The Phospholipid Bilayer and Transport

8/8/2019 The Phospholipid Bilayer and Transport

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The Phospholipid Bilayer The Phospholipid Bilayer (aka the Plasma or Cell(aka the Plasma or Cell

Membrane)Membrane)

Biology IBiology I



Why do cells need water again?Why do cells need water again?

Water is essential to lif eWater is essential to lif e

Lif e evolved in water some 3 billion years agoLif e evolved in water some 3 billion years agoand in a real sense, the cells of organismsand in a real sense, the cells of organisms

have never lef t the watery oceanhave never lef t the watery ocean B/C water can dissolve and carry substances such asB/C water can dissolve and carry substances such as

salts, ions, sugars, proteins, hormones, etcsalts, ions, sugars, proteins, hormones, etc

Essential for chemical reactionsEssential for chemical reactions

Recall properties of water unitRecall properties of water unit

Take Home: without moisture, a cell diesTake Home: without moisture, a cell dies



Why do cells need a membrane?Why do cells need a membrane?

(i.e. what is the purpose of a PM)(i.e. what is the purpose of a PM) Cells need something to keep the inside living ³watery´ partCells need something to keep the inside living ³watery´ part

separated from the ³outside´ world (which may or may notseparated from the ³outside´ world (which may or may notbe water)be water)²²BUT ITS MORE THAN JUST A W ALL!BUT ITS MORE THAN JUST A W ALL!

Cells need something to regulate materials coming in (food,Cells need something to regulate materials coming in (food,OO22, etc) and going out (wastes etc), etc) and going out (wastes etc)

Think of the plasma membrane as the organelle that ³regulates´ theThink of the plasma membrane as the organelle that ³regulates´ thetraffic of chemicals and other materials into and out of the celltraffic of chemicals and other materials into and out of the cell

For example, we need 100X more potassium ions inside cells thanFor example, we need 100X more potassium ions inside cells thanoutside cells for everything to operate smoothlyoutside cells for everything to operate smoothly

Membranes therefore regulate the transport of Membranes therefore regulate the transport of

substances across the boundary, allowing only certainsubstances across the boundary, allowing only certainsubstances to pass.substances to pass. In this way, a membrane maintains a specific chemicalIn this way, a membrane maintains a specific chemical

environment within each compartment it encloses.environment within each compartment it encloses.

Cells need to maintain a constant internal environment despite aCells need to maintain a constant internal environment despite achanging external environment (MAINTAINING ___________)changing external environment (MAINTAINING ___________)



Structure of Plasma MembraneStructure of Plasma Membrane

Composed of Composed of phospholipid moleculesphospholipid molecules f atty acid chains=are nonf atty acid chains=are non--polar ³tails´ polar ³tails´

Like other lipids, these are hydrophobicLike other lipids, these are hydrophobic

Are tucked inside the ³heads´ shielded f rom waterAre tucked inside the ³heads´ shielded f rom water

Phosphate portion= polar (charged) ³head´ Phosphate portion= polar (charged) ³head´ HydrophillicHydrophillic

Face watery inside or outside Face watery inside or outside

Forms a two layer ³sandwich´ of molecules that Forms a two layer ³sandwich´ of molecules that surrounds the cellsurrounds the cell

Composed of proteins that are embedded inComposed of proteins that are embedded inthe phospholipidsthe phospholipids Enzymes are embedded to carry out key reactionsEnzymes are embedded to carry out key reactions

Help cells communicate and recognize each otherHelp cells communicate and recognize each other

Transport Proteins help move certain substances that Transport Proteins help move certain substances that

can¶t easily pass through the membrane on their owncan¶t easily pass through the membrane on their own



Hydrophilic Polar Heads

Hydrophobic Non Polar Tails

Another Animation here«



See Online Activity 6.2See Online Activity 6.2

Pour phopholipids in water«Pour phopholipids in water«



How is the PM Structure Related toHow is the PM Structure Related to

What Can and Cannot Enter The Cell?What Can and Cannot Enter The Cell?

Only small, nonpolar and unchargedOnly small, nonpolar and unchargedsubstances easily cross the hydrophobicsubstances easily cross the hydrophobicregion of the phospholipid bilayer region of the phospholipid bilayer For example: small fats, OFor example: small fats, O22, CO, CO22

Really large things can¶t move through easily atReally large things can¶t move through easily atall (they need help!)all (they need help!)

