chapter 7: membrane structure and function. overview: life at the edge plasma membrane: boundary...

Chapter 7: Membrane Structure and Function

Overview: Life at the Edge

Plasma membrane: boundary that separates the living cell from its surroundings

Exhibits selective permeability allows some

substances to cross it more easily than others

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 7.1: Cellular membranes are fluid mosaics of lipids and

proteinsPhospholipids: most abundant lipid in the plasma

membraneamphipathic molecules: contain hydrophobic and

hydrophilic regionsFluid mosaic model: membrane is a fluid structure

with a “mosaic” of various proteins embedded in it


1935: Hugh Davson & James Danielli proposed sandwich model phospholipid bilayer lies between two

layers of globular proteinsPROBLEM: Placement of membrane

proteins which have hydrophilic and hydrophobic regions

1972: J. Singer & G. Nicolson proposed membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water Freeze-fracture studies of the plasma membrane

supported the fluid mosaic model Freeze-fracture: specialized preparation technique

that splits membrane along the middle of the phospholipid bilayer


History of the Membrane

Fig. 7-4

TECHNIQUE

Extracellularlayer

KnifeProteins Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layerPlasma membrane

The Fluidity of Membranes Phospholipids move within the

bilayer Most lipids, and some proteins, drift

laterally Rarely molecule flip-flop

transversely across membrane Temperatures cool membranes

switch from fluid to solid state Temperature at which

membrane solidifies = types of lipids

Unsaturated fatty acids = more fluid

Saturated fatty acids = more solid Membranes must be fluid to work

properly usually about as fluid as salad oil

Fig. 7-5a

(a) Movement of phospholipids

Lateral movement(107 times per second)

Flip-flop( once per month)

Fig. 7-5b

(b) Membrane fluidity

Fluid

Unsaturated hydrocarbontails with kinks

Viscous

Saturated hydro-carbon tails

Fig. 7-6

RESULTS

Membrane proteins

Mouse cellHuman cell

Hybrid cell

Mixed proteinsafter 1 hour

Steroid cholesterol effects membrane fluidity @ different temperatureswarm temperatures (such as 37°C) cholesterol

restrains movement of phospholipidscool temperatures maintains fluidity by preventing

tight packing

The Fluidity of Membranes

Membrane Proteins and Their Functions

Collage of different proteins embedded in the fluid matrix of the lipid bilayer

Proteins membrane’s specific functionsPeripheral proteins: bound to the surface of the

membraneIntegral proteins:

penetrate hydrophobic core Transmembrane proteins:

Integral proteins that span the membrane

Hydrophobic regions consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices


Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

Six major functions of membrane proteins:TransportEnzymatic activitySignal transductionCell-cell recognition Intercellular joiningAttachment to the

cytoskeleton and extracellular matrix (ECM)

Membrane Functions

Fig. 7-9ac

(a) Transport (b) Enzymatic activity (c) Signal transduction

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

Fig. 7-9df

(d) Cell-cell recognition

Glyco-protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition

Cells recognize each other by binding to surface molecules, often carbohydrates, on plasma membrane

Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

Carbohydrates on external side of plasma membrane vary among species, individuals, and even cell types in an individual


Synthesis and Sidedness of Membranes

Membranes have distinct inside and outside faces

Asymmetrical distribution of proteins, lipids, and carbohydrates in plasma membrane determined when membrane is built by ER and Golgi apparatus

Concept 7.2: Membrane structure results in selective permeability

Cell must exchange materials with its surroundings controlled by the plasma membrane

Plasma membranes = selectively permeable regulating the

cell’s molecular traffic


Permeability of the Lipid BilayerHydrophobic/nonpolar molecules (like

hydrocarbons) can dissolve in lipid bilayer and pass through membrane rapidly

Polar molecules (like sugars) do not cross the membrane easily


Transport ProteinsTransport proteins: allow passage of hydrophilic

substances across the membraneChannel proteins: have a hydrophilic channel that

certain molecules or ions can use as tunnelAquaporins: channel proteins that facilitate

passage of waterCarrier proteins: bind to molecules and change

shape to shuttle them across the membranespecific for

the substance it moves

Concept 7.3: Passive transport is diffusion of a substance across a membrane with no

energy investmentDiffusion: tendency for molecules to spread out

evenly into the available spaceEach molecule moves randomly HOWEVER diffusion

of molecules may exhibit net movement in one direction

Dynamic equilibrium molecules crossing one direction = other direction

Substances diffuse down concentration gradient the difference in concentration of a substance from

one area to another

Passive transport: requires no energy from the cell The diffusion of a substance across biological

membrane


Passive Transport

Effects of Osmosis on Water Balance

Osmosis: diffusion of water across a selectively permeable membrane

Water diffuses across membrane from lower [solute] higher [solute]


Water Balance of Cells Without Walls

Tonicity: ability of solution to cause a cell to gain or lose water

Isotonic solution: Solute concentration is the same as that inside the cell no net water movement across the plasma membrane

Hypertonic solution: Solute concentration is greater than that inside the cell cell loses water

Hypotonic solution: Solute concentration is less than that inside the cell cell gains water


Fig. 7-13

Hypotonic solution

(a) Animal cell

(b) Plant cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

Hypertonic OR hypotonic environments create osmotic problems for organismsWhy? What are some problems?

