Chapter 8Chapter 8

Membrane Structure & FunctionMembrane Structure & Function

Membrane StructureMembrane Structure

Selective permeabilitySelective permeabilityControls trafficControls traffic

Known as the Known as the plasma membraneplasma membraneAmphipathicAmphipathic - hydrophobic & - hydrophobic &

hydrophilic regionshydrophilic regionsSinger-Nicolson developed the Singer-Nicolson developed the

fluid mosaic modelfluid mosaic model

Membranes are FluidMembranes are Fluid

Structures related to properties & Structures related to properties & functionfunction

Membrane is usually about as fluid as salad oil.

Fluid Mosaic ModelFluid Mosaic ModelLipidsLipids

PhospholipidsPhospholipids - - membrane membrane fluidityfluidityCholesterol Cholesterol - - membrane stabilizationmembrane stabilization

““Mosaic” Structure due to:Mosaic” Structure due to:Proteins:Proteins:

Integral proteinsIntegral proteins - - transmembrane proteins transmembrane proteinsPeripheral proteinsPeripheral proteins - - surface /appendages surface /appendages

Attachments-framework for animal cellsAttachments-framework for animal cells

Membrane carbohydratesMembrane carbohydrates - -~ cell to cell recognition; ~ cell to cell recognition; oligosaccharides (cell markers);oligosaccharides (cell markers); glycoproteins; glycolipids; ABO glycoproteins; glycolipids; ABO blood typingblood typing

Sidedness of Plasma MembranesSidedness of Plasma Membranes

Carbohydrates only on the outside surfaceCarbohydrates only on the outside surface

Proteins may be anchoredProteins may be anchored– Inside to cytoskeletonInside to cytoskeleton– Outside to the extracellular matrixOutside to the extracellular matrix

Molecules that start on the Molecules that start on the insidinside e face of Golgi Complex end up on the face of Golgi Complex end up on the

outsideoutside face of the plasma face of the plasma membranemembrane

Know positioning of these structures; polar Know positioning of these structures; polar and nonpolar regions of membraneand nonpolar regions of membrane

Membrane StructureMembrane StructureMembrane protein functions:Membrane protein functions: TransportTransportEnzymatic activityEnzymatic activitySignal transductionSignal transductionIntercellular joiningIntercellular joiningCell-cell recognitionCell-cell recognitionECM attachmentECM attachment

Traffic Across MembranesTraffic Across Membranes

EasilyEasily hydrophobichydrophobic

With AssistanceWith Assistance– Hydrophilc: Polar, chargedHydrophilc: Polar, charged

ChannelsChannels ShuttlesShuttles

TransportTransport PassivePassive

– No energy expenditureNo energy expenditure– Diffuse through membraneDiffuse through membrane– Diffuse aided by proteinDiffuse aided by protein

Facilitated diffusionFacilitated diffusion

ActiveActive– Energy expenditure—Energy expenditure—

ATPATP

– Usually against concentration GradientUsually against concentration Gradient– PumpsPumps

cotransportcotransport

– Bulk TransportBulk Transport EndocytosisEndocytosis ExocytosisExocytosis

Passive TransportPassive Transport diffusion of a substance across a biological diffusion of a substance across a biological

membranemembrane No energy exertedNo energy exerted

DiffusionDiffusion - - tendency of any molecule to tendency of any molecule to spread out into available space spread out into available space

Concentration gradientConcentration gradient – – moves from high to moves from high to lowlow

OsmosisOsmosis - the diffusion of water across - the diffusion of water across a selectively permeable membrane; a selectively permeable membrane; DOWN the concentration gradient.DOWN the concentration gradient.

