffuse freely before jumping toneighboring compartment

Movement o f Subs tances across ce l lmembranes

- net flux indicate movement of substance influxand efflux not balanced

- Diffusion

• substance move from region of higherconcentration to lower to regulate overall

concentration

• depends on random thermal motion ofsolutes; exergonic with increase in entropy

• electrochemical gradient

- solute is an electrolyte (charged)

- charges are opposite, moving process isfavorable

- tendency of electrolyte to diffuse dependson 2 gradients:

• chemical gradient - concentrationdifference

• electric potential gradient- difference incharge (ex: K+ higher concentration

inside cell)

- Diffusion of Substances through membranes

- Diffusion of Water through membrane

• semipermeable

• osmosis- lower solute concentration tohigher solute concentration

• hypertonic - compartment with highsolute concentration; shrink; plasmolysis

• hypotonic- compartment with low soluteconcentration; swell;

- Diffusion of Ions through Membranes

• highly impermeable to charged substance

(Na, K, Ca, Cl)

• conductance- rapid movement play role information of nerve impulse, secretion of

substances, muscle contraction, regulationof cell volume, opening of stomatal pores

• Ion channels - permeable to certain ions

1. voltage-gated- difference in ioniccharge on two sides of membrane

2. Ligand-gated channels- binding ofs p e c i fi c m o l e c u l e ( e x .

neurotransmitters)3. M e c h a n o - g a t e d c h a n n e l s -

mechanical forces(stretch tension)

applied

- KcsA (bacterial K+ ion channel

• Facilitated Diffusion

• Glucose transporter

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- phosphorylating sugar after enteringcytoplasm, lowering intracellular

glucose conc.

- GLUT1 to GLUT5 (isoforms)

- Insulin secretion increase glucosetransporters

• Active Transport

- input of energy

Membrane Potentials and Nerve Impulses

- irritability

- nerve cells (neurons) specialized for collection,conduction, transmission of information - formof fast moving electrical impulses

- cell body,dendrites, axon, myelin sheath

- Resting potential

• membrane potential

• resting potential

• action potential -

- Neurotransmission

• synapses- link between neurons

• synaptic cleft - narrow gap in synapse

• presynaptic cell(receptor) conduct impulsetowards synapse

• postsynaptic cell (neuron, muscle, gland cell)lie on receiving side of synapse

• neuromuscular junction- axon and skeletalmuscle ell

• synaptic vesicles

• neurotransmitters

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chapter 8 cell membrane

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