Polar and charged items DO NOT easily passPolar and charged items DO NOT easily passthrough the membranethrough the membrane

Glucose, alcohol, water, ions (KGlucose, alcohol, water, ions (K++

, Na, Na++

, Cl, Cl--

), proteins), proteins Often these will go through the cell membrane throughOften these will go through the cell membrane through

protein channels that are themselves polar protein channels that are themselves polar (NH(NH22++, COO, COO--))

W ATER IS A NOTABLE EXCEPTION!! EVENW ATER IS A NOTABLE EXCEPTION!! EVENTHOUGH IT IS POLAR, IT IS SMALL ENOUGH TOTHOUGH IT IS POLAR, IT IS SMALL ENOUGH TOPASS THROUGH THE PLASMA MEMBRANE or itPASS THROUGH THE PLASMA MEMBRANE or it

can go through protein channelscan go through protein channels



Movement of Materials intoMovement of Materials into

and out of Cellsand out of Cells

See Online Activity 6.3See Online Activity 6.3



Movement of Materials into and out of Movement of Materials into and out of

CellsCells

Passive TransportPassive Transport No energy expendedNo energy expended

by cellby cell

Fueled by randomFueled by random

movement of movement of moleculesmolecules

Substances moveSubstances movefrom high to lowfrom high to lowconcentrationsconcentrations

Examples includeExamples include DiffusionDiffusion

OsmosisOsmosis

Facilitated diffusionFacilitated diffusion

Active TransportActive Transport Energy IS expendedEnergy IS expended

by the cellby the cell

Usually for charged,Usually for charged,

polar, or largepolar, or largemoleculesmolecules

Transport proteinsTransport proteinsare often involvedare often involved

Moves from low toMoves from low tohigh concentrationhigh concentration Movement ³uphill´ Movement ³uphill´²²

against concentration against concentration gradientgradient

Think of it as a ³pump´ Think of it as a ³pump´



DiffusionDiffusion

Tendency of a substance to move f rom a Tendency of a substance to move f rom a region of high concentration to a region of region of high concentration to a region of low concentrationlow concentration

i.e. travel ³down´ the conc. gradienti.e. travel ³down´ the conc. gradient EX: EX: Released perfume f rom a bottleReleased perfume f rom a bottle

EX: A drop of dye added to waterEX: A drop of dye added to water

The movement of molecules is impacted The movement of molecules is impacted by temperatureby temperature The higher the temperature, the greater the The higher the temperature, the greater the

rate of diffusion b/c molecules are movingrate of diffusion b/c molecules are movingf aster«f aster«



Diffusion of Dye MoleculesDiffusion of Dye Molecules



#1#1OsmosisOsmosis (here)(here) Osmosis is the ³diffusion´ of waterOsmosis is the ³diffusion´ of water

Goes on continuously during lif e of a cell and plays a vitalGoes on continuously during lif e of a cell and plays a vitalrole in the health and functioning of cellsrole in the health and functioning of cells

Rarely are the liquids of cells pure water: theyRarely are the liquids of cells pure water: theyhave lots of salts and other polar substanceshave lots of salts and other polar substances

(solutes) dissolved in them(solutes) dissolved in them The goal of #5The goal of #5osmosisosmosis is to produce equalis to produce equal

solute concentrationssolute concentrations Water moves from low solute concentration to highWater moves from low solute concentration to high

solute concentrationsolute concentration

R EMEMBER: SALT SUCKS!!!R EMEMBER: SALT SUCKS!!!

See animations #4See animations #4herehere and #2and #2herehere

#3#3See animation about hypotonic, hypertonic, and See animation about hypotonic, hypertonic, and isotonicisotonic



OsmosisOsmosis



Passive Transport means«Passive Transport means«



Active Transport« Active Transport«



Active Transport Cont Active Transport Cont..



Animations for Transport of Animations for Transport of

MaterialsMaterials Review OLA 6.3Review OLA 6.3

Membrane transportMembrane transport

Diffusion /OsmosisDiffusion /Osmosis OsmosisOsmosis

Exo and EndodcytosisExo and Endodcytosis

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The Phospholipid Bilayer and Transport