Chapter 7: Membrane Structure and Function. Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundingsThe

Chapter 7:Chapter 7:

Membrane Structure andMembrane Structure and

FunctionFunction

Overview: Life at the EdgeOverview: Life at the Edge

• The plasma membrane is the boundary that The plasma membrane is the boundary that separates the living cell from its separates the living cell from its surroundingssurroundings

The plasma membrane exhibits The plasma membrane exhibits selective selective permeabilitypermeability, allowing some substances to , allowing some substances to cross it more easily than otherscross it more easily than others

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

Cellular membranes are fluid mosaics Cellular membranes are fluid mosaics of lipids and proteinsof lipids and proteins

Phospholipids are the most abundant lipid in Phospholipids are the most abundant lipid in the plasma membranethe plasma membrane

Phospholipids are Phospholipids are amphipathicamphipathic moleculesmolecules, , containing hydrophobic and hydrophilic containing hydrophobic and hydrophilic regionsregions

The The fluid mosaic model fluid mosaic model states that a states that a membrane is a fluid structure with a membrane is a fluid structure with a “mosaic” of various proteins embedded in it“mosaic” of various proteins embedded in it


Hydrophilichead

WATER

Hydrophobictail

WATER

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

(a) Movement of phospholipids

Lateral movement(107 times per second)

Flip-flop( once per month)

• Phospholipids in the plasma membrane can move within the bilayer

As temperatures cool, membranes switch As temperatures cool, membranes switch from a fluid state to a solid statefrom a fluid state to a solid state

The temperature at which a membrane The temperature at which a membrane solidifies depends on the types of lipidssolidifies depends on the types of lipids

Membranes must be fluid to work properly; Membranes must be fluid to work properly; they are usually about as fluid as salad oilthey are usually about as fluid as salad oil


(b) Membrane fluidity

Fluid

Unsaturated hydrocarbontails with kinks

Viscous

Saturated hydro-carbon tails

• Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids

At warm temperatures cholesterol restrains At warm temperatures cholesterol restrains movement of phospholipidsmovement of phospholipids

At cool temperatures, it maintains fluidity by At cool temperatures, it maintains fluidity by preventing tight packingpreventing tight packing


Cholesterol affects in membraneCholesterol affects in membrane

Cholesterol

Cholesterol within the animal cell membrane

Membrane Proteins and Their Membrane Proteins and Their FunctionsFunctions

A membrane is a collage of different A membrane is a collage of different proteins embedded in the fluid matrix of the proteins embedded in the fluid matrix of the lipid bilayerlipid bilayer

Proteins determine most of the membrane’s Proteins determine most of the membrane’s specific functionsspecific functions


Animation: Animation: Plasma Membrane

Fig. 7-7Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

Peripheral proteins Peripheral proteins are bound to the are bound to the surface of the membranesurface of the membrane

Integral proteins Integral proteins penetrate the hydrophobic penetrate the hydrophobic core (often coiled into alpha helicescore (often coiled into alpha helices


Six major functions of membrane proteins:Six major functions of membrane proteins: TransportTransport Enzymatic activityEnzymatic activity Signal transductionSignal transduction Cell-cell recognitionCell-cell recognition Intercellular joiningIntercellular joining Attachment to the cytoskeleton and extracellular Attachment to the cytoskeleton and extracellular

matrix (ECM)matrix (ECM)


(a) Transport

ATP

(b) Enzymatic activity

Enzymes

(c) Signal transduction

Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco-protein

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

Carbohydrates in Cell-Cell RecognitionCarbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to surface Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma molecules, often carbohydrates, on the plasma membranemembrane

Membrane carbohydrates may Membrane carbohydrates may be covalently bonded to lipids be covalently bonded to lipids (forming (forming glycolipidsglycolipids) or more ) or more commonly to proteins (forming commonly to proteins (forming glycoproteinsglycoproteins))


Cell to cell recognition

Glyco-protein

The Permeability of the Lipid The Permeability of the Lipid BilayerBilayer

Hydrophobic (nonpolar) molecules, such as Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidlypass through the membrane rapidly

Polar molecules, such as sugars, do not cross the Polar molecules, such as sugars, do not cross the membrane easilymembrane easily


Transport ProteinsTransport Proteins

Transport proteins Transport proteins allow passage of allow passage of hydrophilic substances hydrophilic substances across the membraneacross the membrane

Channel proteinsChannel proteins, have a , have a hydrophilic channel that certain hydrophilic channel that certain molecules or ions can use as a molecules or ions can use as a tunneltunnel

Aquaporins Aquaporins (channel protein)(channel protein) facilitate the passage of waterfacilitate the passage of water

Carrier proteinsCarrier proteins, bind to molecules , bind to molecules and change shape to shuttle them and change shape to shuttle them across the membraneacross the membrane