Direction determined only by a Direction determined only by a difference in difference in total total solute concentrationsolute concentration

Rate influenced byRate influenced byTemperatureTemperatureSteepness of conc. gradientSteepness of conc. gradient

Water BalanceWater BalanceOsmoregulationOsmoregulation - - control of water control of water

balancebalance

Comparison of 2 solutions:Comparison of 2 solutions:Hypertonic - Hypertonic - higher concentration of solutes higher concentration of solutes

Hypotonic -Hypotonic - lower concentration of solutes lower concentration of solutes

Isotonic -Isotonic - equal concentrations of solutes equal concentrations of solutes

Water BalanceWater BalanceCells with Walls (plants, bacteria, Cells with Walls (plants, bacteria,

fungi):fungi):Require Require hypotonic external environmentshypotonic external environments

to keep their to keep their turgor pressureturgor pressure (water pressure (water pressure pushing cell membrane out against cell wall)pushing cell membrane out against cell wall)

Become limp or Become limp or flaccidflaccid when lose turgor when lose turgor pressurepressure

PlasmolysisPlasmolysis - - plasma membrane pulls away plasma membrane pulls away from cell wallfrom cell wall

Water BalanceWater BalanceCells without Walls (animals, most Cells without Walls (animals, most

protist):protist):Require Require isotonicisotonic external environments external environmentsHypertonic environmentsHypertonic environments – cells swell & – cells swell &

may burst with too much water pressure may burst with too much water pressure ((CytolysisCytolysis))

May have May have contractile vacuolescontractile vacuoles (some (some protists; paramecium also have less porous protists; paramecium also have less porous membrane) to control internal water pressure membrane) to control internal water pressure

Contractile vacuoles & Contractile vacuoles & Osmoregulation in ParameciumOsmoregulation in Paramecium

Specialized TransportSpecialized TransportUtilizingTransport proteinsUtilizingTransport proteins (with or without (with or without

channels)channels)

Facilitated diffusionFacilitated diffusion - - passage of molecules passage of molecules and ions and ions with transport proteinswith transport proteins across a across a membrane down the concentration gradient.membrane down the concentration gradient.

Specific for its substrateSpecific for its substrate

Facilitated diffusionFacilitated diffusion Transport of water and certain hydrophilic Transport of water and certain hydrophilic

solutes across; down conc. gradient.solutes across; down conc. gradient. Transport ProteinsTransport Proteins Most-very specificMost-very specific 2 types pf proteins2 types pf proteins

– 1. Channel Proteins1. Channel Proteins– 2. Transport Proteins2. Transport Proteins

Facilitated diffusionFacilitated diffusion

1. Channel Proteins1. Channel Proteins Permits rapid flow across membrane.

1. Aquaporins: plants and animals. Discovered in plants in 1994

2. Ion Channels– Many of these are Gated

Channels-– Stimulus to open Electrical or Chemical

Stimulus

– Example: Nerve cell stimulated by a neurotransmitter molecules (chemical) , opens gate, allows Na+ into the cell.

Diseases linked to Ion ChannelsDiseases linked to Ion ChannelsA multitude of human and animal diseases A multitude of human and animal diseases are caused by dysfunction of ion channels. This may are caused by dysfunction of ion channels. This may be genetic, i.e. caused directly by mutations in genes be genetic, i.e. caused directly by mutations in genes coding for ion channels. Such diseases are called coding for ion channels. Such diseases are called ‘channelopathies’. Examples of channelopathies are ‘channelopathies’. Examples of channelopathies are cystic fibrosis, epilepsy, and arrhythmias, e.g. the cystic fibrosis, epilepsy, and arrhythmias, e.g. the long QT syndrome. Also, diseases may result from long QT syndrome. Also, diseases may result from defects caused by mutations in genes coding for defects caused by mutations in genes coding for regulatory proteins. regulatory proteins.

http://www.sophion.dk/Technology/ion_channelshttp://www.sophion.dk/Technology/ion_channels

LE 7-15aLE 7-15a

EXTRACELLULARFLUID

Channel protein Solute

CYTOPLASM

Facilitated diffusionFacilitated diffusion

2. Transport Proteins2. Transport Proteins Undergoes subtle change in shape.Undergoes subtle change in shape. Alternates between 2 conformations, moving Alternates between 2 conformations, moving

molecule across as changes.molecule across as changes.

