Chapter 5Chapter 5 Membrane TransportMembrane Transport MechanismsMechanisms

Membrane PermeabilityMembrane Permeability

1) lipid soluble solutes go through 1) lipid soluble solutes go through

fasterfaster

1)1) smaller molecules go fastersmaller molecules go faster

1) uncharged & weakly charged 1) uncharged & weakly charged

go fastergo faster

2) Channels or pores may also 2) Channels or pores may also

exist in membrane to allow exist in membrane to allow

transporttransport

12

How to get other molecules across How to get other molecules across membranesmembranes

There are two ways that the molecules typically move through the membrane:

passive transport and active transport

•Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane.

•Passive transport does not require such an energy expenditure, and occurs spontaneously.

1 Passive Transport1 Passive Transport

Net movement of material from hi to lo cNet movement of material from hi to lo concentrationoncentration• DiffusionDiffusion• OsmosisOsmosis• Facilitated DiffusionFacilitated Diffusion

concentration gradientconcentration gradient

Passive TransportPassive Transport

Simple Simple DiffusionDiffusion- simple movement - simple movement from regions of high concentration to from regions of high concentration to low concentrationlow concentration

OsmosisOsmosis- diffusion of water across a - diffusion of water across a semi-permeable membranesemi-permeable membrane

Facilitated Facilitated diffusiondiffusion 协助扩散协助扩散 -- protein protein transporters which assist in diffusiontransporters which assist in diffusion

Passive TransportPassive Transport

Unaided movement through the phosphUnaided movement through the phospholipid bilayer in response to olipid bilayer in response to concentraticoncentration gradientson gradients..

Large or charged molecules are unable tLarge or charged molecules are unable to pass through the bilayer unassisted.o pass through the bilayer unassisted.

DiffusionDiffusion

Movement generated by random Movement generated by random motion of particles. Caused by motion of particles. Caused by internal thermal energy.internal thermal energy.

Movement always from region of high Movement always from region of high free energy(high concentration) to free energy(high concentration) to regions of low free energy (low conc.)regions of low free energy (low conc.)

DiffusionDiffusion

If a If a concentration gradientconcentration gradient exists, there will be exists, there will be a a net flownet flow of material across the membrane of material across the membrane

OsmosisOsmosis

Movement of water Movement of water across a semi-across a semi-permeable barrier. permeable barrier.

ExampleExample: Salt in water, : Salt in water, cell membrane is cell membrane is barrier. Salt will NOT barrier. Salt will NOT move across move across membrane, water membrane, water will. will.

Text pg 87

OsmosisOsmosis

Refers to movement of waterRefers to movement of water Across a Across a semipermeable membranesemipermeable membrane

• Permeable to waterPermeable to water• Impermeable to dissolved materialsImpermeable to dissolved materials

Water always moves from lo to high soluWater always moves from lo to high solutete• Hypotonic Hypotonic Hypertonic Hypertonic

Osmosis in Osmosis in Hypertonic Hypertonic

mediummedium

cell

Hypertonic solutions- shrink cells

Osmosis in Hypotonic medium

Hypotonic solutions- swell cells

OsmosisOsmosis

Hypotonic Hypertonic

简单扩散与协助扩散的比较简单扩散与协助扩散的比较

Osmotic pressure = Weight of water column

Plant TurgorPlant Turgor

Hypotonic

Central

vacuole

Hypertonic

Water pressure inside cell lends support to plant structure

Plasmolysis due to water deprivation

+

Charged particle cloaked with water molecules, can not get through

Na+

Cl-

K+

H+

Large molecules, also cloaked with water molecules, can not get through

Nucleotides

Amino acids

Sugars

Large molecules can not get through

Sugars

Facilitated Diffusion

Transport ProteinTransport Protein

Shape change resulting from solute interaction with transport protein

Channel

Types of Protein Types of Protein TransportersTransporters

A. A. Facilitated DiffusionFacilitated Diffusion

Assist in diffusion Assist in diffusion process.process.

Solutes go from High Solutes go from High conc conc to Low conc.to Low conc.