Osmoregulation: control of water balancenecessary adaptation for

life in such environmentsProtist Paramecium is

hypertonic to its pond water environment has a contractile vacuole that acts as pump

Effects of Osmosis on Living Cells

Water Balance of Cells with Walls

Cell walls help maintain water balancePlant cell in… hypotonic solution swells until wall opposes

uptake cell now turgid (firm)isotonic no net movement of water into the cell

cell becomes flaccid (limp) plant may wilthypertonic environment plant cells lose water

membrane pulls away from the wall (usually lethal effect) called plasmolysis

Facilitated Diffusion: Passive Transport Aided by ProteinsFacilitated diffusion: transport proteins speed up

passive movement of molecules across plasma membrane

Channel proteins: provide corridors that allow specific molecule/ion to cross membrane

Channel proteins includeAquaporins, for

facilitated diffusion of water

Ion channels that open or close in response to a stimulus (gated channels)

Carrier proteins undergo subtle change in shape that translocates solute-binding site across the membrane

Some diseases caused by malfunctions in specific transport systems Ex: kidney disease, cystinuria

Facilitated Diffusion: Passive Transport Aided by Proteins

Concept 7.4: Active transport uses energy to move solutes against their gradients

Facilitated diffusion still passive because solute moves down its concentration gradient

Some transport proteins can move solutes against their concentration gradients


The Need for Energy in Active TransportActive transport: moves substances against their

concentration gradientrequires energy, usually in the form of ATPperformed by specific proteins embedded in the

membranesallows cells to

maintain concentration gradients that differ from their surroundings

Ex: sodium-potassium pump


Fig. 7-16-1

EXTRACELLULAR

FLUID [Na+] high [K+] low

Na+

Na+

Na+ [Na+] low[K+] high CYTOPLASM

Cytoplasmic Na+ binds tothe sodium-potassium pump. 1

Na+ binding stimulatesphosphorylation by ATP.

Fig. 7-16-2

Na+

Na+

Na+

ATP P

ADP

2

Fig. 7-16-3

Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.

Na+

P

Na+ Na+

3

Fig. 7-16-4

K+ binds on theextracellular side andtriggers release of thephosphate group.

P P

K+

K+

4

Fig. 7-16-5

Loss of the phosphaterestores the protein’s originalshape.

K+

K+

5

Fig. 7-16-6

K+ is released, and thecycle repeats.

K+

K+

6

2

EXTRACELLULAR

FLUID [Na+] high [K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM ATP

ADP P

Na+ Na+

Na+

P 3

K+

K+ 6

K+

K+

5 4

K+

K+

P P

1

Fig. 7-16-7

Fig. 7-17Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

How Ion Pumps Maintain Membrane Potential

Membrane potential: voltage difference across a membranecreated by differences in distribution of positive/negative

ionsElectrochemical gradient: two combined forces that

drive diffusion of ions across membrane:Chemical force: the ion’s concentration gradientElectrical force: the effect of the membrane potential on

the ion’s movementElectrogenic pump: transport protein that generates

voltage across membraneSodium-potassium pump: major electrogenic pump of

animal cellsProton pump: main electrogenic pump of plants, fungi,

and bacteria is a

Fig. 7-18

EXTRACELLULARFLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

Cotransport: Coupled Transport by a Membrane Protein

Cotransport: active transport of a solute indirectly drives transport of another solute

Plants use gradient of hydrogen ionsgenerated by proton pumps to drive active transport of nutrients into the cell

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and

endocytosis

Small molecules & water enter/leave cell through lipid bilayer or transport proteins

Large molecules like polysaccharides and proteins cross membrane in bulk via vesicles*requires energy*


Exocytosis

Exocytosis: transport vesicles migrate to the membrane, fuse with it, and release their contentssecretory cells use exocytosis to export their products


Endocytosis Endocytosis: cell takes in macromolecules by forming vesicles

from plasma membrane reversal of exocytosis, involving

different proteins Three types of endocytosis:

Phagocytosis: cell engulfs a particle in a vacuole then fuses with a lysosome to digest the particle (“cellular eating”)

Pinocytosis: molecules are taken up when extracellular fluid is “gulped” into tiny vesicles (“cellular drinking”)

Receptor-mediated endocytosis: binding of ligands to receptors triggers vesicle formationLigand: any molecule that binds

specifically to receptor site of another molecule

Fig. 7-20a

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULARFLUID

Pseudopodium

“Food” orother particle

Foodvacuole Food vacuole

Bacterium

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Pseudopodiumof amoeba

1 µm

Fig. 7-20b

PINOCYTOSIS

Plasmamembrane

Vesicle

0.5 µm

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)

Fig. 7-20cRECEPTOR-MEDIATED ENDOCYTOSIS

Receptor Coat protein

Coatedpit

Ligand

Coatprotein

Plasmamembrane

0.25 µm

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)

You should now be able to:

1. Define the following terms: amphipathic molecules, aquaporins, diffusion

2. Explain how membrane fluidity is influenced by temperature and membrane composition

3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions


4. Explain how transport proteins facilitate diffusion

5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps

6. Explain how large molecules are transported across a cell membrane


Fig. 7-UN1

Passive transport:Facilitated diffusion

Channelprotein

Carrierprotein

Fig. 7-UN2

Active transport:

ATP

Fig. 7-UN3

Environment:0.01 M sucrose

0.01 M glucose

0.01 M fructose

“Cell”

0.03 M sucrose

0.02 M glucose

Fig. 7-UN4

chapter 7: membrane structure and function. overview: life at the edge plasma membrane: boundary...

Documents

placement of membrane

cell membrane

membrane structure

edgeplasma membrane

mosaic of proteins

membrane hydrophobic

membraneintegral proteins

pearson education