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Membrane Structure and Function

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Plasma MembranePlasma Membrane

•The plasma membrane is the boundaryboundary that separates the living cell from its nonliving surroundings

•Selectively PermeableSelectively Permeable

•Fluid mosaicFluid mosaic of lipids and proteins

•Phospholipid bilayerbilayer

•Contains embeddedContains embedded proteins proteins

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PhospholipidsPhospholipids

•Phospholipids are amphipathicamphipathic, containing both hydrophilic (head) and hydrophobic regions (tails)

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Hydrophilichead

HydrophobicHydrophobic tail tail

WATER

WATER

Phospholipid Bilayer

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Fluid Mosaic ModelFluid Mosaic Model

•A membrane is a fluid fluid structurestructure with a “mosaic” of various proteins embedded in it when viewed from the top

•PhospholipidsPhospholipids can move laterallylaterally a small amount and can “flex” their tails

•Membrane proteinsMembrane proteins also move side to side or laterallylaterally making the membrane fluid

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The Fluidity of Membranes

Phospholipids in the plasma membrane Can move within the bilayer two ways

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

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The type of hydrocarbon tailstype of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-Carbon tails

The Fluidity of Membranes

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The Fluidity of Membranes

The steroid cholesterolsteroid cholesterol Has different effects on membrane fluidity at different temperatures

Figure 7.5

Cholesterol

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Membrane Proteins and Their Functions

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

Fibers of

extracellularmatrix (ECM)

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Types of Membrane Proteins

Integral proteinsIntegral proteinsPenetrate the hydrophobic core of

the lipid bilayerAre often transmembrane proteinstransmembrane proteins,

completely spanning the membrane

EXTRACELLULARSIDE

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Types of Membrane Proteins

Peripheral proteinsPeripheral proteinsAre appendages loosely bound

to the surface of the membrane

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Six Major Functions of Membrane Proteins

Figure 7.9

Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.

Enzymatic activity. A protein built into the membranemay be an enzyme with its active site exposed tosubstances in the adjacent solution. In some cases,several enzymes in a membrane are organized asa team that carries out sequential steps of ametabolic pathway.

Signal transduction. A membrane protein may havea binding site with a specific shape that fits the shapeof a chemical messenger, such as a hormone. Theexternal messenger (signal) may cause aconformational change in the protein (receptor) thatrelays the message to the inside of the cell.

(a)

(b)

(c)

ATP

Enzymes

Signal

Receptor

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Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.

Intercellular joining. Membrane proteins of adjacent cellsmay hook together in various kinds of junctions, such asgap junctions or tight junctions

Attachment to the cytoskeleton and extracellular matrix(ECM). Microfilaments or other elements of thecytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes

(d)

(e)

(f)

Glyco-protein

Six Major Functions of Membrane Proteins

Types of intercellular junctions in animals

Tight junctions prevent fluid from moving across a layer of cells

Tight junction

0.5 µm

1 µm

Spacebetweencells

Plasma membranesof adjacent cells

Extracellularmatrix

Gap junction

Tight junctions

0.1 µm

Intermediatefilaments

Desmosome

Gapjunctions

At tight junctions, the membranes ofneighboring cells are very tightly pressedagainst each other, bound together byspecific proteins (purple). Forming continu-ous seals around the cells, tight junctionsprevent leakage of extracellular fluid acrossA layer of epithelial cells.

Desmosomes (also called anchoringjunctions) function like rivets, fastening cellsTogether into strong sheets. IntermediateFilaments made of sturdy keratin proteinsAnchor desmosomes in the cytoplasm.

Gap junctions (also called communicatingjunctions) provide cytoplasmic channels fromone cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars,amino acids, and other small molecules maypass. Gap junctions are necessary for commu-nication between cells in many types of tissues,including heart muscle and animal embryos.

TIGHT JUNCTIONS

DESMOSOMES

GAP JUNCTIONS

Figure 6.31

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The Role of Membrane Carbohydrates in Cell-Cell

RecognitionCell-cell recognitionCell-cell recognition IIs a cell’s ability to distinguish

one type of neighboring cell from another

Membrane carbohydratesMembrane carbohydratesInteract with the surface

molecules of other cells, facilitating cell-cell recognition

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Synthesis and Sidedness of Membranes

Membranes have distinct distinct inside and outside facesinside and outside faces

This affects the movementaffects the movement of proteins synthesizedproteins synthesized in the endomembrane system endomembrane system (Golgi and ER)(Golgi and ER)

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Synthesis and Sidedness of Membranes

Membrane proteins and lipidsMembrane proteins and lipids are made in the ER ER andand Golgi Golgi apparatusapparatus

ER

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Membrane Permeability

Membrane structurestructure results in selective permeabilityselective permeability

A cell must exchange cell must exchange materials with its materials with its surroundingssurroundings, a process controlled by the plasma membrane

What needs to cross the What needs to cross the membrane?membrane?

Oxygen

Carbon Dioxide

Sucrose

Bacteria

Waste

Sodium+

Potassium+

Glucose

What can diffuse across the What can diffuse across the membrane?membrane?