ExamplesExamples: Glucose : Glucose transporters transporters

Text pg 88http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm

A A Transport ProteinsTransport ProteinsFacilitated Diffusion Facilitated Diffusion & Active Transport& Active Transport

move solutes move solutes

faster across faster across

membranemembrane

highly specific to highly specific to

specific solutesspecific solutes

can be inhibited can be inhibited

by drugs by drugs

Transport Proteins are specificTransport Proteins are specific

Na+

glucose

Cell Controls Movement by Number Cell Controls Movement by Number of Transport Proteins in Membraneof Transport Proteins in Membrane

High Moderate No transport

Gated ChannelsGated Channels

Chemical messenger, e.g. insulin

glucose

Voltage Dependent KVoltage Dependent K++ channel channel

Model of protein shape change in response to change in voltage across a membrane

Roderick MacKinnon, M.D., Howard Hughes Medical Institute Investigator, Youxing Jiang, Ph.D.

CystinuriaCystinuria

Defective transport pDefective transport protein in urinary tracrotein in urinary tract unable to absorb cyt unable to absorb cystine from urinestine from urine

cysteine

cystine

Cystine crystal

Facilitated DiffusionFacilitated DiffusionTheThe Glucose Transporters Glucose Transporters

Transport of glucose into cells mediated by Transport of glucose into cells mediated by proteins in the GLUT (GLUcose Transport) proteins in the GLUT (GLUcose Transport) family of transporters. There are 7 different, family of transporters. There are 7 different, but related, proteins. But, only four (GLUT1-4) but related, proteins. But, only four (GLUT1-4) are known to be involved in glucose transport.are known to be involved in glucose transport.

All GLUT proteins share a set of similar All GLUT proteins share a set of similar structural features and are all about 500 structural features and are all about 500 amino acids in length (giving them a predicted amino acids in length (giving them a predicted molecular weight of about 55,000 Daltons)molecular weight of about 55,000 Daltons)

Glucose uptake shows saturation and glucose Glucose uptake shows saturation and glucose uptake can be inhibited by drugsuptake can be inhibited by drugs

A classic Membrane Transport proteinA classic Membrane Transport protein

Glucose TransporterGlucose Transporter CharacteristicsCharacteristics::

integral protein: spans the integral protein: spans the membranemembrane

12 alpha helices woven into 12 alpha helices woven into membranemembrane

55,000 mol. wt.55,000 mol. wt. Text pg. 88 Text pg. 88

Glucose Transporter:Glucose Transporter:How it worksHow it works....

glucose binds to glucose binds to outside of transporter outside of transporter (exterior side with (exterior side with higher glucose conc.)higher glucose conc.)

glucose binding causes glucose binding causes a conform. change in a conform. change in proteinprotein

glucose drops off inside glucose drops off inside cellcell

protein reassumes 1st protein reassumes 1st configuration configuration

Types of Protein Types of Protein Transporters:Transporters: Ion Channels Ion Channels

work by facilitated diffusion No E! work by facilitated diffusion No E! deal with small molecules... ionsdeal with small molecules... ions open pores are “gated”- Can open pores are “gated”- Can

change shape.change shape.• How?How?• How much gets in?How much gets in?

important in cell communication important in cell communication

B B Ion ChannelsIon Channels Work fast: No conform. changes Work fast: No conform. changes

neededneeded Not simple pores in membrane:Not simple pores in membrane:

• specific to different ions (Na, K, Ca...)specific to different ions (Na, K, Ca...)• gates control openinggates control opening• Toxins, drugs may affect channelsToxins, drugs may affect channels

saxitoxin, tetrodotoxinsaxitoxin, tetrodotoxin cystic fibrosis cystic fibrosis

Toxins…how they workToxins…how they work

Cystic FibrosisCystic Fibrosis Fatal genetic disorderFatal genetic disorder Mucus build-up results in lung and Mucus build-up results in lung and

liver failureliver failure Patients die between 4 and 30 yrs.Patients die between 4 and 30 yrs. Single gene defectSingle gene defect 1 in 25 Caucasians carry 1 bad 1 in 25 Caucasians carry 1 bad

gene copygene copy 1 in 2500 kids has it in Canada 1 in 2500 kids has it in Canada Testing Testing

CF Cont…CF Cont… ~Proteins for diffusion of salt into ~Proteins for diffusion of salt into

the airways don't work. the airways don't work.