Passive transportPassive transport

Diffusion Diffusion is the tendency for molecules to is the tendency for molecules to spread out evenly into the available spacespread out evenly into the available space

Animation: Animation: Diffusion


Molecules of dye Membrane (cross section)

Net diffusionNet diffusion Equilibrium

WATER

Substances diffuse down their Substances diffuse down their concentration concentration gradientgradient, the difference in concentration of a , the difference in concentration of a substance from one area to anothersubstance from one area to another

No work must be done to move substances No work must be done to move substances down the concentration gradientdown the concentration gradient

The diffusion of a substance across a The diffusion of a substance across a biological membrane is biological membrane is passive transport passive transport because it requires no energy from the cell to because it requires no energy from the cell to make it happenmake it happen


Effects of Osmosis on Water BalanceEffects of Osmosis on Water Balance

Osmosis Osmosis is the diffusion of water across a is the diffusion of water across a selectively permeable membraneselectively permeable membrane

Water diffuses across a membrane from the Water diffuses across a membrane from the region of lower solute concentration to the region region of lower solute concentration to the region of higher solute concentrationof higher solute concentration


Lowerconcentrationof solute (sugar)

Fig. 7-12Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Water Balance of Cells Without WallsWater Balance of Cells Without Walls

Tonicity Tonicity is the ability of a solution to cause a cell is the ability of a solution to cause a cell to gain or lose waterto gain or lose water

Isotonic Isotonic solution: Solute concentration is the solution: Solute concentration is the same as that inside the cell (no movement)same as that inside the cell (no movement)

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

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


Fig. 7-13Fig. 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

Water Balance of Cells with WallsWater Balance of Cells with WallsA plant cell in a hypotonic solution swells until the wall A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now opposes uptake; the cell is now turgidturgid (firm) (firm)

If a plant cell and its surroundings are isotonic, there is If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell no net movement of water into the cell; the cell becomes becomes flaccid flaccid (limp), and the plant may wilt(limp), and the plant may wilt

In a hypertonic environment, plant cells lose water; In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a eventually, the membrane pulls away from the wall, a usually lethal effect called usually lethal effect called plasmolysisplasmolysis


Facilitated Diffusion: Passive Transport Facilitated Diffusion: Passive Transport Aided by Proteins (no energy)Aided by Proteins (no energy)

In In facilitated diffusionfacilitated diffusion, transport proteins , transport proteins speed the passive movement of molecules speed the passive movement of molecules across the plasma membrane (down across the plasma membrane (down gradient)gradient)

Channel proteins includeChannel proteins include Aquaporins, for facilitated diffusion of waterAquaporins, for facilitated diffusion of water Ion channels Ion channels that open or close in response to that open or close in response to

a stimulus (a stimulus (gated channelsgated channels))


Animation: Animation: Facilitated diffusion

The Need for Energy in Active The Need for Energy in Active TransportTransport

Active transport Active transport moves substances moves substances against their concentration gradientagainst their concentration gradient

Active transport requires energy, usually in Active transport requires energy, usually in the form of ATPthe form of ATP

Active transport is performed by Active transport is performed by specificspecific proteins embedded in the membranesproteins embedded in the membranes

Animation: Animation: Active Transport


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. 7-16-7Fig. 7-16-7

Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

Bulk transport occurs by exocytosis Bulk transport occurs by exocytosis and endocytosisand endocytosis

Small molecules and water enter or leave Small molecules and water enter or leave the cell through the lipid bilayer or by the cell through the lipid bilayer or by transport proteinstransport proteins

Large molecules, such as polysaccharides Large molecules, such as polysaccharides and proteins, cross the membrane in bulk and proteins, cross the membrane in bulk via vesiclesvia vesicles

Bulk transport requires energyBulk transport requires energy


ExocytosisExocytosis

In In exocytosisexocytosis, transport vesicles migrate to , transport vesicles migrate to the membrane, fuse with it, and release their the membrane, fuse with it, and release their contentscontents


EndocytosisEndocytosis

In In endocytosisendocytosis, the cell takes in , the cell takes in macromolecules by forming vesicles from the macromolecules by forming vesicles from the plasma membraneplasma membrane

There are three types of endocytosis:There are three types of endocytosis: PhagocytosisPhagocytosis (“cellular eating”) (“cellular eating”) Pinocytosis Pinocytosis (“cellular drinking”)(“cellular drinking”) Receptor-mediated endocytosisReceptor-mediated endocytosis: binding of : binding of

ligands to receptors triggers vesicle formationligands to receptors triggers vesicle formation


Animation: Animation: Phagocytosis

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULARFLUID

Pseudopodium

“Food” orother particle Food

vacuole Vesicle

PINOCYTOSIS

Plasmamembrane

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coatedpit

Coat protein

Coatedvesicle

Receptor

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Chapter 7: Membrane Structure and Function. Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundingsThe