Chapter 7: Membrane Chapter 7: Membrane Structure & FunctionStructure & Function

Plasma membranePlasma membraneComposition:Composition: primarily lipids primarily lipids (phospholipids) & proteins with (phospholipids) & proteins with some carbohydrates (glycolipids or some carbohydrates (glycolipids or glycoproteins for cell recognition)glycoproteins for cell recognition)Arranged in a Arranged in a fluid mosaicfluid mosaic

Lipid bi-layer with embedded proteinsLipid bi-layer with embedded proteins

Discovery of plasma Discovery of plasma membrane structuremembrane structure

1915- Red blood cell membranes analyzed; 1915- Red blood cell membranes analyzed; lipid & protein composition discoveredlipid & protein composition discovered

1925- Gorter & Grendel suggest 1925- Gorter & Grendel suggest membrane is phospholipid bi-layermembrane is phospholipid bi-layer

1935- Davson & Danielli suggest proteins 1935- Davson & Danielli suggest proteins sandwich phospholipids (FALSE)sandwich phospholipids (FALSE)

1950s- Electron Microscopes used to study 1950s- Electron Microscopes used to study membrane structuremembrane structure

1972- Singer & Nicolson suggest proteins 1972- Singer & Nicolson suggest proteins are dispersed (“float”) within the lipid bi-are dispersed (“float”) within the lipid bi-layer (further shown by freeze-fracture layer (further shown by freeze-fracture electron microscopyelectron microscopy

Fluidity of membranesFluidity of membranes Membrane held together by weak Membrane held together by weak

hydrophobic interactionshydrophobic interactions; most ; most lipids lipids & some proteins can drift& some proteins can drift within their within their layer of the membranelayer of the membrane

Protein Protein movement/non-movementmovement/non-movement may be may be dependantdependant onon the proteins the proteins connectionconnection/lack of connection /lack of connection to the to the cytoskeletoncytoskeleton

Temperature affects level of fluidityTemperature affects level of fluidity Fluidity affects permeabilityFluidity affects permeability **cholesterol & unsaturated fats **cholesterol & unsaturated fats

increase fluidity (added to membrane increase fluidity (added to membrane to prep for cooler temps)to prep for cooler temps)

Concept CheckConcept Check

What would happen to the fluidity of What would happen to the fluidity of the membrane in the following the membrane in the following scenarios?scenarios? Increase in unsaturated phospholipids?Increase in unsaturated phospholipids? Increase in saturated phospholipids?Increase in saturated phospholipids? A decrease in temperature?A decrease in temperature? An increase in cholesterol levels?An increase in cholesterol levels?

Membrane proteinsMembrane proteins

Determine most of the membrane’s Determine most of the membrane’s specific functionsspecific functions

TypesTypes:: Integral proteins: penetrateIntegral proteins: penetrate through the through the

hydrophobic core of the hydrophobic core of the lipid bi-layerlipid bi-layer (transmembrane proteins)(transmembrane proteins)

Peripheral proteins: not embeddedPeripheral proteins: not embedded in bi- in bi-layer; layer; attached to the surfaceattached to the surface of the of the membranemembrane

Functions of membrane Functions of membrane proteinsproteins

TransportTransport Enzyme activityEnzyme activity Signal transductionSignal transduction Cell-cell recognitionCell-cell recognition Intercellular joiningIntercellular joining Attachment to the cytoskeleton & Attachment to the cytoskeleton &

extracellular matrixextracellular matrix (ECM) (ECM)

Carbohydrates & the Carbohydrates & the membranemembrane

Carbohydrates Carbohydrates in the membrane < in the membrane < 15 sugar units15 sugar units

Types:Types: GlycolipidsGlycolipids GlycoproteinsGlycoproteins

Function: cell-cell recognitionFunction: cell-cell recognition

Synthesis of membranesSynthesis of membranes See text book figure 7.10See text book figure 7.10

1. 1. synthesis of membrane proteins & lipids synthesis of membrane proteins & lipids in the in the ERER; ; Carbohydrate addedCarbohydrate added to make to make

glycoproteinsglycoproteins 2. Inside 2. Inside Golgi apparatus glycolipids are Golgi apparatus glycolipids are

made made and glycoproteins are modifiedand glycoproteins are modified 3. 3. Transmembrane proteins, glycolipids, & Transmembrane proteins, glycolipids, &

secretory proteins are transported in secretory proteins are transported in vessiclesvessicles

4. 4. Vessicles fuse with the membraneVessicles fuse with the membrane releasing releasing secretory proteins & secretory proteins & placing placing glycoproteins & glycoproteins & glycolipids on the glycolipids on the outside of the membraneoutside of the membrane

**Outside of plasma membrane is **Outside of plasma membrane is made from the inside of the ER, & made from the inside of the ER, & Golgi vessicle membranesGolgi vessicle membranes

(When vessicles formed in ER & Golgi (When vessicles formed in ER & Golgi fuse with membrane to release fuse with membrane to release material they become part of the material they become part of the membrane)membrane)

Concept CheckConcept Check

On which side of the membrane are On which side of the membrane are carbohydrates found? How is this carbohydrates found? How is this location useful to the carbohydrate location useful to the carbohydrate function in the membrane?function in the membrane?