~Less salt in the airways means l~Less salt in the airways means less water in the airways. ess water in the airways.

~ Less water in the airways mean~ Less water in the airways means mucus layer is very s mucus layer is very sticky (viscous). sticky (viscous).

~Sticky mucus cannot be easily ~Sticky mucus cannot be easily moved to clear particles from the moved to clear particles from the lungs. lungs.

~Sticky mucus traps bacteria an~Sticky mucus traps bacteria and causes more lung infections. d causes more lung infections.

http://www.the-aps.org/education/lot/cell/HotT.htm

2 2 Active Transport Active Transport

Active transportActive transport- proteins which - proteins which transport transport againstagainst concentration concentration gradient.gradient.

Requires energy inputRequires energy input

T

Types of Protein Types of Protein Transporters:Transporters:

Active Transport Active Transport

carrier proteinscarrier proteins go against the concentration go against the concentration

gradients Low to Highgradients Low to High require Energy to function (ATP, require Energy to function (ATP,

PEP, light energy, electron PEP, light energy, electron transport)transport)

Membrane TransportMembrane Transport Active T Active Transportransport

Move materials from lo to high concentraMove materials from lo to high concentrationtion

Requires cell to expend energyRequires cell to expend energy Equivalent to running diffusion in reverseEquivalent to running diffusion in reverse Simulation 1Simulation 1- how can you put the particl- how can you put the particl

es back into the box?es back into the box?

http://lewis.eeb.uconn.edu/lewishome/applets/Diffusion/diffusion.html

Active TransportActive Transport Sodium-Potassium PumpSodium-Potassium Pump CA-ATP CA-ATP PumpPump

proton proton PumpPump p- p- proton proton PumpPump v -v -proton proton PumpPump H- ATP H- ATP PumpPumpCotransport symportCotransport symport aniportaniport

Active Transport:Active Transport:Sodium-Potassium PumpSodium-Potassium Pump

Na+high

K+low

Na+low

K+high

Balance of the two ions goes hand-in-hand

ATP required for maintenance of the pump

ATPATP-direct depend -direct depend

Electrochemical GradientElectrochemical Gradient

Na+ K+

pump

ATP

ADP

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+K+

K+

K+

K+

K+

negative positive

This gradient powers conduction of signals along nerves

Stored Energy for CotransportStored Energy for Cotransport

proton

pump

ATP

ADP

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

Voltage difference or membrane potential

H+

sucrose

H+

H+

H+

H+

H+

sucrose

sucrose

sucrose

sucrose

sucrose

sucrose

sucrose

The sodium/potassium pumpThe sodium/potassium pump

All nerve and muscle cells have a high internal potassium iAll nerve and muscle cells have a high internal potassium ion concentration and a low internal sodium ion concentraon concentration and a low internal sodium ion concentration. [Ki=166 mM; Ko=5 mM; Nai=18 mM; Nao=135 mM]. tion. [Ki=166 mM; Ko=5 mM; Nai=18 mM; Nao=135 mM].

Early on, it was thought that the nerve and muscle membrEarly on, it was thought that the nerve and muscle membranes were relatively impermeable to these ions and that tanes were relatively impermeable to these ions and that the difference in ionic concentration was set up in early devhe difference in ionic concentration was set up in early development of the cells. The membrane then became imperelopment of the cells. The membrane then became impermeable.meable.

The later availability and use of radioactive Na and K ions The later availability and use of radioactive Na and K ions showed that this was not true and that there was a metabshowed that this was not true and that there was a metabolic pump that pumped Na out of the cell and K in; the ratiolic pump that pumped Na out of the cell and K in; the ratio being 3 Na pumped out of the cell for every 2 K pumped io being 3 Na pumped out of the cell for every 2 K pumped into the cell.nto the cell.

Is a Protein Involved ?Is a Protein Involved ? Experiments showed a Experiments showed a

dependency of both Na and K ions dependency of both Na and K ions for pump to workfor pump to work

Pump was inhibited by ouabain (a Pump was inhibited by ouabain (a drug)drug)

1957: an ATPase enzyme was 1957: an ATPase enzyme was found to be associated with Na/K found to be associated with Na/K pumpingpumping

Studies showed this ATPase Studies showed this ATPase capable of pumping Na/K ions capable of pumping Na/K ions

Sodium/PotassiumSodium/PotassiumATPase ProteinATPase Protein

Made of 2 large and 2 small subunitsMade of 2 large and 2 small subunits 2 large units span membrane 2 large units span membrane

• inside region: contains ATP binding siteinside region: contains ATP binding site• inside: binding sites for Nainside: binding sites for Na• outside: binding site for Koutside: binding site for K

How does it work??How does it work??