Facilated DiffusionFacilated Diffusion

2. Carrier Proteins: Undergo conformational 2. Carrier Proteins: Undergo conformational change, change, solute is transported as the protein solute is transported as the protein changes shape.changes shape.,,

Active transportActive transportPumpPump substance across membranes, substance across membranes,

againstagainst its concentration gradient, its concentration gradient, through a a Carrier Protein, with the help through a a Carrier Protein, with the help of of cellular energy (ATP)cellular energy (ATP)

Example: How cells maintain a higher Example: How cells maintain a higher concentrations of K+ concentrations of K+ insideinside cell cell

Active Transport SystemsActive Transport Systems

1. 1. The sodium-potassium pumpThe sodium-potassium pump.. Specific kind of Active TransportSpecific kind of Active Transport Esp. Esp. important in animalsimportant in animals Exchanges Na+ for K+Exchanges Na+ for K+ Transports Transports

– 3 Na3 Na+ out for every + out for every 2 K2 K+ into the cell.+ into the cell.

Restores normal Restores normal electrochemical gradientelectrochemical gradient following following an action potential. an action potential.

LE 7-16LE 7-16

Cytoplasmic Na+ bonds tothe sodium-potassium pump

CYTOPLASMNa+

[Na+] low[K+] high

Na+

Na+

EXTRACELLULARFLUID

[Na+] high[K+] low

Na+

Na+

Na+

ATP

ADP

P

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

K+

Phosphorylation causesthe protein to change itsconformation, expelling Na+

to the outside.

P

Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.

PP

Loss of the phosphaterestores the protein’soriginal conformation.

K+ is released and Na+

sites are receptive again;the cycle repeats.

K+

K+

K+

K+

K+

Maintenance of Membrane Potential Maintenance of Membrane Potential by Ion Pumpsby Ion Pumps

Membrane PotentialMembrane Potential: The voltage (charge : The voltage (charge separation) across a membraneseparation) across a membrane– Cytoplasm of a cell is negativeCytoplasm of a cell is negative– Extracellular fluid is positiveExtracellular fluid is positive

Ranges from -50- -200mVRanges from -50- -200mV

Favors diffusion of cations into the cell, and anions out of Favors diffusion of cations into the cell, and anions out of cellscells

..

The The electrochemical gradientelectrochemical gradient: a combination : a combination of 2 forces that influence the movement of of 2 forces that influence the movement of ions across membranes.ions across membranes.– A chemical forceA chemical force (the ion’s concentration (the ion’s concentration

gradient)gradient)– An electrical force (the effect of the membrane An electrical force (the effect of the membrane

potential on the ion’s movement).potential on the ion’s movement).

– SO…….ions move across membranes down SO…….ions move across membranes down their electrochemical gradienttheir electrochemical gradient..

• An An electrogenic pumpelectrogenic pump is a transport protein is a transport protein that generates voltage across a membrane.that generates voltage across a membrane.

The main electrogenic pump of The main electrogenic pump of animals is a animals is a sodium-potassium pump.sodium-potassium pump.

The main electrogenic pump of plants, fungi, The main electrogenic pump of plants, fungi,

and bacteria is and bacteria is a proton pump—a proton pump—activelyactively transports H+ out of cells.transports H+ out of cells.

LE 7-18LE 7-18

H+

ATP

CYTOPLASM

EXTRACELLULARFLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

–

–

–

–

–

CotransportCotransport An ATP powered pump indirectly drives the An ATP powered pump indirectly drives the

Active Transport of several other solutes.Active Transport of several other solutes.