Selective permeabilitySelective permeability

Fluid mosaic model explains how Fluid mosaic model explains how membrane can regulate passage of membrane can regulate passage of materials materials Hydrophobic (non-polar) molecules can Hydrophobic (non-polar) molecules can

diffuse through lipid bi-layer easilydiffuse through lipid bi-layer easily Polar molecules & ionsPolar molecules & ions which are which are

impeded by the lipid bi-layer impeded by the lipid bi-layer pass pass through through specificspecific transport proteins transport proteins

Passive TransportPassive Transport

No energyNo energy required required DiffusionDiffusion

Molecules will move from Molecules will move from high to low high to low concentrationconcentration

Diffusion of molecules is Diffusion of molecules is unaffected by unaffected by the concentration of the concentration of otherother substances substances

Rate is determined by membrane Rate is determined by membrane permeabilitypermeability to the molecule to the molecule

Passive Transport (cont’d.)Passive Transport (cont’d.)

OsmosisOsmosis Diffusion of waterDiffusion of water across a selectively across a selectively

permeable membranepermeable membrane Tonicity=ability of a solution toTonicity=ability of a solution to cause a cause a

cell to gain or lose watercell to gain or lose water isotonic: no net movement of waterisotonic: no net movement of water Hypertonic: net loss of waterHypertonic: net loss of water Hypotonic: net gain of waterHypotonic: net gain of water

Passive Transport (cont’d.)Passive Transport (cont’d.) Facilitated diffusionFacilitated diffusion

Movement of molecules down their Movement of molecules down their concentration gradient with the concentration gradient with the assistance of assistance of specificspecific transport proteins transport proteins in the membranein the membrane

Types of transport proteins:Types of transport proteins: Channel proteinsChannel proteins: “corridors” for : “corridors” for passage of passage of

specific ion or moleculespecific ion or molecule Aquaporins (waterAquaporins (water channel proteins) channel proteins) Ion channels/gated channels (electrical or Ion channels/gated channels (electrical or

chemical signal causes opening or closingchemical signal causes opening or closing Carrier proteins: change shape to Carrier proteins: change shape to

translocate substances across the translocate substances across the membranemembrane

Concept CheckConcept Check

What would happen to a Paramecium What would happen to a Paramecium that swam from a hypotonic that swam from a hypotonic environment to an isotonic one?environment to an isotonic one?

Why do water molecules need Why do water molecules need aquaporins to cross the membrane? aquaporins to cross the membrane? Why don’t substances like oxygen Why don’t substances like oxygen and carbon dioxide require transport and carbon dioxide require transport proteins?proteins?

Active TransportActive Transport

Molecules move against the Molecules move against the concentration gradient concentration gradient (low to high)(low to high)

Energy requiredEnergy required Uses carrier transport proteinsUses carrier transport proteins

Active transport: sodium-Active transport: sodium-potassium pumppotassium pump

Na+ in cell binds to proteinNa+ in cell binds to protein ATP binds to proteinATP binds to protein Protein changes shapeProtein changes shape Na+ moves out of cellNa+ moves out of cell K+ outside cell binds to proteinK+ outside cell binds to protein P from ATP is removed P from ATP is removed

(dephosphorylation)(dephosphorylation) Original protein shape is restoredOriginal protein shape is restored

Active transport: electrogenic Active transport: electrogenic pumppump

H+ pumped out through protein with H+ pumped out through protein with the help of ATPthe help of ATP

Outside cell becomes +, inside –Outside cell becomes +, inside – Charge difference across the Charge difference across the

membrane is used to do workmembrane is used to do work

Active transport: Active transport: CotransportCotransport

Same as electrogenic pump but…Same as electrogenic pump but…when H+ moves back into cell by when H+ moves back into cell by diffusion it carries another molecule diffusion it carries another molecule with it (i.e. sucrose)with it (i.e. sucrose)

Passive vs. Active TransportPassive vs. Active Transport

Concept CheckConcept Check Why is the sodium-potassium pump not considered a Why is the sodium-potassium pump not considered a

cotransporter?cotransporter?

Which solute(s) will exhibit net diffusion into the Which solute(s) will exhibit net diffusion into the cell?cell?

Which solute(s) will exhibit net diffusion into the Which solute(s) will exhibit net diffusion into the cell?cell?

Which solution “cell” or environment is hypertonic?Which solution “cell” or environment is hypertonic? In which direction will there be a net osmotic In which direction will there be a net osmotic

movement of water?movement of water? After the cell was placed in the beaker did it become After the cell was placed in the beaker did it become

for flaccid, or more turgid?for flaccid, or more turgid?

Bulk TransportBulk Transport

Exocytosis: cell secretes macromolecules Exocytosis: cell secretes macromolecules through the fusion of vesicles with the through the fusion of vesicles with the plasma membraneplasma membrane

Endocytosis: cell takes in macromolecules Endocytosis: cell takes in macromolecules by forming new vesicles from the plasma by forming new vesicles from the plasma membranemembrane Phagocytosis (“cellular eating”)Phagocytosis (“cellular eating”) Pinocytosis (non-specific “cellular drinking”)Pinocytosis (non-specific “cellular drinking”) Receptor-mediated endocytosis (specificReceptor-mediated endocytosis (specific uptake) uptake)

Concept CheckConcept Check

As a cell grows, its plasma membrane As a cell grows, its plasma membrane expands. Is this a result of exocytosis or expands. Is this a result of exocytosis or endocytosis? Explain. endocytosis? Explain.

After a neuron has been stimulated by After a neuron has been stimulated by neurotransmitters from a neighboring neurotransmitters from a neighboring neuron, the neuron takes in the neuron, the neuron takes in the neurotransmitters by endocytosis. Is it by neurotransmitters by endocytosis. Is it by pinocytosis or receptor-mediated pinocytosis or receptor-mediated endocytosis? Explain. endocytosis? Explain.