Sodium-Potassium PumpSodium-Potassium Pump

http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/sppump.html

Na-K Pump Model: Part INa-K Pump Model: Part I

3 Na+ bind to inner region of protein3 Na+ bind to inner region of protein Na+ binding triggers phosphorylation Na+ binding triggers phosphorylation

of protein. ATP ADP + Piof protein. ATP ADP + Pi Phosphorylation causes conformation Phosphorylation causes conformation

change and Na+ binding site faces change and Na+ binding site faces outsideoutside

3 Na+ released to outside 3 Na+ released to outside

Na-K Active Pump: Part IINa-K Active Pump: Part II 2 K+ ions on outside are able to bind2 K+ ions on outside are able to bind K+ binding causes dephosphorylation K+ binding causes dephosphorylation

and new conformation changeand new conformation change 2K+ ions exposed to inside and 2K+ ions exposed to inside and

releasedreleased

Cyclic process uses ATP energy to Cyclic process uses ATP energy to drive Na & K ion transport drive Na & K ion transport againstagainst conc. Gradient conc. Gradient

钙泵钙泵 Ca++Ca++ 浓度：胞外约浓度：胞外约 1010-3-3MM ，胞质小于或等，胞质小于或等

于于 1010-7-7MM （自由（自由 Ca++Ca++ ）） 肌质网的肌质网的 Ca++Ca++ 泵泵

Transmembrane transport and membranTransmembrane transport and membrane potentiale potential

静息电位静息电位 静息电位静息电位 动作电位动作电位 阳离子通道阳离子通道 突触前膜，突触后膜突触前膜，突触后膜 乙酰胆碱受体（配体控制的闸门）乙酰胆碱受体（配体控制的闸门）

3 Transport of Macromolecules3 Transport of Macromoleculesactive transport active transport

Exocytosis & EndocytosisExocytosis & Endocytosis• Formation of vesicles around large moleculesFormation of vesicles around large molecules

Receptor mediatedReceptor mediatedReceptor proteinsReceptor proteinsCoat proteinsCoat proteins

EndocytosisEndocytosis

PinocytosisPinocytosisphagocytosisphagocytosis

EndocytosisEndocytosis Receptor –mediated endocytosisReceptor –mediated endocytosis No-special endocytosis No-special endocytosis

LDL LDL endocytosisendocytosis movie movie

http://www.blc.arizona.edu/courses/181gh/Lectures_WJG.01/LDL_F.01/atherosclerosis.html

Low Density Lipoprotein

EndocytosisEndocytosis

Transports macromolecules and large pTransports macromolecules and large particles into the cell.articles into the cell.

Part of the membrane engulfs the particlPart of the membrane engulfs the particle and folds inward to “bud off.”e and folds inward to “bud off.”

• Fig. 5.16Fig. 5.16

EndocytosisEndocytosis

Putting Out the GarbagePutting Out the Garbage

Vesicles (lysosomes, other secretory vesiVesicles (lysosomes, other secretory vesicles) can fuse with the membrane and ocles) can fuse with the membrane and open up the the outside…pen up the the outside…

ExocytosisExocytosis (Cellular Secretion) (Cellular Secretion)

ExocytosisExocytosis Constitutive exocytosis pathway(default Constitutive exocytosis pathway(default

pathway)pathway) regulated exocytosis pathwayregulated exocytosis pathway

Cellular MembranesCellular Membranes

REVIEWREVIEW Importance of MembranesImportance of Membranes Membrane StructureMembrane Structure ProteinsProteins Fluid Mosaic modelFluid Mosaic model PermeabilityPermeability Types of Transport Types of Transport

• Passive and ActivePassive and Active

AssignmentAssignment

What is the role of NADWhat is the role of NAD++ and the electron and the electron transport chain in extracting useful enertransport chain in extracting useful energy from food?gy from food?