Example: Sucrose - H+ CotransporterExample: Sucrose - H+ Cotransporter– Sucrose loading in plantsSucrose loading in plants

CO-TRANSPORTCO-TRANSPORT... movement of 2 solutes together ... movement of 2 solutes together - -

                                                                                                  Often moves 1 solute passively & other activelyOften moves 1 solute passively & other actively

      Ex:   1)  H+ pump coupled with sucrose transport       Ex:   1)  H+ pump coupled with sucrose transport ( ( H+symportH+symport** ) )              2)                2)  epithelial transportepithelial transport* * Na+glucose model ( Na+glucose model ( glucose absorptionglucose absorption** ) )

Replenish ions, sugar, fluid following strenuous Replenish ions, sugar, fluid following strenuous exercise, diarrheaexercise, diarrhea

LE 7-18LE 7-18

H+

ATP

CYTOPLASM

EXTRACELLULARFLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

–

–

–

–

–

LE 7-19LE 7-19

H+

ATP

Proton pump

Sucrose-H+

cotransporter

Diffusionof H+

Sucrose

H+

H+

H+

H+

H+

H+

+

+

+

+

+

+

–

–

–

–

–

–

Bulk transport across the Bulk transport across the plasma membrane occurs by plasma membrane occurs by exocytosis and endocytosisexocytosis and endocytosis

Large moleculesLarge molecules, such as polysaccharides and , such as polysaccharides and proteins, cross the membrane via proteins, cross the membrane via vesicles.vesicles.

1. Exocytosis1. Exocytosis In In exocytosisexocytosis: cell secretes : cell secretes

macromolecules.macromolecules. Vessicle buds from Gogi.Vessicle buds from Gogi.

Many secretory cells use this—Many secretory cells use this—– Examples: Pancreas secretes Examples: Pancreas secretes

insulininsulin Neurons secrete Neurons secrete

neurotransmittersneurotransmitters

2. Endocytosis2. Endocytosis Reverse process.Reverse process. The cell takes in macromolecules by forming vesicles The cell takes in macromolecules by forming vesicles

from the plasma membrane.from the plasma membrane.

• Three types of endocytosis:Three types of endocytosis:11. Phagocytosis. Phagocytosis (“cellular eating”): Cell engulfs (“cellular eating”): Cell engulfs particle in a vacuole. Not specific.particle in a vacuole. Not specific.2. 2. PinocytosisPinocytosis (“cellular drinking”): Cell creates (“cellular drinking”): Cell creates vesicle around fluid. Not specific.vesicle around fluid. Not specific.

3. Receptor-mediated endocytosis:3. Receptor-mediated endocytosis: Very SpecificVery Specific Binding Binding of ligandsof ligands to receptors triggers vesicle to receptors triggers vesicle

formationformation

LE 7-20cLE 7-20c

Receptor

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coatedpit

Coatedvesicle

Coat protein

Coat protein

Plasmamembrane

0.25 µm

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).

Familial hypercholesteremiaFamilial hypercholesteremia

Defective gene/genes required for Defective gene/genes required for regulation, synthesis, transport, recycling, or regulation, synthesis, transport, recycling, or turnover of LDL receptors.turnover of LDL receptors.

Exocytosis -Exocytosis - secretion of macromolecules secretion of macromolecules by the fusion of vesicles with the plasma by the fusion of vesicles with the plasma membranemembrane

Endocytosis Endocytosis -- import of macromolecules by import of macromolecules by forming new vesicles with the plasma forming new vesicles with the plasma membranemembranePhagocytosis –Phagocytosis –cell “eating”cell “eating”Pinocytosis – Pinocytosis – cell “drinking”cell “drinking”

Receptor-mediated endocytosisReceptor-mediated endocytosis (ligands) (ligands)

Bulk transit across membranesBulk transit across membranes

Cotransport (symport)Cotransport (symport) involves more than involves more than one type of particle being transported by in one type of particle being transported by in the the same directionsame direction at the same time by the at the same time by the same mechanism From same mechanism From http://http://