Oxygen

Carbon Dioxide

Sucrose

Bacteria

Waste

Sodium+

Potassium+

Glucose

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Permeability of the Lipid Bilayer

Hydrophobic moleculesHydrophobic moleculesAre lipid solublelipid soluble and can

pass through the membrane rapidlyrapidly

Polar moleculesPolar moleculesDo NOT cross the membrane

rapidlyrapidly

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Transport Proteins

Transport proteinsTransport proteinsAllow passage of hydrophilic hydrophilic

substancessubstances across the membrane

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Passive Transport

Passive transportPassive transport is diffusion of a substance across a membrane with no energyno energy investment

COCO22, H, H22O, and OO, and O22 easily diffuse across plasma membranes

Diffusion of water is known as OsmosisOsmosis

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Simple DiffusionDiffusionDiffusion

Is the tendency for molecules of any substance to spread out evenlyspread out evenly into the available space

Move from high to low concentrationhigh to low concentration

DownDown the concentration gradient

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Effects of Osmosis on Water Balance

OsmosisOsmosisIs the movement of water

across a semipermeable membrane

Is affected affected by theby the concentration gradient of concentration gradient of dissolved substances dissolved substances called thecalled the solution’ssolution’s tonicitytonicity

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Water Balance of Cells Without WallsTonicity

Is the ability of a solution to cause a cell to gain or lose water

Has a great impact on cells impact on cells without wallswithout walls

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Three States of Tonicity

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Isotonic Solutions

If a solution is isotonicisotonicThe concentration of solutesconcentration of solutes is

the samesame as it is inside the cellThere will be NO NETNO NET movement

of WATER

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Hypertonic Solution

If a solution is hypertonichypertonicThe concentration of solutesconcentration of solutes is

greatergreater than it is inside the cellThe cell will lose water lose water

(PLASMOLYSIS)(PLASMOLYSIS)

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Hypotonic Solutions

If a solution is hypotonichypotonicThe concentration of solutesconcentration of solutes is

lesless than it is inside the cellThe cell will gain watergain water

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Water Balance in Cells Without Walls

Animal cell. An animal cell fares best in an isotonic isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

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Water Balance of Cells with Walls

Cell WallsCell WallsHelp maintain water balance

Turgor pressureTurgor pressureIs the pressure of water inside a plant

cell pushing outward against the cell membrane

If a plant cell is turgidturgidIt is in a hypotonichypotonic environmentIt is very firm, firm, a healthy state in most a healthy state in most

plantsplants

If a plant cell is flaccidflaccidIt is in an isotonic or hypertonicisotonic or hypertonic

environment

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Facilitated Diffusion

Facilitated diffusionFacilitated diffusionIs a type of PassivePassive Transport Aided by

ProteinsProteins

In facilitated diffusion

Transport proteinsTransport proteins speed the movement of molecules across the plasma membrane

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Facilitated Diffusion & Proteins

Channel proteinsChannel proteinsProvide corridors that allow a specific

molecule or ion to cross the membrane

EXTRACELLULARFLUID

Channel proteinSolute

CYTOPLASM

A channel protein (purple) has a channel through which water molecules or a specific solute can pass.

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Facilitated Diffusion & Proteins

Carrier proteinsCarrier proteinsUndergo a subtle change in shape

that translocates the solute-binding site across the membrane

A carrier proteincarrier protein alternates between two conformationsalternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either directioncan transport the solute in either direction, with the net movement being down the concentration gradientdown the concentration gradient of the solute.

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Active Transport

Active transportUses energyUses energy to move solutes againstagainst their concentration gradients

Requires energy, usually in the form of ATPATP

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The sodium-potassium pumpsodium-potassium pumpIs one type of active transport system

Active Transport

PP i

EXTRACELLULARFLUID

Na+ binding stimulatesphosphorylation by ATP.

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Na+

Cytoplasmic Na+ binds tothe sodium-potassium pump.1

K+ is released and Na+

sites are receptive again; the cycle repeats.

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Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside.

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Extracellular K+ binds to the protein, triggering release of the Phosphate group.

6 Loss of the phosphaterestores the protein’s original conformation.

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CYTOPLASM

[Na+] low[K+] high

Na+

Na+

Na+

Na+

Na+

P ATP

Na+

Na+

Na+

P

ADP

K+

K+

K+

K+ K+

K+

[Na+] high[K+] low

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Comparison of Passive & Active Transport

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

Diffusion. Hydrophobicmolecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,either channel or carrier proteins.

ATP

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Maintenance of Membrane Potential by Ion Pumps

Membrane potentialMembrane potentialIs the voltage difference across a membrane

An electrochemical gradientelectrochemical gradientIs caused by the concentration electrical gradient of ions

across a membrane

An electrogenic pumpelectrogenic pumpIs a transport protein that generates the voltage across a

membrane

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Proton Pump

EXTRACELLULARFLUID

+

H+

H+

H+

H+

H+

H+Proton pump

ATP

CYTOPLASM

+

+

+

+

–

–

–

–

–

+

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Cotransport

CotransportCotransportOccurs when active transportactive transport of

a specific solute indirectly drives the active transport of another solute

Involves transport by a membrane protein membrane protein

Driven by a concentration concentration gradientgradient

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Example of CotransportCotransport: active transport driven driven

by a concentration gradientby a concentration gradient

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Bulk Transport

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

Large proteinsCross the membrane by

different mechanisms

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Exocytosis & Endocytosis

In exocytosisexocytosis

Transport vesiclesTransport vesicles migrate to the plasma membrane, fuse with it, and release their contents

In endocytosisendocytosisThe cell takes in macromolecules

by forming new vesicles from forming new vesicles from the plasma membranethe plasma membrane

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Endocytosis

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Exocytosis

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In phagocytosisphagocytosis, a cellengulfs a particle by engulfs a particle by Wrapping pseudopodiaWrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

Three Types of Endocytosis

PHAGOCYTOSIS

In pinocytosispinocytosis, the cell ““gulps” droplets of gulps” droplets of extracellular fluidextracellular fluid into tinyvesicles. It is not the fluiditself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports.

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0.25 µm

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coatedpit

Coatedvesicle

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

Plasmamembrane

Coatprotein

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.