Membrane Structure & Function cont.I. Membrane Protein FunctionII. Cellular Transport

Integral proteins span lipid bilayer called transmembrane proteins

hydrophobic regions consist of one or more stretches of nonpolar amino acids

often coiled into alpha helices

Visualize and draw membrane with transmembrane protein containing 2 helices

LE 7-8EXTRACELLULARSIDEN-terminusC-terminusCYTOPLASMICSIDEa Helix

Six major functions of membrane proteins:TransportEnzymatic activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton and extracellular matrix (ECM)

LE 7-9aEnzymesSignalReceptorATPTransportEnzymatic activitySignal transduction

LE 7-9bGlyco-proteinCell-cell recognitionIntercellular joining Attachment to thecytoskeleton and extra-cellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell RecognitionCells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane

Carbohydrates covalently bonded to lipids (glycolipids) or more often to proteins (glycoproteins)

Much variability of extracellular carbohydrates among species, individuals, cell types in an individual

Example of Pneumococcus

Synthesis and Sidedness of MembranesMembranes distinct inside and outside faces

Plasma membrane is added to by vesicles from ER & Golgi.

Secreted and integral membrane proteins, lipids and associated carbohydrates transported to membrane by these vesicles.

LE 7-10Plasma membrane:Cytoplasmic faceExtracellular faceTransmembraneglycoproteinPlasma membrane:SecretedproteinVesicleGolgiapparatusGlycolipidSecretoryproteinTransmembraneglycoproteinsER

Transport across cellular membranesTo exchange materials with surroundings in part to take in nutrients and give off waste

Exchange(or transport) regulated: selective permeability

Structure Dictates Membrane PermeabilityHydrophobic (nonpolar) molecules cross membrane rapidlye.g., hydrocarbons, oxygen, CO2 can dissolve in the lipid bilayer and pass through the membrane rapidly

Polar molecules cross slowlye.g. sugars, charged proteins, water

How do hydrophilic substances cross membranes?Transport proteinsSome create hydrophilic channels across membranes for polar molecules or ions to pass through

Example: Aquaporin water channel proteinWith Help!

Carrier proteinsbinds solutes & change the shape of carrier

help to facilitate passage across membrane

highly specific for transported solutes

Examples: glucose transporter is a carrier protein for glucose only

Transport Can be Passive or Active

LE 7-11aMolecules of dyeMembrane (cross section)WATERNet diffusionNet diffusionEquilibriumDiffusion of one solutePassive Transport: Diffusion

Substances diffuse down their concentration gradientHigh to low

Substances reach dynamic equilibrium

No work (no added energy) required

LE 7-11bNet diffusionNet diffusionEquilibriumDiffusion of two solutesNet diffusionNet diffusionEquilibrium

Effects of Osmosis on Water BalanceOsmosis diffusion of water across a selectively permeable membrane

Diffuses across a membrane from the region of lower solute (such as an ion) concentration to the region of higher solute concentration

The direction of osmosis is determined only by a difference in total solute concentration

LE 7-12Lowerconcentrationof solute (sugar)Higherconcentrationof sugarSame concentrationof sugarSelectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules canH2OOsmosis

Water Balance of Cells Without WallsTonicity ability of a solution to cause a cell to gain or lose water

Isotonic solutionsolute concentration is equal inside and outside the cell --> no net water movement cell remains same size

Hypertonic solution external solute concentration is greater than that inside the cell-->cell loses water

Hypotonic solution external solute concentration is less than that inside the cell--> cell gains water

May expand enough to burst!

LE 7-13AnimalcellLysedH2OH2OH2ONormalHypotonic solutionIsotonic solutionHypertonic solutionH2OShriveledH2OH2OH2OH2OPlantcellTurgid (normal)FlaccidPlasmolyzed

Water Balance of Cells with Walls vs No WallsCell walls help maintain water balance

Plant cell in hypotonic solution swells -->turgid (firm)Animal cell?Plant cell and its surroundings isotonic--> no net water movemen; the cell becomes flaccid (limp), and the plant may wiltAnimal cell?In hypertonic environment, plant cells lose water--> membrane pulls away from the wall: plasmolysisLethal

Animal cell?

Passive Transport Aided by ProteinsFacilitated diffusion transport proteins speed movement of molecules across the plasma membraneChannel proteins Carrier proteins

LE 7-15aEXTRACELLULARFLUIDChannel proteinSoluteCYTOPLASM

LE 7-15bCarrier proteinSolute

Active transport

uses energy to move solutes against their gradientsRequires energy, usually ATPPerformed by specific membrane proteins

Examplesodium-potassium pump

LE 7-16 Cytoplasmic Na+ bonds tothe sodium-potassium pumpCYTOPLASMNa+[Na+] low[K+] highNa+Na+EXTRACELLULARFLUID[Na+] high[K+] lowNa+Na+Na+ATPADPP 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 proteinsoriginal conformation. K+ is released and Na+sites are receptive again;the cycle repeats.K+K+K+K+K+

LE 7-17DiffusionFacilitated diffusionPassive transportATPActive transport

Electrogenic pumps is a transport protein that generates a voltage across a membrane--> opposite charges across membrane (membrane potential)

Example: In animals, Na-K pump

In plant fungi and bacteria, proton pumpRequires ATP (active transport)

LE 7-18H+ATPCYTOPLASMEXTRACELLULARFLUIDProton pumpH+H+H+H+H++++++

Cotransport Coupled Transport by a Membrane ProteinWhen active transport of one solute indirectly drives transport of another

ExamplePlants commonly use the proton gradient generated by proton pumps to drive transport of nutrients into the cell

LE 7-19H+ATPProton pumpSucrose-H+cotransporterDiffusionof H+SucroseH+H+H+H+H+H+++++++

How do large molecules move in and out of cells?Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteinsLarge molecules, such as polysaccharides and proteins, cross the membrane via vesicles

ExocytosisTransport vesicles with cargo migrate to the membrane, fuse with it, and are release contents

Example:Many secretory cells use exocytosis to export their products Pancreatic cells (beta-cells) secrete insulin

LE 7-10Plasma membrane:Cytoplasmic faceExtracellular faceTransmembraneglycoproteinPlasma membrane:SecretedproteinVesicleGolgiapparatusGlycolipidSecretoryproteinTransmembraneglycoproteinsER

EndocytosisCell takes in macromolecules by forming vesicles at the plasma membrane

Reversal of exocytosis, involving different proteins

Three types of endocytosis

Phagocytosis (cellular eating): Cell engulfs particle in a vacuole

Pinocytosis (cellular drinking): Cell creates vesicle around fluid

Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation

LE 7-20cReceptorRECEPTOR-MEDIATED ENDOCYTOSISLigandCoatedpitCoatedvesicleCoat proteinCoat proteinPlasmamembrane0.25 mA coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).